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Clouds slow down warming in the short term

November 10, 2016, by Tim Radford

clouds warming

US scientists think the effect clouds have on global warming may be underestimated.
Image: Daniel Spiess via Flickr.

New research shows that clouds block sunlight and reflect radiation back to space, impeding the rate of global warming. But for how long?

LONDON, 10 November, 2016 Scientists may be one step nearer to a solution of the riddle of the clouds: yes, they do damp down global warming, but this feedback effect may not last.

And, if so, the implication is that global warming due to rising carbon dioxide concentrations in the atmosphere has been underestimated, according to new research in the journal Nature Geoscience.

At the heart of the mystery is a puzzle easier to measure than to grasp: the behaviour of low-level clouds. Put crudely, clouds block sunlight and reflect radiation back into space. In a warming world the capacity of the atmosphere to hold water rises, so a warmer world could mean higher densities of cloud cover which would slow the rate of warming.

Uncertain evidence

But is that happening? The evidence so far has been uncertain. One group has reported that greater emissions of pollution have led to cloud formations that may have reduced overall warming. Yet other scientists have reported that they are not so sure.

Another group has calculated that low-level cloud over Greenland may even have accelerated the dramatic thaw of the northern hemisphere’s biggest reserves of ice in 2012. And a comprehensive study earlier this year has concluded that there are no easy answers.

The core of the problem is measurement over a long period of time: global air and sea surface temperatures have been carefully observed and recorded for more than a century. But scientific studies of cloud cover – literally, an overview date only from the satellite age.

Our results indicate that cloud feedback and climate
sensitivity calculated from recently observed trends
may be underestimated, since the warming
pattern during this period is so unique”

Most satellite data start around 1980, so linear trends over the last three decades are often used to make inferences about long-term global warming and to estimate climate sensitivity,” says Chen Zhou of the US Lawrence Livermore National Laboratory, who led the study.

Our results indicate that cloud feedback and climate sensitivity calculated from recently observed trends may be underestimated, since the warming pattern during this period is so unique.”

The US scientists matched satellite observations with climate models to see what could be happening over a long period of time. The message from the latest study is that cloud feedback is likely to be positive in the long term but has been negative over the last 30 years. So clouds have masked the intensity of warming, for now.

Anomalous clouds

The simulations predict that the planet will warm in ways that make low clouds – the ones that reflect most sunlight – less likely. But in the last 30 years, tropical surface temperatures have gone up in places where the air ascends and fallen where the air descends, to keep the lower atmosphere moist and cloudy.

But while increasing concentrations of carbon dioxide, released from car exhausts and factory chimneys in the combustion of fossil fuels, imply ever higher average global temperatures, this is not the only factor at work: volcanic eruptions, aerosol pollution and other agencies introduce natural variability. So short-term trends, the scientists argue, could be highly misleading.

Their study concludes: “Sea surface temperature pattern-induced low cloud anomalies could have contributed to the reduced warming between 1998 and 2013, and offer a physical explanation of why climate sensitivities estimated from recently observed trends are probably biased low.” Climate News Network

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