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Why do we blame climate change on carbon dioxide, when water vapor is a much more common greenhouse gas?

Extra water vapor we put in the atmosphere doesn’t last long enough to change the long-term temperature of our planet. But water does play a major supporting role in climate change.

November 3, 2023

With all the attention given to humans’ climate-warming carbon dioxide (CO₂) emissions, you might be surprised to learn that CO₂ is not the most important greenhouse gas affecting the Earth’s temperature. That distinction belongs to water.

We can thank water vapor for about half of the “greenhouse effect” keeping heat from the sun inside our atmosphere.¹ “It’s the most important greenhouse gas in our climate system, because of its relatively high concentrations,” says Kerry Emanuel, professor emeritus of atmospheric science at MIT. “It can vary from almost nothing to as much as 3% of a volume of air.”

Compare that to CO₂, which today makes up about 420 parts per million of our atmosphere—0.04%—and you can see immediately why water vapor is such a linchpin of our climate system.

So why do we never hear climate scientists raising the alarm about our “water emissions”? It’s not because humans don’t put water into the atmosphere. Even the exhaust coming from a coal power plant—the classic example of a climate-warming greenhouse gas emission—contains almost as much water vapor as CO₂.² It’s why that exhaust forms a visible cloud.

But water vapor differs in one crucial way from other greenhouse gases like CO₂, methane, and nitrous oxide. Those greenhouse gases are always gases (at least when they’re in our atmosphere). Water isn’t. It can turn from a gas to a liquid at temperatures and pressures very common in our atmosphere, and so it frequently does. When it’s colder it falls from the air as rain or snow; when it’s hotter it evaporates and rises up as a gas again.

“This process is so rapid that, on average, a molecule of water resides in the atmosphere for only about two weeks,” says Emanuel.

This means extra water we put into the atmosphere simply doesn’t stick around long enough to alter the climate; you don’t have to worry about warming the Earth every time you boil a kettle. And there’s really no amount of water vapor we could emit that would change this. “If we were to magically double the amount of water vapor in the atmosphere, in roughly two weeks the excess water would rain and snow back into oceans, ice sheets, rivers, lakes, and groundwater,” Emanuel says.

Nonetheless, water vapor is an important part of the climate change story—just in a slightly roundabout way.

At any given temperature, this is a theoretical upper limit to the amount of water vapor the air can hold. The warmer the air, the higher that upper limit. And while the air rarely holds as much water as it could—thanks to rain and snow—Emanuel says that over the long term, rising temperatures steadily raise the average amount of water vapor in the atmosphere at any given time.

And of course, temperatures today are rising, thanks to humans’ emissions of longer-lasting greenhouse gases like CO₂. Water vapor amplifies that effect. “If the temperature rises, the amount of water vapor rises with it,” says Emanuel. “But since water vapor is itself a greenhouse gas, rising water vapor causes yet higher temperatures. We refer to this process as a positive feedback, and it is thought to be the most important positive feedback in the climate system.”

In short, it’s true that water vapor is in some sense the “biggest” greenhouse gas involved in climate change, but it’s not in the driver’s seat. CO₂ is still the main culprit of the global warming we’re experiencing today. Water vapor is just one of the features of our climate that our CO₂ emissions are pushing out of balance—well beyond the stable levels humanity has enjoyed for thousands of years.

Thank you to several readers for sending in related questions, including Arthur Donavan of Reno, Nevada, Jen-shih Lee of Rancho Santa Fe, California, and John Mitchell of Palm Bay, Florida. You can submit your own question to Ask MIT Climate here.