Environment + Climate

UCLA researchers project Southern California rainfall levels through end of century

Models show more precipitation will be rain instead of snow, which would increase flood risk and limit chance to capture water

Rainfall in front of City Hall
Courtesy C-CHANGE.LA

Regional water managers said the study shows the need for increased investment in storm water capture infrastructure.

A UCLA study published today provides the most detailed scientific projections to date of how climate change will affect rainfall and snowfall in Southern California through the end of the 21st century.

The research finds that Southern Californians may face an increased risk for floods and will have smaller windows of time to capture local water because, although the average annual precipitation is expected to remain nearly the same as it has been in recent decades, more of that precipitation will be rain, and less will be snow.

“Will there be rain in L.A.’s future? Unquestionably yes,” said Alex Hall, the report’s lead scientist and a professor in UCLA’s department of atmospheric and oceanic sciences. “Although we don’t expect the total amount of precipitation to change much, we know from a prior snowfall study that warmer temperatures will cause less of that precipitation to fall as snow. Instead, it will fall as rain, which runs off our mountains much more quickly.”

The report was published online in the Journal of Climate and an overview FAQ is posted at C-CHANGE.LA, a website hosted by the Los Angeles Regional Collaborative for Climate Action and Sustainability, of which UCLA is a member. The new research is a companion to an earlier report by the researchers that found local snowfall would decrease dramatically this century.

Hall, who also is a member of UCLA’s Institute of the Environment and Sustainability, has been the lead author of reports by the Intergovernmental Panel on Climate Change assessing global climate change simulations for the United Nations. He collaborated on the new research with UCLA graduate student Neil Berg, who conducted the analysis for the paper; David Neelin, a professor of atmospheric and oceanic sciences; and Fengpeng Sun, Scott Capps, Daniel Walton and Baird Langenbrunner, current and former researchers in atmospheric and oceanic sciences at UCLA.

The researchers found that while average annual precipitation this century will be similar to what they were from 1981 to 2000, very few individual years will be “average”—the model found that the region will continue to swing between wet years and dry years.

“The Los Angeles region resides in between a wetter northern rain regime and a drier southern one,” Hall explained. “These two influences have been in a tug of war for millennia, and our analysis suggests this pattern will continue.”

Although previous global climate models predicted that climate change would reduce rainfall to the south of Southern California, and increase rain to the north, the UCLA study is the first to look in detail at the Los Angeles region and determine the local forecast from those competing influences.

The study did not examine whether climate change has played a role in Southern California’s current historic drought. Other studies suggest that the lack of precipitation is within the bounds of natural variability, but that the drought’s severity is linked to hotter temperatures from global warming that appear to be increasing evaporation and decreasing soil moisture.

The research is associated with UCLA’s first Grand Challenge Project, which aims to move Los Angeles to renewable energy and local water by 2050 while protecting biodiversity.

To arrive at their projections, the researchers employed powerful simulation tools called global climate models, or GCMs. Those models are very low in resolution, so the team developed a novel technique to produce high-resolution projections that take the region’s microclimates into account.

The team produced a simulation of local climate from 1981 to 2000, which was used as a baseline for comparison to future climate conditions. Then, the scientists projected climate conditions for the periods 2041 to 2060 and 2081 to 2100, with the assumption greenhouse gas emissions continue to rise. Some of the GCMs showed a slight decrease in total wet-season precipitation, whereas others show a slight increase. But the scale of the potential changes is small.

Water managers at the Los Angeles Department of Water and Power, Los Angeles County Public Works and the Southern California branch of the Bureau of Reclamation said the study shows the need for increased investment in storm water capture infrastructure to increase the region’s reliance on local water and protect against flooding.

“The study, frankly, helps us move forward with our plans,” said Martin Adams, senior assistant general manager, water system, for the Los Angeles Department of Water and Power. “The study gives us confidence to proceed with our utility’s plans to increase local water supply from 11 percent this year to 36 percent by 2035.”

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