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The term 'Dust Bowl' was coined in 1935 by Robert E. Geiger after Black Sunday, a massive dust storm that blew across the Oklahoma Panhandle on April 14, 1935. Geiger, [...]

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 CO2trends 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. 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. 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 CO2trends and plant physiological effectsDrought events could limit long‐term terrestrial carbon storage across the West African Great Green Wall domain Ecosystem models show that the 20th century Sahelian drought depressed carbon stocks across the West African Great Green Wall domain Carbon fluxes did exhibit some recovery strongly driven atmospheric CO2trends and plant physiological effects Drought events could limit long‐term terrestrial carbon storage across the West African Great Green Wall domain

There is evidence that expected warming trends from increased greenhouse gases have been locally 'masked' by irrigation-induced cooling. Two scientists at the NASA Goddard Institute for Space Studies, Michael Puma [...]

A previous reconstruction back to 800 ceindicated that the 2000–2018 soil moisture deficit in southwestern North America was exceeded during one megadrought in the late-1500s. Here, we show that after exceptional drought severity in 2021, ~19% of which is attributable to anthropogenic climate trends, 2000–2021 was the driest 22-yr period since at least 800. This drought will very likely persist through 2022, matching the duration of the late-1500s megadrought.

Global warming alters surface water availability (precipitation minus evapotranspiration, P–E) and hence freshwater resources. However, the influence of land–atmosphere feedbacks on future P–E changes and the underlying mechanisms remain unclear. Here we demonstrate that soil moisture (SM) strongly impacts future P–E changes, especially in drylands, by regulating evapotranspiration and atmospheric moisture inflow. Using modelling and empirical approaches, we find a consistent negative SM feedback on P–E, which may offset ~60% of the decline in dryland P–E otherwise expected in the absence of SM feedbacks. The negative feedback is not caused by atmospheric thermodynamic responses to declining SM; rather, reduced SM, in addition to limiting evapotranspiration, regulates atmospheric circulation and vertical ascent to enhance moisture transport into drylands. This SM effect is a large source of uncertainty in projected dryland P–E changes, underscoring the need to better constrain future SM changes and improve the representation of SM–atmosphere processes in models.

Plants are expected to generate more global-scale runoff under increasing atmospheric carbon dioxide concentrations through their influence on surface resistance to evapotranspiration. Recent studies using Earth System Models from phase 5 of the Coupled Model Intercomparison Project ostensibly reaffirm this result, further suggesting that plants will ameliorate the dire reductions in water availability projected by other studies that use aridity metrics. Here we complicate this narrative by analysing the change in precipitation partitioning to plants, runoff and storage in multiple Earth system models under both high carbon dioxide concentrations and warming. We show that projected plant responses directly reduce future runoff across vast swaths of North America, Europe and Asia because bulk canopy water demands increase with additional vegetation growth and longer and warmer growing seasons. These runoff declines occur despite increased surface resistance to evapotranspiration and vegetation total water use efficiency, even in regions with increasing or unchanging precipitation. We demonstrate that constraining the large uncertainty in the multimodel ensemble with regional-scale observations of evapotranspiration partitioning strengthens these results. We conclude that terrestrial vegetation plays a large and unresolved role in shaping future regional freshwater availability, one that will not ubiquitously ameliorate future warming-driven surface drying.

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. 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

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. 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

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. 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. 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 Aerosols could suppress greenhouse gases‐forced intensification of rainfall in monsoon and post‐monsoon seasons for several years Aerosols will dominate monsoon and post‐monsoon precipitation patterns until the mid‐21st century under a very high emissions trajectory Local aerosols show a stronger influence on monsoon precipitation patterns throughout the 21st century relative to remote aerosols

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. 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

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. 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

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. Using a large ensemble of simulations from a state‐of‐the‐art Earth System Model, we show that 42% of global vegetated land areas are projected to have “greening” in the form of additional vegetation growth at the same time as “drying” in the form of reduced soil moisture in a business‐as‐usual world. Simultaneous greening and drying is curious and suggests that future ecosystems—which could demand more water due to warmer and longer growing seasons and CO2fertilization—siphon water that historically would have become the runoff that fills rivers and streams, termed “blue water.” We show that warming and changes in plant growth from CO2creates an explicit water trade‐off in which future vegetation directly diminishes runoff relatively or absolutely for nearly half of global land areas. Our results have important implications for future water availability, but also point to the crucial importance of resolving model uncertainties associated with terrestrial vegetation and its response to increasing CO2. Vegetation “greening” and soil “drying” cooccurs over 42% of global vegetated land in a large GCM ensembleThe greening and drying pattern is driven by changes in precipitation partitioning at the land surfaceThese results highlight an intrinsic future trade‐off between terrestrial vegetation (green water) versus total runoff (blue water)

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. 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. 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 Absolute changes in the temperature distribution due to global warming outweigh cooling effects from wet conditions in the Central Plains Meanwhile, wet conditions can still partially constrain extreme atmospheric aridity below historical levels under global warming Differences in heat and aridity responses are mainly dependent on underlying changes in the temperature and aridity distributions

Across the world, wine grape phenology has advanced in recent decades, in step with climate-change-induced trends in temperature—the main driver of fruit maturation—and drought. Fully understanding how climate change contributes to changes in harvest dates, however, requires analysing wine grape phenology and its relationship to climate over a longer-term context, including data predating anthropogenic interference in the climate system. Here, we investigate the climatic controls of wine grape harvest dates from 1600–2007 in France and Switzerland using historical harvest and climate data. Early harvests occur with warmer temperatures (−6 days °C−1) and are delayed by wet conditions (+0.07 days mm−1; +1.68 days PDSI−1) during spring and summer. In recent decades (1981–2007), however, the relationship between harvest timing and drought has broken down. Historically, high summer temperatures in Western Europe, which would hasten fruit maturation, required drought conditions to generate extreme heat. The relationship between drought and temperature in this region, however, has weakened in recent decades and enhanced warming from anthropogenic greenhouse gases can generate the high temperatures needed for early harvests without drought. Our results suggest that climate change has fundamentally altered the climatic drivers of early wine grape harvests in France, with possible ramifications for viticulture management and wine quality.

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, GCMsgenerate 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 GCMhas 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, GCMsproject 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 Change2016, 7:411–432. doi: 10.1002/wcc.394 For further resources related to this article, please visit the WIREs website.

Recent Mediterranean droughts have highlighted concerns that climate change may be contributing to observed drying trends, but natural climate variability in the region is still poorly understood. We analyze 900 years (1100–2012) of Mediterranean drought variability in the Old World Drought Atlas (OWDA), a spatiotemporal tree ring reconstruction of the June‐July‐August self‐calibrating Palmer Drought Severity Index. In the Mediterranean, the OWDA is highly correlated with spring precipitation (April–June), the North Atlantic Oscillation (January–April), the Scandinavian Pattern (January–March), and the East Atlantic Pattern (April–June). Drought variability displays significant east‐west coherence across the basin on multidecadal to centennial timescales and north‐south antiphasing in the eastern Mediterranean, with a tendency for wet anomalies in the Black Sea region (e.g., Greece, Anatolia, and the Balkans) when coastal Libya, the southern Levant, and the Middle East are dry, possibly related to the North Atlantic Oscillation. Recent droughts are centered in the western Mediterranean, Greece, and the Levant. Events of similar magnitude in the western Mediterranean and Greece occur in the OWDA, but the recent 15 year drought in the Levant (1998–2012) is the driest in the record. Estimating uncertainties using a resampling approach, we conclude that there is an 89% likelihood that this drought is drier than any comparable period of the last 900 years and a 98% likelihood that it is drier than the last 500 years. These results confirm the exceptional nature of this drought relative to natural variability in recent centuries, consistent with studies that have found evidence for anthropogenically forced drying in the region. There is large multidecadal drought variability across the Mediterranean over the last 900 yearsDroughts tend to be zonally symmetric, but there is strong north‐south antiphasing in eastern basinThere is an 89%/98% likelihood that the recent Levant drought is the worst of the last 900/500 years

A suite of climate data sets and multiple representations of atmospheric moisture demand are used to calculate many estimates of the self‐calibrated Palmer Drought Severity Index, a proxy for near‐surface soil moisture, across California from 1901 to 2014 at high spatial resolution. Based on the ensemble of calculations, California drought conditions were record breaking in 2014, but probably not record breaking in 2012–2014, contrary to prior findings. Regionally, the 2012–2014 drought was record breaking in the agriculturally important southern Central Valley and highly populated coastal areas. Contributions of individual climate variables to recent drought are also examined, including the temperature component associated with anthropogenic warming. Precipitation is the primary driver of drought variability but anthropogenic warming is estimated to have accounted for 8–27% of the observed drought anomaly in 2012–2014 and 5–18% in 2014. Although natural variability dominates, anthropogenic warming has substantially increased the overall likelihood of extreme California droughts. Warming since 1901 caused a significant trend toward drought in CaliforniaRecent drought was naturally driven and modestly intensified by warmingWarming has rapidly amplified the probability of severe drought

Subtropical marine stratus clouds regulate coastal and global climate, but future trends in these clouds are uncertain. In coastal Southern California (CSCA), interannual variations in summer stratus cloud occurrence are spatially coherent across 24 airfields and dictated by positive relationships with stability above the marine boundary layer (MBL) and MBL height. Trends, however, have been spatially variable since records began in the mid‐1900s due to differences in nighttime warming. Among CSCA airfields, differences in nighttime warming, but not daytime warming, are strongly and positively related to fraction of nearby urban cover, consistent with an urban heat island effect. Nighttime warming raises the near‐surface dew point depression, which lifts the altitude of condensation and cloud base height, thereby reducing fog frequency. Continued urban warming, rising cloud base heights, and associated effects on energy and water balance would profoundly impact ecological and human systems in highly populated and ecologically diverse CSCA. Low clouds regulate temperature and drought in coastal Southern CaliforniaUrban warming has caused substantially increased cloud base height since 1948Feedbacks between warming and decreased summer cloud frequency are expected

During the summer of 1934, over 70% of western North America experienced extreme drought, placing this summer far outside the normal range of drought variability and making 1934 the single worst drought year of the last millennium. Strong atmospheric ridging along the West Coast suppressed cold season precipitation across the Northwest, Southwest, and California, a circulation pattern similar to the winters of 1976–1977 and 2013–2014. In the spring and summer, the drying spread to the Midwest and Central Plains, driven by severe precipitation deficits downwind from regions of major dust storm activity, consistent with previous work linking drying during the Dust Bowl to anthropogenic dust aerosol forcing. Despite a moderate La Niña, contributions from sea surface temperature forcing were small, suggesting that the anomalous 1934 drought was primarily a consequence of atmospheric variability, possibly amplified by dust forcing that intensified and spread the drought across nearly all of western North America. The 1934 drought was the most widespread and severe event of the last millenniumDespite a strong La Niña, SST forcing played only a small roleAtmospheric variability caused winter drying; dust intensified drying in spring

The temporal stationarity of the teleconnection between the tropical Pacific Ocean and North America (NA) is analyzed in atmosphere‐only, and coupled last‐millennium, historical, and control runs from the Coupled Model Intercomparison Project Phase 5 data archive. The teleconnection, defined as the correlation between December‐January‐February (DJF) tropical Pacific sea surface temperatures (SSTs) and DJF 200 mb geopotential height, is found to be nonstationary on multidecadal timescales. There are significant changes in the spatial features of the teleconnection over NA in continuous 56‐year segments of the last millennium and control simulations. Analysis of atmosphere‐only simulations forced with observed SSTs indicates that atmospheric noise cannot account for the temporal variability of the teleconnection, which instead is likely explained by the strength of, and multidecadal changes in, tropical Pacific Ocean variability. These results have implications for teleconnection‐based analyses of model fidelity in simulating precipitation, as well as any reconstruction and forecasting efforts that assume stationarity of the observed teleconnection. The ENSO‐North American hydroclimate teleconnection is non‐stationary in modelsTeleconnection non‐stationarity is linked to the strength of and changes in ENSOThe assumption of teleconnection stationarity is potentially unwarranted

Tropical forests are incredibly dynamic, showing rapid and longer-term changes in growth, mortality and net primary productivity. Tropical species may be highly sensitive to temperature increases associated with climate change because of their narrow thermal tolerances. However, at the ecosystem scale the competing effects of temperature, light and precipitation on tropical forest productivity have been difficult to assess. Here we quantify cloudiness over the past several decades to investigate how clouds, together with temperature and precipitation, affect flower production in two contrasting tropical forests. Our results show that temperature, rather than clouds, is critically important to tropical forest flower production. Warmer temperatures increased flower production over seasonal, interannual and longer timescales, contrary to recent evidence that some tropical forests are already near their temperature threshold. Clouds were primarily important seasonally, and limited production in a seasonally dry forest but enhanced production in an ever-wet forest. A long-term increase in flower production at the seasonally dry forest is not driven by clouds and instead may be tied to increasing temperatures. These relationships show that tropical forest productivity, which is not widely thought to be controlled by temperature, is indeed sensitive to small temperature changes (1–4°C) across multiple timescales.

•The Terminal Warehouse timbers were likely harvested from Georgia or Alabama, US.•Tree-ring analysis provides insight on the logging industry during the 19th century.•Timber preservation is important for history, science, and the environment.

Biophysical vegetation responses to elevated atmospheric carbon dioxide (CO2) affect regional hydroclimate through two competing mechanisms. Higher CO2increases leaf area (LAI), thereby increasing transpiration and water losses. Simultaneously, elevated CO2reduces 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×CO2sensitivity 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. We evaluate the separate and combined biophysical vegetation effects on hydroclimate in a high‐CO2world 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 We evaluate the separate and combined biophysical vegetation effects on hydroclimate in a high‐CO2world using the GISS ModelE global climate model Increased leaf areas enhance soil moisture drying at lower latitudes; reduced stomatal conductance enhances high‐latitude warming Increased leaf area and reduced stomatal conductance also produce complex nonlinear and either competing or mutually amplifying regional hydroclimate responses

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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. 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. 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 We developed tree‐ring reconstructions of central U.S. soil moisture and the atmospheric ridge that causes flash droughts back to 1500 CE Observations capture flash drought's long‐term probability distribution, but reconstructions reveal centennial fluctuations in variability Cycles in reconstructed atmospheric ridging may aid diagnosis and prediction of flash drought on interannual to decadal time scales

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. 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. 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

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. 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. 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

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. 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‐Caribbeandrought, 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. 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

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. New gridded monthly soil‐moisture estimates indicate that the southeast U.S. drought in fall 2016 was the second most severe since at least 1895The driver was low precipitation, but record‐high evaporative demand also contributed. Both countered centennial trends in the regionThese conditions were caused by internal atmospheric variability and were only modestly aided, if at all, by tropical teleconnections

Eastern Australia recently experienced an intense drought (Millennium Drought, 2003–2009) and record-breaking rainfall and flooding (austral summer 2010–2011). There is some limited evidence for a climate change contribution to these events, but such analyses are hampered by the paucity of information on long-term natural variability. Analyzing a new reconstruction of summer (December–January–February) Palmer Drought Severity Index (the Australia-New Zealand Drought Atlas; ANZDA, 1500–2012 Common Era), we find moisture deficits during the Millennium Drought fall within the range of the last 500 years of natural hydroclimate variability. This variability includes periods of multidecadal drought in the 1500s more persistent than any event in the historical record. However, the severity of the Millennium Drought, which was caused by autumn (March-April-May) precipitation declines, may be underestimated in the ANZDA because the reconstruction is biased toward summer and antecedent spring (September-October-November) precipitation. The pluvial in 2011, however, which was characterized by extreme summer rainfall faithfully captured by the ANZDA, is likely the wettest year in the reconstruction for Coastal Queensland. Climate projections (Representative Concentration Pathways (RCP) 8.5 scenario) suggest that eastern Australia will experience long-term drying during the 21st century. While the contribution of anthropogenic forcing to recent extremes remains an open question, these projections indicate an amplified risk of multiyear drought anomalies matching or exceeding the intensity of the Millennium Drought. Recent extremes (the Millennium Drought and 2011 pluvial) are compared to a 500-year soil moisture reconstruction2011 was likely the wettest year in the record for Coastal QueenslandClimate projections indicate substantially increased risk of droughts = the magnitude of the Millennium Drought

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.