Improving effectiveness of solar geoengineering

(Press-News.org) Washington, D.C.— Solar radiation management is a type of geoengineering that would manipulate the climate in order to reduce the impact of global warming caused by greenhouse gasses. Ideas include increasing the amount of aerosols in the stratosphere, which could scatter incoming solar light away from Earth's surface, or creating low-altitude marine clouds to reflect these same rays.

Research models have indicated that the climatic effect of this type of geoengineering will vary by region, because the climate systems respond differently to the reflecting substances than they do to the atmospheric carbon dioxide that traps warmth in Earth's atmosphere. New work from a team including Carnegie's Ken Caldeira uses a climate model to look at maximizing the effectiveness of solar radiation management techniques. Their work is published October 21st by Nature Climate Change.

Attempting to counteract the warming effect of greenhouse gases with a uniform layer of aerosols in the stratosphere, would cool the tropics much more than it affects polar areas. Greenhouse gases tend to suppress precipitation and an offsetting reduction in amount of sunlight absorbed by Earth would not restore this precipitation. Both greenhouse gases and aerosols affect the distribution of heat and rain on this planet, but they change temperature and precipitation in different ways in different places. Varying the amount of sunlight deflected away from the Earth both regionally and seasonally could combat some of this problem.

By tailoring geoengineering efforts by region and by need, the team—led by California Institute of Technology's Douglas MacMartin—was able to explore ways to maximize effectiveness while minimizing the side effects and risks of this type of planetary intervention.

"These results indicate that varying geoengineering efforts by region and over different periods of time could potentially improve the effectiveness of solar geoengineering and reduce climate impacts in at-risk areas," Caldeira said. "For example, these approaches may be able to reverse long-term changes in the Arctic sea ice."

The study used a sophisticated climate model, but the team's model is still much simpler than the real world. Interference in Earth's climate system, whether intentional or unintentional, is likely to produce unanticipated outcomes.

"We have to expect the unexpected," Caldeira added. "The safest way to reduce climate risk is to reduce greenhouse gas emissions."

### David Keith of Harvard and Ben Kravitz, formerly of Carnegie but now at DOE's Pacific Northwest National Lab, are co-authors on the study. The Carnegie Institution for Science is a private, nonprofit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.