“The benefits of climate engineering would be a cooler climate, and therefore likely less severe consequences due to CO2 warming,” said Dr. Trude Storelvmo, an associate professor in the Department of Geology and Geophysics at Yale University.
Dr. Storelvmo and other scientists are studying several approaches to artificially cool the earth. A technique called stratospheric aerosol injection involves adding reflective specks, such as sulfuric acid, to the upper layer of the atmosphere. These specks would reflect sunlight, that would otherwise warm the planet, away from the earth. Approximately 30 percent of incoming sunlight is reflected back into space while the remaining 70 percent is absorbed by the earth. By slightly increasing the reflectivity, less sunlight would be absorbed and the planet would become cooler.
Volcanic eruptions, which emit pollutants that contain sulfur, are a natural example of stratospheric aerosol injection. The 1815 eruption of Mount Tambora in Indonesia, for example, emitted approximately 28 megatons of sulfur and caused a “year without a summer” for many parts of the world. Snow fell in June of 1816 in Albany, New York and ice was observed in rivers and lakes as far south as Pennsylvania in July and August.
Scientists say that if stratospheric aerosol injection were to be used, the aerosols would be released into the stratosphere in the tropics to take advantage of the strong circulation patterns of the upper atmosphere. The aerosols would be carried by the wind towards the poles. The reflective material would be taken about 12 miles up into the stratosphere using aircraft, a balloon, or missiles, and then released.
Two other techniques, cirrus cloud thinning and marine cloud brightening, sound like they contradict each other. Cirrus clouds are high ice clouds that differ from other cloud types by absorbing more sunlight than they reflect, causing a net warming effect. Thinning these cirrus clouds would diminish their heat-trapping ability and help to cool the planet.
Marine cloud brightening, on the other hand, works in an opposite manner. Clouds would be made brighter in order to reflect more of the incoming sunlight. Salt from sea water, for example, would be sprayed in the atmosphere over the oceans, which would allow water droplets to form around the salt particle, forming clouds. Marine cloud brightening is already occurring unintentionally on a small scale from aerosols in the exhaust emitted by ships. But the question that remains is whether marine cloud brightening could work on a much larger scale to combat global warming.
Out of the many climate engineering techniques, stratospheric aerosol injection is “the most promising and achievable way” to reverse global warming, said Dr. Pete Irvine, a post-doctoral research fellow with the John A. Paulson School of Engineering and Applied Sciences at Harvard University.
Dr. Irvine is currently evaluating the potential effectiveness of climate engineering techniques for reducing sea level rise. However, both Dr. Irvine and Dr. Storelvmo from Yale agree that climate engineering would not be a perfect solution to global warming.
“A major concern with all types of climate engineering is that the research on them is in its infancy," said Dr. Storelvmo. "So we have very limited understanding of undesirable consequences and the ‘unknown unknowns’ are an obvious concern.”
One of those “unknowns” is the impact of stratospheric aerosol injection on the water cycle. Climate engineering is a controversial topic and is likely to come with some trade-offs. For example, stratospheric aerosol injection would produce significant cooling, but it is also likely to disturb the water cycle, causing more rainfall in some locations and less rainfall in others. So how will crops fare with the temperature and precipitation changes?
Dr. Storelvmo explained that cirrus cloud thinning would most likely have less of an impact on the water cycle, but it’s unclear whether this technique would work to cool the earth.
Dr. Irvine agreed, saying that global warming is making wet areas wetter, but with climate engineering, “wet areas tend to get a bit drier.” He added that we could, however, go “half way,” injecting a smaller amount of reflective material into the stratosphere. In that case, the water cycle would remain about the same. “It’s a good idea to do some; it’s a bad idea to do too much,” Dr. Irvine said, referring to stratospheric aerosol injection.
In addition, sulfur and sulfuric acid are not harmless; they can have negative effects, such as acid rain, health problems, pollution, and destruction of the ozone layer. Because of this, Dr. Irvine said other, potentially less harmful materials are being considered for stratospheric aerosol injection, such as aluminum, diamond, and calcite.