The western U.S. is locked in the grips of the first human-caused megadrought, study finds – By Andrew Freedman, Darryl Fears (Washington Post) / April 17 2020
A vast region of the western United States, extending from California, Arizona and New Mexico north to Oregon and Idaho, is in the grips of the first climate change-induced megadrought observed in the past 1,200 years, a study shows. The finding means the phenomenon is no longer a threat for millions to worry about in the future, but is already here.
The megadrought has emerged while thirsty, expanding cities are on a collision course with the water demands of farmers and with environmental interests, posing nightmare scenarios for water managers in fast-growing states.
A megadrought is broadly defined as a severe drought that occurs across a broad region for a long duration, typically multiple decades.
Unlike historical megadroughts triggered by natural climate cycles, emissions of heat-trapping gases from human activities have contributed to the current one, the study finds. Warming temperatures and increasing evaporation, along with earlier spring snowmelt, have pushed the Southwest into its second-worst drought in more than a millennium of observations.
The study, published in the journal Science on Thursday, compares modern soil moisture data with historical records gleaned from tree rings, and finds that when compared with all droughts seen since the year 800 across western North America, the 19-year drought that began in 2000 and continued through 2018 (this drought is still ongoing, though the study’s data is analyzed through 2018) was worse than almost all other megadroughts in this region.
The researchers, who painstakingly reconstructed soil moisture records from 1,586 tree-ring chronologies to determine drought severity, found only one megadrought that occurred in the late 1500s was more intense.
Historical megadroughts, spanning vast regions and multiple decades, were triggered by natural fluctuations in tropical ocean conditions, such as La Niña, the cyclic cooling of waters in the tropical Pacific.
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