Antacid for the atmosphere could cool down planet Earth

Could injecting one of the Earth's crust's most abundant compounds into the stratosphere protect against the impacts of global warming?(Credit: NASA)

Geoengineering. You might know it as the dastardly scheme to bail out big polluters, a last-ditch effort to save humanity or a needlessly dangerous attempt to intervene in the Earth's natural systems. In any case, swelling CO2 emissions and rising global temperatures mean that plenty of scientists are paying the concept plenty of attention. Among these is a team of Harvard researchers that has discovered a new kind of aerosol it says could be safely introduced into the atmosphere to bounce heat back out into space, and help repair the ozone layer while it's at it.

A variety of seemingly radical ideas fall under the umbrella of geoengineering. In general terms, it means altering the Earth's environment to push back against the impacts of global warming. Spreading iron throughout the ocean to promote growth of carbon-sucking plankton, launching heat-protecting shields into orbit and adding sun-reflective particles to the atmosphere are a few solutions that have been floated.

This final example has gained a fair bit of attention over the last few years, but that's not to say it is without serious drawbacks. The idea is inspired by erupting volcanoes, which spew particles upwards into the stratosphere where they reflect sunlight back into space, temporarily cooling the planet. While these particles soon fall back down to Earth and allow the planet to heat up again, the thinking with so-called solar geoengineering is that this thin layer of reflective sulfate aerosols would be replenished to help keep it cool.

Though this could help solve one crisis, it would likely create another. Once in the stratosphere, these aerosols produce sulfuric acid that erodes the ozone layer, which plays the critical role of soaking up ultraviolet light from the sun to protect us from skin and other cancers. And with the once-depleted ozone now on the mend courtesy of a 30-year global effort, putting it in harm's way once again would hardly be environmentally prudent.

So scientists have been exploring ways to overcome this dilemma with research into nonreactive aerosols that won't eat away at the ozone with such fervor, with a number of alternatives showing some promise. But researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have found success by taking the opposite approach, by investigating 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 sulfuric acid," says Frank Keutsch, co-author of the new study. "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 and his colleagues knew that the aerosol they were looking for would need to neutralize sulfuric, nitric and hydrochloric acid on its surface, so as to avoid damaging the ozone. By ruling out the unsuitable elements on the periodic table and then through modeling of stratospheric chemistry, the team landed on calcite, a constituent of rocks like limestone, marble and chalk, and one of the Earth's crust's most common compounds.

"Essentially, we ended up with an antacid for the stratosphere," said Keutsch.

The researchers calculated that calcite could not only reflect light into space and cool the planet, but actively repair the ozone layer by neutralizing the acids caused by pollution that kick off harmful chemical reactions. They have started laboratory tests that simulate stratospheric conditions to see if their theory holds up, and emphasize that there are a lot of questions to be answered before such an approach is considered for the real world, and even then, it's unlikely to offer more than a Band-aid-type solution.

"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."

The research was published in the Proceedings of the National Academy of Sciences.

Source: Harvard

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