Moisture sensitive tree ring chronologies of Centrolobium microchaete have been developed from se... more Moisture sensitive tree ring chronologies of Centrolobium microchaete have been developed from seasonally dry forests in the southern Amazon Basin and used to reconstruct wet season rainfall totals from 1799 to 2012, adding over 150 years of rainfall estimates to the short instrumental record for the region. The reconstruction is correlated with the same atmospheric variables that influence the instrumental measurements of wet season rainfall. Anticyclonic circulation over midlatitude South America promotes equatorward surges of cold and relatively dry extratropical air that converge with warm moist air to form deep convection and heavy rainfall over this sector of the southern Amazon Basin. Interesting droughts and pluvials are reconstructed during the preinstrumental nineteenth and early twentieth centuries, but the tree ring reconstruction suggests that the strong multidecadal variability in instrumental and reconstructed wet season rainfall after 1950 may have been unmatched since 1799.
managers. Management must minimize effects of drought on water supplies while also keeping reserv... more managers. Management must minimize effects of drought on water supplies while also keeping reservoir levels low enough to minimize flood risk . California's exposure to hydrological volatility is particularly high because of its geographic setting: the majority of the state's surface water supply falls in the Sierra Nevada during the relatively short cool season of November-March (Dettinger, 2013;, giving this region and season particular importance to California's water resources. While parts of the state, especially southern California, have worked to diversify water portfolios with a combination of surface, imported, and ground water, and to employ conservation measures and water markets , California remains vulnerable to extreme variability of precipitation . Extreme hydrological shortages in the Sierra Nevada can be only moderately buffered by water supplies in regions and seasons not experiencing drought. California's drought and flood risk is amplified by the limited reliability of seasonal forecasts of Sierra Nevada precipitation . While forecast skill beyond a 2-week horizon is inherently poor globally, seasonal forecasting for parts of the western United States is aided by a teleconnection to the El Niño-Southern Oscillation (ENSO) . This is not the case, however, for the Sierra Nevada, which span the neutral point of the ENSO dipole that often drives Abstract California's water resources rely heavily on cool-season (November-March) precipitation in the Sierra Nevada. Interannual variability is highly volatile and seasonal forecasting has little to no skill, making water management particularly challenging. Over 1902-2020, Sierra Nevada cool-season precipitation totals exhibited significant 2.2-and 13-15-year cycles, accounting for approximately 40% of total variability and perhaps signifying potential as seasonal forecasting tools. However, the underlying climate dynamics are not well understood and it is unclear whether these cycles are stable over the long term. We use tree rings to reconstruct Sierra Nevada cool-season precipitation back to 1400. The reconstruction is skillful, accounting for 55%-74% of observed variability and capturing the 20th-century 2.2-and 13-15-year cycles. Prior to 1900, the reconstruction indicates no other century-long periods of significant spectral power in the 2.2-or 13-15-year bands. The reconstruction does indicate significant cyclicity over other extended periods of several decades or longer, however, with dominant periodicities in the ranges of 2.1-2.7 and 3.5-8 years. The late 1700s through 1800s exhibited the highest-amplitude cycles in the reconstruction, with periodicities of 2.4 and 5.7-7.4 years. The reconstruction should serve to caution against extrapolating the observed 2.2-and 13-15-year cycles to guide future expectations. On the other hand, observations and the reconstruction suggest that interannual variability of Sierra Nevada cool-season precipitation is not a purely white noise process and research should aim to diagnose the dynamical drivers of extended periods of cyclicity in this critical natural resource. WILLIAMS ET AL.
The annual ringwidths of trees can be used to search for hypothesized air pollution effects on fo... more The annual ringwidths of trees can be used to search for hypothesized air pollution effects on forests. This search is extremely complicated by the inherent statistical properties of ringwidth data and the high level of uncertainty regarding the sources of variance observed in the ringwidths. A linear aggregate model for ringwidths is described which highlights the general classes of variance which may be found in a tree -ring series. Dendrochronological principles and techniques are des- cribed which can be used to create a tree -ring chronology that is suitable for rigorous statistical analysis and hypothesis testing. The need to model climatic influences on tree growth prior to the search for pollution effects is necessary and a method for achieving this is described. Only after the variance due to age trends, stand dynamics effects and climatic influences has been accounted for can any confidence be placed on inferred pollution effects. An analysis of a red spruce tree -ring chronology indicates that a decline in ringwidths since 1968 cannot be explained by a linear temperature response model using monthly climatic variables. However, threshold responses to climate that could be responsible for the decline need to be considered before the anomalous decline can be attributed to non -climatic influences such as pollution.
Signal extraction in tree -ring research is considered as a general time series decomposition pro... more Signal extraction in tree -ring research is considered as a general time series decomposition problem. A linear aggregate model for a hypothetical ring -width series is proposed, which allows the problem to be reduced to the estimation and extraction of five discrete classes of signals. These classes represent the signals due to trend, climate, endogenous disturbance, exogenous disturbance, and random error. For each class of signal, some mathematical /statistical techniques of estimation are described and reviewed. Except for the exogenous disturbance signal, the techniques only require information contained within the ring -width series, themselves. A unified mathematical framework for solving this decomposition problem has not yet been explicitly formulated. However, the general applicability of ARMA time series models to this problem and the power and flexibility of state space modelling suggest that these techniques will provide the closest thing to a unified framework in the future. Problem der Zeitreihenzerlegung betrachtet. Hierzu wird daher ein linear zusammengesetztes Modell einer hypothetischen Jahrringfolge vorgeschlagen, womit das Problem auf die Schätzung und Extraktion von fünf diskreten Klassen von Signalen reduziert werden kann. Diese Klassen repräsentieren die Signale, die durch einen Trend, durch das Klima, durch endogene sowie exogene Störfaktoren und durch einen Zufallsfehler hervorgerufen werden. Für jede Klasse werden einige mathematisch-statistische Schätzverfahren beschrieben und bewertet. Alle Verfahren benötigen nur solche Informationen, die in den Jahrringfolgen selbst enthalten sind, ausgenommen bei exogenen Störfaktoren. Bislang wurde noch kein einheitliches mathematisches Vorgehen zur Lösung dieses Zerlegungsproblems explizit formuliert. Jedoch könnte die allgemeine Anwendbarkeit von ARMA-Zeitreihenmodellen sowie die Leistungs-und Anpassungsfähigkeit von Zustandsraummodellen einem einheitlichen Vorgehen in der Zukunft sehr nahekommen. L'extraction d'un signal dans la recherche dendrochronologique est considérée comme un problème général de désagrégation en des séries temporelles. Un modèle d'agrégation linéaire valable pour les épaisseurs des cernes est proposé, qui permet de réduire le problème à l'estimation et à l'extraction de cinq classes discontinues de signaux. Ces classes représentent les signaux dus à la tendance, au climat, à des perturbations endogènes et exogènes et à l'erreur aléatoire. Pour chaque classe de signal quelques techhniques mathématiques et statistiques d'estimation sont décrites et commises à critique. A l'exception du signal provenant de perturbations exogènes, les techniques n'exigent que les informations contenues dans les séries d'épaisseur des cernes elles-mémes. Un cadre mathématique unifié pour résoudre ce problème de désagrégation n'a pas encore été formulé de façon explicite. Cependant la possibilité d'application des modèles ARMA à ce problème ainsi que la puissance et la flexibilité de la modélisation spatiale, suggère que ces techniques sont aptes à procurer le meilleur moyen pour utiliser un cadre unifié dans le futur.
Many tree-ring-based climate reconstructions are based on the assumption that the climate reactio... more Many tree-ring-based climate reconstructions are based on the assumption that the climate reaction of trees is independent of their size. Here, we test this assumption for New Zealand kauri (Agathis australis), one of the longest tree ring-based proxies for the El Nin ˜o-Southern Oscillation (ENSO). The most recent kauri chronology contains a large amount of archaeological material, e.g. timber for which the original tree size is often unknown. We analyzed the climate-growth relationship of different-sized kauri in a pristine forest using different temporal scales, i.e. annually, monthly and daily data on tree growth and climate conditions. Trees of different life stages exhibited approximately the same seasonal growth peaks during austral spring (October and November). The dormancy period overlaps with the period where weekly air temperature maxima are below ca. 17-18 °C, and where the corresponding daily minima are below ca. 8 °C. However, both correlation functions between annual growth and seasonal climate as well as Kalman filter regressions between daily growth and climate conditions suggest an influence of tree size on the climate-growth relationship for kauri. Smaller trees (DBH \ 40 cm) contain weaker climate signals than larger trees. Therefore, the precautionary stripping of near-pith material (first 20 cm) from kauri chronologies may result in more uniform responses to climate forcing and thus enhance the reliability of long-term climate reconstructions.
We evaluate a range of blue intensity (BI) tree-ring parameters in eight conifer species (12 site... more We evaluate a range of blue intensity (BI) tree-ring parameters in eight conifer species (12 sites) from Tasmania 15 and New Zealand for their dendroclimatic potential, and as surrogate wood anatomical proxies. Using a dataset of ca. 10-15 trees per site, we measured earlywood maximum blue reflectance intensity (EWB), latewood minimum blue reflectance intensity (LWB) and the associated delta blue intensity (DB) parameter for dendrochronological analysis. No resin extraction was performed, impacting low frequency trends. Therefore, we focused only on the high frequency signal by detrending all tree-ring and climate data using a 20-year cubic smoothing spline. All BI parameters express low relative variance and weak 20 signal strength compared to ring-width. Correlation analysis and principal component regression experiments identified a weak and variable climate response for most ring-width chronologies. However, for most sites, the EWB data, despite weak signal strength, expressed strong calibrations with summer temperatures. Significant correlations for LWB were also noted, but the sign of the relationship for most species is opposite to that reported for all conifer species in the Northern Hemisphere. DB performed well for the Tasmanian sites but explained minimal temperature variance in New Zealand. Using 25 the full multi-species/parameter network, excellent summer temperature calibration was identified for both Tasmania and New Zealand ranging from 52% to 78% explained variance, with equally robust independent validation (Coefficient of Efficiency = 0.41 to 0.77). Comparison of the Tasmanian BI reconstruction with a wood anatomical reconstruction shows that these parameters record essentially the same strong high frequency summer temperature signal. Despite these excellent results, a substantial challenge exists with the capture of potential secular scale climate trends. Although DB, band-pass and 30 other signal processing methods may help with this issue, substantially more experimentation is needed in conjunction with comparative analysis with ring density and quantitative WA measurements.
Indonesia's climate is dominated by the equatorial monsoon system, and has been linked to El Niño... more Indonesia's climate is dominated by the equatorial monsoon system, and has been linked to El Niño-Southern Oscillation (ENSO) events that often result in extensive droughts and floods over the Indonesian archipelago. In this study we investigate ENSO-related signals in a tree-ring δ 18 O record (1900-2007) of Javanese teak. Our results reveal a clear influence of Warm Pool (central Pacific) El Niño events on Javanese tree-ring δ 18 O, and no clear signal of Cold Tongue (eastern Pacific) El Niño events. These results are consistent with the distinct impacts of the two ENSO flavors on Javanese precipitation, and illustrate the importance of considering ENSO flavors when interpreting palaeoclimate proxy records in the tropics.
We evaluate a range of blue intensity (BI) treering parameters in eight conifer species (12 sites... more We evaluate a range of blue intensity (BI) treering parameters in eight conifer species (12 sites) from Tasmania and New Zealand for their dendroclimatic potential, and as surrogate wood anatomical proxies. Using a dataset of ca. 10-15 trees per site, we measured earlywood maximum blue intensity (EWB), latewood minimum blue intensity (LWB), and the associated delta blue intensity (DB) parameter for dendrochronological analysis. No resin extraction was performed, impacting low-frequency trends. Therefore, we focused only on the high-frequency signal by detrending all tree-ring and climate data using a 20-year cubic smoothing spline. All BI parameters express low relative variance and weak signal strength compared to ring width. Correlation analysis and principal component regression experiments identified a weak and variable climate response for most ring-width chronologies. However, for most sites, the EWB data, despite weak signal strength, expressed strong coherence with summer temperatures. Significant correlations for LWB were also noted, but the sign of the relationship for most species is opposite to that reported for all conifer species in the Northern Hemisphere. DB results were mixed but performed better for the Tasmanian sites when combined through principal component regression methods than for New Zealand. Using the full multi-species/parameter network, excellent summer temperature calibration was identified for both Tasmania and New Zealand ranging from 52 % to 78 % explained variance for split periods (1901-1950/1951-1995), with equally robust independent validation (coefficient of efficiency = 0.41 to 0.77). Comparison of the Tasmanian BI reconstruction with a quantitative wood anatomical (QWA) reconstruction shows that these parameters record essentially the same strong high-frequency summer temperature signal. Despite these excellent results, a substantial challenge exists with the capture of potential secular-scale climate trends. Although DB, band-pass, and other signal processing methods may help with this issue, substantially more experimentation is needed in conjunction with comparative analysis with ring density and QWA measurements.
Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atm... more Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atmospheric circulation anomalies in the Atlantic, but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial time scales. Here we analyze the coupled patterns of drought variability on either sides of the North Atlantic Ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin-wide moisture anomalies to ocean and atmosphere processes. A leading mode of broad-scale moisture variability is characterized by distinct patterns of North Atlantic atmosphere circulation and sea surface temperatures. We infer a negative phase of the North Atlantic Oscillation and colder Atlantic sea surface temperatures in the middle of the fifteenth century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age.
To date, no attempt has been made to assess the presence or otherwise of the "Divergence Problem"... more To date, no attempt has been made to assess the presence or otherwise of the "Divergence Problem" (DP) in existing multi-millennial Southern Hemisphere tree-ring chronologies. We have updated the iconic Mt Read Huon pine chronology from Tasmania, southeastern Australia, to now include the warmest decade on record, AD 2000e2010, and used the Kalman Filter (KF) to examine it for signs of divergence against four different temperature series available for the region. Ring-width growth for the past two decades is statistically unprecedented for the past 1048 years. Although we have identified a decoupling between temperature and growth in the past two decades, the relationship between some of the temperature records and growth has varied over time since the start of instrumental records. Rather than the special case of 'divergence', we have identified a more general time-dependence between growth and temperature over the last 100 years. This time-dependence appears particularly problematic at interdecadal time scales. Due to the time-dependent relationships, and uncertainties related to the climate data, the use of any of the individual temperature series examined here potentially complicates temperature reconstruction. Some of the uncertainty in the climate data may be associated with changing climatic conditions, such as the intensification of the sub-tropical ridge (STR) and its impact on the frequency of anticyclonic conditions over the Mt Read site. Increased growth at the site, particularly in the last decade, over and above what would be expected based on a linear temperature model alone, may be consistent with a number of hypotheses. Existing uncertainties in the climate data need to be resolved and independent physiological information obtained before a range of hypotheses for this increased growth can be effectively evaluated.
A detailed understanding of past temporal patterns and spatial expression of temperature variatio... more A detailed understanding of past temporal patterns and spatial expression of temperature variations is important to place recent anthropogenic climate change into a longer term context. In order to fill the current gap in our understanding of northwest European temperature variability, point-by-point principal component regression was used to reconstruct a spatial field of 0.5° temperature grids. A sequence of reconstructions utilizing several combinations of detrending and disturbance correction procedures, and a selection of tree-ring parameters (including ring-width (RW), maximum latewood density (MXD) and blue intensity (BI)) was used in an evaluation of reconstruction skill. The high resolution of the reconstructed field serves also as a diagnostic tool to spatially assess the temperature reconstruction potential of local chronologies. Best reconstruction results, reaching calibration r 2 = 65.8% and verification r 2 = 63.7% in central Scotland over the 1901-1976 period, were achieved using disturbance-corrected and signal free detrended RW chronologies merged with blue intensity data after low-pass (high-pass) filtering the RW (BI) chronologies. Calibration and verification r 2 > 50% was attained for central, north and east Scotland, > 40% in west and northwest, and > 30% in southern Scotland with verification of nearly all grids showing some reconstruction skill. However, the full calibration potential of reconstructions outside central Scotland was reduced either due to residual disturbance trends undetected by the disturbance correction procedure or due to other climatic or non-climatic factors which may have adversely affected the strength of the climate signal.
Droughts, which occur as a part of natural climate variability, are expected to increase in frequ... more Droughts, which occur as a part of natural climate variability, are expected to increase in frequency and/or severity with global climate change. An improved understanding of droughts and their association with atmospheric circulation will add to the knowledge about the controls on drought, and the ways in which changes in climate may impact droughts. In this study, 1) major drought patterns across the United States have been defined, 2) the robustness of these patterns over time using tree-ring-based drought reconstructions have been evaluated, and 3) the drought patterns with respect to global atmospheric pressure patterns have been assessed. From this simple assessment, it is suggested that there are two major drought patterns across North America, which together account for about 30% of the total variance in drought patterns-one resembles the classic ENSO teleconnection, and the other displays an east-west drought dipole. The same two patterns are evident in the instrumental data and the reconstructed drought data for two different periods, 1404-2003 and 900-1350. The 500-mb circulation patterns associated with the two drought patterns suggest that the controls on drought may come from both Northern Hemisphere and tropical sources. The two drought patterns, and presumably their associated circulation patterns, vary in strength over time, indicating the combined effects of the two patterns on droughts over the past millennium.
United Kingdom. He earned a B.Sc. (Honours) degree in Biological Sciences at the University of Ea... more United Kingdom. He earned a B.Sc. (Honours) degree in Biological Sciences at the University of East Anglia (UEA) in 1974 and joined the Climatic Research Unit (CRU) there in 1977. He received his Ph.D. from the UEA School of Environmental Sciences in 1984, successfully defending his thesis "Tree-Climate Relationships and Dendroclimatological Reconstruction in the British Isles." He stayed at CRU for the 40 years of his remarkably productive and significant scientific career, having the title of Emeritus Professor at the time of his passing. Keith showed great breadth of scientific understanding, creativity, rigor, willingness and ability to collaborate and unflinching intellectual honesty. These attributes are clearly evident in all his work, from his Ph.D. dissertation to the most recent of his more than 130 publications. He has been a cen-
Journal of the American Statistical Association, Jan 2, 2016
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 nonclimatic 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, but the magnitude of the resulting temperatures is highly model dependent. Such sensitivity may not be apparent with traditional approaches because the underlying statistical model is often hidden or poorly described. Supplementary materials for this article are available online.
Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict se... more Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict severe socioeconomic and biological consequences. In summer 2021, the Pacific Northwest region of North America (PNW) experienced a 2-week-long extreme heatwave, which contributed to record-breaking summer temperatures. Here, we use tree-ring records to show that summer temperatures in 2021, as well as the rate of summertime warming during the last several decades, are unprecedented within the context of the last millennium for the PNW. In the absence of committed efforts to curtail anthropogenic emissions below intermediate levels (SSP2-4.5), climate model projections indicate a rapidly increasing risk of the PNW regularly experiencing 2021-like extreme summer temperatures, with a 50% chance of yearly occurrence by 2050. The 2021 summer temperatures experienced across the PNW provide a benchmark and impetus for communities in historically temperate climates to account for extreme heat-related impacts in climate change adaptation strategies.
The Monsoon Asia Drought Atlas, version 2 (MADAv2), is a substantially updated and improved year-... more The Monsoon Asia Drought Atlas, version 2 (MADAv2), is a substantially updated and improved year-to-year reconstruction of summer drought and wetness on a 1.0° grid over Monsoon Asia. It has been developed using the same basic methodology as that used in producing the previous version (MADAv1) published in 2010 and is now based on an improved target field of self-calibrating Palmer Drought Severity Indices (scPDSI) and a significantly larger network of 453 tree-ring chronologies compared to the previous effort. Importantly, this expanded tree-ring network also includes several new long tree-ring records that extend back a millennium or more into the past. This has enabled the MADAv2 reconstructions to be usefully extended back to 1000 CE over most of the domain and thus provides estimates of hydroclimatic variability over Monsoon Asia back to medieval times. Doing so reveals occurrences of medieval megadroughts and pluvials over India and Southeast Asia with spatial resolution and d...
Northwestern Europe has experienced a trend of increasingly wet winters over the past 150 years, ... more Northwestern Europe has experienced a trend of increasingly wet winters over the past 150 years, with few explanations for what may have driven this hydroclimatic change. Here we use the Old World Drought Atlas (OWDA), a tree-ring based reconstruction of the self-calibrating Palmer Drought Severity Index (scPDSI), to examine this wetting trend and place it in a longer hydroclimatic context. We find that scPDSI variability in northwestern Europe is strongly correlated with the leading mode of the OWDA during the last millennium (1000-2012). This leading mode, here named the 'English Channel' (EC) mode, has pronounced variability on interannual to centennial timescales and has an expression in scPDSI similar to that of the East Atlantic teleconnection pattern. A shift in the EC mode from a prolonged negative phase to more neutral conditions during the 19th and 20th centuries is associated with the wetting trend over its area of influence in England, Wales, and much of northern continental Europe. The EC mode is the dominant scPDSI mode from approximately 1000-1850, after which its dominance waned in favor of the secondary 'North-South' (NS) mode, which has an expression in scPDSI similar to that of the winter North Atlantic Oscillation (NAO). We examine the dynamical nature of both of these modes and how they vary on interannual to centennial timescales. Our results provide insight into the nature of hydroclimate variability in Europe before the widespread availability of instrumental observations.
The Gilded Age of the late 19th century marked a period of rapid development and urbanization in ... more The Gilded Age of the late 19th century marked a period of rapid development and urbanization in New York City, U.S. To accommodate the high demand in wood products during that time, the timbers used for development of the city were increasingly sourced from locations distant from the northeastern United States. The Terminal Warehouse in the Chelsea neighborhood of New York City was one of many large buildings erected during this period of city expansion, and is an important symbol of New York City commerce during the late 1800s. To determine the history and provenance of timbers used in the construction of the Terminal Warehouse, we used tree-ring analysis on longleaf pine (Pinus palustris Mill.) joists that were original to the building. The ring-width patterns on the joists crossdated well internally, suggesting a common origin of the sampled lumber. Further, our Terminal Warehouse tree-ring chronology (1512-1891 C.E.) correlated strongly with existing treering chronologies from western/central Georgia and eastern Alabama, indicating that the timbers were extracted from this region of the southeastern United States. The provenancing and dating of the Terminal Warehouse timbers underscores the important role that southern pines played in the expansion and development of New York City during the Gilded Age.
BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access t... more BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.
Moisture sensitive tree ring chronologies of Centrolobium microchaete have been developed from se... more Moisture sensitive tree ring chronologies of Centrolobium microchaete have been developed from seasonally dry forests in the southern Amazon Basin and used to reconstruct wet season rainfall totals from 1799 to 2012, adding over 150 years of rainfall estimates to the short instrumental record for the region. The reconstruction is correlated with the same atmospheric variables that influence the instrumental measurements of wet season rainfall. Anticyclonic circulation over midlatitude South America promotes equatorward surges of cold and relatively dry extratropical air that converge with warm moist air to form deep convection and heavy rainfall over this sector of the southern Amazon Basin. Interesting droughts and pluvials are reconstructed during the preinstrumental nineteenth and early twentieth centuries, but the tree ring reconstruction suggests that the strong multidecadal variability in instrumental and reconstructed wet season rainfall after 1950 may have been unmatched since 1799.
managers. Management must minimize effects of drought on water supplies while also keeping reserv... more managers. Management must minimize effects of drought on water supplies while also keeping reservoir levels low enough to minimize flood risk . California's exposure to hydrological volatility is particularly high because of its geographic setting: the majority of the state's surface water supply falls in the Sierra Nevada during the relatively short cool season of November-March (Dettinger, 2013;, giving this region and season particular importance to California's water resources. While parts of the state, especially southern California, have worked to diversify water portfolios with a combination of surface, imported, and ground water, and to employ conservation measures and water markets , California remains vulnerable to extreme variability of precipitation . Extreme hydrological shortages in the Sierra Nevada can be only moderately buffered by water supplies in regions and seasons not experiencing drought. California's drought and flood risk is amplified by the limited reliability of seasonal forecasts of Sierra Nevada precipitation . While forecast skill beyond a 2-week horizon is inherently poor globally, seasonal forecasting for parts of the western United States is aided by a teleconnection to the El Niño-Southern Oscillation (ENSO) . This is not the case, however, for the Sierra Nevada, which span the neutral point of the ENSO dipole that often drives Abstract California's water resources rely heavily on cool-season (November-March) precipitation in the Sierra Nevada. Interannual variability is highly volatile and seasonal forecasting has little to no skill, making water management particularly challenging. Over 1902-2020, Sierra Nevada cool-season precipitation totals exhibited significant 2.2-and 13-15-year cycles, accounting for approximately 40% of total variability and perhaps signifying potential as seasonal forecasting tools. However, the underlying climate dynamics are not well understood and it is unclear whether these cycles are stable over the long term. We use tree rings to reconstruct Sierra Nevada cool-season precipitation back to 1400. The reconstruction is skillful, accounting for 55%-74% of observed variability and capturing the 20th-century 2.2-and 13-15-year cycles. Prior to 1900, the reconstruction indicates no other century-long periods of significant spectral power in the 2.2-or 13-15-year bands. The reconstruction does indicate significant cyclicity over other extended periods of several decades or longer, however, with dominant periodicities in the ranges of 2.1-2.7 and 3.5-8 years. The late 1700s through 1800s exhibited the highest-amplitude cycles in the reconstruction, with periodicities of 2.4 and 5.7-7.4 years. The reconstruction should serve to caution against extrapolating the observed 2.2-and 13-15-year cycles to guide future expectations. On the other hand, observations and the reconstruction suggest that interannual variability of Sierra Nevada cool-season precipitation is not a purely white noise process and research should aim to diagnose the dynamical drivers of extended periods of cyclicity in this critical natural resource. WILLIAMS ET AL.
The annual ringwidths of trees can be used to search for hypothesized air pollution effects on fo... more The annual ringwidths of trees can be used to search for hypothesized air pollution effects on forests. This search is extremely complicated by the inherent statistical properties of ringwidth data and the high level of uncertainty regarding the sources of variance observed in the ringwidths. A linear aggregate model for ringwidths is described which highlights the general classes of variance which may be found in a tree -ring series. Dendrochronological principles and techniques are des- cribed which can be used to create a tree -ring chronology that is suitable for rigorous statistical analysis and hypothesis testing. The need to model climatic influences on tree growth prior to the search for pollution effects is necessary and a method for achieving this is described. Only after the variance due to age trends, stand dynamics effects and climatic influences has been accounted for can any confidence be placed on inferred pollution effects. An analysis of a red spruce tree -ring chronology indicates that a decline in ringwidths since 1968 cannot be explained by a linear temperature response model using monthly climatic variables. However, threshold responses to climate that could be responsible for the decline need to be considered before the anomalous decline can be attributed to non -climatic influences such as pollution.
Signal extraction in tree -ring research is considered as a general time series decomposition pro... more Signal extraction in tree -ring research is considered as a general time series decomposition problem. A linear aggregate model for a hypothetical ring -width series is proposed, which allows the problem to be reduced to the estimation and extraction of five discrete classes of signals. These classes represent the signals due to trend, climate, endogenous disturbance, exogenous disturbance, and random error. For each class of signal, some mathematical /statistical techniques of estimation are described and reviewed. Except for the exogenous disturbance signal, the techniques only require information contained within the ring -width series, themselves. A unified mathematical framework for solving this decomposition problem has not yet been explicitly formulated. However, the general applicability of ARMA time series models to this problem and the power and flexibility of state space modelling suggest that these techniques will provide the closest thing to a unified framework in the future. Problem der Zeitreihenzerlegung betrachtet. Hierzu wird daher ein linear zusammengesetztes Modell einer hypothetischen Jahrringfolge vorgeschlagen, womit das Problem auf die Schätzung und Extraktion von fünf diskreten Klassen von Signalen reduziert werden kann. Diese Klassen repräsentieren die Signale, die durch einen Trend, durch das Klima, durch endogene sowie exogene Störfaktoren und durch einen Zufallsfehler hervorgerufen werden. Für jede Klasse werden einige mathematisch-statistische Schätzverfahren beschrieben und bewertet. Alle Verfahren benötigen nur solche Informationen, die in den Jahrringfolgen selbst enthalten sind, ausgenommen bei exogenen Störfaktoren. Bislang wurde noch kein einheitliches mathematisches Vorgehen zur Lösung dieses Zerlegungsproblems explizit formuliert. Jedoch könnte die allgemeine Anwendbarkeit von ARMA-Zeitreihenmodellen sowie die Leistungs-und Anpassungsfähigkeit von Zustandsraummodellen einem einheitlichen Vorgehen in der Zukunft sehr nahekommen. L'extraction d'un signal dans la recherche dendrochronologique est considérée comme un problème général de désagrégation en des séries temporelles. Un modèle d'agrégation linéaire valable pour les épaisseurs des cernes est proposé, qui permet de réduire le problème à l'estimation et à l'extraction de cinq classes discontinues de signaux. Ces classes représentent les signaux dus à la tendance, au climat, à des perturbations endogènes et exogènes et à l'erreur aléatoire. Pour chaque classe de signal quelques techhniques mathématiques et statistiques d'estimation sont décrites et commises à critique. A l'exception du signal provenant de perturbations exogènes, les techniques n'exigent que les informations contenues dans les séries d'épaisseur des cernes elles-mémes. Un cadre mathématique unifié pour résoudre ce problème de désagrégation n'a pas encore été formulé de façon explicite. Cependant la possibilité d'application des modèles ARMA à ce problème ainsi que la puissance et la flexibilité de la modélisation spatiale, suggère que ces techniques sont aptes à procurer le meilleur moyen pour utiliser un cadre unifié dans le futur.
Many tree-ring-based climate reconstructions are based on the assumption that the climate reactio... more Many tree-ring-based climate reconstructions are based on the assumption that the climate reaction of trees is independent of their size. Here, we test this assumption for New Zealand kauri (Agathis australis), one of the longest tree ring-based proxies for the El Nin ˜o-Southern Oscillation (ENSO). The most recent kauri chronology contains a large amount of archaeological material, e.g. timber for which the original tree size is often unknown. We analyzed the climate-growth relationship of different-sized kauri in a pristine forest using different temporal scales, i.e. annually, monthly and daily data on tree growth and climate conditions. Trees of different life stages exhibited approximately the same seasonal growth peaks during austral spring (October and November). The dormancy period overlaps with the period where weekly air temperature maxima are below ca. 17-18 °C, and where the corresponding daily minima are below ca. 8 °C. However, both correlation functions between annual growth and seasonal climate as well as Kalman filter regressions between daily growth and climate conditions suggest an influence of tree size on the climate-growth relationship for kauri. Smaller trees (DBH \ 40 cm) contain weaker climate signals than larger trees. Therefore, the precautionary stripping of near-pith material (first 20 cm) from kauri chronologies may result in more uniform responses to climate forcing and thus enhance the reliability of long-term climate reconstructions.
We evaluate a range of blue intensity (BI) tree-ring parameters in eight conifer species (12 site... more We evaluate a range of blue intensity (BI) tree-ring parameters in eight conifer species (12 sites) from Tasmania 15 and New Zealand for their dendroclimatic potential, and as surrogate wood anatomical proxies. Using a dataset of ca. 10-15 trees per site, we measured earlywood maximum blue reflectance intensity (EWB), latewood minimum blue reflectance intensity (LWB) and the associated delta blue intensity (DB) parameter for dendrochronological analysis. No resin extraction was performed, impacting low frequency trends. Therefore, we focused only on the high frequency signal by detrending all tree-ring and climate data using a 20-year cubic smoothing spline. All BI parameters express low relative variance and weak 20 signal strength compared to ring-width. Correlation analysis and principal component regression experiments identified a weak and variable climate response for most ring-width chronologies. However, for most sites, the EWB data, despite weak signal strength, expressed strong calibrations with summer temperatures. Significant correlations for LWB were also noted, but the sign of the relationship for most species is opposite to that reported for all conifer species in the Northern Hemisphere. DB performed well for the Tasmanian sites but explained minimal temperature variance in New Zealand. Using 25 the full multi-species/parameter network, excellent summer temperature calibration was identified for both Tasmania and New Zealand ranging from 52% to 78% explained variance, with equally robust independent validation (Coefficient of Efficiency = 0.41 to 0.77). Comparison of the Tasmanian BI reconstruction with a wood anatomical reconstruction shows that these parameters record essentially the same strong high frequency summer temperature signal. Despite these excellent results, a substantial challenge exists with the capture of potential secular scale climate trends. Although DB, band-pass and 30 other signal processing methods may help with this issue, substantially more experimentation is needed in conjunction with comparative analysis with ring density and quantitative WA measurements.
Indonesia's climate is dominated by the equatorial monsoon system, and has been linked to El Niño... more Indonesia's climate is dominated by the equatorial monsoon system, and has been linked to El Niño-Southern Oscillation (ENSO) events that often result in extensive droughts and floods over the Indonesian archipelago. In this study we investigate ENSO-related signals in a tree-ring δ 18 O record (1900-2007) of Javanese teak. Our results reveal a clear influence of Warm Pool (central Pacific) El Niño events on Javanese tree-ring δ 18 O, and no clear signal of Cold Tongue (eastern Pacific) El Niño events. These results are consistent with the distinct impacts of the two ENSO flavors on Javanese precipitation, and illustrate the importance of considering ENSO flavors when interpreting palaeoclimate proxy records in the tropics.
We evaluate a range of blue intensity (BI) treering parameters in eight conifer species (12 sites... more We evaluate a range of blue intensity (BI) treering parameters in eight conifer species (12 sites) from Tasmania and New Zealand for their dendroclimatic potential, and as surrogate wood anatomical proxies. Using a dataset of ca. 10-15 trees per site, we measured earlywood maximum blue intensity (EWB), latewood minimum blue intensity (LWB), and the associated delta blue intensity (DB) parameter for dendrochronological analysis. No resin extraction was performed, impacting low-frequency trends. Therefore, we focused only on the high-frequency signal by detrending all tree-ring and climate data using a 20-year cubic smoothing spline. All BI parameters express low relative variance and weak signal strength compared to ring width. Correlation analysis and principal component regression experiments identified a weak and variable climate response for most ring-width chronologies. However, for most sites, the EWB data, despite weak signal strength, expressed strong coherence with summer temperatures. Significant correlations for LWB were also noted, but the sign of the relationship for most species is opposite to that reported for all conifer species in the Northern Hemisphere. DB results were mixed but performed better for the Tasmanian sites when combined through principal component regression methods than for New Zealand. Using the full multi-species/parameter network, excellent summer temperature calibration was identified for both Tasmania and New Zealand ranging from 52 % to 78 % explained variance for split periods (1901-1950/1951-1995), with equally robust independent validation (coefficient of efficiency = 0.41 to 0.77). Comparison of the Tasmanian BI reconstruction with a quantitative wood anatomical (QWA) reconstruction shows that these parameters record essentially the same strong high-frequency summer temperature signal. Despite these excellent results, a substantial challenge exists with the capture of potential secular-scale climate trends. Although DB, band-pass, and other signal processing methods may help with this issue, substantially more experimentation is needed in conjunction with comparative analysis with ring density and QWA measurements.
Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atm... more Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atmospheric circulation anomalies in the Atlantic, but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial time scales. Here we analyze the coupled patterns of drought variability on either sides of the North Atlantic Ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin-wide moisture anomalies to ocean and atmosphere processes. A leading mode of broad-scale moisture variability is characterized by distinct patterns of North Atlantic atmosphere circulation and sea surface temperatures. We infer a negative phase of the North Atlantic Oscillation and colder Atlantic sea surface temperatures in the middle of the fifteenth century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age.
To date, no attempt has been made to assess the presence or otherwise of the "Divergence Problem"... more To date, no attempt has been made to assess the presence or otherwise of the "Divergence Problem" (DP) in existing multi-millennial Southern Hemisphere tree-ring chronologies. We have updated the iconic Mt Read Huon pine chronology from Tasmania, southeastern Australia, to now include the warmest decade on record, AD 2000e2010, and used the Kalman Filter (KF) to examine it for signs of divergence against four different temperature series available for the region. Ring-width growth for the past two decades is statistically unprecedented for the past 1048 years. Although we have identified a decoupling between temperature and growth in the past two decades, the relationship between some of the temperature records and growth has varied over time since the start of instrumental records. Rather than the special case of 'divergence', we have identified a more general time-dependence between growth and temperature over the last 100 years. This time-dependence appears particularly problematic at interdecadal time scales. Due to the time-dependent relationships, and uncertainties related to the climate data, the use of any of the individual temperature series examined here potentially complicates temperature reconstruction. Some of the uncertainty in the climate data may be associated with changing climatic conditions, such as the intensification of the sub-tropical ridge (STR) and its impact on the frequency of anticyclonic conditions over the Mt Read site. Increased growth at the site, particularly in the last decade, over and above what would be expected based on a linear temperature model alone, may be consistent with a number of hypotheses. Existing uncertainties in the climate data need to be resolved and independent physiological information obtained before a range of hypotheses for this increased growth can be effectively evaluated.
A detailed understanding of past temporal patterns and spatial expression of temperature variatio... more A detailed understanding of past temporal patterns and spatial expression of temperature variations is important to place recent anthropogenic climate change into a longer term context. In order to fill the current gap in our understanding of northwest European temperature variability, point-by-point principal component regression was used to reconstruct a spatial field of 0.5° temperature grids. A sequence of reconstructions utilizing several combinations of detrending and disturbance correction procedures, and a selection of tree-ring parameters (including ring-width (RW), maximum latewood density (MXD) and blue intensity (BI)) was used in an evaluation of reconstruction skill. The high resolution of the reconstructed field serves also as a diagnostic tool to spatially assess the temperature reconstruction potential of local chronologies. Best reconstruction results, reaching calibration r 2 = 65.8% and verification r 2 = 63.7% in central Scotland over the 1901-1976 period, were achieved using disturbance-corrected and signal free detrended RW chronologies merged with blue intensity data after low-pass (high-pass) filtering the RW (BI) chronologies. Calibration and verification r 2 > 50% was attained for central, north and east Scotland, > 40% in west and northwest, and > 30% in southern Scotland with verification of nearly all grids showing some reconstruction skill. However, the full calibration potential of reconstructions outside central Scotland was reduced either due to residual disturbance trends undetected by the disturbance correction procedure or due to other climatic or non-climatic factors which may have adversely affected the strength of the climate signal.
Droughts, which occur as a part of natural climate variability, are expected to increase in frequ... more Droughts, which occur as a part of natural climate variability, are expected to increase in frequency and/or severity with global climate change. An improved understanding of droughts and their association with atmospheric circulation will add to the knowledge about the controls on drought, and the ways in which changes in climate may impact droughts. In this study, 1) major drought patterns across the United States have been defined, 2) the robustness of these patterns over time using tree-ring-based drought reconstructions have been evaluated, and 3) the drought patterns with respect to global atmospheric pressure patterns have been assessed. From this simple assessment, it is suggested that there are two major drought patterns across North America, which together account for about 30% of the total variance in drought patterns-one resembles the classic ENSO teleconnection, and the other displays an east-west drought dipole. The same two patterns are evident in the instrumental data and the reconstructed drought data for two different periods, 1404-2003 and 900-1350. The 500-mb circulation patterns associated with the two drought patterns suggest that the controls on drought may come from both Northern Hemisphere and tropical sources. The two drought patterns, and presumably their associated circulation patterns, vary in strength over time, indicating the combined effects of the two patterns on droughts over the past millennium.
United Kingdom. He earned a B.Sc. (Honours) degree in Biological Sciences at the University of Ea... more United Kingdom. He earned a B.Sc. (Honours) degree in Biological Sciences at the University of East Anglia (UEA) in 1974 and joined the Climatic Research Unit (CRU) there in 1977. He received his Ph.D. from the UEA School of Environmental Sciences in 1984, successfully defending his thesis "Tree-Climate Relationships and Dendroclimatological Reconstruction in the British Isles." He stayed at CRU for the 40 years of his remarkably productive and significant scientific career, having the title of Emeritus Professor at the time of his passing. Keith showed great breadth of scientific understanding, creativity, rigor, willingness and ability to collaborate and unflinching intellectual honesty. These attributes are clearly evident in all his work, from his Ph.D. dissertation to the most recent of his more than 130 publications. He has been a cen-
Journal of the American Statistical Association, Jan 2, 2016
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 nonclimatic 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, but the magnitude of the resulting temperatures is highly model dependent. Such sensitivity may not be apparent with traditional approaches because the underlying statistical model is often hidden or poorly described. Supplementary materials for this article are available online.
Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict se... more Extreme summer temperatures are increasingly common across the Northern Hemisphere and inflict severe socioeconomic and biological consequences. In summer 2021, the Pacific Northwest region of North America (PNW) experienced a 2-week-long extreme heatwave, which contributed to record-breaking summer temperatures. Here, we use tree-ring records to show that summer temperatures in 2021, as well as the rate of summertime warming during the last several decades, are unprecedented within the context of the last millennium for the PNW. In the absence of committed efforts to curtail anthropogenic emissions below intermediate levels (SSP2-4.5), climate model projections indicate a rapidly increasing risk of the PNW regularly experiencing 2021-like extreme summer temperatures, with a 50% chance of yearly occurrence by 2050. The 2021 summer temperatures experienced across the PNW provide a benchmark and impetus for communities in historically temperate climates to account for extreme heat-related impacts in climate change adaptation strategies.
The Monsoon Asia Drought Atlas, version 2 (MADAv2), is a substantially updated and improved year-... more The Monsoon Asia Drought Atlas, version 2 (MADAv2), is a substantially updated and improved year-to-year reconstruction of summer drought and wetness on a 1.0° grid over Monsoon Asia. It has been developed using the same basic methodology as that used in producing the previous version (MADAv1) published in 2010 and is now based on an improved target field of self-calibrating Palmer Drought Severity Indices (scPDSI) and a significantly larger network of 453 tree-ring chronologies compared to the previous effort. Importantly, this expanded tree-ring network also includes several new long tree-ring records that extend back a millennium or more into the past. This has enabled the MADAv2 reconstructions to be usefully extended back to 1000 CE over most of the domain and thus provides estimates of hydroclimatic variability over Monsoon Asia back to medieval times. Doing so reveals occurrences of medieval megadroughts and pluvials over India and Southeast Asia with spatial resolution and d...
Northwestern Europe has experienced a trend of increasingly wet winters over the past 150 years, ... more Northwestern Europe has experienced a trend of increasingly wet winters over the past 150 years, with few explanations for what may have driven this hydroclimatic change. Here we use the Old World Drought Atlas (OWDA), a tree-ring based reconstruction of the self-calibrating Palmer Drought Severity Index (scPDSI), to examine this wetting trend and place it in a longer hydroclimatic context. We find that scPDSI variability in northwestern Europe is strongly correlated with the leading mode of the OWDA during the last millennium (1000-2012). This leading mode, here named the 'English Channel' (EC) mode, has pronounced variability on interannual to centennial timescales and has an expression in scPDSI similar to that of the East Atlantic teleconnection pattern. A shift in the EC mode from a prolonged negative phase to more neutral conditions during the 19th and 20th centuries is associated with the wetting trend over its area of influence in England, Wales, and much of northern continental Europe. The EC mode is the dominant scPDSI mode from approximately 1000-1850, after which its dominance waned in favor of the secondary 'North-South' (NS) mode, which has an expression in scPDSI similar to that of the winter North Atlantic Oscillation (NAO). We examine the dynamical nature of both of these modes and how they vary on interannual to centennial timescales. Our results provide insight into the nature of hydroclimate variability in Europe before the widespread availability of instrumental observations.
The Gilded Age of the late 19th century marked a period of rapid development and urbanization in ... more The Gilded Age of the late 19th century marked a period of rapid development and urbanization in New York City, U.S. To accommodate the high demand in wood products during that time, the timbers used for development of the city were increasingly sourced from locations distant from the northeastern United States. The Terminal Warehouse in the Chelsea neighborhood of New York City was one of many large buildings erected during this period of city expansion, and is an important symbol of New York City commerce during the late 1800s. To determine the history and provenance of timbers used in the construction of the Terminal Warehouse, we used tree-ring analysis on longleaf pine (Pinus palustris Mill.) joists that were original to the building. The ring-width patterns on the joists crossdated well internally, suggesting a common origin of the sampled lumber. Further, our Terminal Warehouse tree-ring chronology (1512-1891 C.E.) correlated strongly with existing treering chronologies from western/central Georgia and eastern Alabama, indicating that the timbers were extracted from this region of the southeastern United States. The provenancing and dating of the Terminal Warehouse timbers underscores the important role that southern pines played in the expansion and development of New York City during the Gilded Age.
BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access t... more BioOne Complete (complete.BioOne.org) is a full-text database of 200 subscribed and open-access titles in the biological, ecological, and environmental sciences published by nonprofit societies, associations, museums, institutions, and presses.
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Papers by Edward Cook