News in Science

Solar shades won't reverse global warming

A proposal to place mirrors in the sky to reflect sunlight away from earth won't give back the climate we had before, says a new study.

Researchers at the University of Bristol in the United Kingdom applied global climate models to predict the effect of using reflective sunshades to reduce the amount of sunlight that enters the earth's atmosphere back.

They compared two future scenarios, both with CO2 levels four times the pre-industrial baseline. In one scenario nothing was done, and in the other, sunlight intensity was reduced to a level that would reduce the global average temperature to pre-industrial levels

The second scenario simulated the use of sunshades, which reduced sunlight by about 4.2%.

"Although we managed to cancel out warming on a global average, what you end up with is some areas that warm up and some that cool down," says Dr Dan Lunt, who led the study, published in Geophysical Research Letters .

Warmer poles

"We found warming in the Arctic and Antarctic regions, and a cooling in the equatorial regions," Lunt says. "We got a decrease in rainfall in a lot of places in the world, decreases in sea ice, and some changes in the El Niño phenomenon."

These changes resulted because the mirrors reduce sunlight more at the equators than near the poles, while CO2 has a warming effect that's more equally distributed, Lunt says.

The sunshades also do nothing to prevent changes caused by increased CO2, such as ocean acidification or changes to plant growth.

Although the sunshade approach won't bring back the same climate as reducing CO2 emissions, "it is highly successful compared to doing nothing," Lunt adds.

Lunt's team's work was not the first to try to understand the effects of sunshades using a climate model.

Work by Dr Ken Caldeira of the Carnegie Institution in Stanford, California, also modelled a 'sunshade world', but with a less complex model that did not include complete accounting for ocean circulation and sea ice.

"The study confirms our earlier findings with a better model," Caldeira says.

Other solutions

Although Lunt's model suggests sunshades are better than nothing, he would rather see efforts focused elsewhere.

"My personal opinion is that we should be focusing our time and money on actually reducing emissions," he says, "rather than some manmade monstrosity in space."

"The biggest problem I have with geoengineering discussions now is that the prospect of it working will reduce efforts to mitigate the problem by reducing fossil fuel emissions," says climatologist Professor Alan Robock of Rutgers University in New Brunswick.

"The solution to the climate problem is mitigation, not geoengineering," he says. "These things are not perfect and there's the potential for unintended consequences."