Study says geoengineering doesn’t offer a win-win solution
By Summit Voice
FRISCO — Trying to hack Earth’s climate by shading the planet could reduce total global precipitation by more than 4 percent, according to researchers with the National Center for Atmospheric Research.
The impacts of trying to deflect incoming solar radiation by injecting sulfates into the atmosphere would be most pronounced in seasonal monsoon precipitation. According to the NCAR-led study, monsoonal rains in North America, East Asia, and other regions could be reduced by 5-7 percent compared to preindustrial conditions. Globally, average precipitation could decrease by about 4.5 percent.
“Geoengineering the planet doesn’t cure the problem,” said NCAR scientist Simone Tilmes, lead author of the study. “Even if one of these techniques could keep global temperatures approximately balanced, precipitation would not return to preindustrial conditions.”
The authors warned that Earth’s climate would not return to its preindustrial state even if the warming itself were successfully mitigated.
“It’s very much a pick-your-poison type of problem,” said NCAR scientist John Fasullo, a co-author. “If you don’t like warming, you can reduce the amount of sunlight reaching the surface and cool the climate. But if you do that, large reductions in rainfall are unavoidable. There’s no win-win option here.”
The study appears in an online issue of the Journal of Geophysical Research: Atmospheres, published this week by the American Geophysical Union. An international team of scientists from NCAR and 14 other organizations wrote the study, which was funded in part by the National Science Foundation,NCAR’s sponsor.
The research team turned to 12 of the world’s leading climate models to simulate global precipitation patterns if the atmospheric level of carbon dioxide, a leading greenhouse gas, reached four times the level of the preindustrial era. They then simulated the effect of reduced incoming solar radiation on the global precipitation patterns.
The scientists chose the artificial scenario of a quadrupling of carbon dioxide levels, which is on the high side of projections for the end of this century, in order to clearly draw out the potential impacts of geoengineering.
In line with other research, they found that an increase in carbon dioxide levels would significantly increase global average precipitation, although there would likely be significant regional variations and even prolonged droughts in some areas.
Much of the reason for the increased rainfall and snowfall has to do with greater evaporation, which would pump more moisture into the atmosphere as a result of more heat being trapped near the surface.
The team then took the research one step further, examining what would happen if a geoengineering approach partially reflected incoming solar radiation high in the atmosphere.
The researchers found that precipitation amounts and frequency, especially for heavy rain events, would decrease significantly. The effects were greater over land than over the ocean, and particularly pronounced during months of heavy, monsoonal rains.
Monsoonal rains in the model simulations dropped by an average of 7 percent in North America, 6 percent in East Asia and South America, and 5 percent in South Africa. In India, however, the decrease was just 2 percent. Heavy precipitation further dropped in Western Europe and North America in summer.
The researchers found two primary reasons for the reduced precipitation.
One reason has to do with evaporation. As Earth is shaded and less solar heat reaches the surface, less water vapor is pumped into the atmosphere through evaporation.
The other reason has to do with plants. With more carbon dioxide in the atmosphere, plants partially close their stomata, the openings that allow them to take in carbon dioxide while releasing oxygen and water into the atmosphere. Partially shut stomata release less water, so the cooled atmosphere would also become even drier over land.
Tilmes stresses that the authors did not address such questions as how certain crops would respond to a combination of higher carbon dioxide and reduced rainfall.
“More research could show both the positive and negative consequences for society of such changes in the environment,” she says. “What we do know is that our climate system is very complex, that human activity is making Earth warmer, and that any technological fix we might try to shade the planet could have unforeseen consequences.”