Western droughts could become norm, say climate scientists

Climate models suggest Colorado to get hotter, drier

Jim Erickson, Rocky Mountain News

Published February 3, 2007 at midnight

Text size

More Local News

More stories »

Average temperatures in the West could rise 7 degrees by the end of the century because of global warming, with drought-like conditions becoming the new norm, climate scientists said Friday.

Some of the world's most advanced climate models suggest that Colorado precipitation levels will remain roughly constant as temperatures climb.

If that happens, the state will get drier, with less mountain snow in the winter, lower stream flows in the summer, and an increased threat of wildfires.

"I think the drought that we've been in since 1999, on and off, is a great preview of what's going to happen in the future - and you won't have to wait too long," said Jonathan Overpeck, a University of Arizona paleoclimatologist and an author of the latest Intergovernmental Panel on Climate Change report.

A 21-page summary of the 11-chapter report was released Friday in Paris. The IPCC's periodic updates are considered the world's most authoritative assessments of climate science.

The new report projects a global temperature increase of 3.2 to 7.2 degrees Fahrenheit by the end of the century.

The planet warmed slightly more than 1 degree Fahrenheit over the past century. IPCC scientists see a greater than 90 percent likelihood that most warming over the last 50 years has occurred because of human-caused emissions of heat-trapping "greenhouse" gases.

"If we don't dial back the greenhouse gas emissions, what we've seen so far is nothing compared to what we'll get," Overpeck said Friday from Paris.

For the first time - because of increased confidence in the more than 20 computerized climate models used in the latest assessment - IPCC included regional projections in its report, said Linda Mearns of the National Center for Atmospheric Research in Boulder.

The models show that the West will warm more than the global average.

Winter warming will be 5.6 to 7.9 degrees, and summer temperatures will rise 5.7 to 8.5 degrees, Mearns said. Average year-round increase is about 7 degrees, said Mearns, one of the study authors.

Those ranges assume modest attempts are made to rein in greenhouse-gas emissions in coming years. If nothing is done to curb emissions, Western warming could hit double digits by the end of the century, the models show.

"The world's scientists have done their job, and now it's up to the rest of us," said Stephen Saunders, president of the Louisville-based Rocky Mountain Climate Organization.

"With the overwhelming evidence in today's consensus statement, it's time for the remaining holdouts to join those of us who want to solve this problem while we still can."Some question models

One of those remaining holdouts - at least when it comes to the topic of climate projections - is former Colorado State Climatologist Roger Pielke Sr.

Pielke said Friday that the models used in the latest IPCC report have no "predictive skill" when it comes to regional climate projections.

"To rely on the numbers they predict on a regional scale I think is overstating the capabilities of these models," said Pielke, now a senior research scientist at the University of Colorado.

"I don't think it's supported by scientific literature."

But the report's authors - some 600 of them from 40 countries - disagree with Pielke.

As far as Colorado's precipitation outlook goes, most of the state lies in a no-man's-land.

Precipitation is expected to decline in the Southwest in coming decades, while Canada and some of the U.S. northern-tier states should see an increase. But Colorado is stuck in the middle and could see little or no annual change, Mearns said.

If precipitation remains constant while temperatures rise in Colorado, the mountain snow season will shrink and summers will get drier.

The snow season contracts on both ends because more of the autumn and spring precipitation falls as rain instead of snow.

That's bad news for Colorado's $2 billion-a-year ski industry, said Auden Schendler, director of environmental affairs for the Aspen Ski Co.

"If you want to see what will happen to the ski industry with warming, visit Europe, where they are covering glaciers with blankets and helicoptering in snow and still having to cancel races due to rain and warm temps in January," he said.

So far, Colorado's resorts have been largely spared because their high altitude shields them from warming's effects, Overpeck said.

"But it's creeping up, and it's just a matter of time before they get hit," he said.

As spring arrives earlier and earlier in the Colorado Rockies, the snow will melt sooner and run down the mountains faster, said Roger Barry, director of the National Snow and Ice Data Center in Boulder and a review editor of the new IPCC report.Higher water prices seen

Springtime snowmelt will come down in a more concentrated pulse, increasing the likelihood of periodic flooding. To guard against those floods, water managers will likely be forced to lower reservoir levels.

"That water that's released in the spring won't be available later in the summer for other uses, and that's what has the water managers worried," Overpeck said.

A shrinking snowpack and earlier runoff leads to lower summer river flows, along with drier soils and forests, said Kevin Trenberth of NCAR, an author of the new climate report.

Dry, stressed trees are more susceptible to wildfires, insects and diseases. Persistently dry soils can succumb to drought.

Reduced summertime river flows are also bad news for irrigation farmers and lawn-watering homeowners, as well as fish and kayakers. Agriculture uses about 80 percent of Western water and relies heavily on summer irrigation.

"We'll be looking at more demand for water from a growing population when there will be less water available," Barry said.

"That could mean higher prices for water and water rationing more often.

"We've got probably 20 years to get things turned around" by reducing greenhouse-gas emissions, he said. "If we don't, the changes will be exacerbated."

A 2005 study by U.S. Geological Survey hydrologist Christopher Milly found runoff from Western rivers could drop 10 to 20 percent by 2050.

Recent work by Boulder climate scientist Martin Hoerling suggests future U.S. droughts could last an average of 12 years, spanning half the region and reducing Colorado River flows that supply millions of people.

"Climate change is moving us in the direction of a perpetual state that is of the Dust Bowl type," Hoerling said in September.

ericksonj@RockyMountainNews.com or 303-954-5129

Subscribe to the Rocky Mountain News