By Catherine Brahic
Planetary engineering projects to cool the planet could backfire quite spectacularly: a new model shows that a “sulphate sunshade” would punch huge holes through the ozone layer above the Arctic.
To make matters worse, it would also delay the full recovery of the Antarctic ozone hole by up to 70 years.
Pumping tiny sulphate particles into the atmosphere to create a sunshield that would keep the planet cool was first suggested as a solution to global warming by Edward Teller, a physicist was best known for his involvement in the development of the hydrogen bomb.
Simone Tilmes of the National Center for Atmospheric Research in Colorado, US, used computer models to see how a sulphate sunshade would affect the ozone layer, which protects us from harmful UV rays. She says it could have “a drastic impact”.
Tilmes modelled two different scenarios: one in which “large” particles measuring 0.43 microns in diameter are used, and one where the particles are two-and-a-half times smaller.
Sulphate particles catalyse the breakdown of ozone molecules by chlorine atoms. Western economies have almost entirely stopped using chlorine-based coolants called CFCs, thanks to the Montreal Protocol. However, such substances are increasingly being used in Asia and the atmosphere is still full of CFCs emitted during the 20th century.
In January 2008, researchers described how much of each type of sulphate particle would need to be injected into the stratosphere in order to compensate for a doubling in atmospheric carbon dioxide concentrations (Geophysical Research Letters, DOI: 10.1029/2007GL032179). Tilmes used these volumes in her computer models.
She found that injections of small particles over the next 20 years could thin the wintertime ozone layer over the Arctic by between 22 and 76%. Large particles, which would have less of a cooling effect, according to previous research, would still reduce Arctic ozone by 15 to 50% during the winter.
In the Antarctic, the injections would delay the recovery of the existing ozone hole by 30 to 70 years.
A thinner ozone layer – popularly known as an ozone “hole” – lets more UV rays through, which can cause an increase in the incidence of various cancers. According to NASA, a 1% decrease in the ozone layer can cause an estimated 2% increase in UV-B irradiation, leading to a 4% increase in basal carcinomas – the most common form of skin cancer.
In 2007, Ken Caldeira of the Carnegie Institution of Washington in the US found that if a sulphate sunshield were deployed and then removed – for instance because of a change in governments – the effects of global warming after the removal would be far worse than before the sunshield.
Caldeira has also found that a sunshade could cause severe drought.