California Drought Linked to Human-Caused Warming: Study

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In parched California, rainfall patterns are essentially the same as they were 120 years ago, but humans have made it warmer and that added heat is driving the state's crippling drought, according to a new study. Warm temperatures dry out soils and cause precipitation to fall as rain rather than snow, thus reducing annual snowpack essential for irrigation, for example. What snow does fall then melts earlier in the spring.

Recent studies have yielded mixed results on whether the dry weather behind California's current drought is driven by climate change. The new study, reported Monday in the Proceedings of the National Academy of Sciences, shows that "it doesn't require any change in precipitation probability to alter the risk of severe drought conditions in California," said lead author Noah Diffenbaugh, a climate scientist at Stanford University in California.

"Human emissions of greenhouse gases have increased the probability that when low precipitation years occur, that they occur with a warm environment," he said. As temperatures continue to creep higher over the next few decades, he added, they'll guarantee that every dry year will coincide with conditions conducive to severe drought.


— John Roach, NBC News contributor