The use of space mirrors as an anti-global warming measure is a proposed technology for climate change mitigation by deflection of sunlight. It was one of a series of proposals for controlling global warming made to the United States government in 2001.
At the "Response Options to Rapid or Severe Climate Change" round-table meeting organized by the President's Climate Change Technology Program in September 2001 to gather ideas for averting climate change, one of the proposals was to station one or more wire-mesh "mirrors" in orbit to deflect sunlight back into space or to filter it. The idea was proposed by Lowell Wood, a senior staff scientist at Lawrence Livermore National Laboratory, who calculated that deflecting 1% of sunlight would restore climatic stability, and that that would require either a single mirror 600,000 square miles (1,600,000 km2) in area or several smaller ones. Wood had been researching the idea for more than ten years but considered it so infeasible that it should only be a back-up plan for solving the global warming problem.
In January 2007, The Guardian reported that the US government was recommending that research on sunlight deflection, including space mirrors, be continued as "insurance" and that the next United Nations Report on Climate Change advocate such a strategy. In addition to the space mirror, suggested sunlight-reducing techniques included launching thousands of highly reflective balloons and pumping sulphate droplets into the upper atmosphere to emulate volcanic emissions.
Space mirrors were first considered in the 1980s as a way to cool the climate of Venus. James Early, also at Livermore, in 1989 proposed using a "space shade" 2,000 kilometers in diameter orbiting at Lagrangian Point L1. He estimated the cost at between one and ten trillion US dollars and suggested manufacturing it on the moon using moon rock.
Using space mirrors as a space sunshade to reduce the impact of sunlight falls into the category of geoengineering: deliberately modifying the earth's climate. At a conference on the topic organized by Daniel Schrag of Harvard University and David Keith of the University of Calgary in November 2007, the consensus was that it was worth studying such ideas further despite their high cost, the doubtful feasibility of some including the space mirror, and the risk of their distracting attention from reduction of greenhouse gas emissions.