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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.

Linked to major volcanic eruptions around 536 and 540 CE, the onset of the Late Antique Little Ice Age has been described as the coldest period of the past two millennia. The exact timing and spatial extent of this exceptional cold phase are, however, still under debate because of the limited resolution and geographical distribution of the available proxy archives. Here, we use 106 wood anatomical thin sections from 23 forest sites and 20 tree species in both hemispheres to search for cell-level fingerprints of ephemeral summer cooling between 530 and 550 CE. After cross-dating and double-staining, we identified 89 Blue Rings (lack of cell wall lignification), nine Frost Rings (cell deformation and collapse), and 93 Light Rings (reduced cell wall thickening) in the Northern Hemisphere. Our network reveals evidence for the strongest temperature depression between mid-July and early-August 536 CE across North America and Eurasia, whereas more localised cold spells occurred in the summers of 532, 540-43, and 548 CE. The lack of anatomical signatures in the austral trees suggests limited incursion of stratospheric volcanic aerosol into the Southern Hemisphere extra-tropics, that any forcing was mitigated by atmosphere-ocean dynamical responses and/or concentrated outside the growing season, or a combination of factors. Our findings demonstrate the advantage of wood anatomical investigations over traditional dendrochronological measurements, provide a benchmark for Earth system models, support cross-disciplinary studies into the entanglements of climate and history, and question the relevance of global climate averages.

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 autumn–winter 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.

Across the Upper Missouri River Basin, the recent drought of 2000 to 2010, known as the “turn-of-the-century drought,” was likely more severe than any in the instrumental record including the Dust Bowl drought. However, until now, adequate proxy records needed to better understand this event with regard to long-term variability have been lacking. Here we examine 1,200 y of streamflow from a network of 17 new tree-ring–based reconstructions for gages across the upper Missouri basin and an independent reconstruction of warm-season regional temperature in order to place the recent drought in a long-term climate context. We find that temperature has increasingly influenced the severity of drought events by decreasing runoff efficiency in the basin since the late 20th century (1980s) onward. The occurrence of extreme heat, higher evapotranspiration, and associated low-flow conditions across the basin has increased substantially over the 20th and 21st centuries, and recent warming aligns with increasing drought severities that rival or exceed any estimated over the last 12 centuries. Future warming is anticipated to cause increasingly severe droughts by enhancing water deficits that could prove challenging for water management.

Cool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

Recent climate extremes in Mongolia have ignited a renewed interest in understanding past climate variability over centennial and longer time scales across north-central Asia. Tree-ring width records have been extensively studied in Mongolia as proxies for climate reconstruction, however, the climate and environmental signals of tree-ring stable isotopes from this region need to be further explored. Here, we evaluated a 182-year record of tree-ring δ13C and δ18O from Siberian Pine (Pinus sibirica Du Tour) from a xeric site in central Mongolia (Khorgo Lava) to elucidate the environmental factors modulating these parameters. First, we analyzed the climate sensitivity of tree-ring δ13C and δ18O at Khorgo Lava for comparison with ring-width records, which have been instrumental in reconstructing hydroclimate in central Mongolia over two millennia. We also compared stable isotope records of trees with partial cambial dieback ('strip-bark morphology'), a feature of long-lived conifers growing on resource-limited sites, and trees with a full cambium ('whole-bark morphology'), to assess the inferred leaf-level physiological behavior of these trees. We found that interannual variability in tree-ring δ13C and δ18O reflected summer hydroclimatic variability, and captured recent, extreme drought conditions, thereby complementing ring-width records. The tree-ring δ18O records also had a spring temperature signal and thus expanded the window of climate information recorded by these trees. Over longer time scales, strip-bark trees had an increasing trend in ring-widths, δ13C (and intrinsic water-use efficiency, iWUE) and δ18O, relative to whole-bark trees. Our results suggest that increases in iWUE at this site might be related to a combination of leaf-level physiological responses to increasing atmospheric CO2, recent drought, and stem morphological changes. Our study underscores the potential of stable isotopes for broadening our understanding of past climate in north-central Asia. However, further studies are needed to understand how stem morphological changes might impact stable isotopic trends.

Funding: Rob Wilson was funded through the University of Melbourne Dyason Fellowship in 2014 to undertake preliminary measurement and analyses for this study. We also acknowledge NSF-NERC funding (NE/W007223/1). Kathy Allen was supported by the Australian Research Council grants DP1201040320 and LP12020811 to Patrick Baker. We evaluate a range of blue intensity (BI) tree-ring 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. Publisher PDF

Instrumental records indicate a century-long trend towards drying over western North America and wetting over eastern North America. A continuation of these trends into the future would have significant hydroclimatic and socioeconomic consequences in both the semi-arid Southwest and humid East. Using tree-ring reconstructions and hydrologic simulations of summer soil moisture, we evaluate and contextualize the modern summer aridity gradient within its natural range of variability established over the past 600 years and evaluate the effects of observed and anthropogenic precipitation, temperature, and humidity trends. The 2001–2020 positive (wet east-dry west) aridity gradient was larger than any 20 year period since 1400 CE, preceded by the most negative (wet west-dry east) aridity gradient during 1976–1995, leading to a strong multi-decade reversal in aridity gradient anomalies that was rivaled only by a similar event in the late-16th century. The 2001–2020 aridity gradient was dominated by long-term summer precipitation increases in the Midwest and Northeast, with smaller contributions from more warming in the West than the East and spring precipitation decreases in the Southwest. Multi-model mean climate simulations from Coupled Model Intercomparison Project 6 experiments suggest anthropogenic climate trends should not have strongly affected the aridity gradient thus far. However, there is high uncertainty due to inter-model disagreement on anthropogenic precipitation trends. The recent strengthening of the observed aridity gradient, its increasing dependence on precipitation variability, and disagreement in modeled anthropogenic precipitation trends reveal significant uncertainties in how water resource availability will change across North America in the coming decades.

We present the European Russia Drought Atlas (ERDA) that covers the East European Plain to the Ural Mountains from 1400–2016 CE. Like the Old World Drought Atlas (OWDA) for the Euro-Mediterranean region, the ERDA is a one-half degree gridded reconstruction of summer Palmer Drought Severity Indices estimated from a network of annual tree-ring chronologies. Ensemble point-by-point regression is used to generate the ERDA with the identical protocols used for developing the OWDA. Split calibration/validation tests of the ERDA indicate that it has significant skill over most of its domain and is much more skillful than the OWDA where they overlap in the western part of ERDA domain. Comparisons to historical droughts over European Russia additionally support the ERDA’s overall validity. The ERDA has been spatially smoothed and infilled using a local regression method to yield a spatially complete drought atlas back to 1400 CE. EOF analysis indicates that there are three principal modes of hydroclimatic variability in the ERDA. After Varimax rotation, these modes correlate significantly with independent climate data sets extending back to the late nineteenth century in a physically interpretable way and relate to atmospheric circulation dynamics of droughts and heatwaves over European Russia based on more recent instrumental data.

This research was funded by grants from the US National Science Foundation P2C2 program (AGS-1501856, and AGS-1502224). BIC was supported by the NASA Modeling, Analysis, and Prediction program (NASA #80NSSC17K0265). 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 timescales. Here we analyze the coupled patterns of drought variability on either side 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 15th century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age. Publisher PDF

National Science FoundationNational Science Foundation (NSF); Paleo Perspective on Climate Change (P2C2) [1401698, 1404188]; National Science Foundation, Water Sustainability and Climate (WSC)National Science Foundation (NSF) [1360446]; U.S. Department of EnergyUnited States Department of Energy (DOE) [DE-SC0018124]; U.S. Bureau of Reclamation WaterSMART Program (Sustain and Manage America's Resources for Tomorrow); state of Montana Department of Natural Resources and Conservation; U.S. Geological Survey Land Resources Mission Area; North Central Climate Adaptation Science Center; Lamont-Doherty Earth Observatory [8349]

This work was funded by the US National Science Foundation (NSF) Grants AGS 1159430, AGS 1502186, AGS 1502150, PLR 15-04134, PIRE 1743738, AGS-15-167 and PLR16-03473. In north-western North America, the so-called divergence problem (DP) is expressed in tree ring width (RW) as an unstable temperature signal in recent decades. Maximum latewood density (MXD), from the same region, shows minimal evidence of DP. While MXD is a superior proxy for summer temperatures, there are very few long MXD records from North America. Latewood blue intensity (LWB) measures similar wood properties as MXD, expresses a similar climate response, is much cheaper to generate and thereby could provide the means to profoundly expand the extant network of temperature sensitive tree-ring (TR) chronologies in North America. In this study, LWB is measured from 17 white spruce sites (Picea glauca) in south-western Yukon to test whether LWB is immune to the temporal calibration instabilities observed in RW. A number of detrending methodologies are examined. The strongest calibration results for both RW and LWB are consistently returned using age-dependent spline (ADS) detrending within the signal-free (SF) framework. RW data calibrate best with June–July maximum temperatures (Tmax), explaining up to 28% variance, but all models fail validation and residual analysis. In comparison, LWB calibrates strongly (explaining 43–51% of May–August Tmax) and validates well. The reconstruction extends to 1337 CE, but uncertainties increase substantially before the early 17th century because of low replication. RW-, MXD- and LWB-based summer temperature reconstructions from the Gulf of Alaska, the Wrangell Mountains and Northern Alaska display good agreement at multi-decadal and higher frequencies, but the Yukon LWB reconstruction appears potentially limited in its expression of centennial-scale variation. While LWB improves dendroclimatic calibration, future work must focus on suitably preserved sub-fossil material to increase replication prior to 1650 CE. Postprint

Tree-ring reconstructions of temperature often target trees at altitudinal or latitudinal tree line where annual growth is broadly expected to be limited by and respond to temperature variability. Based on this principal, regions with sparse tree line would seem to be restricted in their potential to reconstruct past temperatures. In the northeastern United States, there are only two published temperature reconstructions. Previous work in the region reconstructing moisture availability, however, has shown that using a greater diversity of species can improve reconstruction model skill. Here, we use a network of 228 tree-ring records composed of 29 species to test the hypothesis that an increase in species diversity among the pool of predictors improves reconstructions of past temperatures. Chamaecyparis thyoides alone explained 31% of the variability in observed cool-season minimum temperatures, but a multispecies model increased the explained variance to 44%. Liriodendron tulipifera, a species not previously used for temperature reconstructions, explained a similar amount of variance as Chamaecyparis thyoides (12.9% and 20.8%, respectively). Increasing the species diversity of tree proxies has the potential for improving reconstruction of paleotemperatures in regions lacking latitudinal or elevational tree lines provided that long-lived hardwood records can be located.

The association between climate variability and episodic events, such as the antecedent moisture conditions prior to wildfire or the cooling following volcanic eruptions, is commonly assessed using Superposed Epoch Analysis (SEA). In SEA the epochal response is typically calculated as the average climate conditions prior to and following all event years or their deviation from climatology. However, the magnitude and significance of the inferred climate association may be sensitive to the selection or omission of individual key years, potentially resulting in a biased assessment of the relationship between these events and climate. Here we describe and test a modified double-bootstrap SEA that generates multiple unique draws of the key years and evaluates the sign, magnitude, and significance of event-climate relationships within a probabilistic framework. This multiple resampling helps quantify multiple uncertainties inherent in conventional applications of SEA within dendrochronology and paleoclimatology. We demonstrate our modified SEA by evaluating the volcanic cooling signal in a Northern Hemisphere tree-ring temperature reconstruction and the link between drought and wildfire events in the western United States. Finally, we make our Matlab and R code available to be adapted for future SEA applications.

Significance The variability of the Asian summer monsoon (ASM) is important for the functioning of ecological and societal systems at regional to continental scales, but the long-term evolution and interannual variability of this system is not well understood. Here, we present a stable isotope–based reconstruction of ASM variability covering 4680 BCE to 2011 CE. Superimposed on a gradual drying trend, a rapid drop in mean annual precipitation (>40%) toward persistently drier conditions occurred in ∼1675 BCE. This megadrought caused regional forest deterioration and enhanced aeolian activity affecting Chinese ecosystems. We argue that this abrupt aridification starting ∼2000 BCE triggered waves of human migration and societal transformation in northern China, which contributed to the alteration of spatial pattern of ancient civilizations., Asian summer monsoon (ASM) variability and its long-term ecological and societal impacts extending back to Neolithic times are poorly understood due to a lack of high-resolution climate proxy data. Here, we present a precisely dated and well-calibrated tree-ring stable isotope chronology from the Tibetan Plateau with 1- to 5-y resolution that reflects high- to low-frequency ASM variability from 4680 BCE to 2011 CE. Superimposed on a persistent drying trend since the mid-Holocene, a rapid decrease in moisture availability between ∼2000 and ∼1500 BCE caused a dry hydroclimatic regime from ∼1675 to ∼1185 BCE, with mean precipitation estimated at 42 ± 4% and 5 ± 2% lower than during the mid-Holocene and the instrumental period, respectively. This second-millennium–BCE megadrought marks the mid-to late Holocene transition, during which regional forests declined and enhanced aeolian activity affected northern Chinese ecosystems. We argue that this abrupt aridification starting ∼2000 BCE contributed to the shift of Neolithic cultures in northern China and likely triggered human migration and societal transformation.

The lower Brahmaputra River in Bangladesh and Northeast India often floods during the monsoon season, with catastrophic consequences for people throughout the region. While most climate models predict an intensified monsoon and increase in flood risk with warming, robust baseline estimates of natural climate variability in the basin are limited by the short observational record. Here we use a new seven-century (1309–2004 C.E) tree-ring reconstruction of monsoon season Brahmaputra discharge to demonstrate that the early instrumental period (1956–1986 C.E.) ranks amongst the driest of the past seven centuries (13th percentile). Further, flood hazard inferred from the recurrence frequency of high discharge years is severely underestimated by 24–38% in the instrumental record compared to previous centuries and climate model projections. A focus on only recent observations will therefore be insufficient to accurately characterise flood hazard risk in the region, both in the context of natural variability and climate change., This study investigates flood hazards of the Brahmaputra River, Bangladesh. Based on a tree ring reconstruction of seasonal river discharge, climate modelling, and historic documentation of flood events, the authors suggest flood hazard risk is underestimated by ~24–38% in the present day compared to the past 700 years.

Significance The SADA is an annually-resolved hydroclimate atlas in South America that spans the continent south of 12°S from 1400 to 2000 CE. Based on 286 tree ring records and instrumentally-based estimates of soil moisture, the SADA complements six drought atlases worldwide filling a geographical gap in the Southern Hemisphere. Independently validated with historical records, SADA shows that the frequency of widespread severe droughts and extreme pluvials since the 1960s is unprecedented. Major hydroclimate events expressed in the SADA are associated with strong El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) anomalies. Coupled ENSO-SAM anomalies together with subtropical low-level jet intensification due to increasing greenhouse gas emissions may cause more extreme droughts and pluvials in South America during the 21st century., South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions.

We develop a new reconstruction of the winter North Atlantic Oscillation (NAO) index using a network of 97 Euro-Mediterranean tree-ring series. The reconstruction covers the period 910–2018 C.E., making it the longest annually resolved estimate of winter NAO variability available. We use nested correlation-weighted principal components regression and the Maximum Entropy Bootstrap method to generate a 2400-member ensemble of reconstructions for estimating the final reconstruction and its quantile uncertainties. Extensive validation testing of the new reconstruction against data withheld from the calibration exercise demonstrates its skill. The skill level of the new reconstruction is also an improvement over two NAO reconstructions published earlier. Spectral analyses indicate that the new reconstruction behaves like a ‘white noise’ process with intermittent band-limited power, suggesting that the winter NAO is stochastically forced. The ‘white noise’ properties of our reconstruction are also shown to be consistent with the spectral properties of long instrumental NAO indices extending back to 1781 and NAO indices extracted from a large number of forced climate model runs covering the last millennium. In contrast, an annually resolved multi-proxy NAO reconstruction of comparable length, based in part on speleothem data, behaves more like externally forced ‘red noise’ process, which is inconsistent with our reconstruction, long observations, and forced model runs.

At the global scale, reconstructions of cool season temperature over past centuries are relatively rare. Here we present 277-year reconstructions of cool season (July–October) temperatures for southern Australia based on three different data sets: a spatial field reconstruction based on highly resolved temperature data from the Australian Water Availability Product data; reconstructions for the four southeast Australian states based on the Berkeley Earth mean temperature data for each state; and reconstructions for individual stations in southeastern Australia from the Australian Bureau of Meteorology’s Australian Climate Observations Reference Network–Surface Air Temperature data. Our reconstructions typically capture 25–50% of the variation over the late twentieth Century calibration period and are strongest for the southern state of Tasmania and the southeastern part of mainland Australia. All three use Tasmanian tree-rings sensitive to cool season temperatures and display similar variability. In the context of our reconstructions, the persistent warming in the observed record since ~ 1950 is unprecedented. While the low frequency variability of winter temperatures is generally in step with that in summer (December–February) temperatures, high frequency variability is not, illustrating the need for seasonal reconstructions to help improve understanding of variability in inter-seasonal dynamics and the historical importance of this on the environment. The reconstructions covary with changes in the Southern Annular Mode and may be useful for future reconstructions of this phenomenon.

From 1875 to 1878, concurrent multiyear droughts in Asia, Brazil, and Africa, referred to as the Great Drought, caused widespread crop failures, catalyzing the so-called Global Famine, which had fatalities exceeding 50 million people and long-lasting societal consequences. Observations, paleoclimate reconstructions, and climate model simulations are used 1) to demonstrate the severity and characterize the evolution of drought across different regions, and 2) to investigate the underlying mechanisms driving its multiyear persistence. Severe or record-setting droughts occurred on continents in both hemispheres and in multiple seasons, with the “Monsoon Asia” region being the hardest hit, experiencing the single most intense and the second most expansive drought in the last 800 years. The extreme severity, duration, and extent of this global event is associated with an extraordinary combination of preceding cool tropical Pacific conditions (1870–76), a record-breaking El Niño (1877–78), a record strong Indian Ocean dipole (1877), and record warm North Atlantic Ocean (1878) conditions. Composites of historical analogs and two sets of ensemble simulations—one forced with global sea surface temperatures (SSTs) and another forced with tropical Pacific SSTs—were used to distinguish the role of the extreme conditions in different ocean basins. While the drought in most regions was largely driven by the tropical Pacific SST conditions, an extreme positive phase of the Indian Ocean dipole and warm North Atlantic SSTs, both likely aided by the strong El Niño in 1877–78, intensified and prolonged droughts in Australia and Brazil, respectively, and extended the impact to northern and southeastern Africa. Climatic conditions that caused the Great Drought and Global Famine arose from natural variability, and their recurrence, with hydrological impacts intensified by global warming, could again potentially undermine global food security.

This study presents a 259-year (1750–2008 CE) reconstruction for October to February (ONDJF) standardized streamflow index of the Chichiawan Stream, the sole habitat of the critically endangered Formosan landlocked salmon (Oncorhynchus formosanus) remaining natural population. The reconstruction was based on the earlywood ring-width variations of Taiwan Douglas-fir (Pseudotsuga wilsoniana), using ensemble empirical mode decomposition for chronology development. Results showed that a higher average ONDJF streamflow in the 1968–2008 period was positively associated with a wider Taiwan Douglas-fir earlywood ring. During that period, the average ONDJF streamflow was related to the Pacific Decadal Oscillation and El Nino-Southern Oscillation development. Spectral analysis indicated that the reconstructed index had significant peaks around 2∼7 years and less significant peaks around 20–35 years. Based on the derived and reconstructed indices, most of the average ONDJF streamflow between 1750 and 2008 was within the normal range. Furthermore, there was no statistical evidence to suggest that the flow regime of the last few decades had changed, which refuted the speculation that the dry season streamflow natural variability in the 1960s and 1970s might have contributed toward the critically endangered status of Formosan landlocked salmon. The results of this study will be useful for developing short- to mid-term strategies to protect the endangered species and manage water resources.

Sparse long-term Asian monsoon (AM) records have limited our ability to understand and accurately model low-frequency AM variability. Here we present a gridded 544-yr (from 1470 to 2013) reconstructed Asian summer precipitation (RAP) dataset by weighted merging of two complementary proxies including 453 tree-ring-width chronologies and 71 historical documentary records. The RAP dataset provides substantially improved data quality when compared with single-proxy-type reconstructions. Skillful reconstructions are obtained in East and North China, northern India and Pakistan, the Indochina Peninsula, midlatitude Asia, the Maritime Continent, and southern Japan. The RAP faithfully illustrates large-scale regional rainfall variability but has more uncertainties in representing small-scale local rainfall anomalies. The RAP reproduces a realistic climatology and captures well the year-to-year rainfall variability averaged over monsoon Asia, arid central Asia, and all of Asia during the twentieth century. It also shows a general agreement with other proxies (speleothems and ice cores) during the period of 1470–1920. The RAP captures the remarkably abrupt change during the 1600s recorded in the upwelling proxy over the Arabian Sea. Four major modes of variability of the Asian summer precipitation are identified with the long record of the RAP, including a biennial El Niño–Southern Oscillation (ENSO) mode, a low-frequency ENSO mode, a central Pacific El Niño–like decadal mode, and an interdecadal mode. In sum, the RAP provides a valuable dataset for study of the large-scale Asian summer precipitation variability, especially the decadal–centennial variability that is caused by external forcing and internal feedback processes within the Earth climate system.

Our understanding of the full range of natural variability in streamflow, including how modern flow compares to the past, is poorly understood for the Upper Indus Basin (UIB) because of short instrumental gauge records. To help address this challenge, we use Hierarchical Bayesian Regression (HBR) with partial pooling to develop six centuries long (1394–2008 C.E.) streamflow reconstructions at three UIB gauges (Doyian, Gilgit, and Kachora), concurrently demonstrating that HBR can be used to reconstruct short records with interspersed missing data. At one gauge (Partab Bridge), with a longer instrumental record (47 years), we develop reconstructions using both Bayesian Regression (BR) and the more conventionally used Principal Components Regression (PCR). The reconstructions produced by PCR and BR at Partab Bridge are nearly identical and yield comparable reconstruction skill statistics, highlighting that the resulting tree-ring reconstruction of streamflow is not dependent on the choice of statistical method. Reconstructions at all four reconstructions indicate flow levels in the 1990s were higher than mean flow for the past six centuries. While streamflow appears most sensitive to accumulated winter (January-March) precipitation and summer (MJJAS) temperature, with warm summers contributing to high flow through increased melt of snow and glaciers, shifts in winter precipitation and summer temperatures cannot explain the anomalously high flow during the 1990s. Regardless, the sensitivity of streamflow to summer temperatures suggests that projected warming may increase streamflow in coming decades, though long-term water risk will additionally depend on changes in snowfall and glacial mass balance.

This study develops a reconstruction of the frequency of extreme cold-season precipitation and the occurrence of landfalling atmospheric river (AR) storm tracks across the southwestern Unites States using a network of tree ring chronologies and the Living Blended Drought Atlas (LBDA), a 500-year tree ring based reconstruction of the summer Palmer Drought Severity Index. The first two rotated empirical orthogonal functions of the LBDA across the Southwest are shown to relate well to previously identified patterns of regional AR activity. Accordingly, the rotated empirical orthogonal functions also record patterns of extreme precipitation associated with those ARs, albeit with some uncertainty introduced by nonextreme precipitation. A network of chronologies sensitive to cold-season precipitation is then used to reconstruct the occurrence of landfalling ARs and extreme precipitation along the southern Californian coast, demonstrating for the first time the feasibility of reconstructing AR landfalls and extreme events in the Southwest based on spatial patterns in a network of dendroclimatic proxies.

Tree ring science is a new discipline in Bhutan but has contributed substantially to our understanding of climate history and informed sustainable forest management practices in the country. This paper describes dendroecological contributions to the second aspect for Blue Pine using three case studies. i) The effects of livestock grazing impact on Blue Pine radial growth were quantified. Radial growth increment was tendentially higher after three years of livestock exclosure, as compared to continued grazing. However, differences remained statistically not significant, likely due to the brevity of the treatment period. ii) Radial growth rates of Blue Pine were characterized across a 400 m elevation gradient. Cumulative radial growth over 40 years differed by a factor of more than three between the low and the high end of the gradient. However, below 2300 m, radial growth showed a continuous decline from 1990, likely as a results of drought due to climate change. iii) Effects of three levels of prescribed thinning of pole stage (DBH 30–50 cm) Blue Pine in central Bhutan showed distinct response to thinning. Heavy thinning lead to a thinning shock in the year after harvest and did not lead to significantly higher radial growth as compared to moderate thinning, which is thus recommended for the species. A positive thinning effect remained for seven years post operation. The case studies were incorporated into national guidelines on sustainable forest management in Bhutan and prove the demand for tree ring based research to inform policy and practice.

Newly available paleoclimate data and a re-evaluation of the historical and archaeological evidence regarding the Uyghur Empire (744–840)—one of several nomadic empires to emerge on the Inner Asian steppe—suggests that the assumption of a direct causal link between drought and the stability of nomadic societies is not always justified. The fact that a severe drought lasting nearly seven decades did not cause the Uyghur Empire to collapse, to wage war, or to disintegrate gives rise to speculations about which of its characteristics enabled it to withstand unfavorable climatic conditions and environmental change. More broadly, it raises questions about the complex suite of strategies and responses that may have been available to steppe societies in the face of environmental stress.

Since its establishment, tree-ring analysis has benefitted several scientific fields. Because of its many advantages, dendrochronology is a first choice to reconstruct past environmental variability. Two major concerns about the current tree-ring reconstruction paradigm are the subjective choices of detrending functions and the lack of fidelity to data of chronology generation methods. It is difficult to recover the original tree-ring data once they have been detrended and standardized. In this study, ensemble empirical mode decomposition (EEMD) is introduced as an objective high-fidelity stand-alone approach for developing tree-ring chronologies. Basic concepts of EEMD, recommended steps in developing chronologies, and available public domain programs are discussed. To demonstrate the potentials of EEMD for chronology development, two examples are provided, one for climate and the other for streamflow reconstructions. In both examples, EEMD chronologies show higher correlations with the instrume...

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 tree-ring 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.

Few Southern Hemisphere tree-ring chronologies exceed 1000 years in length. We present a ca. 1700 years of indexed values for the long-lived conifer Athrotaxis selaginoides at Cradle Mt in southeastern Australia and compare it with the only other published millennial-plus length tree-ring chronology for Australia: the nearby Mt Read Lagarostrobos franklinii . We use simple correlation function and pointer year analyses to compare the climate responses of the two species (temperature, precipitation and growing degree days). Both chronologies show accelerated growth at their modern ends, but this growth acceleration is not synchronous, beginning approximately a quarter of a century earlier at the Cradle Mt site. This discrepancy may highlight the relevance of chronology composition and/or physiological differences in the species. Although the seasonality of the climatic responses of the two species is similar, that of A. selaginoides is generally weaker than that of L. franklinii . Somewhat paradoxically, the only pointer years in common between the chronologies are 1898 and 1908 CE. The periods from 600 to 900 CE and ∼1200–1450 CE are conspicuous for their absence of positive pointer years while no negative pointer years occur for either site from ∼1200–1350 CE. It is possible that differing patterns of pointer years can be partially explained by a peak in establishment from ∼1150–1850 CE at the Mt Read L. franklinii site compared to continuous establishment at Cradle Mt. Although statistically significant and time-stable climate responses for the A. selaginoides chronology are too weak to base a single-chronology climate reconstruction on, the long chronology will likely make an important contribution to future multi-proxy temperature reconstructions for southeastern Australia.

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 1799-2012, 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 mid-latitude 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 pre-instrumental 19th and early 20th centuries, but the tree-ring reconstruction suggests that the strong multi-decadal variability in instrumental and reconstructed wet season rainfall after 1950 may have been unmatched since 1799.