Knowledge of how climate has changed in the past is essential for understanding present climate c... more Knowledge of how climate has changed in the past is essential for understanding present climate changes and for separating anthropogenic influence from natural variability. The amplitude and geographical coherency of the late-Holocene pre-industrial temperature variability have been much discussed and the occurrence of a distinct Medieval Climate Anomaly (c. AD 800-1300) and a Little Ice Age (c. AD 1300-1900) on
ABSTRACT In this study, wood anatomy, tree-ring width and wood density of Pinus sylvestris at the... more ABSTRACT In this study, wood anatomy, tree-ring width and wood density of Pinus sylvestris at the northern timberline in Fennoscandia were used to identify relationships among the parameters and to screen them for their climatic signals. Furthermore we investigated the influence of the juvenile wood section for all parameters developed. The measurements of wood anatomy were conducted with confocal laser scanning microscopy (CLSM) while the density profiles were produced using an Itrax MultiScanner. We developed chronologies of ring width, wood density and anatomy for a period between 1940 and 2010. Correlations between wood density and wood anatomy were strong in the latewood part. For some wood anatomy and density chronologies youth trends were found in the juvenile part. Wood density decreased from the pith up to the 9th ring and stabilized afterwards, while cell lumen diameter and lumen area increased simultaneously up to the 15th ring. All chronologies contained strong summer temperature signals. The wood anatomical variables provided additional information about seasonal precipitation which could not be found in wood density and tree-ring widths. Our study confirmed previous results stating that the parameter maximum density contains the strongest climate signal, that is, summer temperatures at the northern timberline. Nevertheless, the intra-annual data on tracheid dimensions showed good potential to supply seasonal climatic information and improve our understanding of climatic effects on tree growth and wood formation.
The challenges of archaeological wooden treasures recovered from the seabed Archaeological wood c... more The challenges of archaeological wooden treasures recovered from the seabed Archaeological wood constitutes an important part of our cultural heritage, and the finds recovered from excavation sites may represent a broad range of different objects. This is particularly true for well-preserved shipwrecks, such as the famous Vasa (1628) (Figure 1) of Sweden and the British Mary Rose (1545). The wreck site context of such vessels offers a rich source of archaeological information, and the shipwrecks themselves usually constitute strong historical symbols. 1,2 Waterlogged archaeological wood is often found relatively well-preserved in reduced aquatic environments, or embedded in sediments or in bogs. In particular, the Baltic Sea is well-known for the many shipwrecks preserved in its low salinity water, which prevents invasion by the infamous Teredo navalis (shipworm). 3 However, the conservation procedures of these magnificent finds from the seabed usually consume a lot of resources in ...
A set of global climate model (GCM) simulations for the last thousand years developed by the Max ... more A set of global climate model (GCM) simulations for the last thousand years developed by the Max Planck Institute is compared with palaeoclimate proxy data and instrumental data. This GCM/reality intercomparison utilizes a newly developed statistical framework using optimized quadratic distance and correlation based statistical measures of goodness-of-fit. An advantage of this statistical framework is that a range of regions with different data quality, seasonal representativeness and time periods covered can be used to evaluate the performance of GCM simulations. Moreover, it includes a significance test of whether a forced simulation performs better than unforced (control) simulations. A selection of high quality proxy series and instrumental records are used to compare with corresponding model simulation output. Given the present uncertainty in solar forcing history over the last millennium, it is helpful to attempt to constrain these estimates by comparing simulations over this ...
Sulfur and iron concentrations in wood from three 17(th) century shipwrecks in the Baltic Sea, th... more Sulfur and iron concentrations in wood from three 17(th) century shipwrecks in the Baltic Sea, the Ghost wreck, the Crown and the Sword, were obtained by X-ray fluorescence (XRF) scanning. In near anaerobic environments symbiotic microorganisms degrade waterlogged wood, reduce sulfate and promote accumulation of low-valent sulfur compounds, as previously found for the famous wrecks of the Vasa and Mary Rose. Sulfur K-edge X-ray absorption near-edge structure (XANES) analyses of Ghost wreck wood show that organic thiols and disulfides dominate, together with elemental sulfur probably generated by sulfur-oxidizing Beggiatoa bacteria. Iron sulfides were not detected, consistent with the relatively low iron concentration in the wood. In a museum climate with high atmospheric humidity oxidation processes, especially of iron sulfides formed in the presence of corroding iron, may induce post-conservation wood degradation. Subject to more general confirmation by further analyses no severe c...
High-resolution proxies of past climate are essential for a better understanding of the climate s... more High-resolution proxies of past climate are essential for a better understanding of the climate system. Tree rings are routinely used to reconstruct Holocene climate variations at high temporal resolution, but only rarely have they offered insight into climate variability during earlier periods. Fitzroya cupressoides-a South American conifer which attains ages up to 3,600 years-has been shown to record summer temperatures in northern Patagonia during the past few millennia. Here we report a floating 1,229-year chronology developed from subfossil stumps of F. cupressoides in southern Chile that dates back to approximately 50,000 14C years before present. We use this chronology to calculate the spectral characteristics of climate variability in this time, which was probably an interstadial (relatively warm) period. Growth oscillations at periods of 150-250, 87-94, 45.5, 24.1, 17.8, 9.3 and 2.7-5.3 years are identified in the annual subfossil record. A comparison with the power spectra...
... We thank Paula Santillo, Matt Brookhouse, Rob Waterworth, Rod Bale, Giles Young, Andy Baker, ... more ... We thank Paula Santillo, Matt Brookhouse, Rob Waterworth, Rod Bale, Giles Young, Andy Baker, and Mary Gagen for support and helpful comments. Thank you to Régents Instruments Inc. for use of images and technical assistance. ...
... Thomas S. Bartholin,4,5 Philip D. Jones3 and Bernd Kromer6 ... tortuosa) at 600 800 m asl Th... more ... Thomas S. Bartholin,4,5 Philip D. Jones3 and Bernd Kromer6 ... tortuosa) at 600 800 m asl The present-day continuous boreal forest extends to the eastern end of Lake Torneträsk, where a distinct pine tree-line can be identifi ed at 440 m asl, while the pine distribution further to ...
ABSTRACT We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-rin... more ABSTRACT We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-ring width (TRW) data from the Torneträsk region of northern Sweden and the construction of 1500 year chronologies. Some previous work found that MXD and TRW chronologies from Torneträsk were inconsistent over the most recent 200 years, even though they both reflect predominantly summer temperature influences on tree growth. We show that this was partly a result of systematic bias in MXD data measurements and partly a result of inhomogeneous sample selection from living trees (modern sample bias). We use refinements of the simple Regional Curve Standardisation (RCS) method of chronology construction to identify and mitigate these biases. The new MXD and TRW chronologies now present a largely consistent picture of long-timescale changes in past summer temperature in this region over their full length, indicating similar levels of summer warmth in the medieval period (MWP, c. ce 900–1100) and the latter half of the 20th century. Future work involving the updating of MXD chronologies using differently sourced measurements may require similar analysis and appropriate adjustment to that described here to make the data suitable for the production of un-biased RCS chronologies. The use of ‘growth-rate’ based multiple RCS curves is recommended to identify and mitigate the problem of ‘modern sample bias’.
Minimum blue intensity measurements of resin-extracted Pinus sylvestris (L.) samples, conducted u... more Minimum blue intensity measurements of resin-extracted Pinus sylvestris (L.) samples, conducted using a flat-bed scanner and commercially available software, are shown to provide a robust and reliable surrogate for maximum latewood density. Blue intensity data from 15 trees, from three stands, are reported relative to a standard blue-scale in a manner similar to grey-scale calibration in x-ray densitometry. The resulting
ABSTRACT This paper presents results from the first 1100 years of a long stable carbon isotope ch... more ABSTRACT This paper presents results from the first 1100 years of a long stable carbon isotope chronology currently in development from Scots Pine (Pinus sylvestris L.) trees growing in the Torneträsk region of northern Sweden. The isotope record currently comprises a total of 74 trees with a mean annual replication of >12, thereby enabling it to be compared directly with other tree-ring based palæoclimate reconstructions from this region. In developing the reconstruction, several key topics in isotope dendroclimatology (chronology construction, replication, CO2 adjustment and age trends) were addressed.The resulting carbon isotope series is calibrated against instrumental data from the closest meteorological station at Abisko (AD1913–2008) to provide a record of June–August sunshine for northern Fennoscandia. This parameter is closely linked to the direct control of assimilation rate; Photosynthetically Active Radiation (PAR) and the indirect measures; mean July–August temperature and percent cloud cover. The coupled response of summer sunshine and temperature in this region permits a multi-parameter comparison with a local reconstruction of past temperature variability based upon tree growth proxies to explore the stability of this coupling through time.Several periods are identified where the temperature (X-ray density) and sunshine (stable carbon isotope ratio) records diverge. The most significant and sustained of these occur between c AD1200–1380 and c AD1550–1780, providing evidence for a cool, sunny, two-phase “Little Ice Age”. Whilst summer sunshine reconstructed for the 20th century is significantly different from the mean of the last 1100 years (P < 0.01), conditions during the early mediæval period are similar to those experienced in northern Fennoscandia during the 20th century (P > 0.01), so it is the 17th–18th, and to a lesser extent, the 13th centuries rather than the early mediæval period that appear anomalous when viewed within the context of the last 1100 years. The observed departures between temperature and sunshine are interpreted as indicating a change in large-scale circulation associated with a southward migration of the Polar Front. Such a change, affecting the Northern Annular Mode (Arctic Oscillation) would result in more stable anticyclonic conditions (cool, bright, summers) over northern Fennoscandia, thus providing a testable mechanism for the development of a multi-phase, time-transgressive “Little Ice Age” across Europe.
Philosophical Transactions of the Royal Society B: Biological Sciences, 2008
This paper describes variability in trends of annual tree growth at several locations in the high... more This paper describes variability in trends of annual tree growth at several locations in the high latitudes of Eurasia, providing a wide regional comparison over a 2000-year period. The study focuses on the nature of local and widespread tree-growth responses to recent warming seen in instrumental observations, available in northern regions for periods ranging from decades to a century. Instrumental temperature data demonstrate differences in seasonal scale of Eurasian warming and the complexity and spatial diversity of tree-growing-season trends in recent decades. A set of long tree-ring chronologies provides empirical evidence of association between inter-annual tree growth and local, primarily summer, temperature variability at each location. These data show no evidence of a recent breakdown in this association as has been found at other high-latitude Northern Hemisphere locations. Using Kendall's concordance, we quantify the time-dependent relationship between growth trends of the long chronologies as a group. This provides strong evidence that the extent of recent widespread warming across northwest Eurasia, with respect to 100-to 200-year trends, is unprecedented in the last 2000 years. An equivalent analysis of simulated temperatures using the HadCM3 model fails to show a similar increase in concordance expected as a consequence of anthropogenic forcing.
1] Cloud cover is one of the most important factors controlling the radiation balance of the Eart... more 1] Cloud cover is one of the most important factors controlling the radiation balance of the Earth. The response of cloud cover to increasing global temperatures represents the largest uncertainty in model estimates of future climate because the cloud response to temperature is not wellconstrained. Here we present the first regional reconstruction of summer sunshine over the past millennium, based on the stable carbon isotope ratios of pine treerings from Fennoscandia. Comparison with the regional temperature evolution reveals the Little Ice Age (LIA) to have been sunny, with cloudy conditions in the warmest periods of the Medieval at this site. A negative shortwave cloud feedback is indicated at high latitude. A millennial climate simulation suggests that regionally low temperatures during the LIA were mostly maintained by a weaker greenhouse effect due to lower humidity. Simulations of future climate that display a negative shortwave cloud feedback for high-latitudes are consistent with our proxy interpretation. Citation: Gagen, M.,
Received 25 October 2007; revised 11 December 2007; accepted 27 December 2007; published 29 Febru... more Received 25 October 2007; revised 11 December 2007; accepted 27 December 2007; published 29 February 2008. [1] New and well-dated evidence of sulphate deposits in Greenland and Antarctic ice cores indicate a substantial and extensive atmospheric acidic dust veil at AD 533534 ...
Dendroecology, or the use of ring patterns to assess the age of trees and environmental factors c... more Dendroecology, or the use of ring patterns to assess the age of trees and environmental factors controlling their growth, is a well-developed method in climatologic studies. This method holds great potential as a forensic tool for age dating, contamination assessment, and characterization of releases. Moreover, the method is independent of the physical presence of contamination at the time of sampling because it is focused on the effect rather than the cause. This review is one of the very few articles published to date exploring the forensic applicability of dendroecology. This article is organized in two parts: Part I describes the method principles and proposes a practical procedure for forensic applications; Part II exemplifies and validates the method through six case studies of successful forensic application (related to petroleum products and chlorinated solvent spills).
A statistical framework for comparing the output of ensemble simulations from global climate mode... more 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.
This paper presents updated tree-ring width (TRW) and maximum density (MXD) from Torneträsk in no... more This paper presents updated tree-ring width (TRW) and maximum density (MXD) from Torneträsk in northern Sweden, now covering the period AD 500-2004. By including data from relatively young trees for the most recent period, a previously noted decline in recent MXD is eliminated. Non-climatological growth trends in the data are removed using Regional Curve Standardization (RCS), thus producing TRW and MXD chronologies with preserved lowfrequency variability. The chronologies are calibrated using local and regional instrumental climate records. A bootstrapped response function analysis using regional climate data shows that tree growth is forced by April-August temperatures and that the regression weights for MXD are much stronger than for TRW. The robustness of the reconstruction equation is verified by independent temperature data and shows that 63-64% of the instrumental inter-annual variation is captured by the tree-ring data. This is a significant improvement compared to previously published reconstructions based on tree-ring data from Torneträsk. A divergence phenomenon around AD 1800, expressed as an increase in TRW that is not paralleled by temperature and MXD, is most likely an effect of major changes in the density of the pine population at this northern tree-line site. The bias introduced by this TRW phenomenon is assessed by producing a summer temperature reconstruction based on MXD exclusively. The new data show generally higher temperature estimates than previous reconstructions based on Torneträsk tree-ring data. The late-twentieth century, however, is not exceptionally warm in the new record: On decadal-to-centennial timescales, periods around AD 750, 1000, 1400, and 1750 were equally warm, or warmer. The 200-year long warm period centered on AD 1000 was significantly warmer than the late-twentieth century (p \ 0.05) and is supported by other local and regional paleoclimate data. The new tree-ring evidence from Torneträsk suggests that this ''Medieval Warm Period'' in northern Fennoscandia was much warmer than previously recognized.
Knowledge of how climate has changed in the past is essential for understanding present climate c... more Knowledge of how climate has changed in the past is essential for understanding present climate changes and for separating anthropogenic influence from natural variability. The amplitude and geographical coherency of the late-Holocene pre-industrial temperature variability have been much discussed and the occurrence of a distinct Medieval Climate Anomaly (c. AD 800-1300) and a Little Ice Age (c. AD 1300-1900) on
ABSTRACT In this study, wood anatomy, tree-ring width and wood density of Pinus sylvestris at the... more ABSTRACT In this study, wood anatomy, tree-ring width and wood density of Pinus sylvestris at the northern timberline in Fennoscandia were used to identify relationships among the parameters and to screen them for their climatic signals. Furthermore we investigated the influence of the juvenile wood section for all parameters developed. The measurements of wood anatomy were conducted with confocal laser scanning microscopy (CLSM) while the density profiles were produced using an Itrax MultiScanner. We developed chronologies of ring width, wood density and anatomy for a period between 1940 and 2010. Correlations between wood density and wood anatomy were strong in the latewood part. For some wood anatomy and density chronologies youth trends were found in the juvenile part. Wood density decreased from the pith up to the 9th ring and stabilized afterwards, while cell lumen diameter and lumen area increased simultaneously up to the 15th ring. All chronologies contained strong summer temperature signals. The wood anatomical variables provided additional information about seasonal precipitation which could not be found in wood density and tree-ring widths. Our study confirmed previous results stating that the parameter maximum density contains the strongest climate signal, that is, summer temperatures at the northern timberline. Nevertheless, the intra-annual data on tracheid dimensions showed good potential to supply seasonal climatic information and improve our understanding of climatic effects on tree growth and wood formation.
The challenges of archaeological wooden treasures recovered from the seabed Archaeological wood c... more The challenges of archaeological wooden treasures recovered from the seabed Archaeological wood constitutes an important part of our cultural heritage, and the finds recovered from excavation sites may represent a broad range of different objects. This is particularly true for well-preserved shipwrecks, such as the famous Vasa (1628) (Figure 1) of Sweden and the British Mary Rose (1545). The wreck site context of such vessels offers a rich source of archaeological information, and the shipwrecks themselves usually constitute strong historical symbols. 1,2 Waterlogged archaeological wood is often found relatively well-preserved in reduced aquatic environments, or embedded in sediments or in bogs. In particular, the Baltic Sea is well-known for the many shipwrecks preserved in its low salinity water, which prevents invasion by the infamous Teredo navalis (shipworm). 3 However, the conservation procedures of these magnificent finds from the seabed usually consume a lot of resources in ...
A set of global climate model (GCM) simulations for the last thousand years developed by the Max ... more A set of global climate model (GCM) simulations for the last thousand years developed by the Max Planck Institute is compared with palaeoclimate proxy data and instrumental data. This GCM/reality intercomparison utilizes a newly developed statistical framework using optimized quadratic distance and correlation based statistical measures of goodness-of-fit. An advantage of this statistical framework is that a range of regions with different data quality, seasonal representativeness and time periods covered can be used to evaluate the performance of GCM simulations. Moreover, it includes a significance test of whether a forced simulation performs better than unforced (control) simulations. A selection of high quality proxy series and instrumental records are used to compare with corresponding model simulation output. Given the present uncertainty in solar forcing history over the last millennium, it is helpful to attempt to constrain these estimates by comparing simulations over this ...
Sulfur and iron concentrations in wood from three 17(th) century shipwrecks in the Baltic Sea, th... more Sulfur and iron concentrations in wood from three 17(th) century shipwrecks in the Baltic Sea, the Ghost wreck, the Crown and the Sword, were obtained by X-ray fluorescence (XRF) scanning. In near anaerobic environments symbiotic microorganisms degrade waterlogged wood, reduce sulfate and promote accumulation of low-valent sulfur compounds, as previously found for the famous wrecks of the Vasa and Mary Rose. Sulfur K-edge X-ray absorption near-edge structure (XANES) analyses of Ghost wreck wood show that organic thiols and disulfides dominate, together with elemental sulfur probably generated by sulfur-oxidizing Beggiatoa bacteria. Iron sulfides were not detected, consistent with the relatively low iron concentration in the wood. In a museum climate with high atmospheric humidity oxidation processes, especially of iron sulfides formed in the presence of corroding iron, may induce post-conservation wood degradation. Subject to more general confirmation by further analyses no severe c...
High-resolution proxies of past climate are essential for a better understanding of the climate s... more High-resolution proxies of past climate are essential for a better understanding of the climate system. Tree rings are routinely used to reconstruct Holocene climate variations at high temporal resolution, but only rarely have they offered insight into climate variability during earlier periods. Fitzroya cupressoides-a South American conifer which attains ages up to 3,600 years-has been shown to record summer temperatures in northern Patagonia during the past few millennia. Here we report a floating 1,229-year chronology developed from subfossil stumps of F. cupressoides in southern Chile that dates back to approximately 50,000 14C years before present. We use this chronology to calculate the spectral characteristics of climate variability in this time, which was probably an interstadial (relatively warm) period. Growth oscillations at periods of 150-250, 87-94, 45.5, 24.1, 17.8, 9.3 and 2.7-5.3 years are identified in the annual subfossil record. A comparison with the power spectra...
... We thank Paula Santillo, Matt Brookhouse, Rob Waterworth, Rod Bale, Giles Young, Andy Baker, ... more ... We thank Paula Santillo, Matt Brookhouse, Rob Waterworth, Rod Bale, Giles Young, Andy Baker, and Mary Gagen for support and helpful comments. Thank you to Régents Instruments Inc. for use of images and technical assistance. ...
... Thomas S. Bartholin,4,5 Philip D. Jones3 and Bernd Kromer6 ... tortuosa) at 600 800 m asl Th... more ... Thomas S. Bartholin,4,5 Philip D. Jones3 and Bernd Kromer6 ... tortuosa) at 600 800 m asl The present-day continuous boreal forest extends to the eastern end of Lake Torneträsk, where a distinct pine tree-line can be identifi ed at 440 m asl, while the pine distribution further to ...
ABSTRACT We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-rin... more ABSTRACT We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-ring width (TRW) data from the Torneträsk region of northern Sweden and the construction of 1500 year chronologies. Some previous work found that MXD and TRW chronologies from Torneträsk were inconsistent over the most recent 200 years, even though they both reflect predominantly summer temperature influences on tree growth. We show that this was partly a result of systematic bias in MXD data measurements and partly a result of inhomogeneous sample selection from living trees (modern sample bias). We use refinements of the simple Regional Curve Standardisation (RCS) method of chronology construction to identify and mitigate these biases. The new MXD and TRW chronologies now present a largely consistent picture of long-timescale changes in past summer temperature in this region over their full length, indicating similar levels of summer warmth in the medieval period (MWP, c. ce 900–1100) and the latter half of the 20th century. Future work involving the updating of MXD chronologies using differently sourced measurements may require similar analysis and appropriate adjustment to that described here to make the data suitable for the production of un-biased RCS chronologies. The use of ‘growth-rate’ based multiple RCS curves is recommended to identify and mitigate the problem of ‘modern sample bias’.
Minimum blue intensity measurements of resin-extracted Pinus sylvestris (L.) samples, conducted u... more Minimum blue intensity measurements of resin-extracted Pinus sylvestris (L.) samples, conducted using a flat-bed scanner and commercially available software, are shown to provide a robust and reliable surrogate for maximum latewood density. Blue intensity data from 15 trees, from three stands, are reported relative to a standard blue-scale in a manner similar to grey-scale calibration in x-ray densitometry. The resulting
ABSTRACT This paper presents results from the first 1100 years of a long stable carbon isotope ch... more ABSTRACT This paper presents results from the first 1100 years of a long stable carbon isotope chronology currently in development from Scots Pine (Pinus sylvestris L.) trees growing in the Torneträsk region of northern Sweden. The isotope record currently comprises a total of 74 trees with a mean annual replication of >12, thereby enabling it to be compared directly with other tree-ring based palæoclimate reconstructions from this region. In developing the reconstruction, several key topics in isotope dendroclimatology (chronology construction, replication, CO2 adjustment and age trends) were addressed.The resulting carbon isotope series is calibrated against instrumental data from the closest meteorological station at Abisko (AD1913–2008) to provide a record of June–August sunshine for northern Fennoscandia. This parameter is closely linked to the direct control of assimilation rate; Photosynthetically Active Radiation (PAR) and the indirect measures; mean July–August temperature and percent cloud cover. The coupled response of summer sunshine and temperature in this region permits a multi-parameter comparison with a local reconstruction of past temperature variability based upon tree growth proxies to explore the stability of this coupling through time.Several periods are identified where the temperature (X-ray density) and sunshine (stable carbon isotope ratio) records diverge. The most significant and sustained of these occur between c AD1200–1380 and c AD1550–1780, providing evidence for a cool, sunny, two-phase “Little Ice Age”. Whilst summer sunshine reconstructed for the 20th century is significantly different from the mean of the last 1100 years (P < 0.01), conditions during the early mediæval period are similar to those experienced in northern Fennoscandia during the 20th century (P > 0.01), so it is the 17th–18th, and to a lesser extent, the 13th centuries rather than the early mediæval period that appear anomalous when viewed within the context of the last 1100 years. The observed departures between temperature and sunshine are interpreted as indicating a change in large-scale circulation associated with a southward migration of the Polar Front. Such a change, affecting the Northern Annular Mode (Arctic Oscillation) would result in more stable anticyclonic conditions (cool, bright, summers) over northern Fennoscandia, thus providing a testable mechanism for the development of a multi-phase, time-transgressive “Little Ice Age” across Europe.
Philosophical Transactions of the Royal Society B: Biological Sciences, 2008
This paper describes variability in trends of annual tree growth at several locations in the high... more This paper describes variability in trends of annual tree growth at several locations in the high latitudes of Eurasia, providing a wide regional comparison over a 2000-year period. The study focuses on the nature of local and widespread tree-growth responses to recent warming seen in instrumental observations, available in northern regions for periods ranging from decades to a century. Instrumental temperature data demonstrate differences in seasonal scale of Eurasian warming and the complexity and spatial diversity of tree-growing-season trends in recent decades. A set of long tree-ring chronologies provides empirical evidence of association between inter-annual tree growth and local, primarily summer, temperature variability at each location. These data show no evidence of a recent breakdown in this association as has been found at other high-latitude Northern Hemisphere locations. Using Kendall's concordance, we quantify the time-dependent relationship between growth trends of the long chronologies as a group. This provides strong evidence that the extent of recent widespread warming across northwest Eurasia, with respect to 100-to 200-year trends, is unprecedented in the last 2000 years. An equivalent analysis of simulated temperatures using the HadCM3 model fails to show a similar increase in concordance expected as a consequence of anthropogenic forcing.
1] Cloud cover is one of the most important factors controlling the radiation balance of the Eart... more 1] Cloud cover is one of the most important factors controlling the radiation balance of the Earth. The response of cloud cover to increasing global temperatures represents the largest uncertainty in model estimates of future climate because the cloud response to temperature is not wellconstrained. Here we present the first regional reconstruction of summer sunshine over the past millennium, based on the stable carbon isotope ratios of pine treerings from Fennoscandia. Comparison with the regional temperature evolution reveals the Little Ice Age (LIA) to have been sunny, with cloudy conditions in the warmest periods of the Medieval at this site. A negative shortwave cloud feedback is indicated at high latitude. A millennial climate simulation suggests that regionally low temperatures during the LIA were mostly maintained by a weaker greenhouse effect due to lower humidity. Simulations of future climate that display a negative shortwave cloud feedback for high-latitudes are consistent with our proxy interpretation. Citation: Gagen, M.,
Received 25 October 2007; revised 11 December 2007; accepted 27 December 2007; published 29 Febru... more Received 25 October 2007; revised 11 December 2007; accepted 27 December 2007; published 29 February 2008. [1] New and well-dated evidence of sulphate deposits in Greenland and Antarctic ice cores indicate a substantial and extensive atmospheric acidic dust veil at AD 533534 ...
Dendroecology, or the use of ring patterns to assess the age of trees and environmental factors c... more Dendroecology, or the use of ring patterns to assess the age of trees and environmental factors controlling their growth, is a well-developed method in climatologic studies. This method holds great potential as a forensic tool for age dating, contamination assessment, and characterization of releases. Moreover, the method is independent of the physical presence of contamination at the time of sampling because it is focused on the effect rather than the cause. This review is one of the very few articles published to date exploring the forensic applicability of dendroecology. This article is organized in two parts: Part I describes the method principles and proposes a practical procedure for forensic applications; Part II exemplifies and validates the method through six case studies of successful forensic application (related to petroleum products and chlorinated solvent spills).
A statistical framework for comparing the output of ensemble simulations from global climate mode... more 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.
This paper presents updated tree-ring width (TRW) and maximum density (MXD) from Torneträsk in no... more This paper presents updated tree-ring width (TRW) and maximum density (MXD) from Torneträsk in northern Sweden, now covering the period AD 500-2004. By including data from relatively young trees for the most recent period, a previously noted decline in recent MXD is eliminated. Non-climatological growth trends in the data are removed using Regional Curve Standardization (RCS), thus producing TRW and MXD chronologies with preserved lowfrequency variability. The chronologies are calibrated using local and regional instrumental climate records. A bootstrapped response function analysis using regional climate data shows that tree growth is forced by April-August temperatures and that the regression weights for MXD are much stronger than for TRW. The robustness of the reconstruction equation is verified by independent temperature data and shows that 63-64% of the instrumental inter-annual variation is captured by the tree-ring data. This is a significant improvement compared to previously published reconstructions based on tree-ring data from Torneträsk. A divergence phenomenon around AD 1800, expressed as an increase in TRW that is not paralleled by temperature and MXD, is most likely an effect of major changes in the density of the pine population at this northern tree-line site. The bias introduced by this TRW phenomenon is assessed by producing a summer temperature reconstruction based on MXD exclusively. The new data show generally higher temperature estimates than previous reconstructions based on Torneträsk tree-ring data. The late-twentieth century, however, is not exceptionally warm in the new record: On decadal-to-centennial timescales, periods around AD 750, 1000, 1400, and 1750 were equally warm, or warmer. The 200-year long warm period centered on AD 1000 was significantly warmer than the late-twentieth century (p \ 0.05) and is supported by other local and regional paleoclimate data. The new tree-ring evidence from Torneträsk suggests that this ''Medieval Warm Period'' in northern Fennoscandia was much warmer than previously recognized.
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Papers by Håkan Grudd