The megadrought in the Southwest represents the driest period for the region since at least the year 800. Research from the University of California – Los Angeles finds the current drought has exceeded the severity of a late-1500s megadrought that previously had been identified as the driest such drought in the 1,200 years that the scientists studied. A megadrought is classified as a drought lasting two decades or longer. Climate models have shown that the current drought would have been dry even without climate change, but not to the same extent. Human-caused climate change is responsible for about 42 percent of the soil moisture deficit since 2000, according to UCLA. The research says one of the primary reasons climate change is causing more severe droughts is that warmer temperatures are increasing evaporation, which dries out soil and vegetation. The study was a collaboration among researchers from UCLA, NASA and the Columbia Climate School.