Solar Geoengineering

Researchers from Harvard's John A. Paulson School of Engineering and Applied Sciences (SEAS) are rethinking solar geoengineering, a process some argue is a relatively cheap way to help lower the Earth's warming temperatures and offset problems caused by greenhouse gas emissions.

The basic idea behind solar geoengineering is that we inject the stratosphere with reflective aerosols that would reflect some light away from the planet. Based on the aftermath of large volcanic eruptions, we know that large amounts of aerosols in the stratosphere do cool the Earth, but they also produce sulfuric acid, which damages the ozone layer.

In a study published in the Proceedings of the National Academy of Sciences, the researchers from Harvard propose using aerosols that wouldn't just reflect light away from the planet, but that would also help prevent ozone depletion. Talk about a two-in-one solution.

"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," author Frank Keutsch from SEAS told "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 researchers emphasize that, though it would work in theory to temporarily cool the planet, solar geoengineering is not a permanent solution to the problem of 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."

Credit: NASA

An Antacid for the Stratosphere

The new aerosol developed by the team uses calcite, a compound commonly found in the Earth's crust.  Normally, in using aerosols as coolants, one goes for less reactive substances to mitigate ozone layer damage. But with calcite, the researchers took the opposite route. "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," Keutsch explained.

Calcite came out as the most viable substance for this type of aerosol after the researchers extensively studied models of stratospheric chemistry. It works as a coolant, while also neutralizing emissions-borne acids to counter ozone damage. "Essentially, we ended up with an antacid for the stratosphere," said Keutsch.

The team is already testing calcite in lab experiments mimicking stratospheric conditions, but their work still requires much more research. "Stratospheric chemistry is complicated, and we don't understand everything about it," said researcher David Keith.

Solar geoengineering isn't intended to be the solution to global warming. It's merely a stopgap measure to aid other, more encompassing efforts, like those promoted by the Paris climate agreement. Even if proven feasible, deliberately altering the chemistry of our stratosphere carries huge risks that would need to be very carefully weighed.

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