The global average increase for the same period was 1.0 degrees Fahrenheit (0.55 degrees Celsius).

In the Colorado River Basin, which supplies water to big and fast-growing cities like Los Angeles, San Diego, Las Vegas and Denver, the average temperature rose 2.2 degrees Fahrenheit (1.21 degrees Celsius), the US group said.

Most of the river's water comes from melting snow in the mountains, and climate scientists predict hotter temperatures will reduce the snowpack and increase evaporation, the NRDC said in a statement.

"Global warming is hitting the West hard," said Theo Spencer of the NRDC. "It is already taking an economic toll on the region's tourism, recreation, skiing, hunting and fishing activities."

Study author Stephen Saunders of the Rocky Mountain Climate Organization said there were signs of the economic impacts throughout the West.

"Since 2000 we have seen $2.7 billion in crop loss claims due to drought. Global warming is harming valuable commercial salmon fisheries, reducing hunting activity and revenues, and threatening shorter and less profitable seasons for ski resorts," he said.