Western Water Supplies Already Altered by Global Warming
Observations throughout the western U.S. show snowpack is decreasing, rivers are flowing earlier in the year, and spring temperatures are increasing. A formal detection and attribution study of these changes shows the majority of these trends are due to human effects on the climate.
Credit: David W. Pierce, SIO; land image courtesy of NASA's Earth Observatory

When it rains, it sometimes still pours out West. But it's not enough.

Changes in the western U.S. water supply, such as a declining snowpack and rivers running dry in the summer, can mostly be attributed to human-caused climate change, a new study finds. These changes will require a new approach to water management in the West in the future, scientists say.

Water supplies for much of the West come from winter snowfall that accumulates in mountainous regions and melts throughout the spring and early summer, flowing into the rivers that wind throughout the western states.

Over the past 50 years, researchers have noted significant changes in this system of water flow, which a new study, detailed in the Feb. 1 issue of the journal Science, attributes to global warming.

Warmer and drier

Over the last half-century, less snow has accumulated on mountains throughout the winter months. This snowpack is critical to maintaining the main sources of river water in the spring and summer months in the arid regions of the West.

The snowpack that does exist has been melting earlier than it did in the past as the spring thaw creeps backward. This forces all the water that would be available from the melt to flow through the rivers faster and earlier than it used to. So while water might once have flown from western rivers through June, leaving only July and August as dry months, the snowpack might now be gone by April, tacking on three more months to the dry season.

"The rivers get rid of the water they have for the year earlier," said study leader Tim Barnett at the Scripps Institution of Oceanography in San Diego.

The study used climate and hydrological models to pin down the cause of these changes and found that the length and strength of the changes over the last 50 years can't be explained by natural variations, the authors say. Instead, human-caused global warming is the culprit.

Mega-droughts have struck the West in the past, but these droughts have been caused by the onset of climate conditions that decreased the amount of precipitation falling over the area. But global warming has created a wholly different scenario, driven by increasing temperatures, not decreasing precipitation.

The warming atmosphere is causing precipitation to fall over the mountains as rain rather than snow. The rain runs straight off into rivers, so instead of having water stored as snow into the summer, water availability peaks earlier in the year.

Another effect of global warming has been to push back the start of spring so that snows begin to melt earlier in the year.

The models Barnett and his team ran show that up to 60 percent of the trends in water flow observed from 1950 to 1999 can be attributed to human effects, mainly global warming.

Growing drier

The authors write that their "results are not good news for those living in the western United States."

Previous studies using climate models have projected that even more arid conditions will become the norm for the Southwest over the course of the next 100 years, which will only compound water woes, the authors of the new study say.

In another article in the same issue of Science, a group of hydrologists call for the need to revise the way water management is done in the West.

Currently water resources are allocated based on the idea that water flow in the future will be just like it was in the past, with some natural fluctuations. But the observations over the past 50 years and the predictions for the future now require a shift in the way water management is done.

"Water management needs to take on some new ideas and methods in the way it does business," said one of the authors, Christopher Milly of the United States Geological Survey.

Accounting for the shifting distribution of water over the seasons and the region's increasing dryness will require change on a number of fronts, Milly says, including changing the computational tools water managers use, changing some of the existing water infrastructure, shifting water use from agriculture to urban areas, conservation, and possibly even slowing the rapid development of some southwestern cities.