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Scientists who are working on various concepts for "geoengineering" the climate are almost comically eager to stress that they are not trying to come up with a substitute for reducing carbon dioxide emissions, the main cause of man-made global warming. They are just researching back-up systems that we might need if the reductions don't happen fast enough.
"It's hard to imagine a situation except a dire emergency where this will be used, but in order to have that conversation sensibly, we need to provide some evidence-based research," Dr Matt Watson, of Bristol University, told the British Science Festival in Bradford this month. He is planning to test the feasibility of an "artificial volcano" that injects sulphur dioxide into the stratosphere, so where better to try it than in the pancake-flat county of Norfolk?
Interestingly, he has the implicit blessing of the British armed forces, which take the threat of climate change very seriously. His experiment will be carried out at Sculthorpe airfield, a bomber base during World War 2 and still an active Royal Air Force facility. The goal is to find out whether non-rigid balloons (blimps) could be used to spray sulphur dioxide particles into the air and cool the planet's surface.
Why an artificial volcano? Nobel Prize-winning atmospheric chemist Paul Crutzen, who first suggested this method of cooling the planet five years ago, pointed out that big volcanic explosions inject millions of tonnes of sulphur dioxide into the stratosphere, where they remain on average for a couple of years. During that time, they reflect enough incoming sunlight to lower the temperature at the surface appreciably.
The Mount Pinatubo explosion in the Philippines in 1991 put enough sulphur dioxide in the upper atmosphere to lower the average global temperature by half a degree Celsius for two years. No mass extinction or global famine ensued, so we can tentatively assume that human beings could also put sulphur dioxide into the stratosphere without causing instant global disaster.
We cannot assume that there would be no negative side-effects, however, which is why some scientists have been urging that we carry out small-scale experiments to learn more about the process.
They also want to investigate how we could put such a large volume of sulphate particles into the stratosphere, for the task might turn out to be too expensive or just impossible.
That is the goal of a three-year project called Stratospheric Particle Injection for Climate Engineering (Spice), which is being supported by the universities of Bristol, Cambridge, Edinburgh and Oxford. The first phase of the project involves testing a blimp that will lift a reinforced hosepipe to an altitude of 1km and spray water into the air.
It's purely about delivery methods: will a balloon tethered to the ground be stable enough to support such a length of hose, or will it become uncontrollable in the wind? If it passes that test, then the long process begins of scaling up to a blimp big enough to support a 20km hose, for that is the height at which the sulphate particles must be dispersed in order to stay up for a long time.
A blimp big enough to do that would be the size of a football stadium, and the Spice researchers estimate that it would take about 20 of them, moored over the ocean, to cool the planet by 2degC. It would cost, they think, as little as $7 billion or as much as $75 billion, but even the latter sum would look affordable to a government in a panic - the kind of panic that would occur if the planet got 2degC hotter.
This is the first time that a geoengineering idea has moved out of the lab and into the real world, and it is bound to attract some very hostile attention. You still hear the argument that we should not even discuss geo-engineering techniques, for the knowledge that they might exist will lessen the pressure to cut carbon dioxide emissions quickly. But the cat is out of the bag, and the best we can do is to figure out whether they are really a viable option.
Nobody is claiming that geoengineering measures are a substitute for reducing our greenhouse gas emissions. But for all the talk about cutting emissions, they are still increasing, and it is highly unlikely that we will turn that trend around in time to avoid passing through 2degC hotter - after which the risk of uncontrollable, runaway warming gets very big.
The scientists who are investigating geoengineering are not reckless. They are actually the cautious ones, who want to have some fall-back plan available in case all the promises of future emissions cuts do not come true, just like all the past promises of emissions cuts failed to come true. We will get our emissions down eventually, no doubt, but it would be nice not to have a climate catastrophe in the meantime.
Can geoengineering avert catastrophe? Nobody really knows, and that's the point.
"It may turn out that this whole strategy is a bad strategy," as Dr Hugh Hunt of Cambridge University, who is leading the field test at Sculthorpe, told The Independent newspaper.
"That's what we're trying to find out."
- Gwynne Dyer is an independent London journalist.