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. Four hours after the IPCC report’s release, even the White House (historically extremely hostile to the idea of anthropogenic climate change) had unearthed a 2001 remark by President George W. Bush acknowledging that greenhouse-gas increases were largely created by humans. Consequently, while 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.
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. Last year Roger Angel, University of Arizona Regents Professor and the Steward Observatory Mirror Laboratory’s director, offered another plan: to place in orbit at L1 a very great number of small, already assembled objects. Angel presented his concept to the National Academy of Sciences in April 2006, got a NASA grant to fund further research, and then published a detailed paper, “Feasibility of Cooling the Earth with a Cloud of Small Spacecraft near L1.”