WASHINGTON (Reuters) - A water supply crisis is looming in the western United States thanks to human-caused climate change that already has altered the region's river flows, snow pack and air temperatures, scientists said.

Trends over the past half century foreshadow a worsening decline in water, perhaps the region's most valuable natural resource, even as population and demand expands in western states, researchers led by a scientist at the Scripps Institution of Oceanography wrote in the journal Science on Thursday.

...drowned stumps in many other places in the Sierra Nevada. They all fell into two distinct generations, corresponding to two distinct droughts. The first had begun sometime before 900 and lasted over two centuries. There followed several extremely wet decades, not unlike those of the early 20th century. Then the next epic drought kicked in for 150 years, ending around 1350. Stine estimates that the runoff into Sierran lakes during the droughts must have been less than 60 percent of the modern average, and it may have been as low as 25 percent, for decades at a time. "What we have come to consider normal is profoundly wet," Stine said. "We're kidding ourselves if we think that's going to continue, with or without global warming."
*Emphasis mine