I missed a February article from MIT Technology Review on "Cooling the Planet." It asks this question: If we can’t adequately reduce or sequester carbon emissions, are more-radical alternatives like orbital mirrors a solution to climate change?
In the past two decades, various novel planet-cooling technologies have been proposed -- improbable, monumental projects such as putting into orbit giant mirrors with thousand-kilometer diameters or clouds of trillions of wafer-thin, butterfly-light lenses. Until recently, such proposals have remained on the fringes of acceptable scientific speculation. Now, with the Intergovernmental Panel on Climate Change (IPCC) claiming in its report of February 3 that there's a 90 percent probability that the last half-century of global warming has been caused by humans, a milestone moment has apparently arrived. . .
[W]hile mainstream acceptance of climate change means that the battles over what humanity should do about it are just beginning, radical planet-cooling technological possibilities are receiving consideration alongside the standard proposals for capping, reducing, or sequestering carbon emissions.
And just how radical might some of those proposals be?
The notion of interposing a really big mirror between the Sun and Earth, which exploits the fact that our planet already reflects about 30 percent of incoming sunlight back into space by effectively increasing its reflectivity, dates back to the 1980s. Initially, such mirrors were suggested for cooling Venus as part of a theoretical future effort to terraform that planet. But in 1989, James Early of the Lawrence Livermore National Laboratory noted the harbingers of global warming and proposed deflecting a measure of sunlight with a "space shade" located at Lagrangian Point L1 -- an orbit 1.5 million kilometers up, where Earth's gravity and that of the Sun are balanced so an object can remain stationary relative to both bodies.
How big a shield was Early thinking about? One 2,000 kilometers in diameter and about 10 microns thick, with a weight of about 100 megatons under Earth's gravity. Early's shield would have been either opaque or else transparent in the form of a Fresnel lens (the kind of lens used in lighthouses, in which the amount of material required is reduced from that needed in a conventional spherical lens because the lens is broken into concentric annular sections). Early estimated the cost at $1 to $10 trillion. As for assembling his giant mirror and placing it at L1, Early suggested using moon rock for the materials and a manufacturing plant on the lunar surface, then launching the components by a mass driver from the Moon to L1.
Given how arduous even minor assembly work on the International Space Station's exterior has been, and given that NASA will almost certainly be unable to meet its schedule for returning to the Moon by 2020, such a megaconstruction doesn't seem immediately feasible.
Granted, a mirror that big would be a genuine megaconstruction project. It's the sort of planet-scale engineering that could only be made possible, in the near-term at least, by molecular manufacturing.
So, what about smaller, more feasible possibilities? Two plans are floated in the Tech Review article, one from the University of Arizona's Roger Angel, and one from astrophysicist (and science fiction author) Greg Benford. We've reported before on the ideas proposed by Angel for launching trillions of small spacecraft into orbit. He thinks it's practically and economically doable, although it's still a huge project. As the article points out:
Human beings would have to launch a stack of flyers every five minutes for 10 years to put the whole structure in place. . .
Angel stresses that his plan is an emergency option, for use only if climate change so accelerates that global catastrophe looms within a decade or two. It is, he says, "no substitute for developing renewable energy, the only permanent solution."
Greg Benford suggests another alternative:
Benford has a proposal that possesses the advantages of being both one of the simplest planet-cooling technologies so far suggested and being initially testable in a local context. He suggests suspension of tiny, harmless particles (sized at one-third of a micron) at about 80,000 feet up in the stratosphere. These particles could be composed of diatomaceous earth.
"That's silicon dioxide, which is chemically inert, cheap as earth, and readily crushable to the size we want," Benford says. This could initially be tested, he says, over the Arctic, where warming is already considerable and where few human beings live. Arctic atmospheric circulation patterns would mostly confine the deployed particles around the North Pole. An initial experiment could occur north of 70 degrees latitude, over the Arctic Sea and outside national boundaries. "The fact that such an experiment is reversible is just as important as the fact that it's regional," says Benford.
Is Benford's proposal realistic? According to Ken Caldeira [whom we quoted yesterday], a leading climate scientist at Stanford University and the Carnegie Institution's Department of Global Ecology, "It appears as if any small particle would do the trick in the necessary quantities. I've done a number of computer simulations of what the climate response would be of reflecting sunlight, and all of them indicate that it would work quite well." He adds, "I wouldn't look to these geoengineering schemes as part of normal policy response, but if bad things start to happen quickly, then people will demand something be done quickly."
And there is this interesting (scary?) final note from Benford:
Given that our social systems would crash without the economic growth that depends on the existing energy infrastructure that we have, Benford personally believes that governments can't be counted on to develop and deploy alternatives: "Anybody who thinks governments are suddenly going to leap into action is dreaming."
Benford says that one of the advantages of his scheme is that it could be implemented unilaterally by private parties. "Applying these technologies in the Arctic zone or even over the whole planet would be so cheap that many private parties could do it on their own. That's really a dangerous idea because it suggests the primary actor in this drama will not be the nation-state anymore. You could do this for a hundred million bucks a year. You could do the whole planet for a couple of billion. That's amazingly cheap."