January 14, 2014
Currently, one of the most talked about geoengineering ideas is Solar Radiation Management (SRM), which intends to block shortwave solar radiation, thus cooling the Earth to offset rising temperatures. In other words, SRM may be one way in which global temperatures could be artificially stabilized. But a new study in the Journal of Geophysical Research: Atmospheres, finds that while SRM-style geoengineering may succeed in cooling the Earth, it would also disrupt precipitation patterns around the world.
The June 12, 1991 eruption column from Mount Pinatubo taken from Clark Air Base. Image resized. U.S. Geological Survey Photograph taken by Richard P. Hoblitt.
Climate change is generally expected to increase global precipitation because the heat trapped near Earth's surface leads to higher rates of evaporation. But using SRM is expected to decrease rainfall. Therefore to better understand what rainfall may look like in the future under various scenarios, scientists led by Simone Tilmes of the National Center for Atmospheric Research (NCAR) manipulated 12 of the world's leading climate simulation models via the Geoengineering Model Intercomparison Project (GeoMIP).
Model accuracy is important, and those used by this team were vetted against past observations and shown to correctly predict previous long-term climatic observations. However, their accuracy diminished when used for more specific weather events.
"For instance, over the last 100 year period, most of the models showed the observed temperature increase;" said Tilmes, "however there are still uncertainties in the details…Climate models cannot predict specific weather phenomena, but they can predict that for instance, hurricanes become stronger, rainfall extremes increase, and that can be compared if we have long observational records."
The team began by setting global CO2 levels at an extremely high level: 1,120 parts per million (ppm) or four times the pre-industrial level of around 280 ppm. This level is on the extreme side of projections for the end of this century. But the team wanted to test how effective SRM geoengineering would be against a drastic increase in CO2.
The scientists found that global precipitation rates increased by approximately seven percent under the extreme climate change scenario, without geoengineering intervention . However, within the seven percent increase in precipitation, there were regional variations of rainfall and snowfall as well as prolonged droughts in some parts of the world. For instance, precipitation increased 10 percent over East Asia and India while there was a seven percent reduction in the North American summer monsoon. There were also variations in precipitation rates when comparing land (3.7 percent increase) with oceans (7.1 percent increase). In addition, the researchers observed a 50 percent increase in the number of months with high-intensity rainfall.
However, when the scientists ran the models again, this time with SRM-style geoengineering, things changed significantly. The researchers found that intervention with SRM geoenigeering caused a 4.5 percent reduction in global precipitation as opposed to pre-industrial levels. Again, the scientists observed regional variations: monsoon precipitation decreased by six percent in East Asia, seven percent in North America, six percent in South America, five percent in South Africa and two percent in India. Notably, decreases in precipitation rates on land and in oceans were similar, showing only minor variations compared to the extreme climate change conditions. During high-intensity rainfall months there was a 20 percent reduction in the frequency of monsoon-level rainfall events.
The overall impact of SRM on global mean temperatures was cooling in the tropics and warming in the higher latitudes.
The NCAR study shows that climate engineering can be used to return global temperatures to pre-industrial conditions, even when CO2 levels are extremely high. But these methods cannot solve the associated effects on the water cycle.
"Now we know that any geoengineering scheme would only work with drastic mitigation measures in place, because it can be only used as a short-time fix, to buy some time," said Tilmes. "If we would continue, more and more geoengineering would be required, which is not possible. Most scientists agree that the only way geoengineering should ever become an option is to have a strict strategy of greenhouse gas emission reductions go along with it."
- Tilmes, S. et. al (2013). The hydrological impact of geoengineering in the Geoengineering Model Intercomparison Project (GeoMIP). Journal of Geophysical Research: Atmospheres, Vol. 118, 11,036–11,058.
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