Climate change scenarios forecast increased drought exposure for terrestrial vertebrates in the contiguous United States

Anthropogenic climate change is altering patterns of extreme weather events across the planet, with far-ranging consequences for biodiversity. Increases in the frequency, duration, and severity of droughts are anticipated to substantially impact natural ecosystems. Here, we predicted the extent to which 1,221 terrestrial vertebrates in the contiguous United States will be increasingly exposed to annual (12-month) and prolonged (36-month) drought, under three global climate models and two representative concentration pathways. On average, a 377% increase in annual drought exposure and a 579% increase in prolonged drought exposure are anticipated for the study area by 2050–2080, compared to 1950–2005. Species in the southwestern USA could see the largest increases in drought exposure, while this area also has the highest vertebrate diversity and an abundance of drought-threatened species. Our results aid in identifying vertebrates and ecoregions anticipated to see large increases in drought exposure, which can inform conservation strategies aimed at mitigating vertebrate extinction risks.

Methods
We used the standardized precipitation evapotranspiration index (SPEI) to calculate past and future drought occurrence in the contiguous United States. To this end, we used three Global Climate Models (GCM) and two Representative Concentration Pathways (RCP), for a historical and future period. We contextualized these findings using observed data for two recent periods. We analyzed changes in drought exposure across species ranges extracted from IUCN and 84 Level III Ecoregions of the United States.