Fail-safe solar radiation management geoengineering
To avoid dangerous changes to the climate system, the global mean temperature must not rise more than 2 °C from the 19th century level. The German Advisory Council on Global Change recommends maintaining the rate of change in temperature to within 0.2 °C per decade. This paper supposes that a geoengineering option of solar radiation management (SRM) by injecting aerosol into the Earth’s stratosphere becomes applicable in the future to meet those temperature conditions. However, a failure to continue the use of this option could cause a rapid temperature rebound, and thus we propose a principle of SRM use that the temperature conditions must be satisfied even after SRM termination at any time. We present economically optimal trajectories of the amounts of SRM use and the reduction of carbon dioxide (CO 2 ) emissions under our principle by using an economic model of climate change. To meet the temperature conditions described above, the SRM must reduce radiative forcing by slightly more than 1 W/m 2 at most, and industrial CO 2 emissions must be cut by 80 % by the end of the 21st century relative to 2005, assuming a climate sensitivity of 3 °C. Lower-level use of SRM is required for a higher climate sensitivity; otherwise, the temperature will rise faster in the case of SRM termination. Considering potential economic damages of environmental side effects due to the use of SRM, the contribution of SRM would have to be much smaller.