Water vapor, Earth’s most abundant greenhouse gas, has long been recognized for its role in climate change, though the extent of its contribution to global warming has remained a subject of debate. Recent data from NASA satellites has provided more precise estimates of water vapor’s heat-trapping effect, reinforcing its critical role in climate change.
Andrew Dessler and his team at Texas A&M University in College Station confirmed that the warming effect of water vapor is strong enough to double the climate warming caused by increased carbon dioxide levels in the atmosphere.
By utilizing new data from the Atmospheric Infrared Sounder (AIRS) aboard NASA’s Aqua satellite, the scientists were able to measure humidity across the lower 10 miles of the atmosphere with unprecedented accuracy. This data, combined with global temperature shifts, allowed the team to construct a detailed picture of how water vapor interacts with carbon dioxide and other heat-trapping gases. The findings, funded by NASA, were published in Geophysical Research Letters by the American Geophysical Union.
“Everyone agrees that adding carbon dioxide to the atmosphere leads to warming,” said Dessler. “The real question is: how much warming?”
From 2003 to 2008, data reveals the energy trapped by water vapor across southern to northern latitudes, with the highest concentration near the equator.
Credit: Andrew Dessler
The answer lies in understanding the magnitude of water vapor feedback. As temperatures rise, more water vapor is absorbed into the atmosphere, which in turn leads to even warmer conditions, creating a self-reinforcing cycle of warming and water vapor absorption.
Water vapor feedback also amplifies the warming effect of other greenhouse gases. As carbon dioxide increases, more water vapor enters the atmosphere, further intensifying the warming.
“The difference between an atmosphere with strong water vapor feedback and one with weak feedback is enormous,” Dessler added.
While climate models have predicted the strength of water vapor feedback, previous data on water vapor wasn’t detailed enough to offer a complete picture of how it responds to changes in Earth’s surface temperature. Past ground-based instruments and earlier space-based observations couldn’t measure water vapor across all altitudes in Earth’s troposphere—the layer of the atmosphere that extends from the surface up to about 10 miles high.
AIRS is the first instrument capable of distinguishing variations in water vapor at all altitudes within the troposphere. Using data from AIRS, the research team studied how atmospheric water vapor responded to changes in surface temperatures between 2003 and 2008. By analyzing how humidity levels shifted with surface temperature, they were able to calculate the average global strength of water vapor feedback.
“This new data set reveals that as surface temperatures rise, atmospheric humidity also increases,” Dessler explained. “By adding greenhouse gases to the atmosphere, we make it more humid. And since water vapor itself is a greenhouse gas, this increase in humidity amplifies the warming effect of carbon dioxide.”
The team found that if Earth’s temperature increases by 1.8 degrees Fahrenheit, the resulting rise in water vapor will trap an additional 2 watts of energy per square meter (about 11 square feet).
“That number might seem small at first, but when you account for the total energy trapped across Earth’s entire surface, water vapor is holding onto a significant amount of energy,” Dessler said. “We now believe that water vapor feedback is exceptionally strong, capable of doubling the warming caused by carbon dioxide alone.”
Because the new, precise observations align with previous estimates of water vapor’s impact, researchers are now more confident than ever in the model predictions, which suggest that Earth’s primary greenhouse gas will contribute to a temperature rise of several degrees by the end of this century.
“This study confirms that the predictions made by climate models are truly happening in the atmosphere,” said Eric Fetzer, an atmospheric scientist who works with AIRS data at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Water vapor is the dominant factor in the atmosphere when it comes to climate change.”