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The climate may be more sensitive to greenhouse gases than we thought. That鈥檚 because in a warmer world high-altitude clouds may become thinner, causing them to reflect less sunlight back into space.

鈥淭his is a previously unrecognised effect that should be looked into in more detail,鈥� says Richard Allan at the University of Reading in the UK, who was not involved in the study. 鈥淚f it is going on in the real world, you鈥檇 expect climate feedbacks to be more strongly amplified.鈥�

The main thing heating up Earth鈥檚 climate today is our emissions of greenhouse gases like carbon dioxide. However, we are still not sure how much a given amount of carbon dioxide will warm the planet. That鈥檚 because many feedback mechanisms activate as Earth warms up, some causing warming and some cooling.

The usual measure of this is the equilibrium climate sensitivity: how much the climate heats up if the carbon dioxide level doubles and Earth is allowed to settle. The sensitivity is , but it could be anywhere from 1.5 掳C to 4.5 掳C.

A clouded future

One of the biggest sources of uncertainty is the response of clouds to a warmer world, says Ryan Li of Yale University.

Climatologists are starting to understand low clouds, which cool the planet by reflecting sunlight. There is evidence that a warmer world will have fewer low clouds, so their cooling effect will weaken and warming will accelerate.

However, for high clouds like cirrus 鈥渢here isn鈥檛 yet a consensus鈥�, says Li. As well as cooling the planet by reflecting sunlight, they also trap heat.

In the early 2000s, climatologist Richard Lindzen argued that high cloud coverage would reduce in a warmer climate, like the iris of an eye opening. This would allow more heat to escape to space and significantly slow warming. However, this cooling effect has been 鈥渓argely disproved on a number of grounds鈥�, says Allan.

Instead, Li and his colleagues have found that high clouds will either have no effect on the climate or heat it faster. 鈥淭he iris feedback is unlikely to be negative,鈥� he says.

Thinner clouds

The team focused on thunderclouds in the tropics. Such clouds have narrow bases and wide tops, so they look like anvils. Their wide tops contribute a lot to high cloud cover.

The team simulated what happened to tropical thunderclouds if the climate warmed and, as a result, more of the water in clouds fell as rain rather than rising to high altitudes at the top of the 鈥渁nvils鈥�. They found that the high clouds鈥� ability to trap heat remained steady, because the area covered by the wide anvil-like tops did not change much even as more water was lost as rain at lower altitudes. However, if a lot of water was lost as rain, the high clouds became thinner, so they reflected less sunlight back into space.

鈥淚f you get cloud thinning, we show it will be a distinct positive feedback,鈥� says Li.

It鈥檚 an intriguing idea but there are two uncertainties, says Allan. First, the study only used one climate model, so the result 鈥渃ould be a quirk of this particular simulation鈥�. And second, the model must be checked against real-world data. 鈥淭hey need analysis of newer, more sophisticated satellite data to really see if this change in the efficiency of rain and the changes in cirrus cloud with warming are actually going on in the real world,鈥� says Allan.