Atmosphere additive outlined
The planet is warming, and if reducing greenhouse emissions is not enough to counter the risk, we may need more extreme measures.
One idea is solar geoengineering - injecting light-reflecting sulphate aerosols into the stratosphere to cool the planet.
Researchers know that large amounts of aerosols can significantly cool the planet, based on observations after large volcanic eruptions.
But these sulphate aerosols also carry significant risks, including producing sulphuric acid in the stratosphere, which damages the ozone layer.
Harvard researchers have now identified an aerosol for solar geoengineering that may be able to cool the planet while simultaneously repairing ozone damage.
“In solar geoengineering research, introducing sulphuric 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 Harvard’s School of Engineering and Applied Sciences (SEAS) and first author of the paper.
“This research is a turning point and an important step in analysing and reducing certain risks of solar geoengineering.”
This research fundamentally rethinks what kinds of particles should be used for solar geoengineering.
Previous research focused on ways to limit the ozone-damaging reactions produced by nonreactive aerosols, but researchers have now taken a completely different approach, targeting aerosols that are highly reactive.
“Anytime you introduce even initially unreactive surfaces into the stratosphere, you get reactions that ultimately result in ozone destruction as they are coated with sulphuric acid,” said researcher Frank 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.”
In order to keep aerosols from harming the ozone, the particles would need to neutralise sulphuric, 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 modelling of stratospheric chemistry, the team found that calcite, a constituent of limestone, could counter ozone loss by neutralising 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 Dr 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, but Keith and Keutsch caution that anything introduced 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 were keen to point out 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.”