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Future flood risk assessment has primarily focused on heavy rainfall as the main driver, with the assumption that projected increases in extreme rain events will lead to subsequent flooding. However, the presence of and changes in vegetation have long been known to influence the relationship between rainfall and runoff. Here, we extract historical (1850–1880) and projected (2070–2100) daily extreme rainfall events, the corresponding runoff, and antecedent conditions simulated in a prominent large Earth system model ensemble to examine the shifting extreme rainfall and runoff relationship. Even with widespread projected increases in the magnitude (78% of the land surface) and number (72%) of extreme rainfall events, we find projected declines in event‐based runoff ratio (runoff/rainfall) for a majority (57%) of the Earth surface. Runoff ratio declines are linked with decreases in antecedent soil water driven by greater transpiration and canopy evaporation (both linked to vegetation greening) compared to areas with runoff ratio increases. Using a machine learning regression tree approach, we find that changes in canopy evaporation is the most important variable related to changes in antecedent soil water content in areas of decreased runoff ratios (with minimal changes in antecedent rainfall) while antecedent ground evaporation is the most important variable in areas of increased runoff ratios. Our results suggest that simulated interactions between vegetation greening, increasing evaporative demand, and antecedent soil drying are projected to diminish runoff associated with extreme rainfall events, with important implications for society. Plain Language Summary: Climate change is leading to increases in the magnitude and number of extreme rainfall events. These increases in extreme rainfall events are often assumed to lead to an increase in extreme flooding events. However, using a climate model ensemble, our results indicate that changes in vegetation and atmospheric water demand may alter the relationship between extreme rainfall and extreme runoff. Notably, for a majority of the Earth surface, we find that projected changes in atmospheric aridity and vegetation lead to drier soil conditions prior to the extreme rainfall event that reduces the amount of hydrologic runoff generated. These findings have important implications for water resources management. Key Points: The number and magnitude of extreme rainfall events are projected to increase throughout the global land surfaceProjected declines in event‐based runoff ratio are found for a majority of the global land surfaceProjected runoff ratio declines are linked to decreased antecedent soil water from changes in individual evapotranspiration components [ABSTRACT FROM AUTHOR]

Terrestrial ecosystems store more than twice the carbon of the atmosphere, and are critical to climate change mitigation efforts. This has led to a proliferation of land‐based carbon sequestration efforts, such as re/afforestation associated with the Great Green Wall in the West African Sahel (WAS GGW). However, we currently lack comprehensive assessments of the long‐term viability of these ecosystems' carbon storage in the context of increasingly severe climate extremes. The WAS is particularly prone to recurrent and disruptive extremes, exemplified by the persistent and severe late‐20th century drought. We assessed the response and recovery of WAS GGW carbon stocks and fluxes to this late‐20th century drought, and the subsequent rainfall recovery, by leveraging a suite of terrestrial ecosystem models. While multi‐model mean carbon fluxes (e.g., gross primary production, respiration) partly recovered to pre‐drought levels, modeled total (above and below ground) ecosystem carbon stock falls to as much as two standard deviations below pre‐drought levels and does not recover even ∼20 years after the maximum drought anomaly. Furthermore, to the extent that the modeled regional carbon stock recovers, it is nearly entirely driven by atmospheric CO2 trends rather than the precipitation recovery. Uncertainties in regional ecosystem carbon simulation are high, as the models' carbon responses to drought displayed a nearly 10‐standard deviation spread. Nevertheless, the multi‐model average response highlights the strong and persistent impact of drought on terrestrial carbon storage, and the potential risks of relying on terrestrial ecosystems as a "natural climate solution" for climate change mitigation. Plain Language Summary: Land‐based ecosystems store twice as much carbon as the atmosphere, and are important to protect and restore to address climate change. The West African Sahel (WAS) has been identified as an important region for ecosystem restoration partly to help advance climate change mitigation efforts, for example, as part of the Great Green Wall (GGW). However, increasingly‐severe climate extremes, such as drought, can adversely impact ecosystem carbon storage and recovery. To better understand these drought impacts, we use a suite of terrestrial ecosystem models to evaluate the West African Sahel ecosystem carbon response to the late 20th century drought and following rainfall recovery. The multi‐model average shows that the drought substantially reduced the total amount of carbon stored in the WAS GGW vegetation and soils, which do not fully recover even 20 years after the peak drought period. The models do, however, vary widely in their carbon responses to the prolonged drought, owing to major uncertainties in both process and regional ecosystem representation. There are many benefits of restoring ecosystems, particularly in drought‐prone regions. Nevertheless, our study demonstrates that prolonged drought can strongly influence and potentially compromise the WAS ecosystem's carbon storage capacity, therefore possibly limiting the efficacy of the WAS GGW to serve as "natural climate solutions" specifically toward climate change mitigation goals. Key Points: Ecosystem models show that the 20th century Sahelian drought depressed carbon stocks across the West African Great Green Wall domainCarbon fluxes did exhibit some recovery strongly driven atmospheric CO2 trends and plant physiological effectsDrought events could limit long‐term terrestrial carbon storage across the West African Great Green Wall domain [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Regional warming associated with climate change is already making droughts worse in many places. This trend is expected to continue as global temperatures increase in response to continued emissions of greenhouse gases. With increasing temperatures, soils are projected to lose water to the atmosphere as it draws more and more moisture from the land surface, increasing drought risk. At the same time, climate change is also projected to shift precipitation patterns. This means it is important to understand how changes in rainfall will cause and end droughts in the future. Using state‐of‐the‐art climate models, we investigate how future warming will impact the influence of precipitation on soil moisture in the Mediterranean and Western North America. Our results suggest that current levels of monthly precipitation will be insufficient to alleviate drought conditions in a warmer world. Key Points: Warming causes declines in soil moisture in the Mediterranean and Western North AmericaThe probability of a moderate soil moisture drought increases during both low and high precipitation anomalies, even at 1°C of warmingThe ability of large positive precipitation anomalies to terminate soil moisture droughts will be substantially reduced in a warmer world [ABSTRACT FROM AUTHOR]

With continued fossil‐fuel dependence, anthropogenic aerosols over South Asia are projected to increase until the mid‐21st century along with greenhouse gases (GHGs). Using the Community Earth System Model (CESM1) Large Ensemble, we quantify the influence of aerosols and GHGs on South Asian seasonal precipitation patterns over the 21st century under a very high‐emissions (RCP 8.5) trajectory. We find that increasing local aerosol concentrations could continue to suppress precipitation over South Asia in the near‐term, delaying the emergence of precipitation increases in response to GHGs by several decades in the monsoon season and a decade in the post‐monsoon season. Emergence of this wetting signal is expected in both seasons by the mid‐21st century. Our results demonstrate that the trajectory of local aerosols together with GHGs will shape near‐future precipitation patterns over South Asia. Therefore, constraining precipitation response to different trajectories of both forcers is critical for informing near‐term adaptation efforts. Plain Language Summary: Agricultural production, water availability, and the economy in South Asia depend closely on reliable rainfall. While much of this depends on monsoon season rainfall, the pre‐monsoon season and the post‐monsoon season are also important for these sectors. Understanding how and why South Asian rainfall patterns in these seasons are likely to change is, therefore, relevant for adaptation, planning, and infrastructure resilience. Multiple external climate forcers and natural climate variability influence South Asian rainfall. We examine how increasing greenhouse gases (GHGs) and changing anthropogenic aerosol distributions—two key forcers ‐ shape seasonal rainfall patterns across South Asia using large‐ensemble climate simulations for a very high emissions pathway. Our findings show that anthropogenic aerosols, which have a predominantly weakening influence on rainfall, could suppress the enhancement of monsoon and post‐monsoon season rainfall projected in response to GHGs by several decades. Aerosols continue to be important influences on rainfall patterns in the region for at least the next few decades, after which the influence of GHGs will dominate. While aerosols from other regions have historically influenced regional rainfall, we find that local aerosols are primarily responsible for the projected changes in rainfall patterns. Key Points: Aerosols could suppress greenhouse gases‐forced intensification of rainfall in monsoon and post‐monsoon seasons for several yearsAerosols will dominate monsoon and post‐monsoon precipitation patterns until the mid‐21st century under a very high emissions trajectoryLocal aerosols show a stronger influence on monsoon precipitation patterns throughout the 21st century relative to remote aerosols [ABSTRACT FROM AUTHOR]

While the relationships between dry land surface conditions, heat, and aridity have been well‐established, few studies have addressed whether global warming will affect the ability of wet conditions to moderate high temperatures and atmospheric aridity. Using Coupled Model Intercomparison Project Phase 6 models, we demonstrate that absolute changes in the monthly maximum temperature distribution during Central North American summers strongly outweigh the historical cooling effect of high precipitation and soil moisture conditions. Although wet conditions nearly always prevent concurrent extreme temperatures in the baseline period, these conditions are 40%–48% and 96%–98% less effective at 1 and 2° of global warming, respectively. However, high precipitation and soil moisture partially retain the ability to constrain concurrent high vapor pressure deficit conditions below historical thresholds at 1–2° of warming. Our results highlight the growing vulnerability of Central North America to warmer temperatures and drier atmospheric conditions, even during periods of high precipitation and soil moisture. Plain Language Summary: In this study, we examine whether warming global temperatures will affect the ability of wet conditions, which we define with precipitation or soil moisture, to prevent the occurrence of concurrent high temperatures and dry atmospheric conditions. Focusing on the Central Plains region in North America, we show that even at 1° of global warming, high precipitation, and high soil moisture conditions already partially lose the ability to prevent high maximum temperatures in the summer months. In contrast, we show that high precipitation and high soil moisture conditions are able to better retain the ability to prevent dry atmospheric conditions even at 2° of global warming. Our results indicate that the Central Plains region will quickly become more exposed to warmer and drier climate conditions, even during periods with high precipitation and soil moisture, with potentially serious implications across public health, economic, and environmental sectors. Key Points: Absolute changes in the temperature distribution due to global warming outweigh cooling effects from wet conditions in the Central PlainsMeanwhile, wet conditions can still partially constrain extreme atmospheric aridity below historical levels under global warmingDifferences in heat and aridity responses are mainly dependent on underlying changes in the temperature and aridity distributions [ABSTRACT FROM AUTHOR]

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Future changes to the hydrological cycle are projected in a warming world, and any shifts in drought risk may prove extremely consequential for natural and human systems. In addition to long‐term moistening, drying, or warming trends, perturbations to the annual cycle of regional hydroclimate variables may also have substantial impacts. We analyze projected changes in several hydroclimate variables across the continental United States, along with shifts in the amplitude and phase of their annual cycles. We find that even in regions where no robust change in the annual mean is expected, coherent changes to the annual cycle are projected. In particular, we identify robust regional phase shifts toward earlier arrival of peak evaporation in the northern regions, and peak runoff and total soil moisture in the western regions. Changes in the amplitude of the annual cycle of total and surface soil moisture are also projected, and reflect changes to the annual cycle in surface water supply and demand. Whether changes become detectable above the background noise of internal variability depends strongly on the future scenario considered, and significant changes to the annual cycle are largely avoided in the lowest‐forcing scenario. Plain Language Summary: State‐of‐the‐art climate models project substantial changes to the climate of the continental United States (CONUS). Models project changes to the yearly average rainfall, soil moisture, and runoff in many regions. But crucially, they also project changes to the annual cycle, with peaks in evaporation, runoff, and soil moisture projected to shift earlier in the year even if the yearly averages of those quantities do not change. While different climate models disagree on the exact nature of many shifts, a large source of uncertainty is human behavior and policy choices, and many significant changes can be avoided if greenhouse gas emissions fall drastically in the future. Key Points: Coupled Model Intercomparison Project models project changes to the annual cycle of many hydroclimate variables, many of which are more significant than annual mean changesIn the continental United States, there are significant earlier shifts in the annual cycle in a high emissions scenarioSignificant changes to the annual cycle are largely avoided in the lowest‐emissions scenario [ABSTRACT FROM AUTHOR]

Biophysical vegetation responses to elevated atmospheric carbon dioxide (CO2) affect regional hydroclimate through two competing mechanisms. Higher CO2 increases leaf area (LAI), thereby increasing transpiration and water losses. Simultaneously, elevated CO2 reduces stomatal conductance and transpiration, thereby increasing rootzone soil moisture. Which mechanism dominates in the future is highly uncertain, partly because these two processes are difficult to explicitly separate within dynamic vegetation models. We address this challenge by using the GISS ModelE global climate model to conduct a novel set of idealized 2×CO2 sensitivity experiments to: evaluate the total vegetation biophysical contribution to regional climate change under high CO2; and quantify the separate contributions of enhanced LAI and reduced stomatal conductance to regional hydroclimate responses. We find that increased LAI exacerbates soil moisture deficits across the sub‐tropics and more water‐limited regions, but also attenuates warming by ∼0.5–1°C in the US Southwest, Central Asia, Southeast Asia, and northern South America. Reduced stomatal conductance effects contribute ∼1°C of summertime warming. For some regions, enhanced LAI and reduced stomatal conductance produce nonlinear and either competing or mutually amplifying hydroclimate responses. In northeastern Australia, these effects combine to exacerbate radiation‐forced warming and contribute to year‐round water limitation. Conversely, at higher latitudes these combined effects result in less warming than would otherwise be predicted due to nonlinear responses. These results highlight substantial regional variation in CO2‐driven vegetation responses and the importance of improving model representations of these processes to better quantify regional hydroclimate impacts. Key Points: We evaluate the separate and combined biophysical vegetation effects on hydroclimate in a high‐CO2 world using the GISS ModelE global climate modelIncreased leaf areas enhance soil moisture drying at lower latitudes; reduced stomatal conductance enhances high‐latitude warmingIncreased leaf area and reduced stomatal conductance also produce complex nonlinear and either competing or mutually amplifying regional hydroclimate responses [ABSTRACT FROM AUTHOR]

Rapid drought intensification, or flash droughts, is often driven by anomalous atmospheric ridging and can cause severe and complex impacts on water availability and agriculture, but the full range of variability of such events in terms of intensity and frequency is unknown. New tree‐ring reconstructions of May–July mid‐tropospheric ridging and soil moisture anomalies back to 1500 CE in the central United States—a hotspot for flash drought—suggest that over the last five centuries, anomalies in these two variables combined to indicate flash‐drought conditions in ∼17% of years and exceptionally severe flash drought in ∼4% of years, similar to frequencies in recent decades. However, over one‐third of all inferred exceptional flash droughts occurred since 1900, suggesting the 20th century was highly flash‐drought prone. These results may guide future work to diagnose the roles of external, oceanic, and land‐surface forcing of warm‐season atmospheric circulation and hydroclimate over North America. Plain Language Summary: In 2012, the central United States experienced a "flash drought," when rapid soil drying due to persistently low precipitation totals and high temperatures in late spring and summer caused billions of dollars in damages and agricultural losses. Such events are difficult to forecast because flash droughts can develop in a matter of weeks, and only a handful of flash droughts have been observed in recent decades, giving high uncertainty as to their likelihood. Here, we use tree rings to create two independent annual records of central United States soil moisture and the principal atmospheric circulation pattern known to cause flash droughts that extend back to the year 1500. Taken together, these records provide a new five‐century perspective on these crucial components of flash drought and reveal for the first time the long‐term behavior of central United States flash droughts, including frequency and cyclicity of exceptional events. We find that the instrumental record is a good representation of the long‐term likelihood of flash droughts. This apparent agreement with long‐term average conditions is largely by chance; however, as the reconstructions indicate large century‐to‐century variations in flash‐drought frequency and magnitude over the past 500 years. Key Points: We developed tree‐ring reconstructions of central U.S. soil moisture and the atmospheric ridge that causes flash droughts back to 1500 CEObservations capture flash drought's long‐term probability distribution, but reconstructions reveal centennial fluctuations in variabilityCycles in reconstructed atmospheric ridging may aid diagnosis and prediction of flash drought on interannual to decadal time scales [ABSTRACT FROM AUTHOR]

Summary: Temperate forests are shaped by late spring freezes after budburst – false springs – which may shift with climate change. Research to date has generated conflicting results, potentially because few studies focus on the multiple underlying drivers of false spring risk.Here, we assessed the effects of mean spring temperature, distance from the coast, elevation and the North Atlantic Oscillation (NAO) using PEP725 leafout data for six tree species across 11 648 sites in Europe, to determine which were the strongest predictors of false spring risk and how these predictors shifted with climate change.All predictors influenced false spring risk before recent warming, but their effects have shifted in both magnitude and direction with warming. These shifts have potentially magnified the variation in false spring risk among species with an increase in risk for early‐leafout species (i.e. Aesculus hippocastanum, Alnus glutinosa, Betula pendula) compared with a decline or no change in risk among late‐leafout species (i.e. Fagus sylvatica, Fraxinus excelsior, Quercus robur).Our results show how climate change has reshaped the drivers of false spring risk, complicating forecasts of future false springs, and potentially reshaping plant community dynamics given uneven shifts in risk across species. [ABSTRACT FROM AUTHOR]

Long‐range water planning is complicated by factors that are rapidly changing in the 21st century, including climate, population, and water use. Here, we analyze climate factors and drought projections for Texas as an example of a diverse society straddling an aridity gradient to examine how the projections can best serve water stakeholder needs. We find that climate models are robust in projecting drying of summer‐season soil moisture and decreasing reservoir supplies for both the eastern and western portions of Texas during the 21st century. Further, projections indicate drier conditions during the latter half of the 21st century than even the most arid centuries of the last 1,000 years that included megadroughts. To illustrate how accounting for drought nonstationarity may increase water resiliency, we consider generalized case studies involving four key stakeholder groups: agricultural producers, large surface water suppliers, small groundwater management districts, and regional water planning districts. We also examine an example of customized climate information being used as input to long‐range water planning. We find that while stakeholders value the quantitative capability of climate model outputs, more specific climate‐related information better supports resilience planning across multiple stakeholder groups. New suites of tools could provide necessary capacity for both short‐ and long‐term, stakeholder‐specific adaptive planning. Key Points: Water stakeholders should prepare for future droughts that will be unlike past droughtsInformation available from climate projections often does not align with the detailed information needed for water planningBetter awareness of the mismatch between available and needed information will help inform efforts to close this gap [ABSTRACT FROM AUTHOR]

There is strong evidence that the expansion and intensification of irrigation over the twentieth century has affected climate in many regions. However, it remains uncertain if these irrigation effects, including buffered warming trends, will weaken or persist under future climate change conditions. Using a 20‐member climate model ensemble simulation, we demonstrate that irrigation will continue to attenuate greenhouse gas‐forced warming and soil moisture drying in many regions over the 21st century, including Mexico, the Mediterranean, Southwest Asia, and China. Notably, this occurs without any further expansion or intensification of irrigation beyond current levels, even while greenhouse gas forcing steadily increases. However, the magnitude and significance of these moderating irrigation effects vary across regions and are highly sensitive to the background climate state and the degree to which evapotranspiration is supply (moisture) versus demand (energy) limited. Further, limitations on water and land availability may restrict our ability to maintain modern irrigation rates into the future. Nevertheless, it is likely that irrigation, alongside other components of intensive land management, will continue to strongly modulate regional climate impacts in the future. Irrigation should therefore be considered in conjunction with other key regional anthropogenic forcings (e.g., land cover change and aerosols) when investigating the local manifestation of global climate drivers (e.g., greenhouse gases) in model projections. Plain Language Summary: Alongside increasing global greenhouse gas concentrations, climate is also sensitive to regional climate forcings, including changes in land use and land management. During the twentieth century, the expansion and intensification of irrigation strongly diminished warming trends in many regions. Here, we demonstrate that maintaining modern irrigation rates into the future would continue to modulate regional warming and soil moisture drying during the 21st century, even as greenhouse gas forcing continues to increase. However, the magnitude of these irrigation effects is highly sensitive to regional climate differences, and critical limitations on water and land availability may limit our ability to continue irrigation rates at modern levels. Key Points: Using a climate model ensemble, we evaluate irrigation effects on projected climate change‐forced temperature and soil moisture trendsMaintaining current irrigation continues moderating projected 21st century warming and drying, even as greenhouse gas forcing increasesThe magnitude of irrigation effects varies strongly by region, however, and depends heavily on the background evaporative regime [ABSTRACT FROM AUTHOR]

Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atmospheric circulation anomalies in the Atlantic, but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial time scales. Here we analyze the coupled patterns of drought variability on either sides of the North Atlantic Ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin‐wide moisture anomalies to ocean and atmosphere processes. A leading mode of broad‐scale moisture variability is characterized by distinct patterns of North Atlantic atmosphere circulation and sea surface temperatures. We infer a negative phase of the North Atlantic Oscillation and colder Atlantic sea surface temperatures in the middle of the fifteenth century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age. Key Points: Atlantic ocean‐atmosphere circulation links drought in North America, Europe, and the MediterraneanCoupled modes of drought variability can be used to infer atmospheric circulation and ocean temperature anomaliesCoupled terrestrial drought patterns reveal the start of the Little Ice Age to be characterized by a negative NAO and cold Atlantic SSTs [ABSTRACT FROM AUTHOR]

Temperate plants are at risk of being exposed to late spring freezes. These freeze events—often called false springs—are one of the strongest factors determining temperate plants species range limits and can impose high ecological and economic damage. As climate change may alter the prevalence and severity of false springs, our ability to forecast such events has become more critical, and it has led to a growing body of research. Many false spring studies largely simplify the myriad complexities involved in assessing false spring risks and damage. While these studies have helped advance the field and may provide useful estimates at large scales, studies at the individual to community levels must integrate more complexity for accurate predictions of plant damage from late spring freezes. Here, we review current metrics of false spring, and how, when, and where plants are most at risk of freeze damage. We highlight how life stage, functional group, species differences in morphology and phenology, and regional climatic differences contribute to the damage potential of false springs. More studies aimed at understanding relationships among species tolerance and avoidance strategies, climatic regimes, and the environmental cues that underlie spring phenology would improve predictions at all biological levels. An integrated approach to assessing past and future spring freeze damage would provide novel insights into fundamental plant biology and offer more robust predictions as climate change progresses, which are essential for mitigating the adverse ecological and economic effects of false springs. [ABSTRACT FROM AUTHOR]

The contributions of oceanic and atmospheric variability to spatially widespread summer droughts in the contiguous United States (hereafter, pan‐CONUS droughts) are investigated using 16‐member ensembles of the Community Climate Model version 3 (CCM3) forced with observed sea surface temperatures (SSTs) from 1856–2012. The employed SST forcing fields are either (i) global or restricted to the (ii) tropical Pacific or (iii) tropical Atlantic to isolate the impacts of these two ocean regions on pan‐CONUS droughts. Model results show that SST forcing of pan‐CONUS droughts originates almost entirely from the tropical Pacific because of atmospheric highs from the northern Pacific to eastern North America established by La Niña conditions, with little contribution from the tropical Atlantic. Notably, in all three model configurations, internal atmospheric variability influences pan‐CONUS drought occurrence by as much or more than the ocean forcing and can alone cause pan‐CONUS droughts by establishing a dominant high centered over the U.S. montane west. Similar results are found for the Community Atmosphere Model version 5 (CAM5). Model results are compared to the observational record, which supports model‐inferred contributions to pan‐CONUS droughts from La Niñas and internal atmospheric variability. While there may be an additional association with warm Atlantic SSTs in the observational record, this association is ambiguous due to the limited number of observed pan‐CONUS droughts. The ambiguity thus opens the possibility that the observational results are limited by sampling over the twentieth century and not at odds with the suggested dominance of Pacific Ocean forcing in the model ensembles. Key Points: Models forced with observed sea surface temperatures replicate spatially widespread summer droughts over the contiguous United StatesVariability in the tropical Pacific explains almost all the ocean forcing of these droughts in the models; the Atlantic plays an ambiguous roleContributions to these droughts from internal atmospheric variability are roughly equal to or greater than the collective ocean forcing [ABSTRACT FROM AUTHOR]

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 because of short instrumental gauge records. To help address this challenge, we use Hierarchical Bayesian Regression with partial pooling to develop six centuries long (1394–2008 CE) streamflow reconstructions at three Upper Indus Basin gauges (Doyian, Gilgit, and Kachora), concurrently demonstrating that Hierarchical Bayesian Regression 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 and the more conventionally used principal components regression. The reconstructions produced by principal components regression and Bayesian regression 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 that 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 (May–September) 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. Key Points: Tree ring reconstructions of streamflow in the Upper Indus Basin show wetter conditions in the 1990s compared to the last 600 yearsReconstructions are insensitive to the choice of statistical method used (principal components versus Bayesian regression)Streamflow is most sensitive to winter precipitation and summer temperature, but anomalies in these seasons cannot explain recent high flow [ABSTRACT FROM AUTHOR]

Abstract: Present and future freshwater availability and drought risks are physically tied to the responses of surface vegetation to increasing CO2. A single‐model large ensemble identifies the occurrence of colocated warming‐ and CO2‐induced leaf area index increases with summer soil moisture declines. This pattern of “greening” and “drying,” which occurs over 42% of global vegetated land area, is largely attributable to changes in the partitioning of precipitation at the land surface away from runoff and toward terrestrial vegetation ecosystems. Changes in runoff and ecosystem partitioning are inversely related, with changes in runoff partitioning being governed by changes in precipitation (mean and extremes) and ecosystem partitioning being governed by ecosystem water use and surface resistance to evapotranspiration (ET). Projections show that warming‐influenced and CO2‐enriched terrestrial vegetation ecosystems use water that historically would have been partitioned to runoff over 48% of global vegetated land areas, largely in Western North America, the Amazon, and Europe, many of the same regions with colocated greening and drying. These results have implications for how water available for people will change in response to anthropogenic warming and raise important questions about model representations of vegetation water responses to high CO2. [ABSTRACT FROM AUTHOR]

The fall 2016 drought in the southeastern United States (SE U.S.) appeared exceptional based on its widespread impacts, but the current monitoring framework that only extends from 1979 to present does not readily facilitate evaluation of soil-moisture anomalies in a centennial context. A new method to extend monthly gridded soil-moisture estimates back to 1895 is developed, indicating that since 1895, October-November 2016 soil moisture (0-200 cm) in the SE U.S. was likely the second lowest on record, behind 1954. This severe drought developed rapidly and was brought on by low September-November precipitation and record-high September-November daily maximum temperatures (Tmax). Record-high Tmax drove record-high atmospheric moisture demand, accounting for 28% of the October-November 2016 soil-moisture anomaly. Drought and heat in fall 2016 contrasted with 20th century wetting and cooling in the region but resembled conditions more common from 1895-1956. Dynamically, the exceptional drying in fall 2016 was driven by anomalous ridging over the central United States that reduced south-southwesterly moisture transports into the SE U.S. by approximately 75%. These circulation anomalies were partly promoted by a moderate La Niña and warmth in the tropical Atlantic, but these processes accounted for very little of the SE U.S. drying in fall 2016, implying a large role for internal atmospheric variability. The extended analysis back to 1895 indicates that SE U.S. droughts as strong as the 2016 event are more likely than indicated from a shorter 60 year perspective and continued multidecadal swings in precipitation may combine with future warming to further enhance the likelihood of such events. [ABSTRACT FROM AUTHOR]

During the Medieval Climate Anomaly ( MCA), Western North America experienced episodes of intense aridity that persisted for multiple decades or longer. These megadroughts are well documented in many proxy records, but the causal mechanisms are poorly understood. General circulation models ( GCMs) simulate megadroughts, but do not reproduce the temporal clustering of events during the MCA, suggesting they are not caused by the time history of volcanic or solar forcing. Instead, GCMs generate megadroughts through (1) internal atmospheric variability, (2) sea-surface temperatures, and (3) land surface and dust aerosol feedbacks. While no hypothesis has been definitively rejected, and no GCM has accurately reproduced all features (e.g., timing, duration, and extent) of any specific megadrought, their persistence suggests a role for processes that impart memory to the climate system (land surface and ocean dynamics). Over the 21st century, GCMs project an increase in the risk of megadrought occurrence through greenhouse gas forced reductions in precipitation and increases in evaporative demand. This drying is robust across models and multiple drought indicators, but major uncertainties still need to be resolved. These include the potential moderation of vegetation evaporative losses at higher atmospheric [ CO2], variations in land surface model complexity, and decadal to multidecadal modes of natural climate variability that could delay or advance onset of aridification over the the next several decades. Because future droughts will arise from both natural variability and greenhouse gas forced trends in hydroclimate, improving our understanding of the natural drivers of persistent multidecadal megadroughts should be a major research priority. WIREs Clim Change 2016, 7:411-432. doi: 10.1002/wcc.394 For further resources related to this article, please visit the . [ABSTRACT FROM AUTHOR]

Phenology is an integrative science that comprises the study of recurring biological activities or events. In an era of rapidly changing climate, the relationship between the timing of those events and environmental cues such as temperature, snowmelt, water availability, or day length are of particular interest. This article provides an overview of the observer-based plant phenology sampling conducted by the U.S. National Ecological Observatory Network ( NEON), the resulting data, and the rationale behind the design. Trained technicians will conduct regular in situ observations of plant phenology at all terrestrial NEON sites for the 30-yr life of the observatory. Standardized and coordinated data across the network of sites can be used to quantify the direction and magnitude of the relationships between phenology and environmental forcings, as well as the degree to which these relationships vary among sites, among species, among phenophases, and through time. Vegetation at NEON sites will also be monitored with tower-based cameras, satellite remote sensing, and annual high-resolution airborne remote sensing. Ground-based measurements can be used to calibrate and improve satellite-derived phenometrics. NEON's phenology monitoring design is complementary to existing phenology research efforts and citizen science initiatives throughout the world and will produce interoperable data. By collocating plant phenology observations with a suite of additional meteorological, biophysical, and ecological measurements (e.g., climate, carbon flux, plant productivity, population dynamics of consumers) at 47 terrestrial sites, the NEON design will enable continental-scale inference about the status, trends, causes, and ecological consequences of phenological change. [ABSTRACT FROM AUTHOR]

In humid, broadleaf-dominated forests where gap dynamics and partial canopy mortality appears to dominate the disturbance regime at local scales, paleoecological evidence shows alteration at regional-scales associated with climatic change. Yet, little evidence of these broad-scale events exists in extant forests. To evaluate the potential for the occurrence of large-scale disturbance, we used 76 tree-ring collections spanning ∼840 000 km2 and 5327 tree recruitment dates spanning ∼1.4 million km2 across the humid eastern United States. Rotated principal component analysis indicated a common growth pattern of a simultaneous reduction in competition in 22 populations across 61 000 km2. Growth-release analysis of these populations reveals an intense and coherent canopy disturbance from 1775 to 1780, peaking in 1776. The resulting time series of canopy disturbance is so poorly described by a Gaussian distribution that it can be described as "heavy tailed," with most of the years from 1775 to 1780 comprising the heavy-tail portion of the distribution. Historical documents provide no evidence that hurricanes or ice storms triggered the 1775-1780 event. Instead, we identify a significant relationship between prior drought and years with elevated rates of disturbance with an intense drought occurring from 1772 to 1775. We further find that years with high rates of canopy disturbance have a propensity to create larger canopy gaps indicating repeated opportunities for rapid change in species composition beyond the landscape scale. Evidence of elevated, regional-scale disturbance reveals how rare events can potentially alter system trajectory: a substantial portion of old-growth forests examined here originated or were substantially altered more than two centuries ago following events lasting just a few years. Our recruitment data, comprised of at least 21 species and several shade-intolerant species, document a pulse of tree recruitment at the subcontinental scale during the late-1600s suggesting that this event was severe enough to open large canopy gaps. These disturbances and their climatic drivers support the hypothesis that punctuated, episodic, climatic events impart a legacy in broadleaf-dominated forests centuries after their occurrence. Given projections of future drought, these results also reveal the potential for abrupt, meso- to largescale forest change in broadleaf-dominated forests over future decades. [ABSTRACT FROM AUTHOR]

Climate change has brought renewed interest in the study of plant phenology - the timing of life history events. Data on shifting phenologies with warming have accumulated rapidly, yet research has been comparatively slow to explain the diversity of phenological responses observed across latitudes, growing seasons and species. Here, we outline recent efforts to synthesize perspectives on plant phenology across the fields of ecology, climate science and evolution. We highlight three major axes that vary among these disciplines: relative focus on abiotic versus biotic drivers of phenology, on plastic versus genetic drivers of intraspecific variation, and on cross-species versus autecological approaches. Recent interdisciplinary efforts, building on data covering diverse species and climate space, have found a greater role of temperature in controlling phenology at higher latitudes and for early-flowering species in temperate systems. These efforts have also made progress in understanding the tremendous diversity of responses across species by incorporating evolutionary relatedness, and linking phenological flexibility to invasions and plant performance. Future research with a focus on data collection in areas outside the temperate mid-latitudes and across species' ranges, alongside better integration of how risk and investment shape plant phenology, offers promise for further progress. [ABSTRACT FROM AUTHOR]

Phenological events - defined points in the life cycle of a plant or animal - have been regarded as highly plastic traits, reflecting flexible responses to various environmental cues., The ability of a species to track, via shifts in phenological events, the abiotic environment through time might dictate its vulnerability to future climate change. Understanding the predictors and drivers of phenological change is therefore critical., Here, we evaluated evidence for phylogenetic conservatism - the tendency for closely related species to share similar ecological and biological attributes - in phenological traits across flowering plants. We aggregated published and unpublished data on timing of first flower and first leaf, encompassing ˜4000 species at 23 sites across the Northern Hemisphere. We reconstructed the phylogeny for the set of included species, first, using the software program Phylomatic, and second, from DNA data. We then quantified phylogenetic conservatism in plant phenology within and across sites., We show that more closely related species tend to flower and leaf at similar times. By contrasting mean flowering times within and across sites, however, we illustrate that it is not the time of year that is conserved, but rather the phenological responses to a common set of abiotic cues., Our findings suggest that species cannot be treated as statistically independent when modelling phenological responses., Synthesis. Closely related species tend to resemble each other in the timing of their life-history events, a likely product of evolutionarily conserved responses to environmental cues. The search for the underlying drivers of phenology must therefore account for species' shared evolutionary histories. [ABSTRACT FROM AUTHOR]

Premise of the study: The study of how phenology may contribute to the assembly of plant communities has a long history in ecology. Climate change has brought renewed interest in this area, with many studies examining how phenology may contribute to the success of exotic species. In particular, there is increasing evidence that exotic species occupy unique phenological niches and track climate change more closely than native species. Methods: Here, we use long-term records of species' first flowering dates from five northern hemisphere temperate sites (Chinnor, UK and in the United States, Concord, Massachusetts; Fargo, North Dakota; Konza Prairie, Kansas; and Washington, D.C.) to examine whether invaders have distinct phenologies. Using a broad phylogenetic framework, we tested for differences between exotic and native species in mean annual flowering time, phenological changes in response to temperature and precipitation, and longer-term shifts in first flowering dates during recent pronounced climate change ("flowering time shifts"). Key results: Across North American sites, exotic species have shifted flowering with climate change while native species, on average, have not. In the three mesic systems, exotic species exhibited higher tracking of interannual variation in temperature, such that flowering advances more with warming, than native species. Across the two grassland systems, however, exotic species differed from native species primarily in responses to precipitation and soil moisture, not temperature. Conclusions: Our findings Further, they support recent evidence that exotic species may be important drivers of extended growing seasons observed with climate change in North America. [ABSTRACT FROM AUTHOR]

Premise of the study: Numerous long-term studies in seasonal habitats have tracked interannual variation in first flowering date (FFD) in relation to climate, documenting the effect of warming on the FFD of many species. Despite these efforts, long-term phenological observations are still lacking for many species. If we could forecast responses based on taxonomic affinity, however, then we could leverage existing data to predict the climate-related phenological shifts of many taxa not yet studied. Methods: We examined phenological time series of 1226 species occurrences (1031 unique species in 119 families) across seven sites in North America and England to determine whether family membership (or family mean FFD) predicts the sensitivity of FFD to standardized interannual changes in temperature and precipitation during seasonal periods before flowering and whether families differ significantly in the direction of their phenological shifts. Key results: Patterns observed among species within and across sites are mirrored among family means across sites; early-flowering families advance their FFD in response to warming more than late-flowering families. By contrast, we found no consistent relationships among taxa between mean FFD and sensitivity to precipitation as measured here. Conclusions: Family membership can be used to identify taxa of high and low sensitivity to temperature within the seasonal, temperate zone plant communities analyzed here. The high sensitivity of early-flowering families (and the absence of early-flowering families not sensitive to temperature) may reflect plasticity in flowering time, which may be adaptive in environments where early-season conditions are highly variable among years. [ABSTRACT FROM AUTHOR]

Detrended, modelled first leaf dates for 856 sites across North America for the period 1900-2008 are used to examine how the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) separately and together might influence the timing of spring. Although spring (mean March through April) ENSO and PDO signals are apparent in first leaf dates, the signals are not statistically significant (at a 95% confidence level ( p < 0.05)) for most sites. The most significant ENSO/PDO signal in first leaf dates occurs for El Niño and positive PDO conditions. An analysis of the spatial distributions of first leaf dates for separate and combined ENSO/PDO conditions features a northwest-southeast dipole that is significantly (at p < 0.05) different than the distributions for neutral conditions. The nature of the teleconnection between Pacific SST's and first leaf dates is evident in comparable composites for detrended sea level pressure (SLP) in the spring months. During positive ENSO/PDO, there is an anomalous flow of warm air from the southwestern US into the northwestern US and an anomalous northeasterly flow of cold air from polar regions into the eastern and southeastern US. These flow patterns are reversed during negative ENSO/PDO. Although the magnitudes of first leaf date departures are not necessarily significantly related to ENSO and PDO, the spatial patterns of departures are significantly related to ENSO and PDO. These significant relations and the long-lived persistence of SSTs provide a potential tool for forecasting the tendencies for first leaf dates to be early or late. Copyright © 2011 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Forecasting how species and ecosystems will respond to climate change has been a major aim of ecology in recent years. Much of this research has focused on phenology - the timing of life-history events. Phenology has well-demonstrated links to climate, from genetic to landscape scales; yet our ability to explain and predict variation in phenology across species, habitats and time remains poor. Here, we outline how merging approaches from ecology, climate science and evolutionary biology can advance research on phenological responses to climate variability. Using insight into seasonal and interannual climate variability combined with niche theory and community phylogenetics, we develop a predictive approach for species' reponses to changing climate. Our approach predicts that species occupying higher latitudes or the early growing season should be most sensitive to climate and have the most phylogenetically conserved phenologies. We further predict that temperate species will respond to climate change by shifting in time, while tropical species will respond by shifting space, or by evolving. Although we focus here on plant phenology, our approach is broadly applicable to ecological research of plant responses to climate variability. [ABSTRACT FROM AUTHOR]

The article presents a detailed analysis of a rare cross-taxa native species phenology dataset which include plant flowering, insect first sighting, and amphibian first sighting from Mohonk Lake, New York. In each of the phenology series, the flowering and first sighting Julian calendar dates were examined for the existence of temporal trends. It found out that only one of the five animal species demonstrated evidence for an essential trend in first sighting. Moreover, the growing degree-day (GDD) correlations show significant climate sensitivity in all species. However, different magnitudes and timing of maximum correlation mean that plant and animal reactions to climate changes in the future will not be homogeneous for the tested species at the Mohonk Lake.

Dust aerosols can suppress rainfall by increasing the number of cloud condensation nuclei in warm clouds and affecting the surface radiation budget and boundary layer instability. The extent to which atmospheric dust may affect precipitation yields and the hydrologic cycle in semiarid regions remains poorly understood. We investigate the relationship between dust aerosols and rainfall in the West African Sahel where the dust-rainfall feedback has been speculated to contribute to sustained droughts. We find that the amount of dust loadings is negatively correlated with rainfall values, suggesting that dust entrained in the atmosphere can significantly inhibit rainfall in this region. [ABSTRACT FROM AUTHOR]

We present an integrated modeling study designed to investigate changes in ecosystem level phenology over Europe associated with changes in climate pattern, by the North Atlantic Oscillation (NAO). We derived onset dates from processed NDVI data sets and used growing degree day (GDD) summations from the NCEP re-analysis to calibrate and validate a phenology model to predict the onset of the growing season over Europe. In a cross-validation hindcast, the model (PHENOM) is able to explain 63% of the variance in onset date for grid cells containing at least 50% mixed and boreal forest. Using a model developed from previous work we performed climate change scenarios, generating synthetic temperature and GDD distributions under a hypothetically increasing NAO. These new distributions were used to drive PHENOM and project changes in the timing of onset for forested cells over Europe. Results from the climate change scenarios indicate that, if the current trend in the NAO continues, there is the potential for a continued advance to the start of the growing season by as much as 13 days in some areas. [ABSTRACT FROM AUTHOR]

The Fertile Crescent, its hilly flanks and surrounding drylands has been a critical region for studying how climate has influenced societal change, and this review focuses on the region over the last 20,000 years. The complex social, economic, and environmental landscapes in the region today are not new phenomena and understanding their interactions requires a nuanced, multidisciplinary understanding of the past. This review builds on a history of collaboration between the social and natural palaeoscience disciplines. We provide a multidisciplinary, multiscalar perspective on the relevance of past climate, environmental, and archaeological research in assessing present day vulnerabilities and risks for the populations of southwest Asia. We discuss the complexity of palaeoclimatic data interpretation, particularly in relation to hydrology, and provide an overview of key time periods of palaeoclimatic interest. We discuss the critical role that vegetation plays in the human–climate–environment nexus and discuss the implications of the available palaeoclimate and archaeological data, and their interpretation, for palaeonarratives of the region, both climatically and socially. We also provide an overview of how modelling can improve our understanding of past climate impacts and associated change in risk to societies. We conclude by looking to future work, and identify themes of "scale" and "seasonality" as still requiring further focus. We suggest that by appreciating a given locale's place in the regional hydroscape, be it an archaeological site or palaeoenvironmental archive, more robust links to climate can be made where appropriate and interpretations drawn will demand the resolution of factors acting across multiple scales. This article is categorized under:Human Water > Water as Imagined and RepresentedScience of Water > MethodsWater and Life > Nature of Freshwater Ecosystems We discuss the impact of a changing regional hydroscape (pictured) on the proxy archives and people of southwest Asia over the last 20,000 years. [ABSTRACT FROM AUTHOR]

Anthropogenic land use and land cover change is primarily represented in climate model simulations through prescribed transitions from natural vegetation to cropland or pasture. However, recent studies have demonstrated that land management practices, especially irrigation, have distinct climate impacts. Here we disentangle the seasonal climate impacts of land cover change and irrigation across areas of high agricultural intensity using climate simulations with three different land surface scenarios: (1) natural vegetation cover/no irrigation, (2) year 2000 crop cover/no irrigation, and (3) year 2000 crop cover and irrigation rates. We find that irrigation substantially amplifies land cover‐induced climate impacts but has opposing effects across certain regions. Irrigation mostly causes surface cooling, which substantially amplifies land cover change‐induced cooling in most regions except over Central, West, and South Asia, where it reverses land cover change‐induced warming. Despite increases in net surface radiation in some regions, this cooling is associated with enhancement of latent relative to sensible heat fluxes by irrigation. Similarly, irrigation substantially enhances the wetting influence of land cover change over several regions including West Asia and the Mediterranean. The most notable contrasting impacts of these forcings on precipitation occur over South Asia, where irrigation offsets the wetting influence of land cover during the monsoon season. Differential changes in regional circulations and moist static energy induced by these forcings contribute to their precipitation impacts and are associated with differential changes in surface and tropospheric temperature gradients and moisture availability. These results emphasize the importance of including irrigation forcing to evaluate the combined impacts of land surface changes for attributing historical climatic changes and managing future impacts. Plain Language Summary: Several regions have experienced substantial agricultural expansion and intensification to meet the needs of our growing population. While the effects of land cover change associated with agriculture have been extensively studied and included as a standard forcing in simulations of historical and future climate, the influence of a common form of agricultural intensification—irrigation—is not fully understood. Despite mounting evidence of its importance on regional climate, irrigation is still not considered a standard climate forcing. To isolate the influence of irrigation from land cover changes, we conduct a suite of simulations with a state‐of‐the‐art global climate model. Our analysis of nine regions with extensive agriculture and heavy irrigation demonstrates that irrigation has comparable climatic impacts to land cover changes. Across most regions, irrigation amplifies land cover forced changes. However, over parts of Asia, where irrigation rates are highest, irrigation contrasts land cover forced changes and the combined climate response to land cover and irrigation are opposite to what would be expected with land cover changes alone. Our results highlight the importance of including land management decisions in climate simulations for a more accurate understanding of how human activities shape climate, particularly over these regions, and have implications for management of the effects of future climate change. Key Points: Climate impacts of irrigation and land cover change are comparable in magnitude but opposite in sign over some regions, especially AsiaContrasting or amplifying effects of irrigation relative to land cover change extend beyond the growing season and can vary by seasonLocal and remote impacts are linked to changes in moisture flux, surface and upper‐level temperature gradients, and regional circulations [ABSTRACT FROM AUTHOR]

The late 16th‐century North American megadrought was notable for its persistence, extent, intensity, and occurrence after the main interval of megadrought activity during the Medieval Climate Anomaly. Forcing from sea surface temperatures (SSTs) in the tropical Pacific is considered a possible driver of megadroughts, and we investigate this hypothesis for the late 16th‐century event using two new 600‐year long hydroclimate field reconstructions from Mexico and Australia. Areas represented by these reconstructions have strong teleconnections to tropical Pacific SSTs, evidenced by the leading principal component in each region explaining ∼40% of local hydroclimate variability and correlating significantly with the boreal winter (December‐January‐February) NINO 3.4 index. Using these two principal components as predictors, we develop a skillful reconstruction of the December‐January‐February NINO 3.4 index. The reconstruction reveals that the late 16th‐century megadrought likely occurred during one of the most persistent and intense periods of cold tropical Pacific SST anomalies of the last 600 years (1566–1590 C.E.; median NINO 3.4 = −0.79 K). This anomalously cold period coincided with a major filling episode for Kati Thanda‐Lake Eyre in Australia, a hydroclimate response dynamically consistent with the reconstructed SST state. These results offer new evidence that tropical Pacific forcing was an important driver of the late 16th‐century North American megadrought over the Southwest and Mexico, highlighting the large amplitude of natural variability that can occur within the climate system. Key Points: One of the most severe megadroughts in North America occurred in the late 16th‐centuryWe show this event occurred during a 25‐year cold period in the tropical Pacific OceanThis provides evidence linking ocean forcing and megadroughts outside the Medieval Climate Anomaly [ABSTRACT FROM AUTHOR]

The Caribbean islands are expected to see more frequent and severe droughts from reduced precipitation and increased evaporative demand due to anthropogenic climate change. Between 2013 and 2016, the Caribbean experienced a widespread drought due in part to El Niño in 2015–2016, but it is unknown whether its severity was exacerbated by anthropogenic warming. This work examines the role of recent warming on this drought, using a recently developed high‐resolution self‐calibrating Palmer Drought Severity Index data set. The resulting analysis suggest that anthropogenic warming accounted for ~15–17% of the drought's severity and ~7% of its spatial extent. These findings strongly suggest that climate model projected anthropogenic drying in the Caribbean is already underway, with major implications for the more than 43 million people currently living in this region. Plain Language Summary: Climate models project significant drying for the Caribbean as a consequence of increased anthropogenic greenhouse‐gas concentrations. Between 2013 and 2016, virtually, the entire region experienced a Pan‐Caribbean drought, which was unprecedented since at least 1950. We find that human‐caused warming contributed to ~15–17% of drought severity by increasing evapotranspiration rates and accounted for ~7% of land area under drought across the Caribbean. Our results therefore suggest that anthropogenic warming has already increased drought risk in the Caribbean. Key Points: In 2013–2016, the Caribbean experienced its worst drought since 1950, which we named the Pan‐Caribbean droughtEnhanced evaporative demand due to anthropogenic warming contributed to 15–17% of Pan‐Caribbean drought severityAnthropogenic climate change has likely already enhanced drought risk in the Caribbean [ABSTRACT FROM AUTHOR]