Water vapor buoyancy mitigates climate warming in the tropics

While it is widely accepted that hot air rises, a new study from UC Davis has proven that sometimes cold air can rise too. In a tropical climate, the lightness of water vapor allows cold air to rise.

According to the researchers, this water vapor buoyancy effect helps to offset global warming in tropical climates.

The study is one of the first to demonstrate the profound effects that water vapor has on Earth’s climate and energy balance.

Study senior author Da Yang is an assistant professor of Atmospheric Science at UC Davis and a joint faculty scientist with the Lawrence Berkeley National Laboratory.

“It’s well-known that water vapor is an important greenhouse gas that warms the planet,” said Professor Yang. “But on the other hand, water vapor has a buoyancy effect which helps release the heat of the atmosphere to space and reduce the degree of warming. Without this lightness of water vapor, the climate warming would be even worse.”

Even with the same temperature and pressure conditions, humid air is lighter than dry air, which is called the vapor buoyancy effect. 

The research team discovered that cold, humid air rises and forms clouds and thunderstorms in the tropics, while warmer dry air sinks in clear skies. This means that Earth’s atmosphere emits more energy to space than it would without water vapor buoyancy.

The study revealed that the lightness of water vapor increases the planet’s thermal emission by about 1 to 3 watts per square meter over the tropics. This amount of energy is comparable to the energy captured by doubling carbon dioxide in the atmosphere. 

According to the researchers, their calculations indicate that the radiative effects of water vapor buoyancy increase with climate warming. 

The team concluded that a better understanding of the water vapor buoyancy effect and its stabilizing role in the tropics can improve cloud and thunderstorm simulations, as well as climate models.

“Now that we understand how the lightness of water regulates tropical climate, we plan to study whether global climate models accurately represent this effect,” said study lead author Seth Seidel.

The study is published in the journal Science Advances.

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By Chrissy Sexton, Earth.com Staff Writer