This would be a powerful mitigation against any global warming that would need to be combined with a mitigation for ocean acidification. Also, the world would still need to work towards decarbonizing (getting rid of fossil fuel usage) energy production and transportation
“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, the Gordon McKay Professor of Applied Physics at SEAS and professor of public policy at the Harvard Kennedy School, the 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, the Stonington Professor of Engineering and Atmospheric Science at SEAS and professor of chemistry and chemical biology, a co-author of the paper.
Solar geoengineering — injecting light-reflecting sulfate aerosols into the stratosphere to cool the planet. Researchers know that large amounts of aerosols can significantly cool the planet; the effect has been observed after large volcanic eruptions. But these sulfate aerosols also carry significant risks. The biggest known risk is that they produce sulfuric acid in the stratosphere, which damages ozone. Since the ozone layer absorbs ultraviolet light from the sun, its depletion can lead to increased rates of skin cancer, eye damage, and other adverse consequences.
PNAS - Stratospheric solar geoengineering without ozone loss
The combination of emissions cuts and solar geoengineering could reduce climate risks in ways that cannot be achieved by emissions cuts alone: It could keep Earth under the 1.5-degree mark agreed at Paris, and it might stop sea level rise this century. However, this promise comes with many risks. Injection of sulfuric acid into the stratosphere, for example, would damage the ozone layer. Injection of calcite (or limestone) particles rather than sulfuric acid could counter ozone loss by neutralizing acids resulting from anthropogenic emissions, acids that contribute to the chemical cycles that destroy stratospheric ozone. Calcite aerosol geoengineering may cool the planet while simultaneously repairing the ozone layer.
Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO3) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of −1 W⋅m−2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y−1 of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.
In order to keep aerosols from harming the ozone, the particles would need to neutralize sulfuric, nitric, and hydrochloric acid on their surface. 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, which included sodium and calcium carbonate.
“Essentially, we ended up with an antacid for the stratosphere,” said Keutsch.
Through extensive modeling of stratospheric chemistry, the team found that calcite, a constituent of limestone, could counter ozone loss by neutralizing emissions-borne acids in the atmosphere, while also reflecting light and cooling the planet.
“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 that mimic stratospheric conditions. Keith and Keutsch caution that introducing anything into the atmosphere may have unanticipated 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 researchers emphasize that even if all the attendant risks could be reduced to acceptable levels, solar geoengineering is not a solution to climate change.
“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.”