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Watch a video about a climate change experiment in the Rocky Mountains

that two-thirds of 112 future climate projections from a set of global climate models used by the Intergovernmental Panel on Climate Change show drying, while one-third show “no change or increases in Colorado River streamfl ow, a number which varies depending on the global climate models and emission scenarios used.”

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Drought has plagued the Colorado River Basin for more than a decade. A wet year in 2011 helped restore the reservoirs but this year’s extremely dry conditions have given fears that a shortage declaration could come as soon as 2015. In May, the U.S. Geological Survey (USGS) reported that about 20 percent of the Rocky Mountain snowpack has disappeared the past 30 years because of warmer spring tem- peratures, reversing a trend that saw alternating snowpacks in the northern and central and southern Rockies. Runoff from the winter snowpack in the Rockies accounts for 60 to 80 percent of the annual water supply for more than 70 million people living in the western U.S., and is infl uenced by factors such as the snowpack’s water content, known as snow water equiva- lent, and the timing of snowmelt, according to USGS.

“From 1980 on, warmer spring temperatures melted snowpack throughout the Rockies early, regardless of winter precipitation,” said USGS scientist Greg Pederson, lead author of Regional Patterns and Proximal Causes of the Recent Snowpack Decline in the Rocky Mountains. “The model in turn shows temperature as the major driving factor in snowpack declines over the past thirty years.”

Another snowfall study predicts a 42 percent reduction of the snow- pack in the mountains surrounding Los Angeles by 2050. The June study, Mid- and End-of-Century Snowfall in the Los Angeles Region, used two modeling scenarios regarding greenhouse gas emissions: “business-as-usual” and “mit- igation.” In the former scenario, snow- fall sees a “dramatic further reduction” by the end of the century (42 percent,


while the latter showed a “negligible further reduction” (31 percent) from mid-century.

“Climate change has become inevitable, and we’re going to lose a substantial amount of snow by mid- century,” said study author Alex Hall, professor with the University of California, Los Angeles’ Department of Atmospheric and Oceanic Sciences. “But our choices matter. By the end of the century, there will be stark differ- ences in how much snowfall remains, depending on whether we begin to mitigate greenhouse gas emissions.” The prospect of a reduced snow- pack in Southern California is not as daunting as the loss in the larger water- sheds upon which the region depends, Atwater said.

“If you are talking about the Colorado Rockies and the Colorado River and the Sierras, less snowpack and more rainfall is a big deal because if it’s not snowpack and it’s rain, it runs off in January, February and March instead of staying up in the mountains until April or May and it affects the fl ood control operations and the storage and the way we operate the reservoirs,” he said. “The amount of snow we get on Mount Baldy, the San Gabriels and Big Bear isn’t that signifi cant and doesn’t affect the runoff that much.” Meanwhile, a 120-author report prepared for the National Climate Assessment and spearheaded by the University of Arizona and several partner institutions, says there is “high confi dence that the climate of the Southwest will continue to change through the 21st century and beyond in response to human-generated green- house gas emissions, and will continue to vary in ways that can be observed in historic and paleoclimate records.” The report, Assessment of Climate Change in the Southwest United States, notes that “not all aspects of the climate change or variation can be projected with equal confi dence,” but that the Southwest “has a long legacy of human adaptation to climate vari- ability that has enabled society to live within environmental constraints and

Western Water

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