Anthropogenic warming has increased drought risk in California

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.drought | climate extremes | climate change detection | event attribution | CMIP5

Climate change and California drought in the 21st century

“…Diffenbaugh et al (2) seem to solve that mystery in their latest assessment. As noted earlier in Fig.…”

“…Combined with the role that temperature plays in increasing the loss of water from agriculture, soils, surface water bodies, and snowpack, the authors note that 100% of the moderately dry years between 1995 and 2014 co-occurred with a positive temperature anomaly. Diffenbaugh et al (2) note: efforts to understand drought without examining the role of temperature miss a critical contributor to drought risk. Indeed, our results show that even in the absence of trends in mean precipitation-or trends in the occurrence of extremely low-precipitation events-the risk of severe drought in California has already increased due to extremely warm conditions induced by anthropogenic global warming.…”

“…In addition, Diffenbaugh et al (2) highlight model results that suggest the emergence of a climatic regime in which all future dry years coincide with warmer conditions. As they note, the region is moving toward "a transition to a permanent condition of ∼100% risk that any negative-or extremely negative-12-mo precipitation anomaly is also extremely warm" (2).…”

“…Recent analyses have thus established the "fingerprint" of anthropogenic climate change in an increasingly diverse array of meteorological and hydrological phenomena around the world, from heat waves to coastal damages during extreme tides and storms, flooding from more intense precipitation events, and severe drought (1). In a new study published in PNAS, Diffenbaugh et al now add weight to the accumulating evidence that anthropogenic climatic changes are already influencing the frequency, magnitude, and duration of drought in California (2). The authors show that the increasing co-occurrence of dry years with warm years raises the risk of drought despite limited evidence of a trend in precipitation itself, highlighting the critical role of elevated temperatures in altering water availability and increasing overall drought intensity and impact.…”

Water relations and photosynthesis along an elevation gradient for Artemisia tridentata during an historic drought

“…Statewide 2013 precipitation averages for California were less than 34 % of historic averages (Swain et al 2014), with trends continuing into 2014 and 2015. Anthropogenic warming has increased the ongoing risk of drought for California (Diffenbaugh et al 2015;Mann and Gleick 2015). Drought is a major limiting factor for many ecological processes, particularly in arid and semi-arid regions (Zhou et al 2013).…”