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Water confirmed as major greenhouse gas

Climate change

The distribution of atmospheric water vapour

Warm and wet: The distribution of atmospheric water vapour, a significant greenhouse gas, varies across the globe. During mid 2005, this image shows that most collected at tropical latitudes, particularly over south Asia, where monsoon thunderstorms swept it high above the land.

Credit: NASA

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SYDNEY: Using satellite data, researchers have precisely estimated the heat-trapping effect of water in air, validating its role as a critical component of climate change.

Water vapour has been known to be Earth's most abundant greenhouse gas, but the extent of its contribution to global warming has been debated.

Now a team at Texas A&M University have confirmed that the heat-amplifying effect of water vapour is potent enough to double the warming effect of increasing levels of carbon dioxide in the atmosphere.

Effect on carbon dioxide

The experts used data from NASA's Aqua satellite to measure humidity in the lower layers of the atmosphere.

Combined with global observations of temperature shifts, the information allowed them to build a comprehensive picture of the interplay between water vapour and other greenhouse gases.

The results are published in the journal Geophysical Research Letters.

"Everyone agrees that if you add carbon dioxide to the atmosphere, then warming will result," said lead researcher Andrew Dessler. "So the real question is, how much warming?"

The answer can be found by estimating the magnitude of water vapour feedback, he said. Increasing water vapour leads to warmer temperatures, which causes more water vapour to be absorbed into the air. Warming and water absorption therefore increase in a spiralling cycle.

This feedback can also amplify the warming effect of other greenhouse gases. "The difference in an atmosphere with a strong water vapour feedback and one with a weak feedback is enormous," Dessler said.

Feedback mechanism

Climate models have estimated the strength of water vapour feedback, but until now the record of water vapour data was not sophisticated enough to provide a comprehensive view of at how the vapour responds to changes in Earth's surface temperature.

"This new data set shows that as surface temperature increases, so does atmospheric humidity," Dessler said.

"Dumping greenhouse gases into the atmosphere makes the atmosphere more humid. And since water vapour is itself a greenhouse gas, the increase in humidity amplifies the warming from carbon dioxide."

Specifically, the team found that if Earth warms 1ºC, the associated increase in water vapour will trap an extra two watts of energy per square metre.

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