Anthropogenic warming has increased drought risk in California

TLDR

California is experiencing a record‑setting drought that began in 2012, marked by the lowest precipitation, highest temperatures, and extreme drought indicators, leading to severe water shortages, groundwater overdraft, low streamflow, and heightened wildfire risk. The authors analyze historical climate observations to show that precipitation deficits are more than twice as likely to produce drought years when accompanied by warm conditions. The study finds that while the frequency of negative precipitation anomalies has not changed, drought years have increased in the past two decades, the likelihood of warm‑dry co‑occurrence and drought production has risen, and climate models indicate anthropogenic warming raises the probability that dry years are also warm, making such extreme conditions almost certain in the coming decades.

Abstract

California is currently in the midst of a record-setting drought. The drought began in 2012 and now includes the lowest calendar-year and 12-mo precipitation, the highest annual temperature, and the most extreme drought indicators on record. The extremely warm and dry conditions have led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Analyzing historical climate observations from California, we find that precipitation deficits in California were more than twice as likely to yield drought years if they occurred when conditions were warm. We find that although there has not been a substantial change in the probability of either negative or moderately negative precipitation anomalies in recent decades, the occurrence of drought years has been greater in the past two decades than in the preceding century. In addition, the probability that precipitation deficits co-occur with warm conditions and the probability that precipitation deficits produce drought have both increased. Climate model experiments with and without anthropogenic forcings reveal that human activities have increased the probability that dry precipitation years are also warm. Further, a large ensemble of climate model realizations reveals that additional global warming over the next few decades is very likely to create ∼ 100% probability that any annual-scale dry period is also extremely warm. We therefore conclude that anthropogenic warming is increasing the probability of co-occurring warm-dry conditions like those that have created the acute human and ecosystem impacts associated with the "exceptional" 2012-2014 drought in California.