Evidence suggests that Western North America will be drier at the end of the 21st century than any period of the last 1000 years.

Global warming is expected to increase the frequency and intensity of droughts in the twenty-first century, but the relative contributions from changes in moisture supply (precipitation) versus evaporative demand (potential evapotranspiration; PET) have not been comprehensively assessed. Using output from a suite of general circulation model (GCM) simulations from phase 5 of the Coupled Model Intercomparison Project, projected twentyfirst century drying and wetting trends are investigated using two offline indices of surface moisture balance: the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Evapotranspiration Index (SPEI). PDSI and SPEI projections using precipitation and PenmanMonteith based PET changes from the GCMs generally agree, showing robust cross-model drying in western North America, Central America, the Mediterranean, southern Africa, and the Amazon and robust wetting occurring in the Northern Hemisphere high latitudes and east Africa (PDSI only). The SPEI is more sensitive to PET changes than the PDSI, especially in arid regions such as the Sahara and Middle East. Regional drying and wetting patterns largely mirror the spatially heterogeneous response of precipitation in the models, although drying in the PDSI and SPEI calculations extends beyond the regions of reduced precipitation. This expansion of drying areas is attributed to globally widespread increases in PET, caused by increases in surface net radiation and the vapor pressure deficit.Increased PET not only intensifies drying in areas where precipitation is already reduced, it also drives areas into drought that would otherwise experience little drying or even wetting from precipitation trends alone. This PET amplification effect is largest in the Northern Hemisphere mid-latitudes, and is especially pronounced in western North America, Europe, and southeast China. Compared to PDSI projections using precipitation changes only, the projections incorporating both precipitation and PET changes increase the percentage of global land area projected to experience at least moderate drying (PDSI standard deviation of B-1) by the end of the twenty-first century from 12 to 30 %. PET induced moderate drying is even more severe in the SPEI projections (SPEI standard deviation of B-1; 11 to 44 %), although this is likely less meaningful because much of the PET induced drying in the SPEI occurs in the aforementioned arid regions. Integrated accounting of both the supply and demand sides of the surface moisture balance is therefore critical for characterizing the full range of projected drought risks tied to increasing greenhouse gases and associated warming of the climate system.

Severe and persistent 21st-century drought in southwestern North America (SWNA) motivates comparisons to medieval megadroughts and questions about the role of anthropogenic climate change. We use hydrological modeling and new 1200-year tree-ring reconstructions of summer soil moisture to demonstrate that the 2000–2018 SWNA drought was the second driest 19-year period since 800 CE, exceeded only by a late-1500s megadrought. The megadrought-like trajectory of 2000–2018 soil moisture was driven by natural variability superimposed on drying due to anthropogenic warming. Anthropogenic trends in temperature, relative humidity, and precipitation estimated from 31 climate models account for 47% (model interquartiles of 35 to 105%) of the 2000–2018 drought severity, pushing an otherwise moderate drought onto a trajectory comparable to the worst SWNA megadroughts since 800 CE.

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.

There is strong evidence that climate change will increase drought risk and severity, but these conclusions depend on the regions, seasons, and drought metrics being considered. We analyze changes in drought across the hydrologic cycle (precipitation, soil moisture, and runoff) in projections from Phase Six of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble shows robust drying in the mean state across many regions and metrics by the end of the 21st century, even following the more aggressive mitigation pathways (SSP1-2.6 and SSP2-4.5). Regional hotspots with strong drying include western North America, Central America, Europe and the Mediterranean, the Amazon, southern Africa, China, Southeast Asia, and Australia. Compared to SSP3-7.0 and SSP5-8.5, however, the severity of drying in the lower warming scenarios is substantially reduced and further precipitation declines in many regions are avoided. Along with drying in the mean state, the risk of the historically most extreme drought events also increases with warming, by 200-300% in some regions. Soil moisture and runoff drying in CMIP6 is more robust, spatially extensive, and severe than precipitation, indicating an important role for other temperature-sensitive drought processes, including evapotranspiration and snow. Given the similarity in drought responses between CMIP5 and CMIP6, we speculate that both generations of models are subject to similar uncertainties, including vegetation processes, model representations of precipitation, and the degree to which model responses to warming are consistent with observations. These topics should be further explored to evaluate whether CMIP6 models offer reasons to have increased confidence in drought projections. Plain Language SummaryDrought is an important natural hazard in many regions around the world, and there are significant concerns that climate change will increase the frequency or severity of drought events in the future. Compared to a world before anthropogenic climate change, the latest state-of-the-art climate model projections from CMIP6 show robust drying and increases in extreme drought occurrence across many regions by the end of the 21st century, including western North America, Central America, Europe and the Mediterranean, the Amazon, southern Africa, China, Southeast Asia, and Australia. While these changes occur even under the most aggressive climate mitigation pathways, the models show substantial increases in the extent and severity of this drying under higher warming levels, highlighting the value of mitigation for reducing drought-based climate change impacts. Given the significant response to even modest warming, however, and evidence that climate change has already increased drought risk and severity in some regions, adaptation to a new, drier baseline will likely be required even under the most optimistic scenarios.

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Climate records over the last millennium place the twentieth-century warming in a longer historical context. Reconstructions of millennial temperatures show a wide range of variability, raising questions about the reliability of currently available reconstruction techniques and the uniqueness of late-twentieth-century warming. A calibration method is suggested that avoids the loss of low-frequency variance. A new reconstruction using this method shows substantial variability over the last 1500 yr. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate. High variability in reconstructions does not hamper the detection of greenhouse gas–induced climate change, since a substantial fraction of the variance in these reconstructions from the beginning of the analysis in the late thirteenth century to the end of the records can be attributed to external forcing. Results from a detection and attribution analysis show that greenhouse warming is detectable in all analyzed high-variance reconstructions (with the possible exception of one ending in 1925), and that about a third of the warming in the first half of the twentieth century can be attributed to anthropogenic greenhouse gas emissions. The estimated magnitude of the anthropogenic signal is consistent with most of the warming in the second half of the twentieth century being anthropogenic.

A previous reconstruction back to 800 ce indicated 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.Since the year 2000, southwestern North America (SWNA, 30-45° N, 105-125° W) has been unusually dry due to low precipitation totals and heat, punctuated most recently by exceptional drought in 2021 1 . From 2000 to 2021, mean water-year (October-September) SWNA precipitation was 8.3% below the 1950-1999 average and temperature was 0.91 °C above average (Extended Data Fig. 1). No other 22-yr period since at least 1901 was as dry or as hot. While there have been single-year breaks in these anomalous conditions, aridity has dominated the 2000s, as evidenced by declines in two of North America's largest reservoirs, Lakes Mead and Powell, both on the Colorado River. In summer 2021, these reservoirs reached their lowest levels on record, triggering unprecedented restrictions on Colorado River usage 2 , in part because the 2-yr naturalized flow out of Colorado River's upper basin in water-years 2020-2021 was likely the lowest since at least 1906 (Extended Data Fig. 2). Aridity was especially extreme and widespread from summer 2020 through summer 2021 (Extended Data Fig. 3). Despite an active North American monsoon in 2021, the United States Drought Monitor (USDM 3 ) classified >68% of the western United States as under extreme or exceptional drought for nearly all of July-October 2021 (Extended Data Fig. 4), a record-high proportion of drought extent in the USDM's 22-yr history.Soil moisture is a particularly important integrator of drought. Soil moisture impacts runoff ratios and therefore streamflow, agricultural productivity and irrigation demand, ecosystem productivity and health, wildfire activity and land-atmosphere feedbacks such as heatwave intensity. Summer soil moisture is particularly crucial, as summer is when water demand from ecosystems, humans and the atmosphere is generally highest, and also the season of focus in most tree-ring reconsructions of drought severity 4 . According to a bucket-type water-balance model forced by monthly climate data 5 , SWNA 0-200 cm soil moisture in summer (June-August) was below average in 18 of the 22 years from 2000-2021 (Extended Data Fig. 5). This turn-of-the-twenty-first-century drought was last investigated by Williams et al. 5 through 2018, who speculated that the extended drought event may have been terminating in 2019