UW Researchers Show Reliable Predictions of Earth’s Future Require Getting Daily Cycle of Clouds Right

Clouds play an important role in determining the current and future temperature and global warming rate of the Earth. They simultaneously cool the Earth by reflecting sunlight and warm the planet by insulating Earth’s thermal radiation from escaping to space.

 

Hence, clouds and how they are changing with global warming are the leading source of uncertainty in researchers’ ability to predict future climate. Earth system models (ESMs) are the best tool available to predict the state of the Earth decades in the future at a global scale.

 

However, there is still a problem. Errors in the energy balance of ESMs can be traced to the time of day they predict clouds to have peak coverage, according to a group of University of Wyoming researchers.

 

“In this paper, we show that within ESMs there are errors in the time of day that clouds are simulated, which could have a substantial impact on the cooling effect that clouds have on Earth’s surface,” says Travis Aerenson, an associate research scientist in the UW Department of Atmospheric Science. “Ultimately, however, the total error in the cooling effect of clouds in ESMs is small compared to the error in their simulation of the time of day when clouds occur. Hence, there are compensating errors in the way clouds are represented within the ESMs, such that they may be getting the right radiative balance with errors in their underlying physics.”

 

The paper, titled “Global models predict clouds at the wrong time of day: Does it matter for radiation and climate?,” was published today (Dec. 12) in Science Advances. As the American Association for the Advancement of Science’s open-access, multidisciplinary journal, Science Advances publishes impactful research papers and reviews in any area of science, including disciplinary-specific and broad interdisciplinary areas.

 

Aerenson was the paper’s lead author. Other UW contributors were Daniel McCoy, an assistant professor of atmospheric science; Jacqueline Nugent, an associate research scientist, and Hunter Brown, a postdoctoral research associate, both in the Department of Atmospheric Science; and August Mikkelson, a Ph.D. candidate from Mill Creek, Wash., studying atmospheric science.

 

Outside contributors were Gregory Elsaesser, a research scientist in Columbia University’s Department of Applied Physics and Applied Mathematics, and the NASA Goddard Institute for Space Studies (GISS); Jingbo Wu, a senior staff associate II at Columbia University’s Center for Climate Systems Research, who also works at NASA-GISS; and Mark Zelinka, an atmospheric scientist at Lawrence Livermore National Laboratory.

 

Aerenson and his team developed a new analytical model that connects diurnal cloud biases to emergent mean state radiative biases, according to the paper. With the use of satellite observations, the model demonstrated there are errors in the time of day that clouds are occurring on ESMs that would cause bias in shortwave cloud radiative effects greater than 45 percent of the total bias. Shortwave cloud radiative effects depend on the radiative properties of the clouds that are present and the amount of incident sunlight at a given time.

 

However, such errors in the cloud diurnal cycle are masked by other compensating errors, indicating these ESMs are getting the right answer but for the wrong reasons, the paper says.

 

“These errors in clouds within ESMs contribute to uncertainty in our projections of future global warming,” Aerenson says. “However, they do not alter the primary conclusions gleaned from ESMs, such as the occurrence of global warming and the associated changes in the hydrological cycle due to anthropogenic emissions of greenhouse gases. The errors identified in this paper do show uncertainty in the rate at which the Earth is warming.”

 

According to the paper, these new findings open the door to fresh possibilities in geoengineering to mitigate anthropogenic climate change. A prime example would be marine cloud brightening, a proposed method in which aerosols are introduced into weak precipitating cloud regimes to increase the number of cloud condensation nuclei. This, in turn, increases the reflectivity and lifetime of the clouds, causing more sunlight to be reflected by the clouds.

 

Work at UW was supported by the Department of Energy through the Established Program to Stimulate Competitive Research (EPSCoR), Atmospheric Systems Research, and Earth and Environmental Systems Modeling. This research was a part of a larger project titled “The Perturbed Physics Ensemble Regression Optimization Center for ESM Evaluation and Development,” which has leveraged DOE and UW resources to develop a new framework for climate model evaluation.