UCLA researchers have discovered a way to reduce uncertainty in global climate models’ projections of how climate change will alter rain and other precipitation.

Global climate models agree that climate change will cause precipitation to increase on average, with wet regions becoming wetter and dry areas getting drier. However, according to the findings published today in Nature, most models overestimate the amount that precipitation will increase due to climate change. The study, part of a joint project by UCLA and the Lawrence Livermore National Laboratory, shows that the global average of precipitation will go up, but by approximately 40 percent less than global models currently predict.

Not only will the research make the climate models more accurate, it will also decrease variation among the different models by an estimated 35 percent, according to the study. That will give greater clarity to the international policy makers who depend upon the models to plan for the effects of climate change, researchers said. Global climate models are analyzed and applied by scientists for reports used by the United Nations, including at the U.N. COP21 Paris climate talks wrapping up this week.

► More details are available in this Q&A about the study.

“The projected change in the hydrologic cycle, or water cycle, is one of the most important dimensions of climate change apart from warming because it’s critical for understanding the future of things like water resources, agriculture and ecosystem health,” said principal investigator Alex Hall, a professor in UCLA’s department of atmospheric and oceanic sciences and director of UCLA’s Center for Climate Change Solutions. “Changes in water will be one of the key factors reshaping the planet.”

A goal of the research is to increase the accuracy and decrease the variation among different models, said Anthony DeAngelis, the lead author and a postdoctoral researcher in UCLA’s department of atmospheric and oceanic sciences. While global climate models agree on many details, the broad range in the models’ anticipated global precipitation changes attracted UCLA atmospheric scientists’ attention.

“The greater the agreement in the global climate models, the higher our confidence in the projections,” DeAngelis said. “There are physical laws behind important factors controlling precipitation, and the results flowing from these factors should be predictable.”

In examining a variety of potential causes for the variation, the scientists found that different global climate models disagreed on how much sunlight is absorbed in the atmosphere, which influences the amount of precipitation. Comparing model simulations with satellite observations, they found that many models underestimated sunlight absorption. Sunlight — in the form of shortwave solar radiation — is absorbed by water vapor in the atmosphere. The more the sun’s energy is absorbed, the warmer the atmosphere becomes, and the less precipitation falls.

A stupendous number of calculations — enough to tax even the most powerful supercomputers — are needed to project the complex interaction between the sun’s energy and water vapor in the air, DeAngelis said.

“The theory behind sunlight absorption is sound, but the implementation of this theory in global climate models varies significantly,” DeAngelis said. Some models use scientifically outdated or oversimplified calculations, which contributes to the wide variance in projected precipitation, he said.

The UCLA researchers emphasized their confidence in global climate models’ ability to project other key aspects of the planet’s future climate, adding that although the models are not perfect, they are constantly being improved. This study is part of that process.

The research contributes to the goals of UCLA’s Sustainable L.A. Grand Challenge, a campuswide initiative to transition the Los Angeles region to 100 percent renewable energy, 100 percent local water and enhanced ecosystem health by 2050.

The implications of the findings at a local scale remain to be seen, Hall said. The research projects climate change will cause a less dramatic increase in precipitation from rain, snow, storms and other components of the hydrologic cycle, but that relates only to the global average of precipitation, Hall emphasized.

“It’s necessary to paint this bigger picture before we can zoom in,” Hall said. “We still don’t understand what this change in the hydrologic cycle means. We know it probably means more floods and more droughts, among other impacts. Even though our findings show the increased precipitation will be 40 percent smaller than currently projected, it’s a leap to say that we expect 40 percent fewer droughts and floods.”