Human-driven climate change is already happening in the southwestern United States, including in Southern California, and it’s only going to get worse, with further drops in Colorado River water increasing conflicts among water users.

So says a new White House report, Global Climate Change Impacts in the United States, which lays out a variety of potentially harmful effects for various regions of the country.

In the Southwest, including California, droughts are expected to worsen, temperatures to increase and water supplies to further tighten in coming decades, according to the report, compiled by the United States Global Change Research Program. That could increase water conflicts among states and even nations, as well as different types of users, such as farmers and urban dwellers.

“Human-induced climate change appears to be well underway in the Southwest,” the report says. “Recent warming is among the most rapid in the nation, significantly more than the global average in some areas.”

The report calls out two effects potentially significant for Orange County and the rest of Southern California: reduced snow pack, and reduced water flow in the Colorado River.

The river makes up a part of Southern California’s imported water supply; reduced snow pack directly affects the amount of water pumped our way from the river, as well as from the northern part of the state.

The report says effects from global warming are complicated by natural climate variability in the region and the “heat island” effect, in which concrete and asphalt in large cities intensify warming.

“The urban heat-island effect is big — five degrees in the last 100 years,” said Bill Patzert, climate scientist at NASA’s Jet Propulsion Laboratory whose findings parallel those in the report. ” (It’s) five degrees in the last 50 years in Orange County because growth has been so rapid.”

But his study of the state’s 330 monitoring stations showed warming everywhere, not just urban zones.

“A degree or two we see up in the mountains, too, and that’s definitely the global-warming impact,” he said.

The report says increasing wildfire, landscape changes and shifts in animal populations also could become more prevalent over the course of the coming century. Loss of agricultural productivity is also likely for the region, while air quality could worsen in urban areas.

(Photo: Cracked bed in Lake Mead on the Colorado River in 2005; AP file.)

(O.C. Register graphic by Scott Brown based on figure in climate report.)

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