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Study: Humidity, Not Heat Alone, Is Important in Measuring Climate Change

A new study has found that water in the air, not just heat, is important in measuring global warming.

The combination of increasing heat and humidity can lead to increased weather extremes. The researchers said, when considering humidity and heat, the temperature increase was two times greater than earlier estimates.

The researchers said temperature by itself is not the best way to measure the effects of climate change. They said using temperature underestimates conditions in the tropical areas of the world.

The study was published January 31 in the science publication Proceedings¹ of the National Academy of Sciences.

The research team noted² that the energy created in extreme weather, such as storms, is related to the amount of water in the air. For this reason, the team of scientists decided³ to use a special atmospheric⁴ measurement. It is called equivalent potential temperature, or theta-e. The complex measurement is used to show the amount of heat in an area of air. It is expressed in a scientific measurement for temperature known as degrees Kelvin.

Veerabhadran Ramanathan was one of the writers of the study. He is a climate scientist at the University of California San Diego's Scripps Institution of Oceanography. "There are two drivers of climate change: temperature and humidity," Ramanathan said. "And so far, we measured global warming just in terms of temperature."

But by adding the energy from humidity, "the extremes — heat waves, rainfall and other measures of extremes — correlate much better," he said.

Warm air can hold more water, or moisture, than cold air. For every degree Celsius⁵ that air temperature increases, it can hold seven percent more water. When the water vapor⁶ in the air becomes liquid, it releases heat or energy, "that's why when it rains, now it pours," Ramanathan said.

He added that water vapor is a powerful heat-trapping gas in the atmosphere that increases climate change.

From 1980 to 2019, the researchers said the average world surface air temperature increased by 0.79 degrees Celsius. But when they considered energy from humidity, the researchers found their temperature measurement had increased by 1.48 degrees Celsius. In the tropics, they said the warming was as much as 4 degrees Celsius.

Ramanathan said when looking at surface air temperature, it appears that warming is strongest in North America, areas further from the tropics and at the poles.

But that is not the case, he said. The high humidity in the tropics creates more storm activity, from normal storms to severe ocean storms.

Donald Wuebbles is a climate scientist at the University of Illinois. He was not part of the study. But Wuebbles said the idea makes sense because water vapor is important in extreme rainfall. He said, "Both heat and humidity are important."

Katharine Mach is an environmental scientist at the University of Miami. She also was not part of the study. Mach said humidity is important "in shaping the impacts of heat on human health and well-being⁷, at present and into the future."

Words in This Story

humidity – n. the amount of moisture in the air

tropics – n. the part of the world that is near the equator where the weather is very warm

correlate – v. to have a close connection with something

vapor – n. a substance that is in the form of a gas or that consists of very small drops or particles mixed with the air

pole – n. either end of the imaginary line around which something (such as the earth) turns

impact – v. to have a strong and often bad effect on (something or someone)