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AGU Meeting: To geoengineer or not to geoengineer

By Liz Kalaugher

This year’s AGU Fall Meeting session on geoengineering had twice as many submissions as last year – proof that the field is attracting increasing serious attention. But it’s still a highly controversial area. Not only are there ethical issues involved in committing future generations to maintaining the technology and the fact that it may negatively affect some regions of the globe, but also little is known about which approach is best, how effectively it will work or how much it will cost.

One potential method – introducing sulphate aerosols into the atmosphere to reflect sunlight away from Earth – has already been tested to some degree by nature as a result of volcanic eruptions. Alan Robock of Rutgers University, US, outlined his estimates of the costs of injecting 1 Teragram of sulphur (in the form of hydrogen sulphide) into the lower stratosphere. Robock says hundreds of US KC-135 Stratotankers for refuelling jets are about to become obsolete and could be diverted for geoengineering use. That would cost
around $70 million a year, which compares relatively favourably with the $30 billion a year he estimates for using balloons or naval rifles to inject the sulphur, or the $800 billion it could take to develop a space elevator. “Using airplanes would not be costly, especially if we use existing military ones,” concluded Robock, “but there are still many reasons not to.”

David Mitchell of the Desert Research Institute, on the other hand, is looking at modifying cirrus clouds to prevent them trapping so much of the longwave radiation from the surface of the Earth. Seeding the clouds with a compound such as silver iodide leads to the production of larger ice crystals that fall out of the cloud quicker. Ultimately the system could result in less cirrus cloud coverage and lower atmospheric humidity levels, enabling more longwave radiation to escape into space. Mitchell says we could introduce the seeds into the upper troposphere either at mid-latitudes and the poles, where the greenhouse effect is largest, or over the whole globe. One means to do this could be for the airline industry to dope fuel with the seeding compound or introduce it separately into jet engine exhaust fumes.

 Mitchell sees the technique as potentially buying time for a transition to green technologies and stresses that it wouldn’t solve ocean acidification. But it does have the advantage over sulphur injection techniques that it wouldn’t cause acid rain or ozone depletion.

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