Declining winter rainfall coupled with recent prolonged drought poses significant risks to water ... more Declining winter rainfall coupled with recent prolonged drought poses significant risks to water resources and agriculture across southern Australia. While rainfall declines over recent decades are largely consistent with modelled climate change scenarios, particularly for southwest Australia, the significance of these declines is yet to be assessed within the context of long-term hydroclimatic variability. Here, we present a new 668-year (1350-2017 CE) tree-ring reconstruction of autumnwinter rainfall over inland southwest Australia. This record reveals that a recent decline in rainfall over inland southwest Australia (since 2000 CE) is not unusual in terms of either magnitude or duration relative to rainfall variability over the last seven centuries. Drought periods of greater magnitude and duration than those in the instrumental record occurred prior to 1900 CE, including two 'megadroughts' of > 30 years duration in the eighteenth and nineteenth centuries. By contrast, the wettest > decadal periods of the last seven centuries occurred after 1900 CE, making the twentieth century the wettest of the last seven centuries. We conclude that the instrumental rainfall record (since ~ 1900 CE) does not capture the full scale of natural hydroclimatic variability for inland southwest Australia and that the risk of prolonged droughts in the region is likely much higher than currently estimated.
On behalf of the National Science and Technology Council, the U.S. Climate Change Science Program... more On behalf of the National Science and Technology Council, the U.S. Climate Change Science Program (CCSP) is pleased to transmit to the President and the Congress this Synthesis and Assessment Product (SAP) Abrupt Climate Change. This is part of a series of 21 SAPs produced by the CCSP aimed at providing current assessments of climate change science to inform public debate, policy, and operational decisions. These reports are also intended to help the CCSP develop future program research priorities. On behalf of the National Science and Technology Council, the U.S. Climate Change Science Program (CCSP) is pleased to transmit to the President and the Congress this Synthesis and Assessment Product (SAP) Abrupt Climate Change. This is part of a series of 21 SAPs produced by the CCSP aimed at providing current assessments of climate change science to inform public debate, policy, and operational decisions. These reports are also intended to help the CCSP develop future program research priorities.
Tree-ring based paleoclimate reconstructions entail several sequential estimation or processing s... more Tree-ring based paleoclimate reconstructions entail several sequential estimation or processing steps. Consequently, it can be difficult to isolate climatic from non-climatic variability in the raw ring width measurements, estimate the uncertainty associated with a reconstruction, and directly infer how specific techniques used to sequentially fit growth curves or to reconstruct climate influence the final estimates. This paper explores the use of hierarchical regression models to address these problems. The proposed models simultaneously model the entire reconstruction process in a way that is consistent with the existing step-by-step estimation framework, but allow for uncertainty estimation and propagation across steps, which can help determine how best to improve a candidate model. The utility of hierarchical models is tested for an example, the reconstruction of summertime temperatures in northern Sweden in a cross-validated framework relative to 1) a sequential process of growth curve fitting followed by chronology development, 3) an iterative, "signal-free" approach, and 2) a signal-free regional curve standardization (RCS-SF). Further, an exploration of different structures within the unifying hierarchical framework is provided to illustrate how one could easily test a variety of choices of model design. We focus on a subset of choices relevant to recent dendroclimatic studies using hierarchical methods and related to 1) data transformation, 2) the benefits of biological detrending and climate reconstruction in a single step 3) partial pooling of the age model across trees, 4) the homogeneity of variance across tree-ring residuals, 5) the structural form of the age model, and 6) the inclusion of autoregressive processes for the tree-ring residuals. The work described here represents part of a series of ongoing explorations of potential advances over current dendroclimatic reconstruction approaches 3 and commonly implemented ways in which they have and are specifically implemented. The results show that hierarchical modeling appears to offer improved climate reconstructions over the standardization techniques explored in this exercise, substantially so for the non-RCS sequential and iterative methods.
Provenance studies are an increasingly important analog for understanding how trees adapted to pa... more Provenance studies are an increasingly important analog for understanding how trees adapted to particular climatic conditions might respond to climate change. Dendrochronological analysis can illuminate differences among trees from different seed sources in terms of absolute annual growth and sensitivity to external growth factors. We analyzed annual radial growth of 567 36-year-old pitch pine (Pinus rigida Mill.) trees from 27 seed sources to evaluate their performance in a New Jersey Pine Barrens provenance experiment. Unexpectedly, missing rings were prevalent in most trees, and some years-1992, 1999, and 2006-had a particularly high frequency of missing rings across the plantation. Trees from local seed sources (<55 km away from the plantation) had a significantly smaller percentage of missing rings from 1980-2009 (mean: 5.0%), relative to northernmost and southernmost sources (mean: 9.3% and 7.9%, respectively). Some years with a high frequency of missing rings coincide with outbreaks of defoliating insects or dry growing season conditions. The propensity for missing rings synchronized annual variations in growth across all trees and might have complicated the detection of potential differences in interannual variability among seed sources. Average ring width was significantly larger in seed sources from both the southernmost and warmest origins compared to the northernmost and coldest seed sources in most years. Local seed sources had the highest average radial growth. Adaptation to local environmental conditions and disturbances might have influenced the higher growth rate found in local seed sources. These findings underscore the need to understand the integrative impact of multiple environmental drivers, such as disturbance agents and climate change, on tree growth, forest dynamics, and the carbon cycle.
Multidecadal "megadroughts" were a notable feature of the climate of the American Southwest over ... more Multidecadal "megadroughts" were a notable feature of the climate of the American Southwest over the Common era, yet we still lack a comprehensive theory for what caused these megadroughts and why they curiously only occurred before about 1600 CE. Here, we use the Paleo Hydrodynamics Data Assimilation product, in conjunction with radiative forcing estimates, to demonstrate that megadroughts in the American Southwest were driven by unusually frequent and cold central tropical Pacific sea surface temperature (SST) excursions in conjunction with anomalously warm Atlantic SSTs and a locally positive radiative forcing. This assessment of past megadroughts provides the first comprehensive theory for the causes of megadroughts and their clustering particularly during the Medieval era. This work also provides the first paleoclimatic support for the prediction that the risk of American Southwest megadroughts will markedly increase with global warming.
Previous tree-ring studies indicate that tree growth at high elevations is strongly limited by te... more Previous tree-ring studies indicate that tree growth at high elevations is strongly limited by temperatures in the southeastern China, where the climate is dominated by the East Asian monsoon. Based on this result, we built a highly replicated 202-year tree-ring width chronology from high elevation sites in the Dabie Mountains, southeastern China. The most reliable period of the chronology is from 1834 to 2011 according to a subsample signal strength cutoff of 0.85. Based on this chronology, January-July minimum temperature was reconstructed for the last 178 years, with an explained variance of 57.6% during the instrumental period 1956-2010. The reconstructed temperature series matches reasonably well with three other tree-ring
Annual rings are not commonly produced in tropical trees because they grow in a relatively aseaso... more Annual rings are not commonly produced in tropical trees because they grow in a relatively aseasonal environment. However, in the subalpine zones of Hawaiʻi's highest volcanoes, there is often strong seasonal variability in temperature and rainfall. Using classical dendrochronological methods, annual growth rings were shown to occur in Sophora chrysophylla, commonly called māmane, a native tree species on Maunakea, Hawaiʻi. Sampling occurred at three sites on various facing slopes of Maunakea-Puʻulāʻau (west), Pōhakuloa (south), and Puʻumali (north). Chronologies established from nearby non-native, live conifer trees were used to verify the dates from a total of 52 series from 22 S. chrysophylla trees of the Pōhakuloa site, establishing an 86-y chronology (1926-2011) with 2003 rings, 0.255 mean series intercorrelation, 0.096 chronology lag-1 autocorrelation, and 0.303 mean sensitivity. Ring-width patterns were significantly correlated with monthly rainfall from August of the previous year at the Pōhakuloa site (r = 0.377, P = 0.019). This study is the first in the eastern tropical Pacific region to utilize dendrochronological methods to gain a better understanding of the growth dynamics of a native forest tree.
A brief and personal history of the development of dendrochronology in the Hudson Valley of New Y... more A brief and personal history of the development of dendrochronology in the Hudson Valley of New York in the 1970s and the quantitative reconstruction of climate from tree rings there is provided. Two people stand out in allowing that to happen. Marvin Stokes at the Laboratory of Tree-Ring Research sparked within me a deep and enduring interest in dendrochronology, and Daniel Smiley of Mohonk supported my interest in pursuing tree-ring research in the Shawangunk Mountains through his deep and curious love of its natural environment. The discovery of ancient trees growing in the Shawangunk Mountains, and their use in successfully reconstructing past drought there, truly launched my career as a dendroclimatologist and proved beyond doubt that dendroclimatology and the reconstruction of past climate could be successfully conducted in the northeastern United States.
The amplitude of El Niño-Southern Oscillation (ENSO) varies substantially at each phase of its ev... more The amplitude of El Niño-Southern Oscillation (ENSO) varies substantially at each phase of its evolution, affecting the timing and patterns of atmospheric teleconnections around the globe. Instrumental records are too short to capture the full behavior of ENSO variability. Here we use the well-validated Monsoon Asia Drought Atlas (MADA) and North America Drought Atlas (NADA) for the past 700 years, and show that tree-ring records from different regions represent tropical sea surface temperature (SST) conditions at various phases of ENSO. Three modes of tree-ring based summer drought variability are found to be correlated with ENSO: summer droughts over the Maritime Continent and Southwest North America (NA), and a dipole mode between Central and South Asia. A lagged correlation analysis is performed to determine the time when precipitation and temperature anomaly imprints on summer droughts as recorded in tree-rings. Drought anomalies in the Maritime Continent and Southwest NA represent ENSO at the developing and peak phases respectively, while those over Central/South Asia are associated with tropical wide SST anomalies (including the Indian Ocean) at the decay phase of ENSO. Thus proxy records from different regions can provide valuable information on long-term behavior of ENSO at different phases.
A hierarchical Bayesian regression model is presented for reconstructing the average summer strea... more A hierarchical Bayesian regression model is presented for reconstructing the average summer streamflow at five gauges in the Delaware River basin using eight regional tree-ring chronologies. The model provides estimates of the posterior probability distribution of each reconstructed streamflow series considering parameter uncertainty. The vectors of regression coefficients are modeled as draws from a common multivariate normal distribution with unknown parameters estimated as part of the analysis. This leads to a multilevel structure. The covariance structure of the streamflow residuals across sites is explicitly modeled. The resulting partial pooling of information across multiple stations leads to a reduction in parameter uncertainty. The effect of no pooling and full pooling of station information, as end points of the method, is explored. The nopooling model considers independent estimation of the regression coefficients for each streamflow gauge with respect to each tree-ring chronology. The full-pooling model considers that the same regression coefficients apply across all streamflow sites for a particular tree-ring chronology. The cross-site correlation of residuals is modeled in all cases. Performance on metrics typically used by tree-ring reconstruction experts, such as reduction of error, coefficient of efficiency, and coverage rates under credible intervals is comparable to, or better, for the partial-pooling model relative to the no-pooling model, and streamflow estimation uncertainty is reduced. Long record simulations from reconstructions are used to develop estimates of the probability of duration and severity of droughts in the region. Analysis of monotonic trends in the reconstructed drought events do not reject the null hypothesis of no trend at the 90% significance over 1754-2000.
Quantifying long-term historical climate is fundamental to understanding recent climate change. M... more Quantifying long-term historical climate is fundamental to understanding recent climate change. Most instrumentally recorded climate data are only available for the past 200 years, so proxy observations from natural archives are often considered. We describe a model-based approach to reconstructing climate defined in terms of raw tree-ring measurement data that simultaneously accounts for non-climatic and climatic variability. In this approach we specify a joint model for the tree-ring data and climate variable that we fit using Bayesian inference. We consider a range of prior densities and compare the modeling approach to current methodology using an example case of Scots pine from Torneträsk, Sweden to reconstruct growing season temperature. We describe how current approaches translate into particular model assumptions. We explore how changes to various components in the model-based approach affect the resulting reconstruction. We show that minor changes in model specification can have little effect on model fit but lead to large changes in the predictions. In particular, the periods of relatively warmer and cooler temperatures are robust between models,
Very few annually resolved millennial-length temperature reconstructions exist for the Southern H... more Very few annually resolved millennial-length temperature reconstructions exist for the Southern Hemisphere. Here we present four 979-year reconstructions for southeastern Australia for the austral summer months of December–February. Two of the reconstructions are based on the Australian Water Availability Project dataset and two on the Berkeley Earth Surface Temperature dataset. For each climate data set, one reconstruction is based solely on Lagarostrobos franklinii (restricted reconstructions) while the other is based on multiple Tasmanian conifer species (unrestricted reconstructions). Each reconstruction calibrates ~50−60% of the variance in the temperature datasets depending on the number of tree-ring records available for the reconstruction. We found little difference in the temporal variability of the reconstructions, although extremes are amplified in the restricted reconstructions relative to the unrestricted reconstructions. The reconstructions highlight the occurrence of ...
During the 2018 WorldDendro fieldweek in Bhutan, we examined the flood history of the Dhur River.... more During the 2018 WorldDendro fieldweek in Bhutan, we examined the flood history of the Dhur River. Most villages are located along streams, so knowing the flood history of the area will enable managers to prepare for future events. We collected scarred partial cross sections from 29 trees along a two km stretch of the Dhur River, and two cores per tree from 29 other trees from six species (Populus ciliata, Picea spinulosa, Tsuga dumosa, Quercus semecarpifolia, Pinus wallichiana, and Rhododendron arboreum). We identified large flood events in 2009, 1989, and 1967 from at least two trees with flood scars or traumatic rings. Our flood-scar chronology extends to 1940 with five cross-sectioned trees, and back to 1904 with core evidence from two trees. The oldest flood scar occurred in 1967. The 2009 flood scar was recorded in most of our streamside samples and is the result of heavy precipitation from Cyclone Aila at the end of May 2009. Two other storms and proceeding flood events occurred in 1989 and 1967 according to additional scars detected in several samples. This work demonstrates the successful use of density fluctuations in Pinus along with scarring in multiple species to reconstruct past flood events and identifies the effects of Cyclone Aila, as an extreme event for this area, which was unprecedented for the past hundred years.
82 ESPER, COOK, KRUSIC, PETERS, and SCHWEINGRUBER or regional tree-ring chronology used for clima... more 82 ESPER, COOK, KRUSIC, PETERS, and SCHWEINGRUBER or regional tree-ring chronology used for climate reconstruction (Briffa et al. 1990; Briffa et al. 1992). Long tree-ring chronologies are often built by crossdating individual TRW or MXD time series from living trees with ...
The problem of constructing millennia-long tree-ring chronologies from overlapping segments of cr... more The problem of constructing millennia-long tree-ring chronologies from overlapping segments of cross-dated ring-width series is reviewed, with an emphasis on preserving very low-frequency signals potentially due to climate. In so doing, a fundamental statistical problem coined the 'segment length curse' is introduced. This 'curse' is related to the fact that the maximum wavelength of recoverable climatic information is ordinarily related to the lengths of the individual tree-ring series used to construct the millennia-long chronology. Simple experiments with sine waves are used to illustrate this fact. This is followed by more realistic experiments using a long bristlecone pine series that is randomly cut into a number of 1000-, 500- and 200-year segments and standardized using three very conservative methods. When compared against the original, uncut series, the resulting 'chronologies' show the effects of segment length even when the most conservative and n...
This paper describes an approach that gives an estimate of the reconstructive skill of a proxy-ba... more This paper describes an approach that gives an estimate of the reconstructive skill of a proxy-based palaeoclimatological reconstruction. Uncertainties in proxy data are likely to be reflected in variations in reconstructive skill of the proxy-based climatic reconstructions. The method is based on making an ensemble of reconstructions, providing a probability distribution of each reconstruction estimate. The relative breadth of the distribution for a particular reconstructed value should give an indication of the reconstructive skill. The ensemble reconstruction approach draws on the ensemble prediction system as used for operational weather forecasting. The ensemble reconstruction approach is tested using a recent multiproxy reconstruction of the winter North Atlantic Oscillation (NAO) index. It is shown that the ensemble approach provides a representation of the degree of certainty associated with the reconstructed NAO-index values.
Declining winter rainfall coupled with recent prolonged drought poses significant risks to water ... more Declining winter rainfall coupled with recent prolonged drought poses significant risks to water resources and agriculture across southern Australia. While rainfall declines over recent decades are largely consistent with modelled climate change scenarios, particularly for southwest Australia, the significance of these declines is yet to be assessed within the context of long-term hydroclimatic variability. Here, we present a new 668-year (1350-2017 CE) tree-ring reconstruction of autumnwinter rainfall over inland southwest Australia. This record reveals that a recent decline in rainfall over inland southwest Australia (since 2000 CE) is not unusual in terms of either magnitude or duration relative to rainfall variability over the last seven centuries. Drought periods of greater magnitude and duration than those in the instrumental record occurred prior to 1900 CE, including two 'megadroughts' of > 30 years duration in the eighteenth and nineteenth centuries. By contrast, the wettest > decadal periods of the last seven centuries occurred after 1900 CE, making the twentieth century the wettest of the last seven centuries. We conclude that the instrumental rainfall record (since ~ 1900 CE) does not capture the full scale of natural hydroclimatic variability for inland southwest Australia and that the risk of prolonged droughts in the region is likely much higher than currently estimated.
On behalf of the National Science and Technology Council, the U.S. Climate Change Science Program... more On behalf of the National Science and Technology Council, the U.S. Climate Change Science Program (CCSP) is pleased to transmit to the President and the Congress this Synthesis and Assessment Product (SAP) Abrupt Climate Change. This is part of a series of 21 SAPs produced by the CCSP aimed at providing current assessments of climate change science to inform public debate, policy, and operational decisions. These reports are also intended to help the CCSP develop future program research priorities. On behalf of the National Science and Technology Council, the U.S. Climate Change Science Program (CCSP) is pleased to transmit to the President and the Congress this Synthesis and Assessment Product (SAP) Abrupt Climate Change. This is part of a series of 21 SAPs produced by the CCSP aimed at providing current assessments of climate change science to inform public debate, policy, and operational decisions. These reports are also intended to help the CCSP develop future program research priorities.
Tree-ring based paleoclimate reconstructions entail several sequential estimation or processing s... more Tree-ring based paleoclimate reconstructions entail several sequential estimation or processing steps. Consequently, it can be difficult to isolate climatic from non-climatic variability in the raw ring width measurements, estimate the uncertainty associated with a reconstruction, and directly infer how specific techniques used to sequentially fit growth curves or to reconstruct climate influence the final estimates. This paper explores the use of hierarchical regression models to address these problems. The proposed models simultaneously model the entire reconstruction process in a way that is consistent with the existing step-by-step estimation framework, but allow for uncertainty estimation and propagation across steps, which can help determine how best to improve a candidate model. The utility of hierarchical models is tested for an example, the reconstruction of summertime temperatures in northern Sweden in a cross-validated framework relative to 1) a sequential process of growth curve fitting followed by chronology development, 3) an iterative, "signal-free" approach, and 2) a signal-free regional curve standardization (RCS-SF). Further, an exploration of different structures within the unifying hierarchical framework is provided to illustrate how one could easily test a variety of choices of model design. We focus on a subset of choices relevant to recent dendroclimatic studies using hierarchical methods and related to 1) data transformation, 2) the benefits of biological detrending and climate reconstruction in a single step 3) partial pooling of the age model across trees, 4) the homogeneity of variance across tree-ring residuals, 5) the structural form of the age model, and 6) the inclusion of autoregressive processes for the tree-ring residuals. The work described here represents part of a series of ongoing explorations of potential advances over current dendroclimatic reconstruction approaches 3 and commonly implemented ways in which they have and are specifically implemented. The results show that hierarchical modeling appears to offer improved climate reconstructions over the standardization techniques explored in this exercise, substantially so for the non-RCS sequential and iterative methods.
Provenance studies are an increasingly important analog for understanding how trees adapted to pa... more Provenance studies are an increasingly important analog for understanding how trees adapted to particular climatic conditions might respond to climate change. Dendrochronological analysis can illuminate differences among trees from different seed sources in terms of absolute annual growth and sensitivity to external growth factors. We analyzed annual radial growth of 567 36-year-old pitch pine (Pinus rigida Mill.) trees from 27 seed sources to evaluate their performance in a New Jersey Pine Barrens provenance experiment. Unexpectedly, missing rings were prevalent in most trees, and some years-1992, 1999, and 2006-had a particularly high frequency of missing rings across the plantation. Trees from local seed sources (<55 km away from the plantation) had a significantly smaller percentage of missing rings from 1980-2009 (mean: 5.0%), relative to northernmost and southernmost sources (mean: 9.3% and 7.9%, respectively). Some years with a high frequency of missing rings coincide with outbreaks of defoliating insects or dry growing season conditions. The propensity for missing rings synchronized annual variations in growth across all trees and might have complicated the detection of potential differences in interannual variability among seed sources. Average ring width was significantly larger in seed sources from both the southernmost and warmest origins compared to the northernmost and coldest seed sources in most years. Local seed sources had the highest average radial growth. Adaptation to local environmental conditions and disturbances might have influenced the higher growth rate found in local seed sources. These findings underscore the need to understand the integrative impact of multiple environmental drivers, such as disturbance agents and climate change, on tree growth, forest dynamics, and the carbon cycle.
Multidecadal "megadroughts" were a notable feature of the climate of the American Southwest over ... more Multidecadal "megadroughts" were a notable feature of the climate of the American Southwest over the Common era, yet we still lack a comprehensive theory for what caused these megadroughts and why they curiously only occurred before about 1600 CE. Here, we use the Paleo Hydrodynamics Data Assimilation product, in conjunction with radiative forcing estimates, to demonstrate that megadroughts in the American Southwest were driven by unusually frequent and cold central tropical Pacific sea surface temperature (SST) excursions in conjunction with anomalously warm Atlantic SSTs and a locally positive radiative forcing. This assessment of past megadroughts provides the first comprehensive theory for the causes of megadroughts and their clustering particularly during the Medieval era. This work also provides the first paleoclimatic support for the prediction that the risk of American Southwest megadroughts will markedly increase with global warming.
Previous tree-ring studies indicate that tree growth at high elevations is strongly limited by te... more Previous tree-ring studies indicate that tree growth at high elevations is strongly limited by temperatures in the southeastern China, where the climate is dominated by the East Asian monsoon. Based on this result, we built a highly replicated 202-year tree-ring width chronology from high elevation sites in the Dabie Mountains, southeastern China. The most reliable period of the chronology is from 1834 to 2011 according to a subsample signal strength cutoff of 0.85. Based on this chronology, January-July minimum temperature was reconstructed for the last 178 years, with an explained variance of 57.6% during the instrumental period 1956-2010. The reconstructed temperature series matches reasonably well with three other tree-ring
Annual rings are not commonly produced in tropical trees because they grow in a relatively aseaso... more Annual rings are not commonly produced in tropical trees because they grow in a relatively aseasonal environment. However, in the subalpine zones of Hawaiʻi's highest volcanoes, there is often strong seasonal variability in temperature and rainfall. Using classical dendrochronological methods, annual growth rings were shown to occur in Sophora chrysophylla, commonly called māmane, a native tree species on Maunakea, Hawaiʻi. Sampling occurred at three sites on various facing slopes of Maunakea-Puʻulāʻau (west), Pōhakuloa (south), and Puʻumali (north). Chronologies established from nearby non-native, live conifer trees were used to verify the dates from a total of 52 series from 22 S. chrysophylla trees of the Pōhakuloa site, establishing an 86-y chronology (1926-2011) with 2003 rings, 0.255 mean series intercorrelation, 0.096 chronology lag-1 autocorrelation, and 0.303 mean sensitivity. Ring-width patterns were significantly correlated with monthly rainfall from August of the previous year at the Pōhakuloa site (r = 0.377, P = 0.019). This study is the first in the eastern tropical Pacific region to utilize dendrochronological methods to gain a better understanding of the growth dynamics of a native forest tree.
A brief and personal history of the development of dendrochronology in the Hudson Valley of New Y... more A brief and personal history of the development of dendrochronology in the Hudson Valley of New York in the 1970s and the quantitative reconstruction of climate from tree rings there is provided. Two people stand out in allowing that to happen. Marvin Stokes at the Laboratory of Tree-Ring Research sparked within me a deep and enduring interest in dendrochronology, and Daniel Smiley of Mohonk supported my interest in pursuing tree-ring research in the Shawangunk Mountains through his deep and curious love of its natural environment. The discovery of ancient trees growing in the Shawangunk Mountains, and their use in successfully reconstructing past drought there, truly launched my career as a dendroclimatologist and proved beyond doubt that dendroclimatology and the reconstruction of past climate could be successfully conducted in the northeastern United States.
The amplitude of El Niño-Southern Oscillation (ENSO) varies substantially at each phase of its ev... more The amplitude of El Niño-Southern Oscillation (ENSO) varies substantially at each phase of its evolution, affecting the timing and patterns of atmospheric teleconnections around the globe. Instrumental records are too short to capture the full behavior of ENSO variability. Here we use the well-validated Monsoon Asia Drought Atlas (MADA) and North America Drought Atlas (NADA) for the past 700 years, and show that tree-ring records from different regions represent tropical sea surface temperature (SST) conditions at various phases of ENSO. Three modes of tree-ring based summer drought variability are found to be correlated with ENSO: summer droughts over the Maritime Continent and Southwest North America (NA), and a dipole mode between Central and South Asia. A lagged correlation analysis is performed to determine the time when precipitation and temperature anomaly imprints on summer droughts as recorded in tree-rings. Drought anomalies in the Maritime Continent and Southwest NA represent ENSO at the developing and peak phases respectively, while those over Central/South Asia are associated with tropical wide SST anomalies (including the Indian Ocean) at the decay phase of ENSO. Thus proxy records from different regions can provide valuable information on long-term behavior of ENSO at different phases.
A hierarchical Bayesian regression model is presented for reconstructing the average summer strea... more A hierarchical Bayesian regression model is presented for reconstructing the average summer streamflow at five gauges in the Delaware River basin using eight regional tree-ring chronologies. The model provides estimates of the posterior probability distribution of each reconstructed streamflow series considering parameter uncertainty. The vectors of regression coefficients are modeled as draws from a common multivariate normal distribution with unknown parameters estimated as part of the analysis. This leads to a multilevel structure. The covariance structure of the streamflow residuals across sites is explicitly modeled. The resulting partial pooling of information across multiple stations leads to a reduction in parameter uncertainty. The effect of no pooling and full pooling of station information, as end points of the method, is explored. The nopooling model considers independent estimation of the regression coefficients for each streamflow gauge with respect to each tree-ring chronology. The full-pooling model considers that the same regression coefficients apply across all streamflow sites for a particular tree-ring chronology. The cross-site correlation of residuals is modeled in all cases. Performance on metrics typically used by tree-ring reconstruction experts, such as reduction of error, coefficient of efficiency, and coverage rates under credible intervals is comparable to, or better, for the partial-pooling model relative to the no-pooling model, and streamflow estimation uncertainty is reduced. Long record simulations from reconstructions are used to develop estimates of the probability of duration and severity of droughts in the region. Analysis of monotonic trends in the reconstructed drought events do not reject the null hypothesis of no trend at the 90% significance over 1754-2000.
Quantifying long-term historical climate is fundamental to understanding recent climate change. M... more Quantifying long-term historical climate is fundamental to understanding recent climate change. Most instrumentally recorded climate data are only available for the past 200 years, so proxy observations from natural archives are often considered. We describe a model-based approach to reconstructing climate defined in terms of raw tree-ring measurement data that simultaneously accounts for non-climatic and climatic variability. In this approach we specify a joint model for the tree-ring data and climate variable that we fit using Bayesian inference. We consider a range of prior densities and compare the modeling approach to current methodology using an example case of Scots pine from Torneträsk, Sweden to reconstruct growing season temperature. We describe how current approaches translate into particular model assumptions. We explore how changes to various components in the model-based approach affect the resulting reconstruction. We show that minor changes in model specification can have little effect on model fit but lead to large changes in the predictions. In particular, the periods of relatively warmer and cooler temperatures are robust between models,
Very few annually resolved millennial-length temperature reconstructions exist for the Southern H... more Very few annually resolved millennial-length temperature reconstructions exist for the Southern Hemisphere. Here we present four 979-year reconstructions for southeastern Australia for the austral summer months of December–February. Two of the reconstructions are based on the Australian Water Availability Project dataset and two on the Berkeley Earth Surface Temperature dataset. For each climate data set, one reconstruction is based solely on Lagarostrobos franklinii (restricted reconstructions) while the other is based on multiple Tasmanian conifer species (unrestricted reconstructions). Each reconstruction calibrates ~50−60% of the variance in the temperature datasets depending on the number of tree-ring records available for the reconstruction. We found little difference in the temporal variability of the reconstructions, although extremes are amplified in the restricted reconstructions relative to the unrestricted reconstructions. The reconstructions highlight the occurrence of ...
During the 2018 WorldDendro fieldweek in Bhutan, we examined the flood history of the Dhur River.... more During the 2018 WorldDendro fieldweek in Bhutan, we examined the flood history of the Dhur River. Most villages are located along streams, so knowing the flood history of the area will enable managers to prepare for future events. We collected scarred partial cross sections from 29 trees along a two km stretch of the Dhur River, and two cores per tree from 29 other trees from six species (Populus ciliata, Picea spinulosa, Tsuga dumosa, Quercus semecarpifolia, Pinus wallichiana, and Rhododendron arboreum). We identified large flood events in 2009, 1989, and 1967 from at least two trees with flood scars or traumatic rings. Our flood-scar chronology extends to 1940 with five cross-sectioned trees, and back to 1904 with core evidence from two trees. The oldest flood scar occurred in 1967. The 2009 flood scar was recorded in most of our streamside samples and is the result of heavy precipitation from Cyclone Aila at the end of May 2009. Two other storms and proceeding flood events occurred in 1989 and 1967 according to additional scars detected in several samples. This work demonstrates the successful use of density fluctuations in Pinus along with scarring in multiple species to reconstruct past flood events and identifies the effects of Cyclone Aila, as an extreme event for this area, which was unprecedented for the past hundred years.
82 ESPER, COOK, KRUSIC, PETERS, and SCHWEINGRUBER or regional tree-ring chronology used for clima... more 82 ESPER, COOK, KRUSIC, PETERS, and SCHWEINGRUBER or regional tree-ring chronology used for climate reconstruction (Briffa et al. 1990; Briffa et al. 1992). Long tree-ring chronologies are often built by crossdating individual TRW or MXD time series from living trees with ...
The problem of constructing millennia-long tree-ring chronologies from overlapping segments of cr... more The problem of constructing millennia-long tree-ring chronologies from overlapping segments of cross-dated ring-width series is reviewed, with an emphasis on preserving very low-frequency signals potentially due to climate. In so doing, a fundamental statistical problem coined the 'segment length curse' is introduced. This 'curse' is related to the fact that the maximum wavelength of recoverable climatic information is ordinarily related to the lengths of the individual tree-ring series used to construct the millennia-long chronology. Simple experiments with sine waves are used to illustrate this fact. This is followed by more realistic experiments using a long bristlecone pine series that is randomly cut into a number of 1000-, 500- and 200-year segments and standardized using three very conservative methods. When compared against the original, uncut series, the resulting 'chronologies' show the effects of segment length even when the most conservative and n...
This paper describes an approach that gives an estimate of the reconstructive skill of a proxy-ba... more This paper describes an approach that gives an estimate of the reconstructive skill of a proxy-based palaeoclimatological reconstruction. Uncertainties in proxy data are likely to be reflected in variations in reconstructive skill of the proxy-based climatic reconstructions. The method is based on making an ensemble of reconstructions, providing a probability distribution of each reconstruction estimate. The relative breadth of the distribution for a particular reconstructed value should give an indication of the reconstructive skill. The ensemble reconstruction approach draws on the ensemble prediction system as used for operational weather forecasting. The ensemble reconstruction approach is tested using a recent multiproxy reconstruction of the winter North Atlantic Oscillation (NAO) index. It is shown that the ensemble approach provides a representation of the degree of certainty associated with the reconstructed NAO-index values.
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Papers by Edward Cook