It’s becoming increasingly clear that the climate is warming faster and more significantly than expected. This means that soon, simply reducing greenhouse gas emissions alone will not be enough to mitigate these environmental changes.
Emission reduction efforts are still key, and the Paris climate agreement of last year set the goal of maintaining global temperatures at no higher than 1.5 degrees Celsius above historic preindustrial levels. However, it is anticipated that supplemental efforts will also be needed.
(Image courtesy of David Keith/Harvard.)
One of the more contentious ideas is an effort called solar geoengineering, which involves injecting light-reflecting sulfate aerosols into the stratosphere. Researchers know that large amounts of aerosols in the upper atmosphere can contribute to cooling the planet; this effect has been observed after large volcanic eruptions that spew ash and other particulates high into the air.
But using geoengineering methods that involve these sulfate aerosols bring forward a lot risks. The most significant of these risks is that sulfate aerosols can produce sulfuric acid in the stratosphere, which damages ozone. Because the ozone layer absorbs ultraviolet light from the sun, the depletion of the ozone layer can lead to increased rates skin cancer, eye damage and other adverse consequences for humans.
However, recently a research team of professors and graduate students from the School of Engineering and Applied Sciences (SEAS) at Harvard University state they have identified an aerosol that could be used for solar geoengineering that could help cool the planet while simultaneously repairing ozone damage.
“In solar geoengineering research, introducing sulfuric acid into the atmosphere has been the only idea that had any serious traction, until now,” said David Keith, a professor of applied physics at SEAS and first author of the paper. “This research is a turning point and an important step in analyzing and reducing certain risks of solar geoengineering.”
This research fundamentally rethinks what kinds of particles should be used for solar geoengineering, said Frank Keutsch, a professor of engineering and atmospheric science at SEAS and coauthor of the paper.
Most research up until now has focused on finding methods to limit the ozone-damaging reactions produced by nonreactive aerosols. Keutsch and Keith, however, pursued an alternate approach by targeting highly reactive aerosols.
“Anytime you introduce even initially unreactive surfaces into the stratosphere, you get reactions that ultimately result in ozone destruction as they are coated with sulfuric acid,” said Keutsch. “Instead of trying to minimize the reactivity of the aerosol, we wanted a material that is highly reactive, but in a way that would avoid ozone destruction.”
The particles need to neutralize sulfuric, nitric and hydrochloric acids on their surface in order to keep aerosols from harming the ozone. To find such a particle, Keutsch turned to his handy Periodic Table. After eliminating the toxic elements, the finicky and rare metals, the team was left with the alkali and alkaline earth metals, including sodium and calcium carbonate.
“Essentially, we ended up with an antacid for the stratosphere,” said Keutsch.
By extensively modeling stratospheric chemistry, the team determined that calcite, a constituent of limestone, could reflecting enough light to contribute to cooling the planet, while also countering ozone loss by neutralizing emissions-borne acids in the atmosphere.
“Calcite is one of the most common compounds found in the earth’s crust,” said Keith. “The amounts that would be used in a solar geoengineering application are small compared to what’s found in surface dust.”
The researchers have already begun testing calcite in lab experiments mimicking the conditions found in Earth’s stratosphere. However, Keith and Keutsch still advise caution, as anything that gets introduced into the atmosphere could result in unexpected and unintended consequences.
“Stratospheric chemistry is complicated, and we don’t understand everything about it,” Keith said. “There are ways that this approach could increase global ozone, but at the same time, because of the climate dynamics in the polar regions, increase the ozone hole.”
The research team emphasizes that even if every type of risk involved could be reduced to acceptable levels, solar geoengineering is still not a solution to climate change in and of itself.
“Geoengineering is like taking painkillers,” said Keutsch. “When things are really bad, painkillers can help, but they don’t address the cause of a disease and they may cause more harm than good. We really don’t know the effects of geoengineering, but that is why we’re doing this research.”
This research is one of many projects that will be part of Harvard’s Solar Geoengineering Research Program, a new interdisciplinary research effort launching in the spring. Operating within the Harvard University Center for the Environment, the program aims to become one of the largest and most visible solar geoengineering research initiatives.
The research paper is available through the Proceedings of the National Academy of Sciences, or you can watch the authors describe their paper in this video.