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There are indications that the reference climatology underlying meteorological drought has shown nonstationarity at seasonal, decadal, and centennial timescales, impacting the calculation of drought indices and potentially having ecological and economic consequences. Analyzing these trends in meteorological drought climatology beyond 100 years, a time frame which exceeds the available period of observation data, contributes to a better understanding of the nonstationary changes, ultimately determining whether they are within the range of natural variability or outside this range. To accomplish this, our study introduces a novel approach to integrate unevenly scaled tree-ring proxy data from the North American Seasonal Precipitation Atlas (NASPA) with instrumental precipitation datasets by first temporally downscaling the proxy data to produce a regular time series and then modeling climate nonstationarity while simultaneously correcting model-induced bias. This new modeling approach was applied to 14 sites across the continental United States using the 3-month standardized precipitation index (SPI) as a basis. The findings showed that certain locations have experienced recent rapid shifts towards drier or wetter conditions during the instrumental period compared to the past 1000 years, with drying trends generally found in the west and wetting trends in the east. This study also found that seasonal shifts have occurred in some regions recently, with seasonality changes most notable for southern gauges. We expect that our new approach provides a foundation for incorporating various datasets to examine nonstationary variability in long-term precipitation climatology and to confirm the spatial patterns noted here in greater detail.

The Amazon basin is a global center of hydroclimatic variability and biodiversity, but there are only eight instrumental rainfall stations with continuous records longer than 80 years in the entire basin, an area nearly the size of the coterminous US. The first long moisture-sensitive tree-ring chronology has been developed in the eastern equatorial Amazon of Brazil based on dendrochronological analysis of Cedrela cross sections cut during sustainable logging operations near the Rio Paru. The Rio Paru chronology dates from 1786 to 2016 and is significantly correlated with instrumental precipitation observations from 1939 to 2016. The strength and spatial scale of the precipitation signal vary during the instrumental period, but the Rio Paru chronology has been used to develop a preliminary reconstruction of February to November rainfall totals from 1786 to 2016. The reconstruction is related to SSTs in the Atlantic and especially the tropical Pacific, similar to the stronger pattern of association computed for the instrumental rainfall data from the eastern Amazon. The tree-ring data estimate extended drought and wet episodes in the mid- to late-nineteenth century, providing a valuable, long-term perspective on the moisture changes expected to emerge over the Amazon in the coming century due to deforestation and anthropogenic climate change.

Heatwaves and summer droughts across Europe are likely to intensify under anthropogenic global warming thereby affecting ecological and societal systems. To place modern trends and extremes in the context of past natural variability, annually resolved and absolutely dated climate reconstructions are needed. Here, we present a network of 153 yew ( Taxus baccata L.) tree-ring width (TRW) series from 22 sites in southern England that cover the past 310 years. Significant positive correlations were found between TRW chronologies and both April–July precipitation totals (r > 0.7) and July drought indices (r > 0.59) back to 1901 CE ( p  < 0.05). We used a suite of residual and standard TRW chronologies to reconstruct interannual to multi-decadal spring–summer precipitation and mid-summer drought variability over western Europe, respectively. Our yew hydroclimate reconstructions capture the majority of reported summer droughts and pluvials back to 1710 CE. Clusters of severe drought spells occurred in the second half of the 18th and mid-twentieth century. Our study suggests that the frequency and intensity of recent hydroclimate extremes over western Europe are likely still within the range of past natural variability.

Our understanding of pre‐modern El Niño Southern Oscillation (ENSO) variability is reliant on proxy records, often distant from the center of ENSO activity in the equatorial Pacific Ocean. Here, we assess the relationship between reconstructed soil moisture in four distant ENSO‐influenced regions over the past 400 years. A major breakdown in the teleconnection of regional drought conditions in Asia, Eastern Australia, and North America is identified around 1700 CE. The statistically significant decline in inter‐series correlations (p < 0.01) represents a previously unknown aspect of global hydroclimate dynamics. We hypothesize that the disruption was driven by ENSO weakening and/or by a large‐scale multi‐decadal reconfiguration of ocean‐atmosphere circulation. Data assimilation estimates of soil moisture from the same regions fail to produce results of the same magnitude, potentially due to an overreliance of ENSO influence on the boundaries of spatial covariance in the underlying climate models. Plain Language Summary The El Niño Southern Oscillation (ENSO) is a recurring change in sea surface temperatures in the tropical Pacific Ocean that affects climate across much of the globe. ENSO can influence drought conditions on land, all around the Pacific Ocean and beyond. Here, we use tree‐ring estimates of drought to see how coherent dryness and wetness in these regions have been over the past 400 years. Overall, the four regions we study are in agreement but there is a breakdown in the relationship around 1700 CE. During this period, drought conditions in inner Eurasia, eastern Australia, southeast Asia, and southwest North America were unconnected. Climate models for the past millennium highlight how rare such an event is. The breakdown in drought teleconnection we describe may represent an important component of ENSO that so far is not fully understood. Key Points Drought estimates based on tree rings in four El Niño Southern Oscillation (ENSO)‐influenced regions become disconnected in the early 17th century CE Data assimilation and climate model outputs fail to capture a disruption of similar magnitude Future ENSO‐driven drought teleconnections may include previously unknown variability

Historians and paleoclimatologists have long studied the connection between ecoclimatic changes and empire growth, transformation, and decline, but striking cases remain rare. Here, we introduce a tree-ring chronology from southern China to reconstruct changes in April-to-November water balance of the middle reaches of the Yangtze River over the last 464 years. The reconstruction supports a quantitative assessment of the spatiotemporal structure of the late Ming megadrought and potential effects on subsequent dynastic transitions. Our results indicate that the late Ming megadrought from 1625 – 1644 CE occurred in both the northern and southern parts of the East Asian monsoon region in China. However, variations in the onset, duration, and magnitude of this event differ between regions. The combination of factors such as Pacific sea surface temperature anomalies, weakened solar activity, and large-scale volcanic eruptions may have contributed to the occurrence of the late Ming megadrought. These factors are also identified as key drivers of interannual to decadal fluctuations in drought in the middle reaches of the Yangtze River. Our reconstruction provides an historical context for the development of adaptive measures to mitigate future drought impacts in the region.The late Ming megadrought across north-to-south China, lasting from 1625-1644 CE, was influenced by Pacific sea surface temperature anomalies, weakened solar activity, and volcanic eruptions, according to reconstruction of the Yangtze River's water balance using tree-ring chronology.

A large proportion of western North America experiences regular water stress, compounded by high seasonal and interannual variability. In the Intermountain West region, the El Niño/Southern Oscillation (ENSO) is a critical control on winter precipitation, but the nature of this signal is entangled with a combination of orographic effects and long-term climate trends. This study employs a spatially distributed, nonlinear spline model to isolate ENSO impacts from these other factors using gauge-based observations starting in 1871. In contrast to previous modelling approaches, our approach uses original gauge data, without shortening the record to accommodate a common period. This enables more detailed separation of ENSO effects from the confounding influence of topography and long-term trends, whereas the longer time frame permits more robust correlation with the ENSO signal. Here we show that the complex topography of the Intermountain West exaggerates the underlying ENSO signal, producing a 2.3–5.8 times increase in the range of ENSO-induced precipitation changes along high-elevation western slopes relative to lower elevations. ENSO effects on winter precipitation can be as large as ± 100 mm at high elevations. Further, our approach reveals that the previously recognized dipolar pattern of positive (negative) association of ENSO with precipitation in the south (north) manifests as an incremental relationship in the south but as a near-binary switch in effects between El Niño and La Niña in the north. The location and extent of the strongest precipitation differences vary during the positive and negative ENSO phases within each region. The intricacies of these spatial- and elevation-based modulations of ENSO impacts are especially informative for the northern centre of this dipole, where ENSO-precipitation relationships have previously been difficult to resolve.

The Asian Water Towers play a crucial role by storing and releasing vast amounts of freshwater, thereby sustaining the base flow of major Asian rivers and water security for billions of people at sub-continent to hemispheric scales. Instrumental records, though spatially and temporally limited, indicate rapid warming in High Asia. However, the sensitivity and long-term resilience of these Water Towers remain uncertain. Here, we use an 814-year-long tree-ring record (including tree-ring width and maximum latewood density) from Picea likiangensis on the eastern Tibetan Plateau to develop a summer (June-September) temperature reconstruction. Our reconstruction reveals that the series has warmed by 1.5 °C during the modern observational period (1970–2023), which is 0.5 ± 0.4 °C above the pre-industrial baseline (1210–1850 or 1850-1900), making the summer of 2024 the warmest in the past eight centuries. This unprecedented warming amplifies winter runoff in the Brahmaputra, Indus, and Salween headwaters through a cascade of atmosphere-cryosphere feedbacks: enhanced meltwater and spring soil-moisture persistence promote earlier and lusher vegetation growth, which reduces summer albedo and further accelerates regional warming. Detection and attribution analyses identify volcanic and solar forcing as the main drivers of natural, pre-industrial variability before 1850 CE, whereas anthropogenic forcing is detected with high confidence (exceeding the 99% confidence level) after 2020 CE.

Rivers originating from the Andes Mountains are vital water sources for agricultural and societal needs in South America, yet are increasingly threatened by climate change. Here we reconstruct streamflow of the Negro River (Rió Negro) in northern Patagonia over the past 827 years using tree-ring records from the south-central Andes foothills. This reconstruction reveals an unprecedented decline in river flow in recent decades. Moisture from the El Niño–Southern Oscillation and the Southern Annular Mode provides a key water source, but temperature rise has increased atmospheric moisture demand and reduced availability. Antarctic amplification has further disrupted circulation patterns and accelerated warming, intensifying regional aridity. Since the 20th century, the Negro River has experienced a sustained flow reduction of approximately 10% per decade. Projections under various emission scenarios indicate continued decline, highlighting the urgent need for adaptive water management strategies to mitigate escalating water scarcity risks in the region. Climate change has led to a significant decrease in streamflow, particularly in the Negro River in the Andes, posing water scarcity risks and necessitating effective management strategies, according to streamflow reconstruction and tree-ring data analysis.

Anthropogenic climate change affects regional hydrological cycles and poses significant challenges to the sustainable supply of freshwater. The Central China water tower (CCWT) is the key source region feeding the Yangtze and Yellow Rivers, and its runoff is indispensable for the surrounding mega‐city clusters. Here we present a reconstruction of CCWT runoff depth (RD) back to 1595 CE, based on a new dendrochronological network including 100 tree‐ring sampling sites and an ensemble averaging approach that combines multiple regression models. Comparison of this reconstruction with similar records from six water tower regions along the Pacific Rim (Mongolian Plateau, Tibetan Plateau TP, Great Dividing Range, Southern and Northern Rocky Mountains, Andes Mountains) revealed that the CCWT provide the most stable water supply, while the TP to be most susceptible to extreme runoff events. Twenty‐first century projections indicate generally increasing runoff across most Pacific Rim water towers, whereas the Northern Rocky Mountains are projected to decline substantially. We attribute the differences in runoff variability and projected trends across Pacific Rim water towers to their distinct geographies and synoptic climatic conditions. The long‐term runoff reconstructions and projected changes highlighted in this study provide insights for adaptive management strategies in China and all other regions relying on supply from mountain water towers. Human‐caused climate change is creating major challenges for sustainable freshwater supply. The Central China Water Tower (CCWT), a key source for the Yangtze and Yellow Rivers, supports large populations and cities. In this study, we used tree‐ring data from 100 sites to reconstruct CCWT runoff depth back to 1595 CE, offering a long‐term view of water availability. We also compared CCWT runoff with records from six other Pacific Rim water towers, including the Mongolian Plateau, Tibetan Plateau (TP), Great Dividing Range, Northern and Southern Rocky Mountains, and Andes Mountains. The CCWT showed the most stable water supply over time, while the TP had more extreme fluctuations. Looking ahead, model projections suggest increasing runoff in most regions, but a decline in the Northern Rockies. These contrasting trends reflect differences in geography and climate patterns across regions. Our findings highlight the importance of understanding both past and future changes in mountain water resources and the need for region‐specific water management strategies in a changing climate. A new dendrochronological network of 100 tree‐ring sampling sites reconstructs Central China Water Tower (CCWT) runoff depth back to 1595 CE, revealing long‐term hydrological variability Comparative analysis of Pacific Rim water towers shows the CCWT provides the most stable water supply, while the Tibetan Plateau is more prone to extreme runoff events Twenty‐first century projections predict increased runoff across most Pacific Rim water towers, except the Northern Rocky Mountains, emphasizing the need for region‐specific water management strategies

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.