Spraying millions of metric tons of sulfate particles into the atmosphere could reverse some human-caused global warming, a new study shows. But the simulations also reveal that the technique, which mimics the short-term cooling effects of volcanic eruptions, could chill the planet if overdone.
Alarmed by the lack of progress by governments in finding ways to slow emissions of CO2, climate researchers have begun to examine so-called geoengineering schemes (ScienceNOW, 9 November 2007). One method involves injecting large amounts of sulfate particles into the upper atmosphere. Like the particles emitted during volcanic eruptions, sulfate aerosols cool the Earth by blocking a portion of the sun's rays. Previous studies of how such a scheme would lower Earth's temperature used back-of-the-envelope calculations. Instead of simulating the behavior of aerosols, researchers have turned down the intensity of the sun manually in their models.
In the new work, published in Geophysical Research Letters late last month, researchers modified an established climate model to gauge the effects of varying aerosol amount and size. The findings confirmed the previous rough estimates that releasing large amounts of sulfates can reverse some impacts of higher carbon emissions. For example, in a simulated world where the atmospheric CO2 levels were double today's values--a scenario many scientists believe likely--models predict that Earth will warm by more than 2°C. Adding a million metric tons a year of smaller sulfate aerosols to the stratosphere reduced that increase to 1°C and forestalled some predicted effects on precipitation, including more rain in the Southern Ocean and along the equator. "Geoengineering pushes the climate more towards a climate that looks like today's," says atmospheric scientist Philip Rasch of the National Center for Atmospheric Research in Boulder, Colorado, which conducted the study.
But the models also suggest that the scheme could go too far: Adding excess sulfur could increase ice in Antarctica, "overcompensating" for warming, says Rasch, which could affect ecosystems and the global ocean-atmosphere system in a myriad of ways that scientists haven't studied.
Climate modeler Christopher Bretherton of the University of Washington, Seattle, called the study "good, solid, important work … [suggesting] stratospheric aerosol injection is an approach to compensate greenhouse warming that is worthy of further study." However, other researchers see limitations. If nations begin to geoengineer using aerosols, says climate researcher Michael MacCracken of the Climate Institute in Washington D.C., the aerosols would likely be emitted in certain spots across the globe, not spread equally through the atmosphere, as occurred in models the scientists used. Uneven distribution of the aerosols could lead to more cooling in some places than in others, which could cause unknown environmental consequences.
In addition, the study doesn't address larger concerns about geoengineering, says climate modeler Raymond Pierrehumbert of the University of Chicago in Illinois. He says that global temperatures could soar if scientists stopped geoengineering. He also worries that the use of the technique might encourage nations to continue emitting carbon dioxide "essentially into perpetuity."