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  • 1. Anchukaitis, Kevin J.
    et al.
    Wilson, Rob
    Briffa, Keith R.
    Buntgen, Ulf
    Cook, Edward R.
    D'Arrigo, Rosanne
    Davi, Nicole
    Esper, Jan
    Frank, David
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Hegerl, Gabi
    Helama, Samuli
    Klesse, Stefan
    Krusic, Paul J.
    Stockholm Univ, Dept Phys Geog, Stockholm, Sweden.
    Linderholm, Hans W.
    Myglan, Vladimir
    Osborn, Timothy J.
    Zhang, Peng
    Rydval, Milos
    Schneider, Lea
    Schurer, Andrew
    Wiles, Greg
    Zorita, Eduardo
    Last millennium Northern Hemisphere summer temperatures from tree rings: Part II, spatially resolved reconstructions2017In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 163, p. 1-22Article, review/survey (Refereed)
    Abstract [en]

    Climate field reconstructions from networks of tree-ring proxy data can be used to characterize regional scale climate changes, reveal spatial anomaly patterns associated with atmospheric circulation changes, radiative forcing, and large-scale modes of ocean-atmosphere variability, and provide spatiotemporal targets for climate model comparison and evaluation. Here we use a multiproxy network of tree-ring chronologies to reconstruct spatially resolved warm season (May August) mean temperatures across the extratropical Northern Hemisphere (40-90 degrees N) using Point-by-Point Regression (PPR). The resulting annual maps of temperature anomalies (750-1988 CE) reveal a consistent imprint of volcanism, with 96% of reconstructed grid points experiencing colder conditions following eruptions. Solar influences are detected at the bicentennial (de Vries) frequency, although at other time scales the influence of insolation variability is weak. Approximately 90% of reconstructed grid points show warmer temperatures during the Medieval Climate Anomaly when compared to the Little Ice Age, although the magnitude varies spatially across the hemisphere. Estimates of field reconstruction skill through time and over space can guide future temporal extension and spatial expansion of the proxy network.

  • 2. 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.

  • 3. Björklund, Jesper A.
    et al.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Swedish University of Agricultural Sciences, Sweden.
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Grudd, Håkan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Josefsson, Torbjörn
    Östlund, Lars
    Linderholm, Hans W.
    Advances towards improved low-frequency tree-ring reconstructions, using an updated Pinus sylvestris L. MXD network from the Scandinavian Mountains2013In: Journal of Theoretical and Applied Climatology, ISSN 0177-798X, E-ISSN 1434-4483, Vol. 113, no 3-4, p. 697-710Article in journal (Refereed)
    Abstract [en]

    The dendrochronological use of the parameter maximum density (MXD) in Pinus Sylvestris L., at high latitudes, has provided valuable insights into past summer temperature variations. Few long MXD chronologies, from climatically coherent regions, exist today, with the exception being in northern Europe. Five, 500-year-long, Fennoscandian, MXD chronologies were compared with regard to their common variability and climate sensitivity. They were used to test Signal-free standardization techniques, to improve inferences of low-frequency temperature variations. Climate analysis showed that, in accordance with previous studies on MXD in Fennoscandia, the summer temperature signal is robust (R (2) > 50 %) and reliable over this climatically coherent region. A combination of Individual standardization and regional curve standardization is recommended to refine long-term variability from these MXD chronologies and relieve problems arising from low replication and standardization end-effects.

  • 4. Björklund, Jesper
    et al.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Seftigen, Kristina
    Zhang, Peng
    Linderholm, Hans W.
    Using adjusted Blue Intensity data to attain high-quality summer temperature information: A case study from Central Scandinavia2015In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 25, no 3, p. 547-556Article in journal (Refereed)
    Abstract [en]

    The inexpensive Blue Intensity proxy has been considered a complement or surrogate to maximum latewood density (MXD), but is associated with biases from differential staining between sapwood and heartwood and also between deadwood samples and living-wood samples that compromise centennial-scale information. Here, we show that, with some minor adjustments, Blue Intensity (BI) is comparable with MXD or Density (=the difference or contrast between latewood and earlywood density) in dendroclimatological reconstructions of summer temperatures in the Central Scandinavian region, using Pinus sylvestris L. (Scots pine), on annual and multi-centennial timescales. By using BI, this bias is significantly reduced, but the contrast between earlywood and latewood in BI is altered with degree of staining, while for density it is not. Darker deadwood samples have a reduced contrast compared with the lighter living-wood samples that make BI and Density chronologies diverge. Here, we quantify this behaviour in BI and offer an adjustment that can reduce this bias. The adjustment can be derived on independent samples, so in future work on BI, parallel density measurements are not necessary. We apply this methodology to two Central Scandinavian Scots pine chronologies that averaged into a composite is able to reconstruct summer temperatures with an explained variance in excess of 60% in each verification period using a split sample calibration verification procedure. Although the amount of data used to derive this contrast adjustment produces desirable results, more tests are needed to confirm its performance, and we suggest that future work on the BI proxy should aim for a small subset of parallel BI and density measurements while the bulk of the data is only measured with the BI technique. This is to ensure that the adjustment is continuously updated with new data and that the conclusions derived here are robust.

  • 5. Campbell, Rochelle
    et al.
    McCarroll, Danny
    Robertson, Iain
    Loader, Neil J.
    Grudd, Håkan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    BLUE INTENSITY IN PINUS SYLVESTRIS TREE RINGS: A MANUAL FOR A NEW PALAEOCLIMATE PROXY2011In: TREE-RING RESEARCH, ISSN 1536-1098, Vol. 67, no 2, p. 127-134Article in journal (Refereed)
    Abstract [en]

    Minimum blue intensity is a reflected light imaging technique that provides an inexpensive, robust and reliable surrogate for maximum latewood density. In this application it was found that temperature reconstructions from resin-extracted samples of Pious sylvestris (L.) from Fennoscandia provide results equivalent to conventional x-ray densitometry. This paper describes the implementation of the blue intensity method using commercially available software and a flat-bed scanner. A calibration procedure is presented that permits results obtained by different laboratories, or using different scanners, to be compared. In addition, the use of carefully prepared and chemically treated 10-mm-diameter cores are explored; suggesting that it may not be necessary to produce thin laths with the rings aligned exactly perpendicular to the measurement surface.

  • 6. Campell, R
    et al.
    Robertson, I
    McCarrol, D
    Loader, NJ
    Grudd, H
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gunnarson, B
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Calibration proxy-climate relationshis in central Sweden using stable isotopes in tree rings.2008Conference paper (Other academic)
  • 7. Cunningham, Laura K.
    et al.
    Austin, William E. N.
    Knudsen, Karen Luise
    Eiriksson, Jon
    Scourse, James D.
    Wanamaker, Alan D., Jr.
    Butler, Paul G.
    Cage, Alix G.
    Richter, Thomas
    Husum, Katrine
    Hald, Morten
    Andersson, Carin
    Zorita, Eduardo
    Linderholm, Hans W.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Swedish University of Agricultural Sciences, Sweden.
    Sicre, Marie-Alexandrine
    Sejrup, Hans Petter
    Jiang, Hui
    Wilson, Rob J. S.
    Reconstructions of surface ocean conditions from the northeast Atlantic and Nordic seas during the last millennium2013In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 23, no 7, p. 921-935Article in journal (Refereed)
    Abstract [en]

    We undertake the first comprehensive effort to integrate North Atlantic marine climate records for the last millennium, highlighting some key components common within this system at a range of temporal and spatial scales. In such an approach, careful consideration needs to be given to the complexities inherent to the marine system. Composites therefore need to be hydrographically constrained and sensitive to both surface water mass variability and three-dimensional ocean dynamics. This study focuses on the northeast (NE) North Atlantic Ocean, particularly sites influenced by the North Atlantic Current. A composite plus regression approach is used to create an inter-regional NE North Atlantic reconstruction of sea surface temperature (SST) for the last 1000 years. We highlight the loss of spatial information associated with large-scale composite reconstructions of the marine environment. Regional reconstructions of SSTs off the Norwegian and Icelandic margins are presented, along with a larger-scale reconstruction spanning the NE North Atlantic. The latter indicates that the Medieval Climate Anomaly' warming was most pronounced before ad 1200, with a long-term cooling trend apparent after ad 1250. This trend persisted until the early 20th century, while in recent decades temperatures have been similar to those inferred for the Medieval Climate Anomaly'. The reconstructions are consistent with other independent records of sea-surface and surface air temperatures from the region, indicating that they are adequately capturing the climate dynamics of the last millennium. Consequently, this method could potentially be used to develop large-scale reconstructions of SSTs for other hydrographically constrained regions.

  • 8. Farahat, Emad
    et al.
    Zhang, Peng
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Fuentes, Mauricio
    Stridbeck, Petter
    Linderholm, Hans W.
    Are standing dead trees (snags) suitable as climate proxies? A case study from the central Scandinavian Mountains2018In: Scandinavian Journal of Forest Research, ISSN 0282-7581, E-ISSN 1651-1891, Vol. 33, no 2, p. 114-124Article in journal (Refereed)
    Abstract [en]

    Standing dead trees (snags) play important roles in forest ecology by storing carbon as well as providing habitats for many species. Moreover, snags preserved for hundreds of years can provide useful data to extend tree-ring chronologies used for climatological and ecological studies beyond the lifespans of living trees. Here we examined the growth patterns of Scots pine snags from the central Scandinavian Mountains, in relation to still living trees. Using changes point analyses, we showed that a majority (74%) of the snags displayed significant negative growth changes prior (on average 17 years) to death. Change points around the same years were also seen in living trees, but they recovered their growth. The average growth reduction of snags showing negative growth changes before death was 46%. In most cases the final growth change points coincided with very cold summers, or to a lesser degree to period of cool summers. It was suggested that pines ending up as snags were less resilient than the trees which continued living, and thus not able to recover after cold summer events. Since the snag growth reductions prior to death were partly unrelated to climate, care should be taken when using such data in dendroclimatological studies.

  • 9. Fuentes, Mauricio
    et al.
    Salo, Riikka
    Björklund, Jesper
    Seftigen, Kristina
    Zhang, Peng
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Aravena, Juan-Carlos
    Linderholm, Hans W.
    A 970-year-long summer temperature reconstruction from Rogen, west-central Sweden, based on blue intensity from tree rings2018In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 28, no 2, p. 254-266Article in journal (Refereed)
    Abstract [en]

    To assess past climate variability in west-central Scandinavia, a new 972-year-long temperature reconstruction, based on adjusted delta blue intensity (ΔBIadj), was created. Presently, it is the longest blue intensity chronology in Fennoscandia and the third longest in the northern hemisphere. Measurements were obtained from 119 tree line Scots pine (Pinus sylvestris L.) samples from Rogen, in the central Scandinavian Mountains, Sweden. Early and latewood blue intensity absorption data were used to create ΔBIadj. The data were detrended using a signal-free regional curve standardization method (RSFi) to minimize biological noise and maximize low-frequency climate information. The Rogen ΔBIadj chronology has a substantially stronger temperature signal at inter-annual timescales than the corresponding tree-ring width (RW) chronology, and it displays good spatial representation for the south-central parts of Scandinavia. The ΔBIadj summer (June through August) temperature reconstruction, extending back to 1038 CE, exhibits three warm periods in 1040–1190 CE, 1370–1570 CE and the 20th century and one extended cold period between 1570 and 1920 CE. Regional summer temperature anomalies are associated with a Scandinavian–Greenland dipole sea-level pressure pattern, which has been stable for the past several centuries. Major volcanic eruptions produce distinct anomalies of ΔBIadj indices indicating cooling of summer temperatures in the subsequent years. Our results show that ΔBIadj from Pinus sylvestris in Scandinavia is a suitable proxy providing opportunities to explore past temperature variability at various frequencies, atmospheric dynamics and variability in external forcing. Nevertheless, long-term trend differences with RW imply that further research is needed to fully understand the application of this technique in dendroclimatology.

  • 10.
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Nycklar till kunskap  - om människan bruk av naturen2010In: Nycklar till kunskap  - om människan bruk av naturen: Dendrokronologi / [ed] Eds Tunón, H. and Dahlström, A, Stockholm: Kungl. Skogs- och Lantbruksakademin , 2010, p. 357-360Chapter in book (Other academic)
  • 11.
    Gunnarson, Björn E.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Josefsson, Torbjörn
    Linderholm, Hans W.
    Östlund, Lars
    Legacies of pre-industrial land use can bias modern tree-ring climate calibrations2012In: Climate Research (CR), ISSN 0936-577X, E-ISSN 1616-1572, Vol. 53, no 1, p. 63-76Article in journal (Refereed)
    Abstract [en]

    In Scandinavia, dendrochronological reconstructions of past climate have mostly been based on tree-ring data from forests in which there has been, supposedly, very little or no human impact. However, human land use in sub-alpine forests has a substantially longer history and more profound effects on the forest ecosystems than previously acknowledged. Therefore, to assess human influence on tree-ring patterns over the last 500 yr, we have analyzed tree-ring patterns using trees from 2 abandoned Sami settlements and a reference site with no human impact-all situated in the Tjeggelvas Nature Reserve in north-west Sweden. The hypothesis was that land use legacies have affected tree-ring patterns, and in turn, the resulting palaeoclimate inferences that have been made from these patterns. Our results show that climate signals are strongest at the reference site and weakest at one of the settlement sites. From the 1940s to the present, tree growth at this settlement site has been significantly lower than at the reference site. Lower tree growth at old settlements may have resulted from rapid changes in the traditional land use, or following the abrupt change when the settlements were abandoned. Without site-specific know ledge of past land use, there is a high risk of accidently sampling trees that have been affected by human-induced disturbances in the past. This may create bias in the climate signals inferred from such trees, and hence bias the outcome of climate reconstructions. We therefore recommend sampling several separate sites in study areas to improve the robustness of inferences.

  • 12.
    Gunnarson, Björn
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Linderholm, Hans W.
    Moberg, Anders
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Improving a tree-ring reconstruction from west-central Scandinavia: 900 years of warm-season temperatures2011In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 36, no 1-2, p. 97-108Article in journal (Refereed)
    Abstract [en]

    Dendroclimatological sampling of Scots pine (Pinus sylvestris L.) has been made in the province of Jamtland, in the west-central Scandinavian mountains, since the 1970s. The tree-ring width (TRW) chronology spans several thousand years and has been used to reconstruct June August temperatures back to 1632 BC. A maximum latewood density (MXD) dataset, covering the period AD 1107-1827 (with gap 1292-1315) was presented in the 1980s by Fritz Schweingruber. Here we combine these historical MXD data with recently collected MXD data covering AD 1292-2006 into a single reconstruction of April September temperatures for the period AD 1107 2006. Regional curve standardization (RCS) provides more low-frequency variability than non-RCS and stronger correlation with local seasonal temperatures (51% variance explained). The MXD chronology shows a stronger relationship with temperatures than the TRW data, but the two chronologies show similar multi-decadal variations back to AD 1500. According to the MXD chronology, the period since AD 1930 and around AD 1150-1200 were the warmest during the last 900 years. Due to large uncertainties in the early part of the combined MXD chronology, it is not possible to conclude which period was the warmest. More sampling of trees growing near the tree-line is needed to further improve the MXD chronology.

  • 13. Josefsson, Torbjorn
    et al.
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Liedgren, Lars
    Bergman, Ingela
    Ostlund, Lars
    Historical human influence on forest composition and structure in boreal Fennoscandia2010In: Canadian Journal of Forest Research, ISSN 0045-5067, E-ISSN 1208-6037, Vol. 40, no 5, p. 872-884Article in journal (Refereed)
  • 14. Kirchhefer, Andreas
    et al.
    Young, Giles
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Loader, Niel
    McCarroll, Danny
    Climate from a millennial pine chronology from coastal North-Norway.2008Conference paper (Other (popular science, discussion, etc.))
  • 15. Kirchhefer, Andreas
    et al.
    Young, Giles
    Loader, Niel
    McCarrol, Danny
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    : Low-frequency temperature variability extracted from ring widths in Forfjordalen, northern Norway.2008Conference paper (Other (popular science, discussion, etc.))
  • 16. Levanic, T
    et al.
    Gricar, J
    Grudd, H
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gunnarson, B
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Climate sensitivity of European larch (Larix deciduas Mill.) in the southeasternpart of the Alps2008Conference paper (Other academic)
  • 17. Linderholm, Hans
    et al.
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    900 years of west central Scandinavian april-september temperatures2008Conference paper (Other (popular science, discussion, etc.))
  • 18. Linderholm, Hans W.
    et al.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Liu, Yu
    Comparing Scots pine tree-ring proxies and detrending methods among sites in Jamtland, west-central Scandinavia2010In: Dendrochronologia, ISSN 1125-7865, E-ISSN 1612-0051, Vol. 28, no 4, p. 239-249Article in journal (Refereed)
    Abstract [en]

    Scots pine tree-ring width (TRW) data from Jamtland in the Central Scandinavian Mountains has been used to reconstruct summer temperatures back to 1630 BC. However, it was recently shown that this reconstruction was of limited spatial importance. In this paper, we aim to explain this limitation in the TRW data as a temperature proxy, as well as assess the temperature information from new maximum latewood density (MXD) data. Furthermore, the effect of two standardization methods is evaluated: regional curve standardization (RCS) and a more traditional standardization, termed "non-RCS" standardization. Three TRW and two MXD sites were analyzed. Our results showed that despite the proximity to the Norwegian Sea, the MXD data is a powerful temperature proxy. Difference among sites in TRW data, especially on decadal timescales, together with a lower temperature association, suggests that other factors, such as changes in the local climate regimes, weakens the temperature signal. In general the RCS method overestimates pine growth trends in the latter half of the twentieth century, a feature not seen when using "non-RCS" standardization. This is likely due to an age-bias of older trees in most recent parts of the tree-ring chronologies. This effect will have consequences when reconstructing climate with tree-ring data. To overcome this problem, all age-classes should be represented throughout a chronology. If this is not possible, the use of "non-RCS" standardization is recommended, although this method results in a loss of low-frequency variability. (C) 2010 Istituto ltaliano di Dendrocronologia.

  • 19. 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.

  • 20. Löwemark, Ludvig
    et al.
    Chen, Tzu-Tung
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Su, Chih-Chieh
    Yang, Tien-Nan
    Huang, Jyh-Jaan
    Lan, Yung-Hsiang
    Burr, George
    The Tienchi Pond on Lanyu Island (Western Pacific): Lake formation and potential as environmental archive2015In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 114, p. 435-446Article in journal (Refereed)
    Abstract [en]

    The formation, evolution, and potential as a climate archive of Tienchi Pond was evaluated using geomor-phology, sedimentology, geochemistry, and AMS radiocarbon measurements of tree ring cores. Tienchi Pond is a small ephemeral lake situated on the subtropical Lanyu Island in the Western Pacific Ocean about 90 km east of southern Taiwan. The lake is situated on a mountain ridge and is today characterized by numerous dead trees and stumps. The position at the boundary between the two main components of the SE Asian monsoon system, the Asian landmass and the Western Pacific, makes this lake particularly interesting as a potential natural climate archive. Since previous studies have shown that the bedrock underlying the lake formed over 2 my ago, a volcanic crater origin was ruled out. Rather, element ratios of K/Ti and Ca/Fe together with Pb-210 dating suggest a recent origin for the lake, probably as the result of a natural damming event in the southern, narrower, part of the lake basin, or as a result of increased precipitation at the end of the Little Ice Age (or a combination of both). Radiocarbon measurements on a dead tree near the lake perimeter show C-14 levels of more than 1.7% modern radiocarbon near the bark, suggesting that the tree died in the early 1970s. In contrast, by correlating a large number of radiocarbon measurements to a northern hemisphere reference curve, it was shown that trees closer to the center of the lake died in the early 20th century. This agrees with a scenario where a small lake formed by initial damming, and gradually grew larger through erosion of the surrounding shore lines caused by rapid lake level oscillations.

  • 21. McCarroll, Danny
    et al.
    Loader, Neil J.
    Jalkanen, Risto
    Gagen, Mary H.
    Grudd, Håkan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kirchhefer, Andreas J.
    Friedrich, Michael
    Linderholm, Hans W.
    Lindholm, Markus
    Boettger, Tatjana
    Los, Sietse O.
    Remmele, Sabine
    Kononov, Yuri M.
    Yamazaki, Yasuhiro H.
    Young, Giles H. F.
    Zorita, Eduardo
    A 1200-year multiproxy record of tree growth and summer temperature at the northern pine forest limit of Europe2013In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 23, no 4, p. 471-484Article in journal (Refereed)
    Abstract [en]

    Combining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing.

  • 22. Rydval, Milos
    et al.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Loader, Neil J.
    Cook, Edward R.
    Druckenbrod, Daniel L.
    Wilson, Rob
    Spatial reconstruction of Scottish summer temperatures from tree rings2017In: International Journal of Climatology, ISSN 0899-8418, E-ISSN 1097-0088, Vol. 37, no 3, p. 1540-1556Article in journal (Refereed)
    Abstract [en]

    A detailed understanding of past temporal patterns and spatial expression of temperature variations is important to place recent anthropogenic climate change into a longer term context. In order to fill the current gap in our understanding of northwest European temperature variability, point-by-point principal component regression was used to reconstruct a spatial field of 0.5 degrees temperature grids across Scotland. A sequence of reconstructions utilizing several combinations of detrending and disturbance correction procedures, and a selection of tree-ring parameters [including ring width (RW), maximum latewood density (MXD) and blue intensity (BI)] was used in an evaluation of reconstruction skill. The high resolution of the reconstructed field serves also as a diagnostic tool to spatially assess the temperature reconstruction potential of local chronologies. Best reconstruction results, reaching calibration r(2)=65.8% and verification r(2)=63.7% in central Scotland over the 1901-1976 period, were achieved using disturbance-corrected and signal-free detrended RW chronologies merged with BI data after low-pass (high-pass) filtering the RW (BI) chronologies. Calibration and verification r(2)>50% was attained for central, north and east Scotland, >40% in west and northwest, and >30% in southern Scotland with verification of nearly all grids showing some reconstruction skill. However, the full calibration potential of reconstructions outside central Scotland was reduced either due to residual disturbance trends undetected by the disturbance correction procedure or due to other climatic or non-climatic factors which may have adversely affected the strength of the climate signal.

  • 23. Rydval, Milos
    et al.
    Larsson, Lars-Åke
    McGlynn, Laura
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Loader, Neil J.
    Young, Giles H. F.
    Wilson, Rob
    Blue intensity for dendroclimatology: Should we have the blues? Experiments from Scotland2014In: Dendrochronologia, ISSN 1125-7865, E-ISSN 1612-0051, Vol. 32, no 3, p. 191-204Article in journal (Refereed)
    Abstract [en]

    Blue intensity (BI) has the potential to provide information on past summer temperatures of a similar quality to maximum latewood density (MXD), but at a substantially reduced cost. This paper provides a methodological guide to the generation of BI data using a new and affordable BI measurement system; CooRecorder. Focussing on four sites in the Scottish Highlands from a wider network of 42 sites developed for the Scottish Pine Project, BI and MXD data from Scots pine (Pious sylvestris L.) were used to facilitate a direct comparison between these parameters. A series of experiments aimed at identifying and addressing the limitations of BI suggest that while some potential limitations exist, these can be minimised by adhering to appropriate BI generation protocols. The comparison of BI data produced using different resin-extraction methods (acetone vs. ethanol) and measurement systems (CooRecorder vs. WinDendro) indicates that comparable results can be achieved. Using samples from the same trees, a comparison of both BI and MXD with instrumental climate data revealed that overall, BI performs as well as, if not better than, MXD in reconstructing past summer temperatures (BI r(2) = 0.38-0.46; MXD r(2) = 0.34-0.35). Although reconstructions developed using BI and MXD data appeared equally robust, BI chronologies were more sensitive to the choice of detrending method due to differences in the relative trends of non-detrended raw BI and MXD data. This observation suggests that the heartwood-sapwood colour difference is not entirely removed using either acetone or ethanol chemical treatment, which may ultimately pose a potential limitation for extracting centennial and longer timescale information when using BI data from tree species that exhibit a distinct heartwood-sapwood colour difference. Additional research is required in order to develop new methods to overcome this potential limitation. However, the ease with which BI data can be produced should help justify and recognise the role of this parameter as a potential alternative to MXD, particularly when MXD generation may be impractical or unfeasible for financial or other reasons.

  • 24. Rydval, Miloš
    et al.
    Loader, Neil J.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Druckenbrod, Daniel L.
    Linderholm, Hans W.
    Moreton, Steven G.
    Wood, Cheryl V.
    Wilson, Rob
    Reconstructing 800 years of summer temperatures in Scotland from tree rings2017In: Climate Dynamics, ISSN 0930-7575, E-ISSN 1432-0894, Vol. 49, no 9-10, p. 2951-2974Article in journal (Refereed)
    Abstract [en]

    This study presents a summer temperature reconstruction using Scots pine tree-ring chronologies for Scotland allowing the placement of current regional temperature changes in a longer-term context. 'Living-tree' chronologies were extended using 'subfossil' samples extracted from nearshore lake sediments resulting in a composite chronology > 800 years in length. The North Cairngorms (NCAIRN) reconstruction was developed from a set of composite blue intensity high-pass and ring-width low-pass filtered chronologies with a range of detrending and disturbance correction procedures. Calibration against July-August mean temperature explains 56.4% of the instrumental data variance over 1866-2009 and is well verified. Spatial correlations reveal strong coherence with temperatures over the British Isles, parts of western Europe, southern Scandinavia and northern parts of the Iberian Peninsula. NCAIRN suggests that the recent summer-time warming in Scotland is likely not unique when compared to multi-decadal warm periods observed in the 1300s, 1500s, and 1730s, although trends before the mid-sixteenth century should be interpreted with some caution due to greater uncertainty. Prominent cold periods were identified from the sixteenth century until the early 1800s-agreeing with the so-called Little Ice Age observed in other tree-ring reconstructions from Europe-with the 1690s identified as the coldest decade in the record. The reconstruction shows a significant cooling response 1 year following volcanic eruptions although this result is sensitive to the datasets used to identify such events. In fact, the extreme cold (and warm) years observed in NCAIRN appear more related to internal forcing of the summer North Atlantic Oscillation.

  • 25. Wilson, Rob
    et al.
    Anchukaitis, Kevin
    Briffa, Keith R.
    Buentgen, Ulf
    Cook, Edward
    D'Arrigo, Rosanne
    Davi, Nicole
    Esper, Jan
    Frank, Dave
    Gunnarson, Björn
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Hegerl, Gabi
    Helama, Samuli
    Klesse, Stefan
    Krusic, Paul J.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Linderholm, Hans W.
    Myglan, Vladimir
    Osborn, Timothy J.
    Rydval, Milos
    Schneider, Lea
    Schurer, Andrew
    Wiles, Greg
    Zhang, Peng
    Zorita, Eduardo
    Last millennium northern hemisphere summer temperatures from tree rings: Part I2016In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 134, p. 1-18Article, review/survey (Refereed)
    Abstract [en]

    Large-scale millennial length Northern Hemisphere (NH) temperature reconstructions have been progressively improved over the last 20 years as new datasets have been developed. This paper, and its companion (Part II, Anchukaitis et al. in prep), details the latest tree-ring (TR) based NH land air temperature reconstruction from a temporal and spatial perspective. This work is the first product of a consortium called N-TREND (Northern Hemisphere Tree-Ring Network Development) which brings together dendroclimatologists to identify a collective strategy for improving large-scale summer temperature reconstructions. The new reconstruction, N-TREND2015, utilises 54 records, a significant expansion compared with previous TR studies, and yields an improved reconstruction with stronger statistical calibration metrics. N-TREND2015 is relatively insensitive to the compositing method and spatial weighting used and validation metrics indicate that the new record portrays reasonable coherence with large scale summer temperatures and is robust at all time-scales from 918 to 2004 where at least 3 TR records exist from each major continental mass. N-TREND2015 indicates a longer and warmer medieval period (similar to 900 1170) than portrayed by previous TR NH reconstructions and by the CMIP5 model ensemble, but with better overall agreement between records for the last 600 years. Future dendroclimatic projects should focus on developing new long records from data-sparse regions such as North America and eastern Eurasia as well as ensuring the measurement of parameters related to latewood density to complement ring-width records which can improve local based calibration substantially.

  • 26. Wilson, Rob
    et al.
    Wilson, David
    Rydval, Milos
    Crone, Anne
    Buntgen, Ulf
    Clark, Sylvie
    Ehmer, Janet
    Forbes, Emma
    Fuentes, Mauricio
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Linderholm, Hans W.
    Nicolussi, Kurt
    Wood, Cheryl
    Mills, Coralie
    Facilitating tree-ring dating of historic conifer timbers using Blue Intensity2017In: Journal of Archaeological Science, ISSN 0305-4403, E-ISSN 1095-9238, Vol. 78, p. 99-111Article in journal (Refereed)
    Abstract [en]

    Dendroarchaeology almost exclusively uses ring-width (RW) data for dating historical structures and artefacts. Such data can be used to date tree-ring sequences when regional climate dominates RW variability. However, the signal in RW data can be obscured due to site specific ecological influences (natural and anthropogenic) that impact crossdating success. In this paper, using data from Scotland, we introduce a novel tree-ring parameter (Blue Intensity BI) and explore its utility for facilitating dendrohistorical dating of conifer samples. BI is similar to latewood density as they both reflect the combined hemicellulose, cellulose and lignin content in the latewood cell walls of conifer species and the amount of these compounds is strongly controlled, at least for trees growing in temperature limited locations, by late summer temperatures. BI not only expresses a strong climate signal, but is also less impacted by site specific ecological influences. It can be concurrently produced with RW data from images of finely sanded conifer samples but at a significantly reduced cost compared to traditional latewood density. Our study shows that the probability of successfully crossdating historical samples is greatly increased using BI compared to RW. Furthermore, due to the large spatial extent of the summer temperature signal expressed by such data, a sparse multi -species conifer network of long BI chronologies across Europe could be used to date and loosely provenance imported material.

  • 27. Young, Giles H. F.
    et al.
    Demmler, Joanne C.
    Gunnarson, Björn E.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kirchhefer, Andreas J.
    Loader, Neil J.
    McCarroll, Danny
    Age trends in tree ring growth and isotopic archives: A case study of Pinus sylvestris L. from northwestern Norway2011In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 25, p. GB2020-Article in journal (Refereed)
    Abstract [en]

    Measurements of tree ring width and relative density have contributed significantly to many of the large-scale reconstructions of past climatic change, but to extract the climate signal it is first necessary to remove any nonclimatic age-related trends. This detrending can limit the lower-frequency climate information that may be extracted from the archive (the segment length curse). This paper uses a data set of ring widths, maximum latewood density and stable carbon and oxygen isotopes from 28 annually resolved series of known-age Pinus sylvestris L. trees in northwestern Norway to test whether stable isotopes in tree rings require an equivalent statistical detrending. Results indicate that stable oxygen and carbon isotope ratios from tree rings whose cambial age exceeds c.50 years exhibit no significant age trends and thus may be used to reconstruct environmental variability and physiological processes at this site without the potential loss of low-frequency information associated with detrending.

  • 28. 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.

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