A new study could bolster water conservation efforts in L.A., including projects like this wetland park, which treats storm water.

Bri Weldon/Flickr

A 50-year weather forecast for Los Angeles

For decades, Los Angeles has guzzled far more of its water from melted snow in the Sierra Nevada mountain range and the Colorado River than from local, rain-fed rivers and aquifers. But although climate change threatens to make mountain snow less reliable, new research says southern California’s rain won’t dry up in the future. The analysis—one of the most detailed climate change forecasts for any city to date—predicts that Los Angeles’s average rainfall will probably stay roughly the same in decades ahead, despite the current drought.

The city of L.A. commissioned the research 4 years ago as part of a series of studies to help it prepare for the shocks and stresses of climate change. The city couldn’t rely on existing global climate models to churn out the detailed information that, say, a civil engineer or a water utility manager needs. Most global models of climate change place anything the size of L.A. County (less than 150 kilometers wide) into roughly a pixel—meaning one data point and a single prediction for the entire region. But “there’s clearly large variation in L.A. from the coast to the mountains,” says Neil Berg, a climate scientist on the University of California, Los Angeles (UCLA), team that headed the work. Climate change could have different impacts on the many types of terrain that stretch across L.A. “It’s hard to make local policy when you don’t have a clear picture of how various neighborhoods could be hit.”

To make a more detailed model of L.A.’s climate future, Berg and his colleagues logged countless hours on UCLA’s on-campus supercomputer. They combined two common methods of small-scale climate modeling—one using statistical calculations, one using simulations of weather—and weaved together information from 36 global climate models. Their new model forecasts the climate in different parts of L.A. County between 2041 and 2060 (a time frame that also lets the city begin making long-term planning decisions) and is so fine-grained that it can predict the average weather in areas as small as 2 kilometers across.

The study, published online today in the Journal of Climate, describes what future rain will look like in a typical year, but doesn’t comment on how often drought might loom over southern California. Moreover, as Berg points out, “everyone who lives here knows that we rarely have an average wet season.” L.A.’s weather can be capricious, seesawing between drier and stormier winters. That won’t change, according to the study, but local rainfall could be more valuable as other water sources become scarcer. L.A.’s rain-fed rivers and aquifers “would be a good source to conserve even more,” Berg says.

The UCLA team’s approach to developing this model could prove useful to other scientists working to produce fine-scale forecasts of climate change. “The whole idea of using this hybrid approach is new. … It’s very intriguing,” says Robert Oglesby, an earth and atmospheric scientist at the University of Nebraska, Lincoln, who wasn’t involved in the new study. But Oglesby cautions that this approach could be harder to pull off elsewhere in the world, such as in Africa and Latin America, where some types of weather records and climate data aren’t as easily available.

William Gutowski, a meteorologist at Iowa State University in Ames, praises the authors for creating a model that is both rigorous and also practical, with concrete information about water and weather. “That puts it into a context where people who are using that climate data for, say, planning purposes can understand it,” he says.

Berg says he has come to appreciate “how sensitive Los Angeles is to the handful of storms that we get and just how vital those storms are.” He and his colleagues hope the study will be particularly useful for the L.A. region’s local governments, many of which are already trying to collect and use more of the water that falls during local rainstorms and rely less on snow-fed water sources. Los Angeles Mayor Eric Garcetti recently instructed the city to cut its reliance on distant water sources in half by 2024, and the city of Santa Monica plans to use only local water by 2020.

The UCLA researchers have also used the same method to make predictions for future heat waves in the region. They are now working on forecasts of the effects climate change will have on snowfall in the mountain ranges that ring L.A., on local rivers, and on water resources in the Sierra Nevada.