Climate Forecast: Cloudy, but Slowly Clearing
Severity of future global warming due to increasing greenhouse gases is uncertain for several reasons. A key mystery is: what will happen to clouds as climate warms? Clouds reflect sunlight, cooling the Earth, but they also trap heat, warming the Earth. Different clouds do more of one or the other, depending on how bright and how high in the sky they are. Overall, today's Earth is cooler than it would be with no clouds. As climate changes, though, cooling by clouds may either intensify (e.g., if Earth gets cloudier or clouds become brighter) or diminish (if it gets less cloudy, or clouds get thinner or move to higher altitudes). In a warmer world, more water should evaporate from the oceans, but this doesn't mean that Earth will become cloudier, because warmer air can hold more water vapor before it saturates and forms clouds. (Summer, for example, is much warmer than winter in many parts of the world, yet the summer sky is no cloudier than the winter sky.)
In climate models used to predict conditions of the next century, we must estimate when clouds will form, given knowledge only of the average temperature, humidity, pressure and winds over an area the size of New York state. Many current climate models predict only the cloud cover (fraction of the sky covered by clouds) and cloud altitude, assuming that clouds will remain as bright as they are today even as the climate changes. We have developed a cloud model that allows the cloud brightness to change as well (Del Genio et al., 1996) and have used it in climate change simulations with the GISS global climate model (GCM). To do this, we must estimate a variety of things about how clouds behave: not only when they form, but also whether they are liquid or ice and how much of each, how big the cloud droplets or crystals are, whether they produce rain or snow, how quickly they evaporate when dry air from the outside mixes in, and how thick they are.
When the climate warms, our predicted cloud cover decreases slightly, i.e., skies clear a bit overall. This makes the climate warmer than it would be if clouds remained fixed. However, large anvil clouds associated with thunderstorms in the tropics get somewhat brighter, partly offsetting the global decrease of cloud cover. The size of this compensation depends on whether the tropical east Pacific Ocean warms more or less than the waters of the tropical west Pacific, a topic about which climate modelers currently do not agree. In the GISS GCM, the east warms more than the west, and as a result a climate with twice the current carbon dioxide level eventually warms by about 6°F. While uncertainty remains, the advent of climate models that predict more of the features of clouds and how they change brings us one step closer to a reliable forecast of decadal climate changes.
Del Genio, A.D., M.-S. Yao, W. Kovari and K.K.-W. Lo 1996. A prognostic cloud water parameterization for global climate models. J. Climate 9, 270-304.
Please address all inquiries about this research to Dr. Anthony Del Genio.