Role of sunshades in space as a climate control option

Placing reflectors in the stratosphere or in higher-altitude space to shade sunlight is expected to be a promising geoengineering option for use as a countermeasure against climate change. While the sunshade technology has been evaluated from earth-scientific viewpoints, no study has evaluated when and on what scale it can be deployed from a cost-effectiveness viewpoint, presuming that it is used in combination with CO2 abatement options as a pillar of the mitigation of climatic change. Nor have the contributions of geoengineering research and development to the economics of overall climate control measures been evaluated. Such evaluations are attempted in this study, which focuses on a space-based sunshade technology. As a result, it is recommended that radiative forcing be reduced by 0.8 W/m2 through the sunshades over a period of approximately a century from the latter half of the 21st century. In this case, an increase in global CO2 emissions by some 15% is allowed in the former half of the 21st century, and the total climate control cost is reducible by 240 billion constant 2005 US$ on a present value basis compared to the case in which the sunshades are unavailable. To enjoy the above advantages, the mass of the sunshade stock required to offset the increase in radiative forcing due to a doubling of the atmospheric CO2 concentration should be lower than 100 Mt/2×CO2 assuming a continuous decline in the cost of placing (space-launching) sunshades from 6000 to 1000 US$/kg over a half-century.