We may need shades.

For the past year, Angel has been looking at ways to cool the Earth in an emergency. He's been studying the practicality of deploying a space sunshade in a global warming crisis, a crisis where it becomes clear that Earth is unmistakably headed for disastrous climate change within a decade or two. . .

Angel is now publishing a first detailed, scholarly paper, "Feasibility of cooling the Earth with a cloud of small spacecraft near L1," in the Proceedings of the National Academy of Sciences. The plan would be to launch a constellation of trillions of small free-flying spacecraft a million miles above Earth into an orbit aligned with the sun, called the L-1 orbit.

The spacecraft would form a long, cylindrical cloud with a diameter about half that of Earth, and about 10 times longer. About 10 percent of the sunlight passing through the 60,000-mile length of the cloud, pointing lengthwise between the Earth and the sun, would be diverted away from our planet. The effect would be to uniformly reduce sunlight by about 2 percent over the entire planet, enough to balance the heating of a doubling of atmospheric carbon dioxide in Earth's atmosphere. . .

The lightweight flyers designed by Angel would be made of a transparent film pierced with small holes. Each flyer would be two feet in diameter, 1/5000 of an inch thick and weigh about a gram, the same as a large butterfly. It would use "MEMS" technology mirrors as tiny sails that tilt to hold the flyers position in the orbiting constellation. The flyer's transparency and steering mechanism prevent it from being blown away by radiation pressure. Radiation pressure is the pressure from the sun's light itself.

The total mass of all the fliers making up the space sunshade structure would be 20 million tons. At $10,000 a pound, conventional chemical rocket launch is prohibitively expensive. Angel proposes using a cheaper way developed by Sandia National Laboratories for electromagnetic space launches, which could bring cost down to as little as $20 a pound.

The sunshade could be deployed by a total 20 electromagnetic launchers launching a stack of flyers every 5 minutes for 10 years. The electromagnetic launchers would ideally run on hydroelectric power, but even in the worst-case environmental scenario with coal-generated electricity, each ton of carbon used to make electricity would mitigate the effect of 1000 tons of atmospheric carbon.

Once propelled beyond Earth's atmosphere and gravity with electromagnetic launchers, the flyer stacks would be steered to L-1 orbit by solar-powered ion propulsion, a new method proven in space by the European Space Agency's SMART-1 moon orbiter and NASA's Deep Space 1 probe.

"The concept builds on existing technologies," Angel said. "It seems feasible that it could be developed and deployed in about 25 years at a cost of a few trillion dollars. With care, the solar shade should last about 50 years. So the average cost is about $100 billion a year, or about two-tenths of one percent of the global domestic product."

See Social Dunces for an earlier discussion of geoengineering, and Free Fire Zone for an even earlier discussion.

The main point made in Social Dunces was that politicized scientists sought to suppress discussion of geoengineering.

"The knowledge that we maybe could engineer our way out of climate problems inevitably lessens the political will to begin reducing carbon dioxide emissions," observes David Keith from the University of Calgary in Canada. . .

Ken Caldeira agrees that geoengineering is, for the moment, a tempting but illusory quick fix to an intricate system; a much less problematic solution, he says, would be to change our lifestyles by reducing energy consumption and CO2 emissions.

"I think the Earth's system is so complicated that our interfering with it is very likely to screw things up and very unlikely to improve things," he says. "And this is the only planet we have."

Hopefully, political will won't rear its ugly head since that has historically been the road to damnation. Caldeira is utterly naive to think that "a much less problematic solution. . . would be to change our lifestyles by reducing energy consumption and CO2 emissions". That's a sure way to "screw things up". It may be easy to say but a moment's reflection reveals that it is impossible to do, and that the attempt would be destructive, leaving us less able to mount a necessary defense.

If the threat is real then we need sensible responses. We can't reduce energy use, not with a global population set to rise to half again its current size, and the vast majority of humanity still grubbing out a precarious living devoid of education, health care or opportunity to participate in developed society. We need much, much more energy but we need to generate it is better ways, and use it more efficiently. The crunch may be that the old ways damage us too greatly before the new ways come on line.

Having a shade to give us more time to evolve makes perfect sense. There are worrisome considerations - such as the effect on plant growth from reducing insolation "about 2 percent over the entire planet". Will this matter at a time when we also need to double production of food and fiber? Perhaps not since sunlight is seldom the limiting factor, but it needs clarification. Another issue is the cost. It's far, far cheaper than the sort of expenditures being glibly discussed by politicians (see Stern), but still, a few trillion dollars isn't chump change.

What a disappointment this would be for politicians. Their grand opportunity to rule the world may slip away, turn into a rather ordinary project of relatively short duration. Fear not. They'll soon have another scheme.