Limestone Dust: The Latest Climate Geoengineering Favourite

Guest essay by Eric Worrall

A US study has suggested hurling crushed limestone into the stratosphere to halt global warming, on the grounds that it would do less damage than sulphates, and might help repair the ozone layer.

Atmospheric limestone dust injection could halt global warming

Geoengineering using limestone aerosols would also help to stop ozone layer depletion

Distributing limestone particles into the upper atmosphere could apply the brake to global warming while simultaneously repairing the ozone layer, scientists in the US propose. A solid aerosol of limestone or the mineral calcite dispersed 20km up would reflect and scatter incoming solar radiation, slowing greenhouse gas warming, and neutralise three important halide-bearing acids responsible for ozone destruction.

Up until now, atmospheric geoengineering research had focused on injecting sulfates into the stratosphere, but this would generate sulfuric acid and damage the ozone layer. Conversely, calcite or limestone would neutralise the acids HNO2, HCl and HBr, which provide the nitrogen, chlorine and bromine radicals that destroy ozone. ‘This would restore the ozone layer, though not perfectly,’ says senior author David Keith at Harvard University. ‘A base would react with those main acids in the stratosphere, which are dominant in the catalytic cycles that manage ozone.’

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‘We are not suggesting that we go ahead and manipulate the whole planet, but a flight experiment from a stratosphere balloon could make a small cloud of aerosol, with a few hundred grams of material and allow us to make real in situ measurements of stratospheric chemistry,’ says Keith. There are lots of uncertainties about reaction rates at present, he adds.

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Read more: https://www.chemistryworld.com/news/atmospheric-limestone-dust-injection-could-halt-global-warming/2500141.article

The abstract of the study;

Stratospheric solar geoengineering without ozone loss

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.

Read more: http://www.pnas.org/content/early/2016/12/07/1615572113

Its nice that the geoengineering crowd seem to have moved onto less toxic options than sulphates.

Limestone is quite dense, around three times denser than water, so it would be interesting to see how long the particles stayed aloft. But the full text of the study seems to suggest fluffing the particles up a bit, making them porous, to increase surface area available for neutralising ozone destroying acids. This would likely create a pumice like consistency, which could help the particles persist longer in the stratosphere.

The study also sounds a cautious note about the possible impact of their limestone particles on bioavailability of nitrates, and other potential environmental impacts. So maybe limestone might be as damaging as sulphates after all.