The reason for the diminishing water flow in Western United States has been found by a collaborative study. The reason it seems are humans.
Since the Rocky Mountains have warmed by 2 degrees Fahrenheit, the snow pack in the Sierras has dwindled by 20 percent and the temperatures there have heated up by 1.7 degrees Fahrenheit, there is a possibility of dire consequences for the water supply in the Western United States, including California.
Scientists found that water flow in the West has decreased in the last 20 to 30 years, and they could not figure out why it was happening.
Now, researchers from Lawrence Livermore National Laboratory's Program for Climate Model Diagnosis and Intercomparison in collaboration with Scripps Institution of Oceanography have pinpointed the cause of that diminishing water flow on a regional scale: humans.
"We looked at whether there is a human-caused climate change where we live, and in aspects of our climate that we really care about," said Benjamin Santer of LLNL and co-author of the study.
"No matter what we did, we couldn't shake this robust conclusion that human-caused warming is affecting water resources here in the Western United States," he added.
By looking at air temperatures, river flow and snow pack over the last 50 years, the researchers determined that the human-induced increase in greenhouse gases has seriously affected the water supply in the West and the future holds more of the same.
"It's pretty much the same throughout all of the Western United States. The results are being driven by temperature change. And that temperature change is caused by us," said Tim Barnett of Scripps and a co-author of the study.
For the study, the researchers scaled down global climate models to the regional scale and compared the results to observations over the last 50 years.
The findings were solid, giving the researchers confidence that they could use the same models to predict the effects of the global scale increase in greenhouse gases on the Western United States in the future.
The predicted consequences are bleak. By 2040, most of the snow packs in the Sierras and Colorado Rockies would melt by April 1 of each year due to rising air temperatures. The earlier snowmelt would lead to a shift in river flows.
The shift could lead to flooding in California's Central Valley. Presently, state reservoirs are filled during the rainy season. As the water is drawn down, the reservoirs are replenished with snowmelt from the Sierras.
If that snow melts earlier, as climate models predict, the reservoirs could overflow.
"We are headed for a water crisis in the Western United States that has already started. A couple of decades ahead, we might not have that snow pack, making us more susceptible to flooding." Barnett said.
According to Santer, the increase in predicted river flow should be a wake- up call to officials that the water supply infrastructure needs to be updated now, as opposed to waiting until the situation is urgent.
As far as warming is concerned, with the existing greenhouse gas in the atmosphere, the Earth will continue to warm for the next 80-100 years.
The study appears in the Jan. 31 online edition of Science Express.