Discovery.com, July 8, 2008
A proposal to reverse climate change by placing mirrors in the sky to reflect sunlight away from Earth won't give us back the same climate we had before we started emitting so much carbon dioxide, says a new study.
Researchers at the University of Bristol in the United Kingdom applied state-of-the-art global climate models to predict the effect of using reflective sunshades to send a fraction of the sunlight that enters Earth's atmosphere back into space before it can heat things up.
They compared two futures with four times the pre-industrial level of CO2: one where nothing was done, and another where the sunlight intensity was reduced to a level that achieved the lower, pre-industrial global average temperature, despite the higher CO2 levels.
The second case simulated the use of enough sunshades to drop the average temperature to pre-industrial levels, which turned out to be a reduction in sunlight of about 4.2 percent.
"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," said Dan Lunt, who led the study, published in Geophysical Research Letters.
"We found warming in the Arctic and Antarctic regions, and a cooling in the equatorial regions," Lunt said. "We got a decrease in rainfall a lot of places in the world, decreases in sea ice, and some changes in the El Nino 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 said.
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 Ken Caldeira of the Carnegie Institution in Stanford, Calif., also modeled 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 said.
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 said, "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," agreed climatologist Alan Robock of Rutgers University in New Brunswick, N.J.
"The solution to the climate problem is mitigation, not geoengineering," he added. "These things are not perfect and there's the potential for unintended consequences."