The beginning of the 21st century has seen intense drought in the American Southwest (SW), encompassing California, Nevada, Arizona, Utah, New Mexico, and Colorado—a region with over 60 million people and a $4.2 trillion annual gross regional product. This ongoing drought, lasting approximately two decades, has placed unprecedented strain on water resources. The situation is critical, with reservoirs like Lake Powell and Lake Mead at their lowest levels since their creation, leading to water and power supply reductions with widespread economic and ecological consequences. Previous research using paleoclimate and instrumental records, along with climate model simulations, has indicated the severity of the drought and its potential future trajectory. This study builds upon that research by examining the drought through the lens of fundamental climatological drivers—precipitation and temperature—to understand their individual contributions and to assess the likelihood of recovery.

Numerous studies have examined long-term drought conditions in the southwestern US, utilizing paleoclimate and instrumental data. These studies, employing various drought indices (including PDSI and PMDI) and reconstructions, consistently point to the severity of the current drought, often highlighting its unprecedented intensity compared to several centuries of past drought events. Additionally, investigations into precipitation variability in the region, particularly in California, have explored the interplay between moisture supply and the onset and duration of drought periods. However, this research often focuses on precipitation alone, or on an implicit coupling of precipitation and temperature in the context of drought indices. The present study aims to extend the analysis by explicitly examining temperature’s independent effect on drought conditions. This approach allows for a more refined understanding of the current drought and its potential future.

This study uses a multi-faceted approach to analyze the SW drought. First, it examines historical (1571-2021) and instrumental (1896-present) data for temperature and precipitation, separately assessing their roles in the ongoing drought and statistically comparing their current trends to long-term patterns. Second, a new reconstruction of the Standardized Precipitation Evaporation Index (SPEI) is created, extending back to 1571 CE, to complement existing soil moisture-based drought reconstructions (like the PMDI from the Living Blended Drought Atlas). This SPEI reconstruction allows evaluation of the moisture balance (precipitation supply vs. temperature demand). The PMDI data is further utilized to extend the drought analysis back to 600 CE, exploring the intensity and duration of past events. Atmospheric circulation patterns in the Northeast Pacific are also compared between the current drought and a past extreme drought in the late 1500s to determine the potential for future more extreme circulation events. Lastly, a comprehensive analysis of climate model simulations (CMIP6-SSP585 scenario, along with RCP8.5 and RCP4.5 scenarios from CMIP5) is undertaken to project future trends in temperature, precipitation, and aridity, and to gauge the likelihood of reservoir recovery under current water demand.

The analysis reveals that while precipitation exhibits 500-year low bidecadal levels, recent temperatures are exceptional. Five of the ten warmest/driest 20-year periods in the past half-millennium have occurred since 2000. Probabilistic analysis indicates that the current multi-decadal temperature rise is extremely unlikely to be a random occurrence from the long-term temperature record. The SPEI and PMDI reconstructions show that the current drought is the most intense in the Southwestern US since at least 600 CE. Regression analysis reveals that recent warming has reduced the impact of moisture delivery on the SPEI by approximately one-third (34% using INST SPEI data, and 37% using RECON+INST PMDI data). The probability of recovery to normal moisture levels within the next 10–15 years is estimated to be only around 5%, while even mid-century recovery seems unlikely. Climate model simulations project a continuation of these trends, indicating strongly negative moisture balances and an extremely low likelihood of major reservoirs returning to full capacity given current demands. An analysis of atmospheric circulation patterns suggests that the conditions in the Northeast Pacific during the drought of the late 1500s were more extreme than the current drought and that similar or even stronger conditions could occur in the future, further worsening the drought situation.

The findings strongly suggest that the ongoing drought is not an isolated event but rather the beginning of a sustained period of drier conditions in the Southwestern US. The combination of empirical recovery time estimates for precipitation, the statistically significant shift in the regional temperature distribution, and the future climate simulations, strongly indicates ongoing and intensifying aridification. The reduced role of precipitation relative to temperature in the current drought, coupled with the projected further increases in temperature, paints a grim picture for water resources in the region. The relatively white-noise characteristics of precipitation, as seen in both historical and model data, suggest that sustained periods of above-average precipitation needed for significant reservoir recovery are highly unlikely. The potential for more extreme atmospheric circulation patterns in the future, as suggested by comparisons with the late 1500s drought, further increases the risk of prolonged and severe drought conditions.

This study provides compelling evidence that the current drought in the Southwestern US is highly likely to be the start of a prolonged period of drier conditions. The combination of exceptionally low precipitation, significantly elevated temperatures, and projections from climate models paint a concerning picture for water resources and the region's ecosystems. Full reservoir recovery in the foreseeable future seems highly improbable given continued water demand. Future research should focus on developing adaptation strategies to mitigate the impacts of this ongoing and intensifying aridification. Specific focus should also be directed at investigating second-order impacts of less snowpack and more rain including changes in runoff and soil moisture.

The study's conclusions rely on climate model projections, which inherently contain uncertainties. While the models used represent state-of-the-art climate simulations, their projections of precipitation in the Southwestern US remain relatively variable. Additionally, the analysis focuses primarily on large-scale climatic patterns, and local variations in drought intensity and recovery time may not be fully captured. The recovery time analysis assumes that regional precipitation will continue to behave as a white noise process in the future which may not accurately reflect the future climate.