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Should we flood the air with sulfur?

July 12, 2006 By Ned Stafford This article courtesy of Nature News.

Nobel chemist lends weight to geoengineering schemes.

A soon-to-be-published paper by a Nobel laureate will seriously consider injecting sulphur into the stratosphere to combat climate change. His article is already creating a buzz, some of which is highly sceptical.

Paul Crutzen, co-winner of the 1995 Nobel Prize in Chemistry for ozone research, admits that until two years ago he was doubtful that climate-engineering proposals were a good or viable option for our planet. But, he says, he has been driven to explore the idea out of "desperation", given the lack of international action to reduce greenhouse-gas emissions.

The world needs to be prepared for a worst-case scenario, he says.

Cool stuff

The fact that Crutzen is talking about it gives it more legitimacy.
Gavin Schmidt
NASA Goddard Institute for Space Studies
In his paper, due to be published in Climatic Change1, Crutzen argues that it might be possible to cool Earth by injecting sulphate particles into the layer of atmosphere that starts about 10 kilometres above the ground. This would artificially enhance Earth's reflectivity to bounce a larger portion of solar radiation back into space.

Gavin Schmidt, a climate modeller at the NASA Goddard Institute for Space Studies in New York, has highlighted the proposal on a blog called RealClimate. He points out that in principle this is "a relatively old idea". And he doubts that the details of such a scheme could ever be properly worked out.

But, Schmidt adds, "the fact that Crutzen is talking about the idea gives it legitimacy. I see his paper as a request for the scientific community to look at the issue more seriously."

That is exactly what Crutzen intends, although he realizes his entry into the debate will generate some controversy. "I will have friends. I will have enemies. I hope I can spark a good discussion."

Volcanic proposal

In essence, Crutzen suggests we mimic the after-effects of a volcanic eruption. Mount Pinatubo's eruption in June 1991, for example, injected some 10 million tonnes of sulphur as sulphur dioxide into the tropical stratosphere. There it formed submicrometre sulphate particles, which cooled the planet by about 0.5°C.

So to compensate for the warming produced by doubled carbon dioxide levels, for example, Crutzen estimates the atmosphere would need to be loaded with 5.3 million tonnes of sulphur. The particles would only be expected to stay in the air for one to two years, so this would require continuous, sizeable injections at great cost.

Hydrogen sulphide would create suitable sulphate particles, says Crutzen. It might be put in place by balloons or artillery, but "delivery is my weakness", he says.

He admits that negative side effects are possible, such as damage to the protective ozone layer and increased acid rain. Indeed, attempts are currently being made to reduce the 55 million tonnes of suphur that are released into the lower atmosphere each year as a result of burning coal and oil.

Sulphate loading would lead to "some whitening of the sky, but colourful sunsets and sunrises would also occur," adds Crutzen.

Not in my lifetime

Crutzen estimates that it would take 20 years of intensive international research before the idea could be put into effect. "I'm 72 years old," he says. "This will not happen in my lifetime."

Schmidt cautions that such geoengineering schemes can be used as an excuse for inaction. "There are clearly people who don't want to do anything about greenhouse gases, who seize on an idea like this," he says.

The Nobel laureate agrees that cutting emissions is the wisest course of action. "It would be best if emissions of the greenhouse gases could be reduced so much that the stratospheric sulphur-release experiment would not need to take place," he says. "Currently, this looks like a pious wish."

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  1. Crutzen P., Climatic Change, (in the press) (2006).


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