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AGU Meeting: To geoengineer or not to geoengineer
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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