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Abstract We perform a global assessment of dryness and wetness conditions using an ensemble of observational, satellite-informed, and reanalysis-based soil moisture datasets [soil moisture (SM) Global Land Evaporation Amsterdam Model (SM_GLEAM), SM_ERA5-LAND, and SM_MERRA-2] and drought indices [Palmer drought severity index (PDSI_CRU) and standardized precipitation evapotranspiration index (SPEI_CRU)]. Over the short term (1982–2023), four or more products consistently identify drying trends in western North America, South America, the Mediterranean, Europe, central Asia, and southern Australia. Wetting trends appear in southern and southeastern Asia, the Sahel, and parts of southern sub-Saharan Africa, while disagreements across products are most evident in central Africa, parts of South America, the Middle East, and Southeast Asia. The year 2023 ranks among the driest on record since 1982: driest in PDSI_CRUand SM_ERA5-LAND, second driest in SM_GLEAMand SPEI_CRU, and eighth in SM_MERRA-2. A longer-term analysis using PDSI_CRUfrom 1903 to 2023 estimates that 2023 was the most severe and spatially extensive drought year in over a century, affecting more than 27% of the global reporting land area. Moreover, the 2021–23 period marks the longest consecutive stretch of extreme drought conditions globally since the early twentieth century. The global patterns of droughts over 2021–23 align with long-term drying trends, consistent with the broader influence of anthropogenic climate change on global hydroclimate. Despite the presence of long-term wetting trends, the wetness of the past decades has not produced events of comparable magnitude to those in the mid-twentieth century, a time that remains unmatched in both the severity and extent of wet extremes. Significance StatementOn a global scale, the years 2021–23 have seen some of the most widespread and severe drought conditions in over a century. By analyzing five major datasets that characterize changes in soil moisture conditions, we find that 2023 was the driest year on record in over 120 years and that 2021–23 was the driest 3-yr period over the same time interval. We show that these recent twenty-first-century extremes are consistent with long-term drying trends and highlight a broad intensification of hydroclimate extremes across much of the globe. While previous work has emphasized regional drought events over shorter intervals, this study uses multiple datasets in a global framework to provide strong evidence that anthropogenic climate change is a key driver of an intensifying global pattern of more widespread and severe drought. 

Abstract Droughts over the last century in Southwestern North America (SWNA) have had severe consequences for people and ecosystems across the region, most recently during the early 21st‐century megadrought (2000–2022). The 20^thcentury, however, was bracketed by two extended pluvials that also had significant impacts in the region. We use a 1,224 years (800–2023 CE) record of observed and reconstructed soil moisture, in concert with a paleoclimate reanalysis product, to place the 20th‐century pluvials in a longer‐term context and investigate the occurrence and dynamics of similar events in the Common Era. Analyses of the soil moisture reconstruction demonstrate that pluvials and megapluvials are as ubiquitous as droughts and megadroughts over the last millennium. The early (19 years; 1905–1923) and late (22 years; 1978–1999) 20th‐century pluvials rank as the second and first wettest in the record, respectively, positioning these as events on par with the most extreme megadroughts. Pluvials show a strong association with tropical Pacific (warm) sea surface temperatures (SSTs) during the 20^thcentury and over the prior millennium, though the role of the tropical Atlantic is much more uncertain and ambiguous. Using a Bayesian hierarchical modeling approach trained on the pre‐industrial period (800–1849 CE), we find that the record setting late 20^th‐century megapluvial likely occurred as a consequence of anomalously strong Pacific sea surface temperature forcing. This work establishes pluvial and megapluvial events as intrinsic components of Common Era hydroclimate variability in SWNA, comparable in importance to droughts and megadroughts. 

Abstract The North American Southwest (NASW) and South American Southwest (SASW) are regions susceptible to prolonged and intense droughts that can span a decade or more (i.e., megadroughts). Although the drivers and impacts of megadroughts in each region and their co-occurrence have been examined in paleoclimate reconstructions, it is not known whether climate models simulate co-occurring megadroughts in these regions with characteristics and drivers that are similar to the real world. We compare the temporal characteristics of concurrent megadroughts and the Pacific Ocean conditions associated with these events in the Paleo Hydrodynamics Data Assimilation (PHYDA) product and the Community Earth System Model Last Millennium Ensemble (CESM-LME). We find that concurrent megadroughts in PHYDA and CESM-LME have similar temporal characteristics, but the relationship between hydroclimate conditions in the NASW and SASW is different between proxy-based estimates and the climate model. Further analyses reveal that changes in the tropical Pacific Ocean are weaker during concurrent megadroughts in the CESM-LME compared to those in PHYDA and that their teleconnection patterns and strengths are different. Reconstruction methodology is also found to be a factor in how the relationship between the tropical Pacific and each region is characterized. These results together indicate that while the CESM-LME simulates concurrent megadroughts with temporal characteristics similar to PHYDA, it does so for different reasons; this result leaves open the question of whether climate models used for future projections can accurately capture the risk of concurrent megadroughts in future projections. 

Abstract In summer 2021, 90% of the western United States (WUS) experienced drought, with over half of the region facing extreme or exceptional conditions, leading to water scarcity, crop loss, ecological degradation, and significant socio‐economic consequences. Beyond the established influence of oceanic forcing and internal atmospheric variability, this study highlights the importance of land‐surface conditions in the development of the 2020–2021 WUS drought, using observational data analysis and novel numerical simulations. Our results demonstrate that the soil moisture state preceding a meteorological drought, due to its intrinsic memory, is a critical factor in the development of soil droughts. Specifically, wet soil conditions can delay the transition from meteorological to soil droughts by several months or even nullify the effects of La Niña‐driven meteorological droughts, while drier conditions can exacerbate these impacts, leading to more severe soil droughts. For the same reason, soil droughts can persist well beyond the end of meteorological droughts. Our numerical experiments suggest a relatively weak soil moisture‐precipitation coupling during this drought period, corroborating the primary contributions of the ocean and atmosphere to this meteorological drought. Additionally, drought‐induced vegetation losses can mitigate soil droughts by reducing evapotranspiration and slowing the depletion of soil moisture. This study highlights the importance of soil moisture and vegetation conditions in seasonal‐to‐interannual drought predictions. Findings from this study have implications for regions like the WUS, which are experiencing anthropogenically‐driven soil aridification and vegetation greening, suggesting that future soil droughts in these areas may develop more rapidly, become more severe, and persist longer. 

Abstract We critically reexamine the question of whether volcanic eruptions cause surface warming over Eurasia in winter, in the light of recent modeling studies that have suggested internal variability may overwhelm any forced volcanic response, even for the very largest eruptions during the Common Era. Focusing on the last millennium, we combine model output, instrumental observations, tree-ring records, and ice cores to build a new temperature reconstruction that specifically targets the boreal winter season. We focus on 20 eruptions over the last millennium with volcanic stratospheric sulfur injections (VSSIs) larger than the 1991 Pinatubo eruption. We find that only 7 of these 20 large events are followed by warm surface temperature anomalies over Eurasia in the first posteruption winter. Examining the 13 events that show cold posteruption anomalies, we find no correlation between the amplitude of winter cooling and VSSI mass. We also find no evidence that the North Atlantic Oscillation is correlated with VSSI in winter, a key element of the proposed mechanism through which large, low-latitude eruptions might cause winter warming over Eurasia. Furthermore, by inspecting individual eruptions rather than combining events into a superposed epoch analysis, we are able to reconcile our findings with those of previous studies. Analysis of two additional paleoclimatic datasets corroborates the lack of posteruption Eurasian winter warming. Our findings, covering the entire last millennium, confirm the findings of most recent modeling studies and offer important new evidence that large, low-latitude eruptions are not, in general, followed by significant surface wintertime warming over Eurasia. 

Abstract Anthropogenic climate change has already affected drought severity and risk across many regions, and climate models project additional increases in drought risk with future warming. Historically, droughts are typically caused by periods of below‐normal precipitation and terminated by average or above‐normal precipitation. In many regions, however, soil moisture is projected to decrease primarily through warming‐driven increases in evaporative demand, potentially affecting the ability of negative precipitation anomalies to cause drought and positive precipitation anomalies to terminate drought. Here, we use climate model simulations from Phase Six of the Coupled Model Intercomparison Project (CMIP6) to investigate how different levels of warming (1, 2, and 3°C) affect the influence of precipitation on soil moisture drought in the Mediterranean and Western North America regions. We demonstrate that the same monthly precipitation deficits (25th percentile relative to a preindustrial baseline) at a global warming level of 2°C increase the probability of both surface and rootzone soil moisture drought by 29% in the Mediterranean and 32% and 6% in Western North America compared to the preindustrial baseline. Furthermore, the probability of a dry (25th percentile relative to a preindustrial baseline) surface soil moisture month given a high (75th percentile relative to a preindustrial baseline) precipitation month is 6 (Mediterranean) and 3 (Western North America) times more likely in a 2°C world compared to the preindustrial baseline. For these regions, warming will likely increase the risk of soil moisture drought during low precipitation periods while simultaneously reducing the efficacy of high precipitation periods to terminate droughts. 

Abstract Climate field reconstructions (CFRs) combine modern observational data with paleoclimatic proxies to estimate climate variables over spatiotemporal grids during time periods when widespread observations of climatic conditions do not exist. The Common Era (CE) has been a period over which many seasonally‐ and annually‐resolved CFRs have been produced on regional to global scales. CFRs over the CE were first produced in the 1970s using dendroclimatic records and linear regression‐based approaches. Since that time, many new CFRs have been produced using a wide range of proxy data sets and reconstruction techniques. We assess the early history of research on CFRs for the CE, which provides context for our review of advances in CFR research over the last two decades. We review efforts to derive gridded hydroclimatic CFRs over continental regions using networks of tree‐ring proxies. We subsequently explore work to produce hemispheric‐ and global‐scale CFRs of surface temperature using multi‐proxy data sets, before specifically reviewing recently‐developed data assimilation techniques and how they have been used to produce simultaneous reconstructions of multiple climatic fields globally. We then review efforts to develop standardized and digitized databases of proxy networks for use in CFR research, before concluding with some thoughts on important next steps for CFR development. 

Common Era temperature variability has been a prominent component in Intergovernmental Panel on Climate Change reports over the last several decades and was twice featured in their Summary for Policymakers. A single reconstruction of mean Northern Hemisphere temperature variability was first highlighted in the 2001 Summary for Policymakers, despite other estimates that existed at the time. Subsequent reports assessed many large-scale temperature reconstructions, but the entirety of Common Era temperature history in the most recent Sixth Assessment Report of the Intergovernmental Panel on Climate Change was restricted to a single estimate of mean annual global temperatures. We argue that this focus on a single reconstruction is an insufficient summary of our understanding of temperature variability over the Common Era. We provide a complementary perspective by offering an alternative assessment of the state of our understanding in high-resolution paleoclimatology for the Common Era and call for future reports to present a more accurate and comprehensive assessment of our knowledge about this important period of human and climate history. 

Abstract Large volcanic eruptions are known to significantly impact global climate for several years, yet a comprehensive comparison across the growing number of paleoclimate datasets has not been performed. Here, we assess the impacts of major eruptions over the Last Millennium on surface air temperature (SAT), Palmer drought severity index (PDSI), and 500-hPa geopotential height using tree-ring reconstructions, nine data assimilation (DA) products, and two climate model ensembles. We confirm robust global SAT cooling but find large differences in magnitude and persistence: Reconstructions based on tree-ring density show shorter, physically consistent cooling, whereas products dominated by tree-ring widths show longer persistence, likely reflecting biological memory. PDSI responses reveal coherent wetting over the western United States, the Mediterranean Basin, and southern South America, and coherent drying over northern and European Russia, central Asia, and southern Siberia, with divergence elsewhere. El Niño–Southern Oscillation responses differ across products, suggesting that any volcanically forced signal is weak relative to internal variability and highly sensitive to the background climate state. Geopotential height anomalies reveal widespread posteruption tropospheric contraction and robust extratropical circulation shifts, including negative height anomalies over mid- to high latitudes and wave-like patterns in the Southern Hemisphere. These anomalies are dynamically consistent with the spatial patterns of wetting and drying in PDSI, suggesting that volcanic forcing reorganizes large-scale atmospheric circulation in ways that influence hydroclimate. Together, these findings provide a comprehensive framework for interpreting volcanic impacts, strengthen confidence in regions with robust signals, and identify priority areas—particularly in the tropics and the Southern Hemisphere—where additional proxy coverage could reduce current uncertainties. Significance StatementThis study presents the first comprehensive intercomparison of volcanic impacts on temperature, hydroclimate, and atmospheric circulation from tree-ring reconstructions, data assimilation products, and climate model ensembles. Reconstructions based on tree-ring density are the physically most consistent benchmark, showing shorter and more realistic duration of cooling, whereas products dominated by tree-ring widths overestimate persistence resulting from biological memory. Hydroclimate responses reflect coherent wetting in the western United States, the Mediterranean, and southern South America, and coherent drying in northern and European Russia, central Asia, and southern Siberia, with minimal agreement elsewhere. Responses of the El Niño–Southern Oscillation phenomenon remain inconsistent between products. We also connect hydroclimate signals with posteruption atmospheric height anomalies and associated extratropical circulation changes to provide a clearer framework for interpreting how volcanoes impact interactions between the ocean and atmosphere. This linkage also helps pinpoint priority regions—especially across the tropics and the Southern Hemisphere—where expanded, high-resolution proxy records could most effectively narrow current uncertainties. 

Across western North America (WNA), 20th-21st century anthropogenic warming has increased the prevalence and severity of concurrent drought and heat events, also termed hot droughts. However, the lack of independent spatial reconstructions of both soil moisture and temperature limits the potential to identify these events in the past and to place them in a long-term context. We develop the Western North American Temperature Atlas (WNATA), a data-independent 0.5° gridded reconstruction of summer maximum temperatures back to the 16th century. Our evaluation of the WNATA with existing hydroclimate reconstructions reveals an increasing association between maximum temperature and drought severity in recent decades, relative to the past five centuries. The synthesis of these paleo-reconstructions indicates that the amplification of the modern WNA megadrought by increased temperatures and the frequency and spatial extent of compound hot and dry conditions in the 21st century are likely unprecedented since at least the 16th century.