August 22, 2016, by Tim Radford
Cloudy sunset in Phuket, Thailand: what clouds do to climate is still unsettled.
Image: 29cm via Wikimedia Commons
The perennial question of how clouds affect the Earth’s climate takes another twist, with one study expecting cooling and another the opposite.
LONDON, 22 August, 2016 – Scientists have just been presented with new evidence on how tropical clouds’ climate impact affects rates of global warming, and therefore need to be factored into computer simulations of climate change over the next century.
Confusingly, one study says thin tropical clouds at 5km height are far more common than thought, and have a substantial cooling effect on climate.
The other suggests that as the world warms there will be fewer low-level clouds, which will therefore reflect less sunlight back into space and possibly push global temperatures to 2.3°C above the average for most of human history.
The findings are not contradictory: the first, in Nature Communications, addresses evidence derived from space-borne study of clouds at mid-level right now. The second, in the American Meteorological Society’s Journal of Climate, examines changes with time in the prevalence of clouds at the lower levels.
What both papers do is deliver a reminder that climate is a complex bit of machinery and that, as the great Danish physicist Niels Bohr is supposed to have said, prediction is very difficult, especially about the future.
For the Nature Communications study, Quentin Bourgeois of the Bolin Centre for Climate Research in Stockholm University and colleagues used space-borne instruments and numerical models to look at the mid-level clouds in the tropics and found that the cooling effect of these could be as large as the warming induced by the cirrus clouds at high level.
Since clouds cover more than 70% of the Earth’s surface at any time, and since different types of clouds impact climate differently, there should be no surprise that the overall effect of cloud cover in climate is a puzzle.
One study has suggested researchers over-estimate the cooling effect of clouds. Another has attributed the dramatic thaw of Greenland’s icecap in 2012 to clouds, while other studies have concentrated on exploring the mechanisms and the dynamics of cloud formation. So both studies represent small pieces of a giant atmospheric jigsaw puzzle.
“Climate sensitivity is more likely situated in the upper half of previous estimates, probably around four degrees”
For the Journal of Climate study, two scientists from the Swiss Federal Institute of Technology, usually known as ETH Zurich, studied 15 years of radiometer data from Nasa satellites. These continuously measure how much sunlight is reflected back into space, and the variations in the data showed that, in the past, there have been fewer low-lying clouds than in the cooler years.
It follows that, as the world warms, the cloud cover at this altitude will tend to thin. The study suggests, not for the first time, that the 195 nations which in Paris in December agreed to contain global warming to less than 2°C have set an ambitious target.
All the observational data from this study suggest that if carbon dioxide in the atmosphere doubles then average global temperatures will rise substantially. Researchers like to call this climate sensitivity.
“It’s very unlikely that the climate sensitivity is less than 2.3°C,” said Tapio Schneider, one of the authors. “Climate sensitivity is more likely situated in the upper half of previous estimates, probably around four degrees.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.