Billionaire Richard Branson will announce today in London a prize of $25 million to the inventor of a device that effectively reduces greenhouse gas concentrations. Although the participants are under a media embargo, American climatologist James Hansen — who will serve as a judge of the potential inventions, along with English scientist James Lovelock and Australian author Tim Flannery — did discuss the topic of geoengineering a solution to global warming this week in front of a large crowd at U.C. Santa Barbara, as part of a lecture he gave on the dangers of human-caused climate change.

Hansen was asked what he thought about a U.S. government suggestion (PDF) that the IPCC should look into engineering to “modify solar radiance,” if attempts to reduce greenhouse gas emissions fail. Hansen said that this was a reference to an idea floated last year by an atmospheric chemist named Paul Crutzen. Crutzen won the Nobel Prize in l995 for his work on ozone depletion, and proposed to blanket the earth with sulphur particles, to reduce the amount of sunlight reaching the atmosphere. Hansen said:

I think Paul was trying to wake people up. These schemes have disadvantages; they’re very expensive, and so maybe he was doing a public service, but I think obviously the first thing we should be doing is trying to reduce greenhouse gas emissions.

In fact, when Crutzen spoke about this idea last year, he commented:

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Importantly, its possibility should not be used to justify inadequate climate policies, but merely to create a possibility to combat potentially drastic climate heating. The very best would be if emissions of the greenhouse gases could be reduced. Currently, this looks like a pious wish.

Estimates are that to blanket the earth with sulphur particles would cost in the range of $25-50 billion for a two-year stretch. Another noteworthy scientist at the lecture, Ralph Keeling of Scripps Institution of Oceanography, had doubts about the idea, pointing out that:

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It’s very hard to be sure we’re not doing tremendous harm, which would be more intentional than what we’re doing with greenhouse gases.

The two also discussed another scheme by a Columbia physicist named Klaus Lackner to use what some call “synthetic trees” to reduce the concentrations of CO2 in the atmosphere. In theory, 250,000 such devices could remove the 22 billion tons of carbon dioxide deposited in the air annually.

Hansen did say that two weeks ago he got the call about the prize from Branson. With a smile, he added:

Maybe that will encourage someone to come up with a solution to remove these gases, but I still say it would be easier not to put them there in the first place.

Hmmm. Pardon my skepticism, but isn’t $25 million how much the U.S. government offered for the capture of Osama Bin Laden?