Climate models

Lightness of water vapor adds weight to global climate models

Clouds are notoriously difficult to pin down, especially in climatology.

A study from the University of California at Davis and published in the journal Nature Geoscience shows that air temperature and cloud cover are strongly influenced by the buoyancy effect of water vapor, an effect currently overlooked in some models leading global climates.

Global climate models are the primary tools used to study the Earth’s climate, predict its future changes, and inform climate policy making. However, climate models often differ on the precise degree of future warming, largely due to their depiction of clouds.

“Climate models are the best tool we have for predicting future climate change,” said lead author Da Yang, an assistant professor of atmospheric science at UC Davis and a researcher at the Lawrence Berkeley National Lab. “It is important that we actively try to improve them.”

Clouds form over the California coast in 2002. (Jacques Descloitres, NASA/GSFC)

Cold air rising?

While conventional wisdom holds that warm air rises, the reverse is true in the tropical atmosphere, the study notes. Previous research by Yang and his colleagues has proposed that cold air rises in the tropics because moist air is lighter than dry air. This effect is known as vapor buoyancy and regulates the amount of low cloud over the subtropical ocean.

“Steam buoyancy influences the distribution of low clouds – the type of clouds we have off the California coast, which contribute greatly to the global energy balance,” Yang said. “The biggest challenge in accurately predicting future climate change is clouds, so we need to get the vapor buoyancy right.”

The biggest challenge in accurately predicting future climate change is clouds, so we need to get the vapor buoyancy right. —Da Yang

The study reported that six of 23 widely used climate models analyzed do not yet include this effect because water vapor is a trace gas, so its buoyancy effect was considered negligible. But the study shows that the buoyancy effect of vapor is more important than previously thought. In climate models without vapor buoyancy, low cloud cover may be shifted by about 50% in some regions.

How Clouds Affect Climate Change

Low clouds are among the most important clouds for climate change and the energy balance of the planet because they reflect a lot of sunlight. Fewer low clouds can lead to more sunlight absorption and a warmer planet. Lower clouds can create a cooler landscape.

“In a warmer climate, the buoyancy effect of water vapor would be increasingly important due to more atmospheric water vapor,” Yang said. “It is worth putting more effort into understanding how the buoyancy of water vapor regulates the Earth’s climate.”

Additional co-authors on the study include UC Davis graduate student Seth Seidel and Wenyu Zhou, a former member of Yang’s group, now at Pacific Northwest National Laboratory.

The study was funded by Packard Fellowship for Science and Engineering, National Science Foundation, Lawrence Berkeley National Laboratory and the US Department of Energy.

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