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Publications (10 of 15) Show all publications
Lashgari, K., Brattström, G., Moberg, A. & Sundberg, R. (2022). Evaluation of simulated responses to climate forcings: a flexible statistical framework using confirmatory factor analysis and structural equation modelling – Part 1: Theory. Advances in Statistical Climatology, Meteorology and Oceanography, 8(2), 225-248
Open this publication in new window or tab >>Evaluation of simulated responses to climate forcings: a flexible statistical framework using confirmatory factor analysis and structural equation modelling – Part 1: Theory
2022 (English)In: Advances in Statistical Climatology, Meteorology and Oceanography, E-ISSN 2364-3587, Vol. 8, no 2, p. 225-248Article in journal (Refereed) Published
Abstract [en]

Evaluation of climate model simulations is a crucial task in climate research. Here, a new statistical framework is proposed for evaluation of simulated temperature responses to climate forcings against temperature reconstructions derived from climate proxy data for the last millennium. The framework includes two types of statistical models, each of which is based on the concept of latent (unobservable) variables: confirmatory factor analysis (CFA) models and structural equation modelling (SEM) models. Each statistical model presented is developed for use with data from a single region, which can be of any size. The ideas behind the framework arose partly from a statistical model used in many detection and attribution (D&A) studies. Focusing on climatological characteristics of five specific forcings of natural and anthropogenic origin, the present work theoretically motivates an extension of the statistical model used in D&A studies to CFA and SEM models, which allow, for example, for non-climatic noise in observational data without assuming the additivity of the forcing effects. The application of the ideas of CFA is exemplified in a small numerical study, whose aim was to check the assumptions typically placed on ensembles of climate model simulations when constructing mean sequences. The result of this study indicated that some ensembles for some regions may not satisfy the assumptions in question.

National Category
Probability Theory and Statistics Earth and Related Environmental Sciences
Research subject
Mathematical Statistics
Identifiers
urn:nbn:se:su:diva-214598 (URN)10.5194/ascmo-8-225-2022 (DOI)2-s2.0-85145563123 (Scopus ID)
Funder
Swedish Research Council, C0592401
Available from: 2023-02-06 Created: 2023-02-06 Last updated: 2023-02-10Bibliographically approved
Lashgari, K., Moberg, A. & Brattström, G. (2022). Evaluation of simulated responses to climate forcings: a flexible statistical framework using confirmatory factor analysis and structural equation modelling – Part 2: Numerical experiment. Advances in Statistical Climatology, Meteorology and Oceanography, 8(2), 249-271
Open this publication in new window or tab >>Evaluation of simulated responses to climate forcings: a flexible statistical framework using confirmatory factor analysis and structural equation modelling – Part 2: Numerical experiment
2022 (English)In: Advances in Statistical Climatology, Meteorology and Oceanography, E-ISSN 2364-3587, Vol. 8, no 2, p. 249-271Article in journal (Refereed) Published
Abstract [en]

The performance of a new statistical framework, developed for the evaluation of simulated temperature responses to climate forcings against temperature reconstructions derived from climate proxy data for the last millennium, is evaluated in a so-called pseudo-proxy experiment, where the true unobservable temperature is replaced with output data from a selected simulation with a climate model. Being an extension of the statistical model used in many detection and attribution (D&A) studies, the framework under study involves two main types of statistical models, each of which is based on the concept of latent (unobservable) variables: confirmatory factor analysis (CFA) models and structural equation modelling (SEM) models. Within the present pseudo-proxy experiment, each statistical model was fitted to seven continental-scale regional data sets. In addition, their performance for each defined region was compared to the performance of the corresponding statistical model used in D&A studies. The results of this experiment indicated that the SEM specification is the most appropriate one for describing the underlying latent structure of the simulated temperature data in question. The conclusions of the experiment have been confirmed in a cross-validation study, presuming the availability of several simulation data sets within each studied region. Since the experiment is performed only for zero noise level in the pseudo-proxy data, all statistical models, chosen as final regional models, await further investigation to thoroughly test their performance for realistic levels of added noise, similar to what is found in real proxy data for past temperature variations.

National Category
Probability Theory and Statistics Earth and Related Environmental Sciences
Research subject
Mathematical Statistics
Identifiers
urn:nbn:se:su:diva-214600 (URN)10.5194/ascmo-8-249-2022 (DOI)2-s2.0-85145548029 (Scopus ID)
Funder
Swedish Research Council, C0592401
Available from: 2023-02-06 Created: 2023-02-06 Last updated: 2023-02-10Bibliographically approved
Sharma, S., Barrie, L. A., Magnusson, E., Brattström, G., Leaitch, W. R., Steffen, A. & Landsberger, S. (2019). A Factor and Trends Analysis of Multidecadal Lower Tropospheric Observations of Arctic Aerosol Composition, Black Carbon, Ozone, and Mercury at Alert, Canada. Journal of Geophysical Research - Atmospheres, 124(24), 14133-14161
Open this publication in new window or tab >>A Factor and Trends Analysis of Multidecadal Lower Tropospheric Observations of Arctic Aerosol Composition, Black Carbon, Ozone, and Mercury at Alert, Canada
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2019 (English)In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 124, no 24, p. 14133-14161Article in journal (Refereed) Published
Abstract [en]

Observations from 1980 to 2013 of 20 aerosol constituents, ozone and mercury at Alert, Canada (82.50 degrees N, 62.35 degrees W), were analyzed for trends and dominant factors of the Arctic haze during winter and spring. Trends reflect changing emissions in Eurasia, the main source region for surface pollution in the high Arctic. SO42-, H+, NH4,+ K+, Cu, Ni, Pb, Zn, nonsoil V, nonsoil Mn, and equivalent black carbon decreased between 23% and 80% as emissions declined rapidly in northern Eurasia during the early 1990s. NO3- increased by 20% as aerosol acidity declined. Metals were linked to emissions from smelting and fossil fuel combustion. In winter, ozone increased by 5% over 23 years, consistent with other observations and global modeling. Twelve PMF factors emerged for the dark period (November to February) and 13 for the light period (March to May). Eleven PMF factors are common to both dark and light, a twelfth factor was associated with sulfate in the dark and nitrate in the light, and the thirteenth (light period) was related to ozone and gaseous mercury depletion near Alert. IODINE and NITRATE factors, important for Arctic chemistry, changed with sunlight. In the light, 50% of all NO3- was on the NITRATE factor, while in the dark, most was associated with MODIFIED SEA SALT and equivalent black carbon. In the dark (light), 90% (28%) of iodine were found on the factor IODINE and 58% associated with SEA-SALT and MODIFIED SEA-SALT. These results help in understanding the role of atmospheric chemistry in weather and climate processes.

Keywords
Arctic haze, Arctic aerosol components, sources and trends in Arctic pollution, black carbon, polar sunrise chemistry, detection of climate change
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-178669 (URN)10.1029/2019JD030844 (DOI)000504282500001 ()
Available from: 2020-02-20 Created: 2020-02-20 Last updated: 2022-03-23Bibliographically approved
Charpentier Ljungqvist, F., Zhang, Q., Brattström, G., Krusic, P. J., Seim, A., Li, Q., . . . Moberg, A. (2019). Centennial-Scale Temperature Change in Last Millennium Simulations and Proxy-Based Reconstructions. Journal of Climate, 32(9), 2441-2482
Open this publication in new window or tab >>Centennial-Scale Temperature Change in Last Millennium Simulations and Proxy-Based Reconstructions
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2019 (English)In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 32, no 9, p. 2441-2482Article in journal (Refereed) Published
Abstract [en]

Systematic comparisons of proxy-based reconstructions and climate model simulations of past millennium temperature variability offer insights into climate sensitivity and feedback mechanisms, besides allowing model evaluation independently from the period covered by instrumental data. Such simulation-reconstruction comparisons can help to distinguish more skillful models from less skillful ones, which may subsequently help to develop more reliable future projections. This study evaluates the low-frequency simulation-reconstruction agreement within the past millennium through assessing the amplitude of temperature change between the Medieval Climate Anomaly (here, 950-1250 CE) and the Little Ice Age (here, 1450-1850 CE) in PMIP3 model simulations compared to proxy-based local and continental-scale reconstructions. The simulations consistently show a smaller temperature change than the reconstructions for most regions in the Northern Hemisphere, but not in the Southern Hemisphere, as well as a partly different spatial pattern. A cost function analysis assesses how well the various simulations agree with reconstructions. Disregarding spatial correlation, significant differences are seen in the agreement with the local temperature reconstructions between groups of models, but insignificant differences are noted when compared to continental-scale reconstructions. This result points toward a limited possibility to rank models by means of their low-frequency temperature variability alone. The systematically lower amplitude of simulated versus reconstructed temperature change indicates either too-small simulated internal variability or that the analyzed models lack some critical forcing or have missing or too-weak feedback mechanisms. We hypothesize that too-cold initial ocean conditions in the models-in combination with too-weak internal variability and slow feedbacks over longer time scales-could account for much of the simulation-reconstruction disagreement.

Keywords
Paleoclimate, Surface temperature, Ranking methods, Climate models, Model comparison
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-168313 (URN)10.1175/JCLI-D-18-0525.1 (DOI)000464467700001 ()
Available from: 2019-05-26 Created: 2019-05-26 Last updated: 2022-03-23Bibliographically approved
Charpentier Ljungqvist, F., Krusic, P. J., Sundqvist, H. S., Zorita, E., Brattström, G. & Frank, D. (2016). Northern Hemisphere hydroclimate variability over the past twelve centuries. Nature, 532(7597), 94-98
Open this publication in new window or tab >>Northern Hemisphere hydroclimate variability over the past twelve centuries
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2016 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 532, no 7597, p. 94-98Article in journal (Refereed) Published
Abstract [en]

Accurate modelling and prediction of the local to continental-scale hydroclimate response to global warming is essential given the strong impact of hydroclimate on ecosystem functioning, crop yields, water resources, and economic security. However, uncertainty in hydroclimate projections remains large, in part due to the short length of instrumental measurements available with which to assess climate models. Here we present a spatial reconstruction of hydroclimate variability over the past twelve centuries across the Northern Hemisphere derived from a network of 196 at least millennium-long proxy records. We use this reconstruction to place recent hydrological changes and future precipitation scenarios in a long-term context of spatially resolved and temporally persistent hydroclimate patterns. We find a larger percentage of land area with relatively wetter conditions in the ninth to eleventh and the twentieth centuries, whereas drier conditions are more widespread between the twelfth and nineteenth centuries. Our reconstruction reveals that prominent seesaw patterns of alternating moisture regimes observed in instrumental data across the Mediterranean, western USA, and China have operated consistently over the past twelve centuries. Using an updated compilation of 128 temperature proxy records, we assess the relationship between the reconstructed centennial-scale Northern Hemisphere hydroclimate and temperature variability. Even though dry and wet conditions occurred over extensive areas under both warm and cold climate regimes, a statistically significant co-variability of hydroclimate and temperature is evident for particular regions. We compare the reconstructed hydroclimate anomalies with coupled atmosphere-ocean general circulation model simulations and find reasonable agreement during pre-industrial times. However, the intensification of the twentieth-century-mean hydroclimate anomalies in the simulations, as compared to previous centuries, is not supported by our new multi-proxy reconstruction. This finding suggests that much work remains before we can model hydroclimate variability accurately, and highlights the importance of using palaeoclimate data to place recent and predicted hydroclimate changes in a millennium-long context.

National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-129893 (URN)10.1038/nature17418 (DOI)000373555500040 ()
Available from: 2016-05-06 Created: 2016-05-03 Last updated: 2022-02-23Bibliographically approved
Hind, A., Zhang, Q. & Brattström, G. (2016). Problems encountered when defining Arctic amplification as a ratio. Scientific Reports, 6, Article ID 30469.
Open this publication in new window or tab >>Problems encountered when defining Arctic amplification as a ratio
2016 (English)In: Scientific Reports, E-ISSN 2045-2322, Vol. 6, article id 30469Article in journal (Refereed) Published
Abstract [en]

In climate change science the term 'Arctic amplification' has become synonymous with an estimation of the ratio of a change in Arctic temperatures compared with a broader reference change under the same period, usually in global temperatures. Here, it is shown that this definition of Arctic amplification comes with a suite of difficulties related to the statistical properties of the ratio estimator itself. Most problematic is the complexity of categorizing uncertainty in Arctic amplification when the global, or reference, change in temperature is close to 0 over a period of interest, in which case it may be impossible to set bounds on this uncertainty. An important conceptual distinction is made between the 'Ratio of Means' and 'Mean Ratio' approaches to defining a ratio estimate of Arctic amplification, as they do not only possess different uncertainty properties regarding the amplification factor, but are also demonstrated to ask different scientific questions. Uncertainty in the estimated range of the Arctic amplification factor using the latest global climate models and climate forcing scenarios is expanded upon and shown to be greater than previously demonstrated for future climate projections, particularly using forcing scenarios with lower concentrations of greenhouse gases.

National Category
Other Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:su:diva-133387 (URN)10.1038/srep30469 (DOI)000380798500001 ()27461918 (PubMedID)2-s2.0-84979780094 (Scopus ID)
Available from: 2016-09-07 Created: 2016-09-06 Last updated: 2022-09-15Bibliographically approved
Charpentier Ljungqvist, F., Krusic, P. J., Brattström, G. & Sundqvist, H. S. (2012). Northern Hemisphere temperature patterns in the last 12 centuries. Climate of the Past, 8, 227-249
Open this publication in new window or tab >>Northern Hemisphere temperature patterns in the last 12 centuries
2012 (English)In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 8, p. 227-249Article in journal (Refereed) Published
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.

Keywords
Temperature reconstruction, spatio-temporal patterns of temperature variability, Little Ice Age, Medieval Warm Period, climate change, global warming
National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-72188 (URN)10.5194/cp-8-227-2012 (DOI)000300878100015 ()
Available from: 2012-02-06 Created: 2012-02-03 Last updated: 2022-03-23Bibliographically approved
Moberg, A. & Brattström, G. (2011). Prediction intervals for climate reconstructions with autocorrelated noise: An analysis of ordinary least squares and measurement error methods. Palaeogeography, Palaeoclimatology, Palaeoecology, 308(3-4), 313-329
Open this publication in new window or tab >>Prediction intervals for climate reconstructions with autocorrelated noise: An analysis of ordinary least squares and measurement error methods
2011 (English)In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, E-ISSN 1872-616X, Vol. 308, no 3-4, p. 313-329Article in journal (Refereed) Published
Abstract [en]

The estimation of past climate variations is a statistical prediction problem, where climate proxy data are calibrated against instrumental observations. Although noise is always present in both instrumental and proxy data, motivating the use of so-called errors-in-variables or measurement error methods, such methods have not yet been widely accepted by palaeoclimatologists. We define a univariate measurement error model that allows for white noise in instrumental and red noise in proxy data, and derive new formulae to construct prediction intervals for past climate values. The new method can be applied to either unsmoothed data or to data smoothed after calibration. Using synthetic simulated data, we demonstrate that the new formulae perform well for noise levels and calibration period lengths typical of many palaeoclimate series, in particular tree-rings and other annually resolved data. With an example, using a recently published 500-year long temperature reconstruction, we demonstrate that conclusions about the statistical significance of the difference between the present and past climates may be incorrect if the noise is not adequately modeled.

Keywords
Climate reconstruction, Statistical calibration, Measurement error models, Prediction interval, Autocorrelation, Red noise
National Category
Mathematics Climate Research
Research subject
Physical Geography; Mathematical Statistics
Identifiers
urn:nbn:se:su:diva-60361 (URN)10.1016/j.palaeo.2011.05.035 (DOI)000294396900006 ()
Projects
Millennium (EU 017008)Climate in the last millennium (VR 621-2007-45642)Reconstructing climate in the last millennium (VR 622-2009-7515)
Available from: 2011-08-16 Created: 2011-08-16 Last updated: 2022-02-24Bibliographically approved
Sundqvist, H. S., Zhang, Q., Moberg, A., Holmgren, K., Körnich, H., Nilsson, J. & Brattström, G. (2010). Climate change between the mid and late Holocene in the northern high latitudes: Part I: Survey of temperature and precipitation proxy data. Climate of the Past, 6, 591-608
Open this publication in new window or tab >>Climate change between the mid and late Holocene in the northern high latitudes: Part I: Survey of temperature and precipitation proxy data
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2010 (English)In: Climate of the Past, ISSN 1814-9324, E-ISSN 1814-9332, Vol. 6, p. 591-608Article in journal (Refereed) Published
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.

Keywords
climate change, Holocene, proxy data, northern high latitudes
National Category
Climate Research
Research subject
Physical Geography
Identifiers
urn:nbn:se:su:diva-43497 (URN)10.5194/cp-6-739-2010 (DOI)000283667900004 ()
Projects
Holocene Climate Variability over Scandinavia
Available from: 2010-10-19 Created: 2010-10-18 Last updated: 2022-02-24Bibliographically approved
Brattström, G. (2008). Leonardo da Pisa: Inger Christensen och Fibonacci. Lyrikvännen, 55(6), 33-38
Open this publication in new window or tab >>Leonardo da Pisa: Inger Christensen och Fibonacci
2008 (Swedish)In: Lyrikvännen, ISSN 0460-0762, Vol. 55, no 6, p. 33-38Article in journal (Other (popular science, discussion, etc.)) Published
Abstract [sv]

Den danska poeten Inger Christensen byggde sin diktsamling Alfabet på Fibonaccis talserie. Artikeln är en kort historisk essä över Fibonacci, och beskriver även hur talserien är konstruerad.

Keywords
Fibonacci, Fibonaccital, matematikhistoria, Inger Chistensen
National Category
Other Mathematics Discrete Mathematics
Identifiers
urn:nbn:se:su:diva-16289 (URN)
Available from: 2008-12-16 Created: 2008-12-16 Last updated: 2022-02-25Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-6180-2873

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