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  • 1. Agnini, Claudia
    et al.
    Spofforth, David J. A.
    Dickens, Gerald R.
    Rio, Domenico
    Pälike, Heiko
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Backman, Jan
    Muttoni, Giovanni
    Dallanave, Edoardo
    Stable isotope and calcareous nannofossil assemblage record of the late Paleocene and early Eocene (Cicogna section)2016In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 12, no 4, p. 883-909Article in journal (Refereed)
    Abstract [en]

    We present records of stable carbon and oxygen isotopes, CaCO3 content, and changes in calcareous nannofossil assemblages across an 81 m thick section of upper Paleocene lower Eocene marine sedimentary rocks now exposed along the Cicogna Stream in northeast Italy. The studied stratigraphic section represents sediment accumulation in a bathyal hemipelagic setting from approximately 57.5 to 52.2 Ma, a multi-million-year time interval characterized by perturbations in the global carbon cycle and changes in calcareous nannofossil assemblages. The bulk carbonate delta C-13 profile for the Cicogna section, once placed on a common timescale, resembles that at several other locations across the world, and includes both a long-term drop in delta C-13 and multiple short-term carbon isotope excursions (CIEs). This precise correlation of widely separated delta C-13 records in marine sequences results from temporal changes in the carbon composition of the exogenic carbon cycle. However, diagenesis has likely modified the delta C-13 record at Cicogna, an interpretation supported by variations in bulk carbonate 8180, which do not conform to expectations for a primary signal. The record of CaCO3 content reflects a combination of carbonate dilution and dissolution, as also inferred at other sites. Our detailed documentation and statistical analysis of calcareous nannofossil assemblages show major differences before, during and after the Paleocene Eocene Thermal Maximum. Other CIEs in our lower Paleogene section do not exhibit such a distinctive change; instead, these events are sometimes characterized by variations restricted to a limited number of taxa and transient shifts in the relative abundance of primary assemblage components. Both long-lasting and short-lived modifications to calcareous nannofossil assemblages preferentially affected nannoliths or holococcoliths such as Discoaster,, Fasciculithus, Rhomboaster/Tribrachiatus, Sphenolithus and Zygrhablithus, which underwent distinct variations in abundance as well as permanent evolutionary changes in terms of appearances and disappearances. By contrast, placoliths such as Coccolithus and Tow eius, which represent the main component of the assemblages, were characterized by a gradual decline in abundance over time. Comparisons of detailed nannofossil assemblage records at the Cicogna section and at ODP Site 1262 support the idea that variations in the relative and absolute abundances, even some minor changes, were globally synchronous. An obvious link is through climate forcing and carbon cycling, although the linkages between variations in calcareous nannoplankton, changes in delta C-13 records and oceanography will need additional work.

  • 2.
    Armstrong McKay, David I.
    et al.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre. University of Southampton, UK.
    Lenton, Timothy M.
    Reduced carbon cycle resilience across the Palaeocene-Eocene Thermal Maximum2018In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 14, no 10, p. 1515-1527Article in journal (Refereed)
    Abstract [en]

    Several past episodes of rapid carbon cycle and climate change are hypothesised to be the result of the Earth system reaching a tipping point beyond which an abrupt transition to a new state occurs. At the Palaeocene-Eocene Thermal Maximum (PETM) at similar to 56 Ma and at subsequent hyperthermal events, hypothesised tipping points involve the abrupt transfer of carbon from surface reservoirs to the atmosphere. Theory suggests that tipping points in complex dynamical systems should be preceded by critical slowing down of their dynamics, including increasing temporal auto-correlation and variability. However, reliably detecting these indicators in palaeorecords is challenging, with issues of data quality, false positives, and parameter selection potentially affecting reliability. Here we show that in a sufficiently long, high-resolution palaeorecord there is consistent evidence of destabilisation of the carbon cycle in the similar to 1.5 Myr prior to the PETM, elevated carbon cycle and climate instability following both the PETM and Eocene Thermal Maximum 2 (ETM2), and different drivers of carbon cycle dynamics preceding the PETM and ETM2 events. Our results indicate a loss of resilience (weakened stabilising negative feedbacks and greater sensitivity to small shocks) in the carbon cycle before the PETM and in the carbon-climate system following it. This pre-PETM carbon cycle destabilisation may reflect gradual forcing by the contemporaneous North Atlantic Volcanic Province eruptions, with volcanism-driven warming potentially weakening the organic carbon burial feedback. Our results are consistent with but cannot prove the existence of a tipping point for abrupt carbon release, e.g. from methane hydrate or terrestrial organic carbon reservoirs, where as we find no support for a tipping point in deep ocean temperature.

  • 3.
    Ballarotta, Maxime
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Laurent, Brodeau
    Stockholm University, Faculty of Science, Department of Meteorology .
    Jenny, Brandefelt
    Lundberg, Peter
    Stockholm University, Faculty of Science, Department of Meteorology .
    Döös, Kristofer
    Stockholm University, Faculty of Science, Department of Meteorology .
    Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions: do eddies contribute to a better consistency between models and palaeoproxies?2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, no 6, p. 2669-2686Article in journal (Refereed)
    Abstract [en]

    Most state-of-the-art climate models include a coarsely resolved oceanic component, which hardly captures detailed dynamics, whereas eddy-permitting and eddy-resolving simulations are developed to reproduce the observed ocean. In this study, an eddy-permitting and a coarse resolution numerical experiment are conducted to simulate the global ocean state for the period of the Last Glacial Maximum (LGM, ~26 500 to 19 000 yr ago) and to investigate the improvements due to taking into account the smaller spatial scales. The ocean state from each simulation is confronted with a data set from the Multiproxy Approach for the Reconstruction of the Glacial Ocean (MARGO) sea surface temperatures (SSTs), some reconstructions of the palaeo-circulations and a number of sea-ice reconstructions. The western boundary currents and the Southern Ocean dynamics are better resolved in the high-resolution experiment than in the coarse simulation, but, although these more detailed SST structures yield a locally improved consistency between model predictions and proxies, they do not contribute sig- nificantly to the global statistical score. The SSTs in the tropical coastal upwelling zones are also not significantly improved by the eddy-permitting regime. The models perform in the mid-latitudes but as in the majority of the Paleo- climate Modelling Intercomparison Project simulations, the modelled sea-ice conditions are inconsistent with the palaeo-reconstructions. The effects of observation locations on the comparison between observed and simulated SST suggest that more sediment cores may be required to draw reliable conclusions about the improvements introduced by the high resolution model for reproducing the global SSTs. One has to be careful with the interpretation of the deep ocean state which has not reached statistical equilibrium in our simula-tions. However, the results indicate that the meridional overturning circulations are different between the two regimes, suggesting that the model parametrizations might also play a key role for simulating past climate states. 

  • 4. Belt, S. T.
    et al.
    Brown, T. A.
    Ampel, Linda
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Cabedo-Sanz, P.
    Fahl, K.
    Kocis, J. J.
    Masse, G.
    Navarro-Rodriguez, A.
    Ruan, J.
    Xu, Y.
    An inter-laboratory investigation of the Arctic sea ice biomarker proxy IP25 in marine sediments: key outcomes and recommendations2014In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 10, no 1, p. 155-166Article in journal (Refereed)
    Abstract [en]

    We describe the results of an inter-laboratory investigation into the identification and quantification of the Arctic sea ice biomarker proxy IP25 in marine sediments. Seven laboratories took part in the study, which consisted of the analysis of IP25 in a series of sediment samples from different regions of the Arctic, sub-Arctic and Antarctic, additional sediment extracts and purified standards. The results obtained allowed 4 key outcomes to be determined. First, IP25 was identified by all laboratories in sediments from the Canadian Arctic with inter-laboratory variation in IP25 concentration being substantially larger than within individual laboratories. This greater variation between laboratories was attributed to the difficulty in accurately determining instrumental response factors for IP25, even though laboratories were supplied with appropriate standards. Second, the identification of IP25 by 3 laboratories in sediment from SW Iceland that was believed to represent a blank, was interpreted as representing a better limit of detection or quantification for such laboratories, contamination or mis-identification. These alternatives could not be distinguished conclusively with the data available, although it is noted that the precision of these data was significantly poorer compared with the other IP25 concentration measurements. Third, 3 laboratories reported the occurrence of IP25 in a sediment sample from the Antarctic Peninsula even though this biomarker is believed to be absent from the Southern Ocean. This anomaly is attributed to a combined chromatographic and mass spectrometric interference that results from the presence of a di-unsaturated highly branched isoprenoid (HBI) pseudo-homologue of IP25 that occurs in Antarctic sediments. Finally, data are presented that suggest that extraction of IP25 is consistent between Accelerated Solvent Extraction (ASE) and sonication methods and that IP25 concentrations based on 7-hexylnonadecane as an internal standard are comparable using these methods. Recoveries of some more unsaturated HBIs and the internal standard 9-octylheptadecene, however, were lower with the ASE procedure, possibly due to partial degradation of these more reactive chemicals as a result of higher temperatures employed with this method. For future measurements, we recommend the use of reference sediment material with known concentration(s) of IP25 for determining and routinely monitoring instrumental response factors. Given the significance placed on the presence (or otherwise) of IP25 in marine sediments, some further recommendations pertaining to quality control are made that should also enable the two main anomalies identified here to be addressed.

  • 5.
    Berger, M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brandefelt, J.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    The sensitivity of the Arctic sea ice to orbitally induced insolation changes: a study of the mid-Holocene Paleoclimate Modelling Intercomparison Project 2 and 3 simulations2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, no 2, p. 969-982Article in journal (Refereed)
    Abstract [en]

    In the present work the Arctic sea ice in the mid-Holocene and the pre-industrial climates are analysed and compared on the basis of climate-model results from the Paleoclimate Modelling Intercomparison Project phase 2 (PMIP2) and phase 3 (PMIP3). The PMIP3 models generally simulate smaller and thinner sea-ice extents than the PMIP2 models both for the pre-industrial and the mid-Holocene climate. Further, the PMIP2 and PMIP3 models all simulate a smaller and thinner Arctic summer sea-ice cover in the mid-Holocene than in the pre-industrial control climate. The PMIP3 models also simulate thinner winter sea ice than the PMIP2 models. The winter sea-ice extent response, i.e. the difference between the mid-Holocene and the pre-industrial climate, varies among both PMIP2 and PMIP3 models. Approximately one half of the models simulate a decrease in winter sea-ice extent and one half simulates an increase. The model-mean summer sea-ice extent is 11% (21 %) smaller in the mid-Holocene than in the pre-industrial climate simulations in the PMIP2 (PMIP3). In accordance with the simple model of Thorndike (1992), the sea-ice thickness response to the insolation change from the pre-industrial to the mid-Holocene is stronger in models with thicker ice in the pre-industrial climate simulation. Further, the analyses show that climate models for which the Arctic sea-ice responses to increasing atmospheric CO2 concentrations are similar may simulate rather different sea-ice responses to the change in solar forcing between the mid-Holocene and the pre-industrial. For two specific models, which are analysed in detail, this difference is found to be associated with differences in the simulated cloud fractions in the summer Arctic; in the model with a larger cloud fraction the effect of insolation change is muted. A sub-set of the mid-Holocene simulations in the PMIP ensemble exhibit open water off the north-eastern coast of Greenland in summer, which can provide a fetch for surface waves. This is in broad agreement with recent analyses of sea-ice proxies, indicating that beach-ridges formed on the north-eastern coast of Greenland during the early-to mid-Holocene.

  • 6. Björklund, J. A.
    et al.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Seftigen, K.
    Esper, J.
    Linderholm, H. W.
    Blue intensity and density from northern Fennoscandian tree rings, exploring the potential to improve summer temperature reconstructions with earlywood information2014In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 10, no 2, p. 877-885Article in journal (Refereed)
    Abstract [en]

    Here we explore two new tree-ring parameters, derived from measurements of wood density and blue intensity (BI). The new proxies show an increase in the inter-annual summer temperature signal compared to established proxies, and present the potential to improve long-term performance. At high latitudes, where tree growth is mainly limited by low temperatures, radiodensitometric measurements of wood density, specifically maximum latewood density (MXD), provides a temperature proxy that is superior to that of tree-ring widths. The high cost of developing MXD has led to experimentation with a less expensive method using optical flatbed scanners to produce a new proxy, herein referred to as maximum latewood blue absorption intensity (abbreviated MXBI). MXBI is shown to be very similar to MXD on annual timescales but less accurate on centennial timescales. This is due to the fact that extractives, such as resin, stain the wood differentially from tree to tree and from heartwood to sapwood. To overcome this problem, and to address similar potential problems in radiodensitometric measurements, the new parameters Delta blue intensity (Delta BI) and Delta density are designed by subtracting the ambient BI/ density in the earlywood, as a background value, from the latewood measurements. As a case-study, based on Scots pine trees from Northern Sweden, we show that Delta density can be used as a quality control of MXD values and that the reconstructive performance of warm-season mean temperatures is more focused towards the summer months (JJA - June, July, August), with an increase by roughly 20% when also utilising the interannual information from the earlywood. How-ever, even though the new parameter Delta BI experiences an improvement as well, there are still puzzling dissimilarities between Delta density and Delta BI on multicentennial timescales. As a consequence, temperature reconstructions based on Delta BI will presently only be able to resolve information on decadal-to- centennial timescales. The possibility of trying to calibrate BI into a measure of lignin content or density, similarly to how radiographic measurements are calibrated into density, could be a solution. If this works, only then can Delta BI be used as a reliable proxy in multicentennial-scale climate reconstructions.

  • 7. Bothe, O.
    et al.
    Jungclaus, J. H.
    Zanchettin, D.
    Zorita, E.
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Climate of the last millennium: ensemble consistency of simulations and reconstructions2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, no 3, p. 1089-1110Article in journal (Refereed)
    Abstract [en]

    Are simulations and reconstructions of past climate and its variability consistent with each other? We assess the consistency of simulations and reconstructions for the climate of the last millennium under the paradigm of a statistically indistinguishable ensemble. In this type of analysis, the null hypothesis is that reconstructions and simulations are statistically indistinguishable and, therefore, are exchangeable with each other. Ensemble consistency is assessed for Northern Hemisphere mean temperature, Central European mean temperature and for global temperature fields. Reconstructions available for these regions serve as verification data for a set of simulations of the climate of the last millennium performed at the Max Planck Institute for Meteorology. Consistency is generally limited to some sub-domains and some sub-periods. Only the ensemble simulated and reconstructed annual Central European mean temperatures for the second half of the last millennium demonstrates unambiguous consistency. Furthermore, we cannot exclude consistency of an ensemble of reconstructions of Northern Hemisphere temperature with the simulation ensemble mean. If we treat simulations and reconstructions as equitable hypotheses about past climate variability, the found general lack of their consistency weakens our confidence in inferences about past climate evolutions on the considered spatial and temporal scales. That is, our available estimates of past climate evolutions are on an equal footing but, as shown here, inconsistent with each other.

  • 8. Bothe, Oliver
    et al.
    Hind, Alistair
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Mathematics.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Zorita, Eduardo
    Continental-scale temperature variability in PMIP3 simulations and PAGES 2k regional temperature reconstructions over the past millennium2015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, no 12, p. 1673-1699Article in journal (Refereed)
    Abstract [en]

    Estimated external radiative forcings, model results, and proxy-based climate reconstructions have been used over the past several decades to improve our understanding of the mechanisms underlying observed climate variability and change over the past millennium. Here, the recent set of temperature reconstructions at the continental-scale generated by the PAGES 2k project and a collection of state-of-the-art model simulations driven by realistic external forcings are jointly analysed. The first aim is to estimate the consistency between model results and reconstructions for each continental-scale region over the time and frequency domains. Secondly, the links between regions are investigated to determine whether reconstructed global-scale covariability patterns are similar to those identified in model simulations. The third aim is to assess the role of external forcings in the observed temperature variations. From a large set of analyses, we conclude that models are in relatively good agreement with temperature reconstructions for Northern Hemisphere regions, particularly in the Arctic. This is likely due to the relatively large amplitude of the externally forced response across northern and high-latitude regions, which results in a clearly detectable signature in both reconstructions and simulations. Conversely, models disagree strongly with the reconstructions in the Southern Hemisphere. Furthermore, the simulations are more regionally coherent than the reconstructions, perhaps due to an underestimation of the magnitude of internal variability in models or to an overestimation of the response to the external forcing in the Southern Hemisphere. Part of the disagreement might also reflect large uncertainties in the reconstructions, specifically in some Southern Hemisphere regions, which are based on fewer palaeoclimate records than in the Northern Hemisphere.

  • 9. Brandefelt, J.
    et al.
    Kjellstrom, E.
    Naslund, J. -O
    Strandberg, G.
    Voelker, A. H. L.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    A coupled climate model simulation of Marine Isotope Stage 3 stadial climate2011In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 7, no 2, p. 649-670Article in journal (Refereed)
    Abstract [en]

    We present a coupled global climate model (CGCM) simulation, integrated for 1500 yr to quasi-equilibrium, of a stadial (cold period) within Marine Isotope Stage 3 (MIS 3). The simulated Greenland stadial 12 (GS12; similar to 44 ka BP) annual global mean surface temperature (T(s)) is 5.5 degrees C lower than in the simulated recent past (RP) climate and 1.3 degrees C higher than in the simulated Last Glacial Maximum (LGM; 21 ka BP) climate. The simulated GS12 is evaluated against proxy data and previous modelling studies of MIS3 stadial climate. We show that the simulated MIS 3 climate, and hence conclusions drawn regarding the dynamics of this climate, is highly model-dependent. The main findings are: (i) Proxy sea surface temperatures (SSTs) are higher than simulated SSTs in the central North Atlantic, in contrast to earlier simulations of MIS 3 stadial climate in which proxy SSTs were found to be lower than simulated SST. (ii) The Atlantic Meridional Overturning Circulation (AMOC) slows down by 50% in the GS12 climate as compared to the RP climate. This slowdown is attained without freshwater forcing in the North Atlantic region, a method used in other studies to force an AMOC shutdown. (iii) El-Nino-Southern Oscillation (ENSO) teleconnections in mean sea level pressure (MSLP) are significantly modified by GS12 and LGM forcing and boundary conditions. (iv) Both the mean state and variability of the simulated GS12 is dependent on the equilibration. The annual global mean T(s) only changes by 0.10 degrees C from model years 500-599 to the last century of the simulation, indicating that the climate system may be close to equilibrium already after 500 yr of integration. However, significant regional differences between the last century of the simulation and model years 500-599 exist. Further, the difference between simulated and proxy SST is reduced from model years 500-599 to the last century of the simulation. The results of the ENSO variability analysis is also shown to depend on the equilibration.

  • 10. Brugnara, Y.
    et al.
    Auchmann, R.
    Brönnimann, S.
    Allan, R. J.
    Auer, I.
    Barriendos, M.
    Bergström, H.
    Bhend, J.
    Brázdil, R.
    Compo, G. P.
    Cornes, R. C.
    Dominguez-Castro, F.
    van Engelen, A. F. V.
    Filipiak, J.
    Holopainen, J.
    Jourdain, S.
    Kunz, M.
    Luterbacher, J.
    Maugeri, M.
    Mercalli, L.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Mock, C. J.
    Pichard, G.
    Řezníčková, L.
    van der Schrier, G.
    Slonosky, V.
    Ustrnul, Z.
    Valente, M. A.
    Wypych, A.
    Yin, X.
    A collection of sub-daily pressure and temperature observations for the early instrumental period with a focus on the "year without a summer" 18162015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, no 8, p. 1027-1047Article in journal (Refereed)
    Abstract [en]

    The eruption of Mount Tambora (Indonesia) in April 1815 is the largest documented volcanic eruption in history. It is associated with a large global cooling during the following year, felt particularly in parts of Europe and North America, where the year 1816 became known as the "year without a summer". This paper describes an effort made to collect surface meteorological observations from the early instrumental period, with a focus on the years of and immediately following the eruption (1815–1817). Although the collection aimed in particular at pressure observations, correspondent temperature observations were also recovered. Some of the series had already been described in the literature, but a large part of the data, recently digitised from original weather diaries and contemporary magazines and newspapers, is presented here for the first time. The collection puts together more than 50 sub-daily series from land observatories in Europe and North America and from ships in the tropics. The pressure observations have been corrected for temperature and gravity and reduced to mean sea level. Moreover, an additional statistical correction was applied to take into account common error sources in mercury barometers. To assess the reliability of the corrected data set, the variance in the pressure observations is compared with modern climatologies, and single observations are used for synoptic analyses of three case studies in Europe. All raw observations will be made available to the scientific community in the International Surface Pressure Databank.

  • 11.
    Carlson, Henrik
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Atmospheric circulation and hydroclimate impacts of alternative warming scenarios for the Eocene2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 8, p. 1037-1048Article in journal (Refereed)
    Abstract [en]

    Recent work in modelling the warm climates of the early Eocene shows that it is possible to obtain a reasonable global match between model surface temperature and proxy reconstructions, but only by using extremely high atmospheric CO2 concentrations or more modest CO2 levels complemented by a reduction in global cloud albedo. Understanding the mix of radiative forcing that gave rise to Eocene warmth has important implications for constraining Earth's climate sensitivity, but progress in this direction is hampered by the lack of direct proxy constraints on cloud properties. Here, we explore the potential for distinguishing among different radiative forcing scenarios via their impact on regional climate changes. We do this by comparing climate model simulations of two end-member scenarios: one in which the climate is warmed entirely by CO2 (which we refer to as the greenhouse gas (GHG) scenario) and another in which it is warmed entirely by reduced cloud albedo (which we refer to as the low CO2-thin clouds or LCTC scenario). The two simulations have an almost identical global-mean surface temperature and equator-to-pole temperature difference, but the LCTC scenario has similar to 11% greater global-mean precipitation than the GHG scenario. The LCTC scenario also has cooler midlatitude continents and warmer oceans than the GHG scenario and a tropical climate which is significantly more El Nino-like. Extremely high warm-season temperatures in the subtropics are mitigated in the LCTC scenario, while cool-season temperatures are lower at all latitudes. These changes appear large enough to motivate further, more detailed study using other climate models and a more realistic set of modelling assumptions.

  • 12.
    Charpentier Ljungqvist, Fredrik
    et al.
    Stockholm University, Faculty of Humanities, Department of History.
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Brattström, Gudrun
    Stockholm University, Faculty of Science, Department of Mathematics.
    Sundqvist, Hanna S.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Northern Hemisphere temperature patterns in the last 12 centuries2012In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 8, p. 227-249Article in journal (Refereed)
    Abstract [en]

    We analyse the spatio-temporal patterns of temperature variability over Northern Hemisphere land areas, on centennial time-scales, for the last 12 centuries using an unprecedentedly large network of temperature-sensitive proxy records. Geographically widespread positive temperature anomalies are observed from the 9th to 11th centuries, similar in extent and magnitude to the 20th century mean. A dominance of widespread negative anomalies is observed from the 16th to 18th centuries. Though we find the amplitude and spatial extent of the 20th century warming is within the range of natural variability over the last 12 centuries, we also find that the rate of warming from the 19th to the 20th century is unprecedented in the context of the last 1200 yr. The positive Northern Hemisphere temperature change from the 19th to the 20th century is clearly the largest between any two consecutive centuries in the past 12 centuries. These results remain robust even after removing a significant number of proxies in various tests of robustness showing that the choice of proxies has no particular influence on the overall conclusions of this study.

  • 13.
    Christiansen, Bo
    et al.
    Danish Meteorological Institute.
    Charpentier Ljungqvist, Fredrik
    Stockholm University, Faculty of Humanities, Department of History.
    The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability2012In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 8, no 2, p. 765-786Article in journal (Refereed)
    Abstract [en]

    We present two new multi-proxy reconstructions of the extra-tropical Northern Hemisphere (30–90° N) mean temperature: a two-millennia long reconstruction reaching back to 1 AD and a 500-yr long reconstruction reaching back to 1500 AD.  The reconstructions are based on compilations of 32 and 91 proxies, respectively, of which only little more than half pass a screening procedure and are included in the actual reconstructions.  The proxies are of different types and of different resolutions (annual, annual-to-decadal, and decadal) but all have previously been shown to relate to local or regional temperature. We use a reconstruction method, LOCal (LOC), that recently has been shown to confidently reproduce low-frequency variability. Confidence intervals are obtained by an ensemble pseudo-proxy method that both estimates the variance and the bias of the reconstructions.  The two-millennia long reconstruction shows a well defined Medieval Warm Period, with a peak warming ca. 950–1050 AD reaching 0.6 °C relative to the reference period 1880–1960 AD. The 500-yr long reconstruction confirms previous results obtained with the LOC method applied to a smaller proxy compilation; in particular it shows the Little Ice Age cumulating in 1580–1720 AD with a temperature minimum of −1.0 °C below the reference period.  The reconstructed local temperatures, the magnitude of which are subject to wide confidence intervals, show a rather geographically homogeneous Little Ice Age, while more geographical inhomogeneities are found for the Medieval Warm Period. Reconstructions based on different subsets of proxies show only small differences, suggesting that LOC reconstructs 50-yr smoothed extra-tropical NH mean temperatures well and that low-frequency noise in the proxies is a relatively small problem.

  • 14. Cronin, Thomas M.
    et al.
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gemery, Laura
    Toomey, Michael
    Semiletov, Igor
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Deglacial sea level history of the East Siberian Sea and Chukchi Sea margins2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 9, p. 1097-1110Article in journal (Refereed)
    Abstract [en]

    Deglacial (12.8-10.7 ka) sea level history on the East Siberian continental shelf and upper continental slope was reconstructed using new geophysical records and sediment cores taken during Leg 2 of the 2014 SWERUS-C3 expedition. The focus of this study is two cores from Herald Canyon, piston core SWERUS-L2-4-PC1 (4-PC1) and multicore SWERUS-L2-4-MC1 (4-MC1), and a gravity core from an East Siberian Sea transect, SWERUS-L2-20-GC1 (20GC1). Cores 4-PC1 and 20-GC were taken at 120 and 115m of modern water depth, respectively, only a few meters above the global last glacial maximum (LGM; similar to 24 kiloannum or ka) minimum sea level of similar to 125-130 meters below sea level (m b.s.l.). Using calibrated radiocarbon ages mainly on molluscs for chronology and the ecology of benthic foraminifera and ostracode species to estimate paleodepths, the data reveal a dominance of river-proximal species during the early part of the Younger Dryas event (YD, Greenland Stadial GS-1) followed by a rise in river-intermediate species in the late Younger Dryas or the early Holocene (Preboreal) period. A rapid relative sea level rise beginning at roughly 11.4 to 10.8 ka (similar to 400 cm of core depth) is indicated by a sharp faunal change and unconformity or condensed zone of sedimentation. Regional sea level at this time was about 108m b.s.l. at the 4-PC1 site and 102m b.s.l. at 20-GC1. Regional sea level near the end of the YD was up to 42-47m lower than predicted by geophysical models corrected for glacio-isostatic adjustment. This discrepancy could be explained by delayed isostatic adjustment caused by a greater volume and/or geo-graphical extent of glacial-age land ice and/or ice shelves in the western Arctic Ocean and adjacent Siberian land areas.

  • 15. Davies, Frazer J.
    et al.
    Renssen, Hans
    Blaschek, Michael
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The impact of Sahara desertification on Arctic cooling during the Holocene2015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, p. 571-586Article in journal (Refereed)
    Abstract [en]

    Since the start of the Holocene, temperatures in the Arctic have steadily declined. This has been accredited to the orbitally forced decrease in summer insolation reconstructed over the same period. However, here we present climate modelling results from an Earth model of intermediate complexity (EMIC) that indicate that 17–40% of the cooling in the Arctic, over the period 9–0 ka, was a direct result of the desertification that occurred in the Sahara after the termination of the African Humid Period. We have performed a suite of sensitivity experiments to analyse the impact of different combinations of forcings, including various vegetation covers in the Sahara. Our simulations suggest that over the course of the Holocene, a strong increase in surface albedo in the Sahara as a result of desertification led to a regional increase in surface pressure, a weakening of the trade winds, the westerlies and the polar easterlies, which in turn reduced the meridional heat transported by the atmosphere to the Arctic. We conclude that during interglacials, the climate of the Northern Hemisphere is sensitive to changes in Sahara vegetation type.

  • 16.
    Donges, Jonathan F.
    et al.
    Stockholm University, Faculty of Science, Stockholm Resilience Centre. Potsdam Institute for Climate Impact Research, Germany.
    Donner, R. V.
    Marwan, N.
    Breitenbach, S. F. M.
    Rehfeld, K.
    Kurths, J.
    Non-linear regime shifts in Holocene Asian monsoon variability: potential impacts on cultural change and migratory patterns2015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, no 5, p. 709-741Article in journal (Refereed)
    Abstract [en]

    The Asian monsoon system is an important tipping element in Earth's climate with a large impact on human societies in the past and present. In light of the potentially severe impacts of present and future anthropogenic climate change on Asian hydrology, it is vital to understand the forcing mechanisms of past climatic regime shifts in the Asian monsoon domain. Here we use novel recurrence network analysis techniques for detecting episodes with pronounced non-linear changes in Holocene Asian monsoon dynamics recorded in speleothems from caves distributed throughout the major branches of the Asian monsoon system. A newly developed multi-proxy methodology explicitly considers dating uncertainties with the COPRA (COnstructing Proxy Records from Age models) approach and allows for detection of continental-scale regime shifts in the complexity of monsoon dynamics. Several epochs are characterised by non-linear regime shifts in Asian monsoon variability, including the periods around 8.5-7.9, 5.7-5.0, 4.1-3.7, and 3.0-2.4 ka BP. The timing of these regime shifts is consistent with known episodes of Holocene rapid climate change (RCC) and high-latitude Bond events. Additionally, we observe a previously rarely reported non-linear regime shift around 7.3 ka BP, a timing that matches the typical 1.0-1.5 ky return intervals of Bond events. A detailed review of previously suggested links between Holocene climatic changes in the Asian monsoon domain and the archaeological record indicates that, in addition to previously considered longer-term changes in mean monsoon intensity and other climatic parameters, regime shifts in monsoon complexity might have played an important role as drivers of migration, pronounced cultural changes, and the collapse of ancient human societies.

  • 17. Gemery, Laura
    et al.
    Cronin, Thomas M.
    Poirier, Robert K.
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences. Aarhus University, Denmark.
    Barrientos, Natalia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Johansson, Carina
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Koshurnikov, Andrey
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Central Arctic Ocean paleoceanography from similar to 50 ka to present, on the basis of ostracode faunal assemblages from the SWERUS 2014 expedition2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 11, p. 1473-1489Article in journal (Refereed)
    Abstract [en]

    Late Quaternary paleoceanographic changes at the Lomonosov Ridge, central Arctic Ocean, were reconstructed from a multicore and gravity core recovered during the 2014 SWERUS-C3 Expedition. Ostracode assemblages dated by accelerator mass spectrometry (AMS) indicate changing sea-ice conditions and warm Atlantic Water (AW) inflow to the Arctic Ocean from similar to 50 ka to present. Key taxa used as environmental indicators include Acetabulastoma arcticum (perennial sea ice), Polycope spp. (variable sea-ice margins, high surface productivity), Krithe hunti (Arctic Ocean deep water), and Rabilimis mirabilis (water mass change/AWinflow). Results indicate periodic seasonally sea-ice-free conditions during Marine Isotope Stage (MIS) 3 (similar to 57-29 ka), rapid deglacial changes in water mass conditions (15-11 ka), seasonally sea-ice-free conditions during the early Holocene (similar to 10-7 ka) and perennial sea ice during the late Holocene. Comparisons with faunal records from other cores from the Mendeleev and Lomonosov ridges suggest generally similar patterns, although sea-ice cover during the Last Glacial Maximum may have been less extensive at the new Lomonosov Ridge core site (similar to 85.15 degrees N, 152 degrees E) than farther north and towards Greenland. The new data provide evidence for abrupt, large-scale shifts in ostracode species depth and geographical distributions during rapid climatic transitions.

  • 18. Goldner, A.
    et al.
    Huber, M.
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Does Antarctic glaciation cool the world?2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, no 1, p. 173-189Article in journal (Refereed)
    Abstract [en]

    In this study, we compare the simulated climatic impact of adding an Antarctic ice sheet (AIS) to the greenhouse world of the Eocene and removing the AIS from the modern world. The modern global mean surface temperature anomaly (Delta T) induced by Antarctic Glaciation depends on the background CO2 levels and ranges from -1.22 to -0.18 K. The Eocene Delta T is nearly constant at similar to-0.25 K. We calculate an climate sensitivity parameter S[Antarctica] which we define as Delta T divided by the change in effective radiative forcing (Delta Q(Antarctica)) which includes some fast feedbacks imposed by prescribing the glacial properties of Antarctica. The main difference between the modern and Eocene responses is that a negative cloud feedback warms much of the Earth's surface as a large AIS is introduced in the Eocene, whereas this cloud feedback is weakly positive and acts in combination with positive sea-ice feedbacks to enhance cooling introduced by adding an ice sheet in the modern. Because of the importance of cloud feedbacks in determining the final temperature sensitivity of the AIS, our results are likely to be model dependent. Nevertheless, these model results suggest that the effective radiative forcing and feedbacks induced by the AIS did not significantly decrease global mean surface temperature across the Eocene-Oligocene transition (EOT -34.1 to 33.6 Ma) and that other factors like declining atmospheric CO2 are more important for cooling across the EOT. The results illustrate that the efficacy of AIS forcing in the Eocene is not necessarily close to one and is likely to be model and state dependent. This implies that using EOT paleoclimate proxy data by itself to estimate climate sensitivity for future climate prediction requires climate models and consequently these estimates will have large uncertainty, largely due to uncertainties in modelling low clouds.

  • 19. Goldner, Aaron
    et al.
    Huber, Matthew
    Diffenbaugh, Noah
    Caballero, Rodrigo
    University College Dublin, Ireland.
    Implications of the permanent El Niño teleconnection "blueprint" for past global and North American hydroclimatology2011In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 7, p. 723-743Article in journal (Refereed)
    Abstract [en]

    Substantial evidence exists for wetter-than-modern continental conditions in North America during the pre-Quaternary warm climate intervals. This is in apparent conflict with the robust global prediction for future climate change of a northward expansion of the subtropical dry zones that should drive aridification of many semiarid regions.  Indeed,      areas of expected future aridification include much of western North America, where extensive paleoenvironmental records are documented to have been much wetter before the onset of Quaternary ice ages. It has also been proposed that climates previous to the Quaternary may have been characterized as being in a state with warmer-than-modern eastern equatorial sea surface temperatures (SSTs).  Because equatorial Pacific SSTs exert strong controls on midlatitude atmospheric circulation and the global hydrologic cycle,      the teleconnected response from this permanent El Niño-like mean state has been proposed as a useful analogue model, or "blueprint", for understanding global climatological anomalies in the past.  The present study quantitatively explores the implications of this blueprint for past climates with a specific focus on the Miocene and Pliocene, using a global climate model (CAM3.0) and a nested high-resolution climate model (RegCM3) to study the      hydrologic impacts on global and North American climate  of a change in mean SSTs resembling that which occurs during modern El Niño events. We find that the global circulation response to a permanent El Niño resembles a large, long El Niño event. This state also exhibits equatorial super-rotation, which would represent a fundamental change to the tropical circulations.  We also find a southward shift in winter storm tracks in the Pacific and Atlantic, which affects precipitation and temperature over the mid-latitudes.  In addition, summertime precipitation increases over the majority of the continental United States.  These increases in precipitation are controlled by shifts in the subtropical jet and secondary atmospheric feedbacks. Based on these results and the data proxy comparison, we conclude that a permanent El Niño like state is one potential explanation of wetter-than-modern conditions observed in paleoclimate-proxy records, particularly over the western United States.

  • 20.
    Hind, Alistair
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Sundberg, Rolf
    Stockholm University, Faculty of Science, Department of Mathematics.
    Statistical framework for evaluation of climate model simulations by use of climate proxy data from the last millennium – Part 2: A pseudo-proxy study addressing the amplitude of solar forcing2012In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 8, no 4, p. 1355-1365Article in journal (Refereed)
    Abstract [en]

    The statistical framework of Part 1 (Sundberg et al., 2012), for comparing ensemble simulation surface temperature output with temperature proxy and instrumental records, is implemented in a pseudo-proxy experiment. A set of previously published millennial forced simulations (Max Planck Institute – COSMOS), including both "low" and "high" solar radiative forcing histories together with other important forcings, was used to define "true" target temperatures as well as pseudo-proxy and pseudo-instrumental series. In a global land-only experiment, using annual mean temperatures at a 30-yr time resolution with realistic proxy noise levels, it was found that the low and high solar full-forcing simulations could be distinguished. In an additional experiment, where pseudo-proxies were created to reflect a current set of proxy locations and noise levels, the low and high solar forcing simulations could only be distinguished when the latter served as targets. To improve detectability of the low solar simulations, increasing the signal-to-noise ratio in local temperature proxies was more efficient than increasing the spatial coverage of the proxy network. The experiences gained here will be of guidance when these methods are applied to real proxy and instrumental data, for example when the aim is to distinguish which of the alternative solar forcing histories is most compatible with the observed/reconstructed climate.

  • 21. Huber, M.
    et al.
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    The early Eocene equable climate problem revisited2011In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 7, no 2, p. 603-633Article in journal (Refereed)
    Abstract [en]

    The early Eocene quable climate problem, i.e. warm extratropical annual mean and above-freezing winter temperatures evidenced by proxy records, has remained as one of the great unsolved problems in paleoclimate. Recent progress in modeling and in paleoclimate proxy development provides an opportunity to revisit this problem to ascertain if the current generation of models can reproduce the past climate features without extensive modification. Here we have compiled early Eocene terrestrial temperature data and compared with climate model results using a consistent and rigorous methodology. We test the hypothesis that equable climates can be explained simply as a response to increased greenhouse gas forcing within the framework of the atmospheric component of the Community Climate System Model (version 3), a climate model in common use for predicting future climate change. We find that, with suitably large radiative forcing, the model and data are in general agreement for annual mean and cold month mean temperatures, and that the pattern of high latitude amplification recorded by proxies can be largely, but not perfectly, reproduced.

  • 22.
    Hutchinson, David K.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    de Boer, Agatha M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Coxall, Helen K.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Baatsen, Michiel
    Climate sensitivity and meridional overturning circulation in the late Eocene using GFDL CM2.12018In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 14, no 6, p. 789-810Article in journal (Refereed)
    Abstract [en]

    The Eocene-Oligocene transition (EOT), which took place approximately 34 Ma ago, is an interval of great interest in Earth's climate history, due to the inception of the Antarctic ice sheet and major global cooling. Climate simulations of the transition are needed to help interpret proxy data, test mechanistic hypotheses for the transition and determine the climate sensitivity at the time. However, model studies of the EOT thus far typically employ control states designed for a different time period, or ocean resolution on the order of 3 degrees. Here we developed a new higher resolution palaeoclimate model configuration based on the GFDL CM2.1 climate model adapted to a late Eocene (38 Ma) palaeogeography reconstruction. The ocean and atmosphere horizontal resolutions are 1 degrees similar to 1.5 degrees and 3 degrees 3.75 ffi respectively. This represents a significant step forward in resolving the ocean geography, gateways and circulation in a coupled climate model of this period. We run the model under three different levels of atmospheric CO2: 400, 800 and 1600 ppm. The model exhibits relatively high sensitivity to CO2 compared with other recent model studies, and thus can capture the expected Eocene high latitude warmth within observed estimates of atmospheric CO2. However, the model does not capture the low meridional temperature gradient seen in proxies. Equatorial sea surface temperatures are too high in the model (3037 degrees C) compared with observations (max 32 degrees C), although observations are lacking in the warmest regions of the western Pacific. The model exhibits bipolar sinking in the North Pacific and Southern Ocean, which persists under all levels of CO2. North Atlantic surface salinities are too fresh to permit sinking (25-30 psu), due to surface transport from the very fresh Arctic (similar to 20 psu), where surface salinities approximately agree with Eocene proxy estimates. North Atlantic salinity increases by 1-2 psu when CO2 is halved, and similarly freshens when CO2 is doubled, due to changes in the hydrological cycle.

  • 23.
    Jakobsson, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences. Aarhus University, Denmark.
    Cronin, Thomas M.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Anderson, Leif G.
    Barrientos, Natalia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Björk, Göran
    Coxall, Helen
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    de Boer, Agatha
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mayer, Larry A.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Rattray, Jayne E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of New Hampshire, USA.
    Semiletov, Igor
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Post-glacial flooding of the Bering Land Bridge dated to 11 cal ka BP based on new geophysical and sediment records2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 8, p. 991-1005Article in journal (Refereed)
    Abstract [en]

    The Bering Strait connects the Arctic and Pacific oceans and separates the North American and Asian landmasses. The presently shallow (similar to 53 m) strait was exposed during the sea level lowstand of the last glacial period, which permitted human migration across a land bridge today referred to as the Bering Land Bridge. Proxy studies (stable isotope composition of foraminifera, whale migration into the Arctic Ocean, mollusc and insect fossils and paleobotanical data) have suggested a range of ages for the Bering Strait reopening, mainly falling within the Younger Dryas stadial (12.9-11.7 cal ka BP). Here we provide new information on the deglacial and post-glacial evolution of the Arctic-Pacific connection through the Bering Strait based on analyses of geological and geophysical data from Herald Canyon, located north of the Bering Strait on the Chukchi Sea shelf region in the western Arctic Ocean. Our results suggest an initial opening at about 11 cal ka BP in the earliest Holocene, which is later than in several previous studies. Our key evidence is based on a well-dated core from Herald Canyon, in which a shift from a near-shore environment to a Pacific-influenced open marine setting at around 11 cal ka BP is observed. The shift corresponds to meltwater pulse 1b (MWP1b) and is interpreted to signify relatively rapid breaching of the Bering Strait and the submergence of the large Bering Land Bridge. Although the precise rates of sea level rise cannot be quantified, our new results suggest that the late deglacial sea level rise was rapid and occurred after the end of the Younger Dryas stadial.

  • 24. Jasechko, S.
    et al.
    Lechler, A.
    Pausata, Francesco S. R.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Fawcett, P. J.
    Gleeson, T.
    Cendon, D. I.
    Galewsky, J.
    LeGrande, A. N.
    Risi, C.
    Sharp, Z. D.
    Welker, J. M.
    Werner, M.
    Yoshimura, K.
    Late-glacial to late-Holocene shifts in global precipitation delta O-182015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, no 10, p. 1375-1393Article in journal (Refereed)
    Abstract [en]

    Reconstructions of Quaternary climate are often based on the isotopic content of paleo-precipitation preserved in proxy records. While many paleo-precipitation isotope records are available, few studies have synthesized these dispersed records to explore spatial patterns of late-glacial precipitation delta O-18. Here we present a synthesis of 86 globally distributed groundwater (n = 59), cave calcite (n = 15) and ice core (n = 12) isotope records spanning the late-glacial (defined as similar to 50 000 to similar to 20 000 years ago) to the late-Holocene (within the past similar to 5000 years). We show that precipitation delta O-18 changes from the late-glacial to the late-Holocene range from -7.1% (delta O-18(late-Holocene) > delta O-18(late-glacial)) to + 1.7% (delta O-18(late-glacial) > delta O-18(late-Holocene)), with the majority (77 %) of records having lower late-glacial delta O-18 than late-Holocene delta O-18 values. High-magnitude, negative precipitation delta O-18 shifts are common at high latitudes, high altitudes and continental interiors (delta O-18(late-Holocene) > delta O-18(late-glacial) by more than 3 %). Conversely, low-magnitude, positive precipitation delta O-18 shifts are concentrated along tropical and subtropical coasts (delta O-18(late-glacial) > delta O-18(late-Holocene) by less than 2 %). Broad, global patterns of late-glacial to late-Holocene precipitation delta O-18 shifts suggest that stronger-than-modern isotopic distillation of air masses prevailed during the late-glacial, likely impacted by larger global temperature differences between the tropics and the poles. Further, to test how well general circulation models reproduce global precipitation delta O-18 shifts, we compiled simulated precipitation delta O-18 shifts from five isotope-enabled general circulation models simulated under recent and last glacial maximum climate states. Climate simulations generally show better intermodel and model-measurement agreement in temperate regions than in the tropics, highlighting a need for further research to better understand how inter-model spread in convective rainout, seawater delta O-18 and glacial topography parameterizations impact simulated precipitation delta O-18. Future research on paleo-precipitation delta O-18 records can use the global maps of measured and simulated late-glacial precipitation isotope compositions to target and prioritize field sites.

  • 25.
    Jonsson, C. E.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Andersson, S.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Rosqvist, G.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Leng, M.
    NERC Isotope Geosciences Laboratory, BGS, UK.
    Reconstructing past atmosperic circulation changes using oxygen isotopes in lake sediments from Sweden2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, no 1, p. 49-62Article in journal (Refereed)
    Abstract [en]

    Here we use lake sediment studies from Sweden to illustrate how Holocene-aged oxygen isotope records from lakes located in different hydrological settings, can provide information about climate change. In particular changes in precipitation, atmospheric circulation and water balance. We highlight the importance of understanding the present lake hydrology, and the relationship between climate variables and the oxygen isotopic composition of precipitation (d18Op)and lake waters (d18Olakewater) for interpretation of the oxygen isotopic record from the sediments (d18O). Both precipitation reconstructions from northern Sweden and water balance reconstructions from south and central Sweden show that the atmospheric circulation changed from zonal to a more meridional airflow over the Holocene. Superimposed on this Holocene trend are δ18Op minima resembling intervals of the negative phase of the North Atlantic Oscillation (NAO), thus suggesting that the climate of Northern Europe is strongly influenced by atmospheric and oceanic circulation changes over the North Atlantic.

  • 26.
    Keskitalo, Kirsi
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Tesi, Tommaso
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. CNR-National Research Council of Italy, Italy.
    Bröder, Lisa
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Andersson, August
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sköld, Martin
    Stockholm University, Faculty of Science, Department of Mathematics.
    Semiletov, Igor P.
    Dudarev, Oleg V.
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Sources and characteristics of terrestrial carbon in Holocene-scale sediments of the East Siberian Sea2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 9, p. 1213-1226Article in journal (Refereed)
    Abstract [en]

    Thawing of permafrost carbon (PF-C) due to climate warming can remobilise considerable amounts of terrestrial carbon from its long-term storage to the marine environment. PF-C can be then be buried in sediments or remineralised to CO2 with implications for the carbon-climate feedback. Studying historical sediment records during past natural climate changes can help us to understand the response of permafrost to current climate warming. In this study, two sediment cores collected from the East Siberian Sea were used to study terrestrial organic carbon sources, composition and degradation during the past similar to 9500 cal yrs BP. CuO-derived lignin and cutin products (i.e., compounds solely biosynthesised in terrestrial plants) combined with delta C-13 suggest that there was a higher input of terrestrial organic carbon to the East Siberian Sea between similar to 9500 and 8200 cal yrs BP than in all later periods. This high input was likely caused by marine transgression and permafrost destabilisation in the early Holocene climatic optimum. Based on source apportionment modelling using dual-carbon isotope (Delta C-14, Delta C-13) data, coastal erosion releasing old Pleistocene permafrost carbon was identified as a significant source of organic matter translocated to the East Siberian Sea during the Holocene.

  • 27.
    Kleman, Johan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Fastook, Jim
    Ebert, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Pre-LGM Northern Hemisphere ice sheet topography2013In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 9, p. 2365-2378Article in journal (Refereed)
    Abstract [en]

    We here reconstruct the paleotopography of Northern Hemisphere ice sheets during the glacial maxima of marine isotope stages (MIS) 5b and 4.We employ a combined approach, blending geologically based reconstruction and numerical modeling, to arrive at probable ice sheet extents and topographies for each of these two time slices. For a physically based 3-D calculation based on geologically derived 2-D constraints, we use the University of Maine Ice Sheet Model (UMISM) to calculate ice sheet thickness and topography. The approach and ice sheet modeling strategy is designed to provide robust data sets of sufficient resolution for atmospheric circulation experiments for these previously elusive time periods. Two tunable parameters, a temperature scaling function applied to a spliced Vostok–GRIP record, and spatial adjustment of the climatic pole position, were employed iteratively to achieve a good fit to geological constraints where such were available. The model credibly reproduces the first-order pattern of size and location of geologically indicated ice sheets during marine isotope stages (MIS) 5b (86.2 kyr model age) and 4 (64 kyr model age). From the interglacial state of two north–south obstacles to atmospheric circulation (Rocky Mountains and Greenland), by MIS 5b the emergence of combined Quebec–central Arctic and Scandinavian–Barents-Kara ice sheets had increased the number of such highland obstacles to four. The number of major ice sheets remained constant through MIS 4, but the merging of the Cordilleran and the proto-Laurentide Ice Sheet produced a single continent-wide North American ice sheet at the LGM.

  • 28. Linan, I. Dorado
    et al.
    Buentgen, U.
    Gonzalez-Rouco, F.
    Zorita, Eduardo
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Montavez, J. P.
    Gomez-Navarro, J. J.
    Brunet, M.
    Heinrich, I.
    Helle, G.
    Gutierrez, E.
    Estimating 750 years of temperature variations and uncertainties in the Pyrenees by tree ring reconstructions and climate simulations2012In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 8, no 3, p. 919-933Article in journal (Refereed)
    Abstract [en]

    Past temperature variations are usually inferred from proxy data or estimated using general circulation models. Comparisons between climate estimations derived from proxy records and from model simulations help to better understand mechanisms driving climate variations, and also offer the possibility to identify deficiencies in both approaches. This paper presents regional temperature reconstructions based on tree-ring maximum density series in the Pyrenees, and compares them with the output of global simulations for this region and with regional climate model simulations conducted for the target region. An ensemble of 24 reconstructions of May-to-September regional mean temperature was derived from 22 maximum density tree-ring site chronologies distributed over the larger Pyrenees area. Four different tree-ring series standardization procedures were applied, combining two detrending methods: 300-yr spline and the regional curve standardization (RCS). Additionally, different methodological variants for the regional chronology were generated by using three different aggregation methods. Calibration verification trials were performed in split periods and using two methods: regression and a simple variance matching. The resulting set of temperature reconstructions was compared with climate simulations performed with global (ECHO-G) and regional (MM5) climate models. The 24 variants of May-to-September temperature reconstructions reveal a generally coherent pattern of inter-annual to multi-centennial temperature variations in the Pyrenees region for the last 750 yr. However, some reconstructions display a marked positive trend for the entire length of the reconstruction, pointing out that the application of the RCS method to a suboptimal set of samples may lead to unreliable results. Climate model simulations agree with the tree-ring based reconstructions at multi-decadal time scales, suggesting solar variability and volcanism as the main factors controlling preindustrial mean temperature variations in the Pyrenees. Nevertheless, the comparison also highlights differences with the reconstructions, mainly in the amplitude of past temperature variations and in the 20th century trends. Neither proxy-based reconstructions nor model simulations are able to perfectly track the temperature variations of the instrumental record, suggesting that both approximations still need further improvements.

  • 29. Linderholm, Hans W.
    et al.
    Nicolle, Marie
    Francus, Pierre
    Gajewski, Konrad
    Helama, Samuli
    Korhola, Atte
    Solomina, Olga
    Yu, Zicheng
    Zhang, Peng
    D'Andrea, William J.
    Debret, Maxime
    Divine, Dmitry V.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Loader, Neil J.
    Massei, Nicolas
    Seftigen, Kristina
    Thomas, Elizabeth K.
    Werner, Johannes
    Andersson, Sofia
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Berntsson, Annika
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Luoto, Tomi P.
    Nevalainen, Liisa
    Saarni, Saija
    Väliranta, Minna
    Arctic hydroclimate variability during the last 2000 years: current understanding and research challenges2018In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 14, no 4, p. 473-514Article in journal (Refereed)
    Abstract [en]

    Reanalysis data show an increasing trend in Arctic precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydro-climate changes are expected to continue and possibly accelerate in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through the atmospheric and ocean circulations. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand Arctic hydroclimate and its variability prior to the instrumental record, climate proxy records are needed. The purpose of this review is to summarise the current understanding of Arctic hydroclimate during the past 2000 years. First, the paper reviews the main natural archives and proxies used to infer past hydroclimate variations in this remote region and outlines the difficulty of disentangling the moisture from the temperature signal in these records. Second, a comparison of two sets of hydroclimate records covering the Common Era from two data-rich regions, North America and Fennoscandia, reveals inter- and intra-regional differences. Third, building on earlier work, this paper shows the potential for providing a high-resolution hydroclimate reconstruction for the Arctic and a comparison with last-millennium simulations from fully coupled climate models. In general, hydroclimate proxies and simulations indicate that the Medieval Climate Anomaly tends to have been wetter than the Little Ice Age (LIA), but there are large regional differences. However, the regional coverage of the proxy data is inadequate, with distinct data gaps in most of Eurasia and parts of North America, making robust assessments for the whole Arctic impossible at present. To fully assess pan-Arctic hydroclimate variability for the last 2 millennia, additional proxy records are required.

  • 30. Luciani, Valeria
    et al.
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rice University, USA.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Fornaciari, Eliana
    Giusberti, Luca
    Agnini, Claudia
    D'Onofrio, Roberta
    Major perturbations in the global carbon cycle and photosymbiont-bearing planktic foraminifera during the early Eocene2016In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 12, no 4, p. 981-1007Article in journal (Refereed)
    Abstract [en]

    A marked switch in the abundance of the planktic foraminiferal genera Morozovella and Acarinina occurred at low-latitude sites near the start of the Early Eocene Climatic Optimum (EECO), a multi-million-year interval when Earth surface temperatures reached their Cenozoic maximum. Stable carbon and oxygen isotope data of bulk sediment are presented from across the EECO at two locations: Possagno in northeast Italy and Deep Sea Drilling Project (DSDP) Site 577 in the northwest Pacific. Relative abundances of plank tic foraminifera are presented from these two locations, as well as from Ocean Drilling Program (ODP) Site 1051 in the northwest Atlantic. All three sections have good strati graphic markers, and the delta C-13 records at each section can be correlated amongst each other and to delta C-13 records at other locations across the globe. These records show that a series of negative carbon isotope excursions (CIEs) occurred before, during and across the EECO, which is defined here as the interval between the J event and the base of Discoaster sublodoensis. Significant though ephemeral modifications in planktic foraminiferal assemblages coincide with some of the short-term CIEs, which were marked by increases in the relative abundance ofAcarinina, similar to what happened across established hyperthermal events in Tethyan settings prior to the EECO. Most crucially, a temporal link exists between the onset of the EECO, carbon cycle changes during this time and the decline in Morozovella. Possible causes are mani- fold and may include temperature effects on photosymbiont-bearing planktic foraminifera and changes in ocean chemistry.

  • 31.
    Löfverström, Marcus
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Caballero, Rodrigo
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kleman, Johan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Evolution of the large-scale atmospheric circulation in response to changing ice sheets over the last glacial cycle2014In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 10, no 4, p. 1453-1471Article in journal (Refereed)
    Abstract [en]

    We present modelling results of the atmospheric circulation at the cold periods of marine isotope stage 5b (MIS 5b), MIS 4 and the Last Glacial Maximum (LGM), as well as the interglacial. The palaeosimulations are forced by ice-sheet reconstructions consistent with geological evidence and by appropriate insolation and greenhouse gas concentrations. The results suggest that the large-scale atmospheric winter circulation remained largely similar to the interglacial for a significant part of the glacial cycle. The proposed explanation is that the ice sheets were located in areas where their interaction with the mean flow is limited. However, the LGM Laurentide Ice Sheet induces a much larger planetary wave that leads to a zonalisation of the Atlantic jet. In summer, the ice-sheet topography dynamically induces warm temperatures in Alaska and central Asia that inhibits the expansion of the ice sheets into these regions. The warm temperatures may also serve as an explanation for westward propagation of the Eurasian Ice Sheet from MIS 4 to the LGM.

  • 32.
    Moberg, Anders
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Renssen, Hans
    Sundqvist, Hanna S.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology .
    Preface "Holocene climate variability over Scandinavia – A special issue originating from a workshop organized by the Bert Bolin Centre for Climate Research"2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, p. 719-721Article in journal (Other academic)
  • 33.
    Moberg, Anders
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Sundberg, Rolf
    Stockholm University, Faculty of Science, Department of Mathematics.
    Grudd, Håkan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Hind, Alistair
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Mathematics.
    Statistical framework for evaluation of climate model simulations by use of climate proxy data from the last millennium - Part 3: Practical considerations, relaxed assumptions, and using tree-ring data to address the amplitude of solar forcing2015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, no 3, p. 425-448Article in journal (Refereed)
    Abstract [en]

    A statistical framework for evaluation of climate model simulations by comparison with climate observations from instrumental and proxy data (part 1 in this series) is improved by the relaxation of two assumptions. This allows autocorrelation in the statistical model for simulated internal climate variability and enables direct comparison of two alternative forced simulations to test whether one fits the observations significantly better than the other. The extended framework is applied to a set of simulations driven with forcings for the pre-industrial period 1000-1849 CE and 15 tree-ring-based temperature proxy series. Simulations run with only one external forcing (land use, volcanic, small-amplitude solar, or large-amplitude solar) do not significantly capture the variability in the tree-ring data - although the simulation with volcanic forcing does so for some experiment settings. When all forcings are combined (using either the small- or large-amplitude solar forcing), including also orbital, greenhouse-gas and non-volcanic aerosol forcing, and additionally used to produce small simulation ensembles starting from slightly different initial ocean conditions, the resulting simulations are highly capable of capturing some observed variability. Nevertheless, for some choices in the experiment design, they are not significantly closer to the observations than when unforced simulations are used, due to highly variable results between regions. It is also not possible to tell whether the small-amplitude or large-amplitude solar forcing causes the multiple-forcing simulations to be closer to the reconstructed temperature variability. Proxy data from more regions and of more types, or representing larger regions and complementary seasons, are apparently needed for more conclusive results from model-data comparisons in the last millennium.

  • 34.
    O'Regan, Matt
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Barrientos, Natalia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Cronin, Thomas M.
    Gemery, Laura
    Kirchner, Nina
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Mayer, Larry A.
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Noormets, Riko
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences. Aarhus University, Denmark.
    Semiletov, Igor
    Stranne, Christian
    Stockholm University, Faculty of Science, Department of Geological Sciences. University of New Hampshire, USA.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The De Long Trough: A newly discovered glacial trough on the East Siberian continental margin2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 9, p. 1269-1284Article in journal (Refereed)
    Abstract [en]

    Ice sheets extending over parts of the East Siberian continental shelf have been proposed for the last glacial period and during the larger Pleistocene glaciations. The sparse data available over this sector of the Arctic Ocean have left the timing, extent and even existence of these ice sheets largely unresolved. Here we present new geophysical mapping and sediment coring data from the East Siberian shelf and slope collected during the 2014 SWERUS-C3 expedition (SWERUS-C3: Swedish - Russian - US Arctic Ocean Investigation of Climate-Cryosphere-Carbon Interactions). The multibeam bathymetry and chirp sub-bottom profiles reveal a set of glacial landforms that include grounding zone formations along the outer continental shelf, seaward of which lies a > 65m thick sequence of glacio-genic debris flows. The glacial landforms are interpreted to lie at the seaward end of a glacial trough - the first to be reported on the East Siberian margin, here referred to as the De Long Trough because of its location due north of the De Long Islands. Stratigraphy and dating of sediment cores show that a drape of acoustically laminated sediments covering the glacial deposits is older than similar to 50 cal kyr BP. This provides direct evidence for extensive glacial activity on the Siberian shelf that predates the Last Glacial Maximum and most likely occurred during the Saalian (Marine Isotope Stage (MIS) 6).

  • 35.
    Pearce, Christof
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Aarhus University, Denmark.
    Varhelyi, Aron
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wastegård, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Muschitiello, Francesco
    Stockholm University, Faculty of Science, Department of Geological Sciences. Columbia University, USA; Bjerknes Centre for Climate Research, Norway.
    Barrientos, Natalia
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Cronin, Thomas M.
    Gemery, Laura
    Semiletov, Igor
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    The 3.6 ka Aniakchak tephra in the Arctic Ocean: a constraint on the Holocene radiocarbon reservoir age in the Chukchi Sea2017In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 13, no 4, p. 303-316Article in journal (Refereed)
    Abstract [en]

    The caldera-forming eruption of the Aniakchak volcano in the Aleutian Range on the Alaskan Peninsula at 3.6 cal kyr BP was one of the largest Holocene eruptions worldwide. The resulting ash is found as a visible sediment layer in several Alaskan sites and as a cryptotephra on Newfoundland and Greenland. This large geographic distribution, combined with the fact that the eruption is relatively well constrained in time using radiocarbon dating of lake sediments and annual layer counts in ice cores, makes it an excellent stratigraphic marker for dating and correlating mid-late Holocene sediment and paleoclimate records. This study presents the outcome of a targeted search for the Aniakchak tephra in a marine sediment core from the Arctic Ocean, namely Core SWERUS-L2-2-PC1 (2PC), raised from 57m water depth in Herald Canyon, western Chukchi Sea. High concentrations of tephra shards, with a geochemical signature matching that of Aniakchak ash, were observed across a more than 1.5m long sediment sequence. Since the primary input of volcanic ash is through atmospheric transport, and assuming that bioturbation can account for mixing up to ca. 10 cm of the marine sediment deposited at the coring site, the broad signal is interpreted as sustained reworking at the sediment source input. The isochron is therefore placed at the base of the sudden increase in tephra concentrations rather than at the maximum concentration. This interpretation of major reworking is strengthened by analysis of grain size distribution which points to ice rafting as an important secondary transport mechanism of volcanic ash. Combined with radiocarbon dates on mollusks in the same sediment core, the volcanic marker is used to calculate a marine radiocarbon reservoir age offset Delta R = 477 +/- 60 years. This relatively high value may be explained by the major influence of typically carbon-old Pacific waters, and it agrees well with recent estimates of Delta R along the northwest Alaskan coast, possibly indicating stable oceanographic conditions during the second half of the Holocene. Our use of a volcanic absolute age marker to obtain the marine reservoir age offset is the first of its kind in the Arctic Ocean and provides an important framework for improving chronologies and correlating marine sediment archives in this region. Core 2PC has a high sediment accumulation rate averaging 200 cm kyr(-1) throughout the last 4000 years, and the chronology presented here provides a solid base for high-resolution reconstructions of late Holocene climate and ocean variability in the Chukchi Sea.

  • 36. Rickaby, R. E. M.
    et al.
    Henderiks, orijntje
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Young, J. N.
    Perturbing phytoplankton: response and isotopic fractionation with changing carbonate chemistry in two coccolithophore species2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, no 6, p. 771-785Article in journal (Refereed)
    Abstract [en]

    All species of coccolithophore appear to respond to perturbations of carbonate chemistry in a different way. Here, we show that the degree of malformation, growth rate and stable isotopic composition of organic matter and carbonate produced by two contrasting species of coccolithophore (Gephyrocapsa oceanica and Coccolithus pelagicus ssp. braarudii) are indicative of differences between their photosynthetic and calcification response to changing DIC levels (ranging from similar to 1100 to similar to 7800 mu mol kg(-1)) at constant pH (8.13 +/- 0.02). Gephyrocapsa oceanica thrived under all conditions of DIC, showing evidence of increased growth rates at higher DIC, but C. braarudii was detrimentally affected at high DIC showing signs of malformation, and decreased growth rates. The carbon isotopic fractionation into organic matter and the coccoliths suggests that C. braarudii utilises a common internal pool of carbon for calcification and photosynthesis but G. oceanica relies on independent supplies for each process. All coccolithophores appear to utilize bicarbonate as their ultimate source of carbon for calcification resulting in the release of a proton. But, we suggest that this proton can be harnessed to enhance the supply of CO(2)(aq) for photosynthesis either from a large internal HCO(3)(-) pool which acts as a pH buffer (C. braarudii), or pumped externally to aid the diffusive supply of CO(2) across the membrane from the abundant HCO(3)(-) (G. oceanica), likely mediated by an internal and external carbonic anhydrase respectively. Our simplified hypothetical spectrum of physiologies may provide a context to understand different species response to changing pH and DIC, the species-specific epsilon(p) and calcite vital effects, as well as accounting for geological trends in coccolithophore cell size.

  • 37. Schollaen, K.
    et al.
    Karamperidou, C.
    Krusic, Paul
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Cook, E.
    Helle, G.
    ENSO flavors in a tree-ring delta O-18 record of Tectona grandis from Indonesia2015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, no 10, p. 1325-1333Article in journal (Refereed)
    Abstract [en]

    Indonesia's climate is dominated by the equatorial monsoon system, and has been linked to El Nino-Southern Oscillation (ENSO) events that often result in extensive droughts and floods over the Indonesian archipelago. In this study we investigate ENSO-related signals in a tree-ring delta O-18 record (1900-2007) of Javanese teak. Our results reveal a clear influence of Warm Pool (central Pacific) El Nino events on Javanese tree-ring delta O-18, and no clear signal of Cold Tongue (eastern Pacific) El Nino events. These results are consistent with the distinct impacts of the two ENSO flavors on Javanese precipitation, and illustrate the importance of considering ENSO flavors when interpreting palaeoclimate proxy records in the tropics, as well as the potential of palaeoclimate proxy records from appropriately selected tropical regions for reconstructing past variability of. ENSO flavors.

  • 38. Sjolte, Jesper
    et al.
    Sturm, Christophe
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Adolphi, Florian
    Vinther, Bo M.
    Werner, Martin
    Lohmann, Gerrit
    Muscheler, Raimund
    Solar and volcanic forcing of North Atlantic climate inferred from a process-based reconstruction2018In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 14, no 8, p. 1179-1194Article in journal (Refereed)
    Abstract [en]

    The effect of external forcings on atmospheric circulation is debated. Due to the short observational period, the analysis of the role of external forcings is hampered, making it difficult to assess the sensitivity of atmospheric circulation to external forcings, as well as persistence of the effects. In observations, the average response to tropical volcanic eruptions is a positive North Atlantic Oscillation (NAO) during the following winter. However, past major tropical eruptions exceeding the magnitude of eruptions during the instrumental era could have had more lasting effects. Decadal NAO variability has been suggested to follow the 11-year solar cycle, and linkages have been made between grand solar minima and negative NAO. However, the solar link to NAO found by modeling studies is not unequivocally supported by reconstructions, and is not consistently present in observations for the 20th century. Here we present a reconstruction of atmospheric winter circulation for the North Atlantic region covering the period 1241-1970 CE. Based on seasonally resolved Greenland ice core records and a 1200-year-long simulation with an isotope-enabled climate model, we reconstruct sea level pressure and temperature by matching the spatiotemporal variability in the modeled isotopic composition to that of the ice cores. This method allows us to capture the primary (NAO) and secondary mode (Eastern Atlantic Pattern) of atmospheric circulation in the North Atlantic region, while, contrary to previous reconstructions, preserving the amplitude of observed year-to-year atmospheric variability. Our results show five winters of positive NAO on average following major tropical volcanic eruptions, which is more persistent than previously suggested. In response to decadal minima of solar activity we find a highpressure anomaly over northern Europe, while a reinforced opposite response in pressure emerges with a 5-year time lag. On centennial timescales we observe a similar response of circulation as for the 5-year time-lagged response, with a high-pressure anomaly across North America and south of Greenland. This response to solar forcing is correlated to the second mode of atmospheric circulation, the Eastern Atlantic Pattern. The response could be due to an increase in blocking frequency, possibly linked to a weakening of the subpolar gyre. The long-term anomalies of temperature during solar minima shows cooling across Greenland, Iceland and western Europe, resembling the cooling pattern during the Little Ice Age (1450-1850 CE). While our results show significant correlation between solar forcing and the secondary circulation pattern on decadal (r = 0 : 29, p < 0 : 01) and centennial timescales (r = 0 : 6, p < 0 : 01), we find no consistent relationship between solar forcing and NAO. We conclude that solar and volcanic forcing impacts different modes of our reconstructed atmospheric circulation, which can aid in separating the regional effects of forcings and understanding the underlying mechanisms.

  • 39. Slotnick, B. S.
    et al.
    Lauretano, V.
    Backman, Jan
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rice University, USA.
    Sluijs, A.
    Lourens, L.
    Early Paleogene variations in the calcite compensation depth: new constraints using old borehole sediments from across Ninetyeast Ridge, central Indian Ocean2015In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 11, no 3, p. 473-493Article in journal (Refereed)
    Abstract [en]

    Major variations in global carbon cycling occurred between 62 and 48 Ma, and these very likely related to changes in the total carbon inventory of the ocean-atmosphere system. Based on carbon cycle theory, variations in the mass of the ocean carbon should be reflected in contemporaneous global ocean carbonate accumulation on the seafloor and, thereby, the depth of the calcite compensation depth (CCD). To better constrain the cause and magnitude of these changes, the community needs early Paleogene carbon isotope and carbonate accumulation records from widely separated deep-sea sediment sections, especially including the Indian Ocean. Several CCD reconstructions for this time interval have been generated using scientific drill sites in the Atlantic and Pacific oceans; however, corresponding information from the Indian Ocean has been extremely limited. To assess the depth of the CCD and the potential for renewed scientific drilling of Paleogene sequences in the Indian Ocean, we examine lithologic, nannofossil, carbon isotope, and carbonate content records for late Paleocene - early Eocene sediments recovered at three sites spanning Ninetyeast Ridge: Deep Sea Drilling Project (DSDP) Sites 213 (deep, east), 214 (shallow, central), and 215 (deep, west). The disturbed, discontinuous sediment sections are not ideal, because they were recovered in single holes using rotary coring methods, but remain the best Paleogene sediments available from the central Indian Ocean. The delta C-13 records at Sites 213 and 215 are similar to those generated at several locations in the Atlantic and Pacific, including the prominent high in delta C-13 across the Paleocene carbon isotope maximum (PCIM) at Site 215, and the prominent low in delta C-13 across the early Eocene Climatic Optimum (EECO) at both Site 213 and Site 215. The Paleocene-Eocene thermal maximum (PETM) and the K/X event are found at Site 213 but not at Site 215, presumably because of coring gaps. Carbonate content at both Sites 213 and 215 drops to < 5% shortly after the first occurrence of Discoaster lodoensis and the early Eocene rise in delta C-13 (similar to 52 Ma). This reflects a rapid shoaling of the CCD, and likely a major decrease in the net flux of C-13-depleted carbon to the ocean. Our results support ideas that major changes in net fluxes of organic carbon to and from the exogenic carbon cycle occurred during the early Paleogene. Moreover, we conclude that excellent early Paleogene carbonate accumulation records might be recovered from the central Indian Ocean with future scientific drilling.

  • 40.
    Steinthorsdottir, Margret
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Porter, Amanda S.
    Holohan, Aidan
    Kunzmann, Lutz
    Collinson, Margaret
    McElwain, Jennifer C.
    Fossil plant stomata indicate decreasing atmospheric CO2 prior to the Eocene-Oligocene boundary2016In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 12, no 2, p. 439-454Article in journal (Refereed)
    Abstract [en]

    A unique stratigraphic sequence of fossil leaves of Eotrigonobalanus furcinervis (extinct trees of the beech family, Fagaceae) from central Germany has been used to derive an atmospheric pCO(2) record with multiple data points spanning the late middle to late Eocene, two sampling levels which may be earliest Oligocene, and two samples from later in the Oligocene. Using the inverse relationship between the density of stomata and pCO(2), we show that pCO(2) decreased continuously from the late middle to late Eocene, reaching a relatively stable low value before the end of the Eocene. Based on the subsequent records, pCO(2) in parts of the Oligocene was similar to latest Eocene values. These results suggest that a decrease in pCO(2) preceded the large shift in marine oxygen isotope records that characterizes the Eocene-Oligocene transition and that when a certain threshold of pCO(2) change was crossed, the cumulative effects of this and other factors resulted in rapid temperature decline, ice build up on Antarctica and hence a change of climate mode.

  • 41.
    Strandberg, Gustav
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Kjellström, Erik
    Stockholm University, Faculty of Science, Department of Meteorology . Swedish Meteorological and Hydrological Institute, Sweden.
    Poska, A.
    Wagner, S.
    Gaillard, M. -J
    Trondman, A. -K
    Mauri, A.
    Davis, B. A. S.
    Kaplan, J. O.
    Birks, H. J. B.
    Bjune, A. E.
    Fyfe, R.
    Giesecke, T.
    Kalnina, L.
    Kangur, M.
    van der Knaap, W. O.
    Kokfelt, U.
    Kunes, P.
    Latalowa, M.
    Marquer, L.
    Mazier, F.
    Nielsen, A. B.
    Smith, B.
    Seppa, H.
    Sugita, S.
    Regional climate model simulations for Europe at 6 and 0.2 k BP: sensitivity to changes in anthropogenic deforestation2014In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 10, no 2, p. 661-680Article in journal (Refereed)
    Abstract [en]

    This study aims to evaluate the direct effects of anthropogenic deforestation on simulated climate at two contrasting periods in the Holocene, similar to 6 and similar to 0.2 k BP in Europe. We apply We apply the Rossby Centre regional climate model RCA3, a regional climate model with 50 km spatial resolution, for both time periods, considering three alternative descriptions of the past vegetation: (i) potential natural vegetation (V) simulated by the dynamic vegetation model LPJ-GUESS, (ii) potential vegetation with anthropogenic land use (deforestation) from the HYDE3.1 (History Database of the Global Environment) scenario (V + H3.1), and (iii) potential vegetation with anthropogenic land use from the KK10 scenario (V + KK10). The climate model results show that the simulated effects of deforestation depend on both local/regional climate and vegetation characteristics. At similar to 6 k BP the extent of simulated deforestation in Europe is generally small, but there are areas where deforestation is large enough to produce significant differences in summer temperatures of 0.5-1 degrees C. At similar to 0.2 k BP, extensive deforestation, particularly according to the KK10 model, leads to significant temperature differences in large parts of Europe in both winter and summer. In winter, deforestation leads to lower temperatures because of the differences in albedo between forested and unforested areas, particularly in the snow-covered regions. In summer, deforestation leads to higher temperatures in central and eastern Europe because evapotranspiration from unforested areas is lower than from forests. Summer evaporation is already limited in the southernmost parts of Europe under potential vegetation conditions and, therefore, cannot become much lower. Accordingly, the albedo effect dominates in southern Europe also in summer, which implies that deforestation causes a decrease in temperatures. Differences in summer temperature due to deforestation range from -1 degrees C in south-western Europe to +1 degrees C in eastern Europe. The choice of anthropogenic land-cover scenario has a significant influence on the simulated climate, but uncertainties in palaeoclimate proxy data for the two time periods do not allow for a definitive discrimination among climate model results.

  • 42.
    Sundberg, Rolf
    et al.
    Stockholm University, Faculty of Science, Department of Mathematics.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hind, Alistair
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Statistical framework for evaluation of climate model simulations by use of climate proxy data from the last millennium – Part 1: Theory2012In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 8, no 4, p. 1339-1353Article in journal (Refereed)
    Abstract [en]

    A statistical framework for comparing the output of ensemble simulations from global climate models with networks of climate proxy and instrumental records has been developed, focusing on near-surface temperatures for the last millennium. This framework includes the formulation of a joint statistical model for proxy data, instrumental data and simulation data, which is used to optimize a quadratic distance measure for ranking climate model simulations. An essential underlying assumption is that the simulations and the proxy/instrumental series have a shared component of variability that is due to temporal changes in external forcing, such as volcanic aerosol load, solar irradiance or greenhouse gas concentrations. Two statistical tests have been formulated. Firstly, a preliminary test establishes whether a significant temporal correlation exists between instrumental/proxy and simulation data. Secondly, the distance measure is expressed in the form of a test statistic of whether a forced simulation is closer to the instrumental/proxy series than unforced simulations. The proposed framework allows any number of proxy locations to be used jointly, with different seasons, record lengths and statistical precision. The goal is to objectively rank several competing climate model simulations (e.g. with alternative model parameterizations or alternative forcing histories) by means of their goodness of fit to the unobservable true past climate variations, as estimated from noisy proxy data and instrumental observations.

  • 43.
    Sundqvist, Hanna S.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kaufman, D. S.
    McKay, N. P.
    Balascio, N. L.
    Briner, J. P.
    Cwynar, L. C.
    Sejrup, H. P.
    Seppa, H.
    Subetto, D. A.
    Andrews, J. T.
    Axford, Y.
    Bakke, J.
    Birks, H. J. B.
    Brooks, S. J.
    de Vernal, A.
    Jennings, A. E.
    Charpentier Ljungqvist, Fredrik
    Stockholm University, Faculty of Humanities, Department of History.
    Ruehland, K. M.
    Saenger, C.
    Smol, J. P.
    Viau, A. E.
    Arctic Holocene proxy climate database - new approaches to assessing geochronological accuracy and encoding climate variables2014In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 10, no 4, p. 1605-1631Article in journal (Refereed)
    Abstract [en]

    We present a systematic compilation of previously published Holocene proxy climate records from the Arctic. We identified 170 sites from north of 58 degrees N latitude where proxy time series extend back at least to 6 cal ka (all ages in this article are in calendar years before present - BP), are resolved at submillennial scale (at least one value every 400 +/- 200 years) and have age models constrained by at least one age every 3000 years. In addition to conventional meta-data for each proxy record (location, proxy type, reference), we include two novel parameters that add functionality to the database. First, climate interpretation is a series of fields that logically describe the specific climate variable(s) represented by the proxy record. It encodes the proxy-climate relation reported by authors of the original studies into a structured format to facilitate comparison with climate model outputs. Second, geochronology accuracy score (chron score) is a numerical rating that reflects the overall accuracy of C-14-based age models from lake and marine sediments. Chron scores were calculated using the original author-reported C-14 ages, which are included in this database. The database contains 320 records (some sites include multiple records) from six regions covering the circumpolar Arctic: Fennoscandia is the most densely sampled region (31% of the records), whereas only five records from the Russian Arctic met the criteria for inclusion. The database contains proxy records from lake sediment (60 %), marine sediment (32 %), glacier ice (5 %), and other sources. Most (61 %) reflect temperature (mainly summer warmth) and are primarily based on pollen, chironomid, or diatom assemblages. Many (15 %) reflect some aspect of hydroclimate as inferred from changes in stable isotopes, pollen and diatom assemblages, humification index in peat, and changes in equilibrium-line altitude of glaciers. This comprehensive database can be used in future studies to investigate the spatio-temporal pattern of Arctic Holocene climate changes and their causes. The Arctic Holocene data set is available from NOAA Paleoclimatology.

  • 44.
    Sundqvist, Hanna S.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Meteorology .
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Brattström, Gudrun
    Stockholm University, Faculty of Science, Department of Mathematics.
    Climate change between the mid and late Holocene in the northern high latitudes: Part I: Survey of temperature and precipitation proxy data2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, p. 591-608Article in journal (Refereed)
    Abstract [en]

    We undertake a study in two parts, where theoverall aim is to quantitatively compare results from climateproxy data with results from several climate model simulationsfrom the Paleoclimate Modelling IntercomparisonProject for the mid-Holocene period and the pre-industrial,conditions for the pan-arctic region, north of 60 N. In thisfirst paper, we survey the available published local temperatureand precipitation proxy records. We also discuss andquantifiy some uncertainties in the estimated difference inclimate between the two periods as recorded in the availabledata. The spatial distribution of available published localproxies has a marked geographical bias towards land areassurrounding the North Atlantic sector, especially Fennoscandia.The majority of the reconstructions are terrestrial, andthere is a large over-representation towards summer temperaturerecords. The available reconstructions indicate that thenorthern high latitudes were warmer in both summer, winterand the in annual mean temperature at the mid-Holocene(6000 BP±500 yrs) compared to the pre-industrial period(1500AD±500 yrs). For usage in the model-data comparisons(in Part 1), we estimate the calibration uncertainty andalso the internal variability in the proxy records, to derive acombined minimum uncertainty in the reconstructed temperaturechange between the two periods. Often, the calibrationuncertainty alone, at a certain site, exceeds the actual reconstructedclimate change at the site level. In high-density regions,however, neighbouring records can be merged into aCorrespondence to: H. S. Sundqvist(hanna.sundqvist@natgeo.su.se)composite record to increase the signal-to-noise ratio. Thechallenge of producing reliable inferred climate reconstructionsfor the Holocene cannot be underestimated, consideringthe fact that the estimated temperature and precipitationfluctuations during this period are in magnitude similar to, orlower than, the uncertainties the reconstructions. We advocatea more widespread practice of archiving proxy recordsas most of the potentially available reconstructions are notpublished in digital form.

  • 45. Werner, Johannes P.
    et al.
    Divine, Dmitry V.
    Charpentier Ljungqvist, Fredrik
    Stockholm University, Faculty of Humanities, Department of History. Bolin Centre for Climate Research, Sweden.
    Nilsen, Tine
    Francus, Pierre
    Spatio-temporal variability of Arctic summer temperatures over the past 2 millennia2018In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 14, no 4, p. 527-557Article in journal (Refereed)
    Abstract [en]

    In this article, the first spatially resolved and millennium-length summer (June August) temperature reconstruction over the Arctic and sub-Arctic domain (north of 60 degrees N) is presented. It is based on a set of 44 annually dated temperature-sensitive proxy archives of various types from the revised PAGES2k database supplemented with six new recently updated proxy records. As a major advance, an extension of the Bayesian BARCAST climate field (CF) reconstruction technique provides a means to treat climate archives with dating uncertainties. This results not only in a more precise reconstruction but additionally enables joint probabilistic constraints to be imposed on the chronologies of the used archives. The new seasonal CF reconstruction for the Arctic region can be shown to be skilful for the majority of the terrestrial nodes. The decrease in the proxy data density back in time, however, limits the analyses in the spatial domain to the period after 750 CE, while the spatially averaged reconstruction covers the entire time interval of 1-2002 CE. The centennial to millennial evolution of the reconstructed temperature is in good agreement with a general pattern that was inferred in recent studies for the Arctic and its subregions. In particular, the reconstruction shows a pronounced Medieval Climate Anomaly (MCA; here ca. 920-1060 CE), which was characterised by a sequence of extremely warm decades over the whole domain The medieval warming was followed by a gradual cooling into the Little Ice Age (LIA), with 1766-1865 CE as the longest centennial -scale cold period, culminating around 1811-1820 CE for most of the target region. In total over 600 independent realisations of the temperature CF were generated. As showcased for local and regional trends and temperature anomalies, operating in a probabilistic framework directly results in comprehensive uncertainty estimates, even for complex analyses. For the presented multi -scale trend analysis, for example, the spread in different paths across the reconstruction ensemble prevents a robust analysis of features at timescales shorter than ca. 30 years. For the spatial reconstruction, the benefit of using the spatially resolved reconstruction ensemble is demonstrated by focusing on the regional expression of the recent warming and the MCA. While our analysis shows that the peak MCA summer temperatures were as high as in the late 20th and early 21st centuries, the spatial coherence of extreme years over the last decades of the reconstruction (1980s onwards) seems unprecedented at least back until 750 CE. However, statistical testing could not provide conclusive support of the contemporary warming to exceed the peak of the MCA in terms of the pan-Arctic mean summer temperatures: the reconstruction cannot be extended reliably past 2002 CE due to lack of proxy data and thus the most recent warming is not captured.

  • 46. Zhang, Peng
    et al.
    Linderholm, Hans W.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Björklund, Jesper
    Chen, Deliang
    1200 years of warm-season temperature variability in central Scandinavia inferred from tree-ring density2016In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 12, no 6, p. 1297-1312Article in journal (Refereed)
    Abstract [en]

    Despite the emergence of new high-resolution temperature reconstructions around the world, only a few cover the Medieval Climate Anomaly (MCA). Here we present C-Scan, a new Scots pine tree-ring density-based reconstruction of warm-season (April-September) temperatures for central Scandinavia back to 850aEuro-CE, extending the previous reconstruction by 250aEuro-years. C-Scan is based on samples collected in a confined mountain region, adjusted for their differences in altitude and local environment, and standardised using the new RSFi algorithm to preserve low-frequency signals. In C-Scan, the warm peak of MCA occurs ca. 1000-1100aEuro-CE, and the Little Ice Age (LIA) between 1550 and 1900aEuro-CE. Moreover, during the last millennium the coldest decades are found around 1600aEuro-CE, and the warmest 10 and 30aEuro-years occur in the most recent century. By comparing C-Scan with other millennium-long temperature reconstructions from Fennoscandia, regional differences in multi-decadal temperature variability, especially during the warm period of the last millennium are revealed. Although these differences could be due to methodological reasons, they may indicate asynchronous warming patterns across Fennoscandia. Further investigation of these regional differences and the reasons and mechanisms behind them are needed.

  • 47.
    Zhang, Qiong
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Sundqvist, Hanna
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Nilsson, Johan
    Stockholm University, Faculty of Science, Department of Meteorology .
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Climate change between the mid and late Holocene in northern high latitudes: Part 2: Model-data comparisons2010In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, p. 609-626Article in journal (Refereed)
    Abstract [en]

    The climate response over northern high latitudesto the mid-Holocene orbital forcing has been investigated inthree types of PMIP (Paleoclimate Modelling IntercomparisonProject) simulations with different complexity of themodelled climate system. By first undertaking model-datacomparison, an objective selection method has been appliedto evaluate the capability of the climate models to reproducethe spatial response pattern seen in proxy data. The possiblefeedback mechanisms behind the climate response havebeen explored based on the selected model simulations. Subsequentmodel-model comparisons indicate the importanceof including the different physical feedbacks in the climatemodels. The comparisons between the proxy-based reconstructionsand the best fit selected simulations show that overthe northern high latitudes, summer temperature change followsclosely the insolation change and shows a commonfeature with strong warming over land and relatively weakwarming over ocean at 6 ka compared to 0 ka. Furthermore,the sea-ice-albedo positive feedback enhances this response.The reconstructions of temperature show a strongerresponse to enhanced insolation in the annual mean temperaturethan winter and summer temperature. This is verified inthe model simulations and the behaviour is attributed to thelarger contribution from the large response in autumn. Despitea smaller insolation during winter at 6 ka, a pronouncedwarming centre is found over Barents Sea in winter in thesimulations, which is also supported by the nearby northernEurasian continental and Fennoscandian reconstructions.This indicates that in the Arctic region, the response of theocean and the sea ice to the enhanced summer insolationis more important for the winter temperature than the synchronousdecrease of the insolation.

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