Scientists who are working on various concepts for "geo-engineering" the climate are almost comically eager to explain 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 backup 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, England, last week.
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 at Sculthorpe air force base in the pancake-flat county of Norfolk?
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 reflect enough incoming sunlight to lower the temperature at the surface.
So Watson wants to find out whether nonrigid balloons (blimps) could be used to spray sulphur dioxide particles into the air and cool the planet's surface.
The three-year project is called Stratospheric Particle Injection for Climate Engineering (SPICE), and is sponsored by the universities of Bristol, Cambridge, Edinburgh and Oxford.
The first phase of the project involves testing a blimp that will lift a reinforced hose pipe to an altitude of one km and spray water into the air. 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 20-km 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 2 degrees C.
The cost, they think, might be as little as $7 billion or as much as $75 billion, but even the latter would look affordable to a government in a panic -- the kind of panic that would occur if the planet gets 2 degrees C (3.5 degrees F) hotter.
This is the first time that a geo-engineering 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. Can geo-engineering 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 a London-based independent journalist whose articles are published in 45 countries.