Paul J. Krusic

Experimental Dendroclimatic Reconstruction of the Southern Oscillation

Abstract Exactly dated tree-ring chronologies from ENSO-sensitive regions in subtropical North America and Indonesia together register the strongest ENSO signal yet detected in tree-ring data worldwide and have been used to reconstruct the winter Southern Oscillation index (SOI) from 1706 to 1977. This reconstruction explains 53% of the variance in the instrumental winter SOI during the boreal cool season (December-February) and was verified in the time, space, and frequency domains by comparisons with independent instrumental SOI and sea surface temperature (SST) data. The large-scale SST anomaly patterns associated with ENSO in the equatorial and North Pacific during the 1879-1977 calibration period are reproduced in detail by this reconstruction. Cross-spectral analyses indicate that the reconstruction reproduces over 70% of the instrumental winter SOI variance at periods between 3.5 and 5.6 yr, and over 88% in the 4-yr frequency band. Oscillatory modes of variance identified with singular spectrum ana...

Old World megadroughts and pluvials during the Common Era

An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability. Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability.

Tree-ring reconstructed summer temperature anomalies for temperate East Asia since 800 C.E.

We develop a summer temperature reconstruction for temperate East Asia based on a network of annual tree-ring chronologies covering the period 800–1989 C.E. The East Asia reconstruction is the regional average of 585 individual grid point summer temperature reconstructions produced using an ensemble version of point-by-point regression. Statistical calibration and validation tests indicate that the regional average possesses sufficient overall skill to allow it to be used to study the causes of temperature variability and change over the region. The reconstruction suggests a moderately warm early medieval epoch (ca. 850–1050 C.E.), followed by generally cooler ‘Little Ice Age’ conditions (ca. 1350–1880 C.E.) and 20th century warming up to the present time. Since 1990, average temperature has exceeded past warm epochs of comparable duration, but it is not statistically unprecedented. Superposed epoch analysis reveals a volcanic forcing signal in the East Asia summer temperature reconstruction, resulting in pulses of cooler summer conditions that may persist for several years. Substantial uncertainties remain, however, particularly at lower frequencies, thus requiring caution and scientific prudence in the interpretation of this record.

IDENTIFYING FUNCTIONAL GROUPS OF TREES IN WEST GULF COAST FORESTS (USA): A TREE-RING APPROACH

A dendroclimatic study of important tree species in the west Gulf Coastal Plain region, USA, was conducted to evaluate how climate affects tree radial growth in this southeasternmost section of the Eastern Deciduous Forest/Southern Evergreen Forest. We established an east–west transect from western Louisiana to central Texas that crossed the western range limits of each of 16 species and developed a network of 104 annual tree ring-width chronologies from 38 sites. Of the 104 chronologies, 99 series from the genera Pinus, Quercus, and Fagus were analyzed using rotated principal components analysis (RPCA). The RPCA revealed the presence of three robust phylogenetic signals in the tree-ring patterns, which partitioned the data into the Pinus species (PISP), the oak species in the black oak subgenus Erythrobalanus (QUBO), and the oak species in the white oak subgenus Leucobalanus (QUWO). The Fagus chronologies (FAGR) also loaded most highly with the QUWO series, resulting in a combined QUWO/FAGR factor. This partitioning occurred even though tree species within each phylogenetic group came from contrasting xeric and mesic sites and, in the case of the QUWO/FAGR factor, from different genera. Only in the xeric western range limits of the transect did site location begin to override the phylogenetic groupings. Consequently, responses to climate based on genetics appeared to be more important than ecological and site characteristics in determining the tree-ring patterns of the sampled species overall. We tested this hypothesis by independently modeling the dendroclimate signals in the tree-ring chronologies using monthly precipitation and maximum temperature data. The resulting climate correlation functions were subjected to RPCA as before. As we did so, the same phylogenetic groups emerged. All of the chronologies were drought sensitive. However, the phylogenetic differences in climate response were related to differences in the timing of the peak monthly responses to climate and to the differing patterns of climate response in the months prior to the current growing season. The findings of this study indicate that there is an underlying organizing principle based on genetics that determines how certain phylogenetic groups of trees respond to climate in a way that is largely independent of the site environment. At a coarse level, these phylogenetic distinctions persist even at the most stressed sites near tree range limits, though distinctions within genera start to break down. These findings therefore suggest functional groupings of tree species, which can be used in vegetation/climate models that attempt to predict realistically how such forests will respond to future climate changes.

Northern Hemisphere hydroclimate variability over the past twelve centuries

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

Progress in Dendroclimatic Studies of Mountain Pine in Northern Thailand

New data added to the existing tree-ring width chronologies of mountain pine (Pinus kesiya and Pinus merkusii) result in a total of seven chronologies for these species for Thailand. The oldest (1647-1993) is from a P. merkusii site at Phu Kradung, north central Thailand. An analysis of the three longest P. kesiya chronologies, from north central Thailand, with Phetchabun rainfall (1951-1992) reveals correspondence between years oflow growth and below average rainfall (drought) during the wet season (July-November). The lowest growth year averaged over these three sites during the period of rainfall data (1951-1992) occurred in 1979, coinciding with the lowest wet season rainfall on record. For the common period of tree-ring record prior to 1951 (1830-1950), the level of drought severity in 1979 appears to have been exceeded only twice previously, in 1832 and 1894. A P. merkusii record from Thung Salaeng Luang is most significantly correlated with temperatures during May-June, considered a critical period for the subsequent evolution of the Asian monsoon.

Tree-Ring Amplification of the Early Nineteenth-Century Summer Cooling in Central Europe

Annually resolved and absolutely dated tree-ring chronologies are the most important proxy archives to reconstruct climate variability over centuries to millennia. However, the suitability of tree- ...

Continental-Scale Temperature Variability during the Past Two Millennia: Supplementary Information

Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.

Six hundred thirty‐eight years of summer temperature variability over the Bhutanese Himalaya

High-resolution tree ring reconstructions from the Himalaya provide essential context for assessing impacts of future climate change on regional water reserves and downstream agriculture. Here we e ...

LONG-TERM SUMMER TEMPERATURE VARIATIONS IN THE PYRENEES FROM DETRENDED STABLE CARBON ISOTOPES

Abstract Substantial effort has recently been put into the development of climate reconstructions from tree-ring stable carbon isotopes, though the interpretation of long-term trends retained in such timeseries remains challenging. Here we use detrended δ13C measurements in Pinus uncinata tree-rings, from the Spanish Pyrenees, to reconstruct decadal variations in summer temperature back to the 13th century. The June-August temperature signal of this reconstruction is attributed using decadally as well as annually resolved, 20th century δ13C data. Results indicate that late 20th century warming has not been unique within the context of the past 750 years. Our reconstruction contains greater am-plitude than previous reconstructions derived from traditional tree-ring density data, and describes particularly cool conditions during the late 19th century. Some of these differences, including early warm periods in the 14th and 17th centuries, have been retained via δ13C timeseries detrending - a novel approach in tree-ring stable isotope chronology development. The overall reduced variance in earlier studies points to an underestimation of pre-instrumental summer temperature variability de-rived from traditional tree-ring parameters.