Despite national and international efforts to reduce anthropogenic emissions, growing concentrations of atmospheric carbon dioxide will yield planetary warming and associated impacts for the foreseeable future. Concerns that mitigation may be too slow in coming have led to renewed dialogue within the scientific community regarding potential strategies for counteracting global warming through geoengineering. Climate geoengineering has been defined as "the intentional large-scale manipulation" of the climate system with the intent to counteract the global warming effects of anthropogenic CO2 emissions. While several methods for counteracting climate change with geoengineering are considered feasible, injecting sulfates or other fine aerosols into the stratosphere, thereby increasing planetary albedo, is a leading contender.

Several recent modeling studies have examined the global and regional temperature and hydrological response to geoengineering-type climate forcings. Building on these studies, this experiment will investigate the impacts of geoengineering simulations run using HadCM3L. While the limited number of past modeling studies using AOGCMs to examine the potential impacts of geoengineering activities have generally examined the effects of applying a constant geoengineering forcing, this study applies transient forcings designed to counter variable net projected anthropogenic radiative forcings. In addition, a wide range of forcing schemes designed to span the approximate range of uncertainties associated with anthropogenic climate forcing estimates were generated and implemented in order to assess what differences in effects exist between the "best guess" counter-anthropogenic geoengineering forcing scheme and other plausible schemes.

Twin ensembles will be investigated for responses to IPCC A1B emissions scenario between 2000 and 2080, both with and without geoengineering activities starting in 2005. All models use identical parameter inputs, with the exception of an initial condition parameter. The only variations are in the volcanic forcing files. Geoengineering activities are mimicked in the models by modifying the volcanic aerosol radiative inputs, applied as variations in stratospheric optical depth over four zonal bands bounded by the equator, 30°N and 30°S. The impacts of 135 geoengineering scenarios will be examined, corresponding with forcings equivalent and opposite to the forcings associated with long-lived greenhouse gases, tropospheric sulfur aerosols and tropospheric ozone. The 135 schemes are designed to span the uncertainties associated with these anthropogenic forcings.

This experiment has been designed in collaboration of EPP, Carnegie Mellon University and AOPP, Oxford and was released on October 1, 2008.