Geoengineering, or deliberately modifying the Earth’s environment to reverse climate change, is increasingly being proposed as a bridge to a carbon-neutral future. The possibility that geoengineering will be used to combat global warming raises a host of environmental questions. Will such approaches actually work? Who will control the thermostat for the Earth? What other environmental consequences will arise and where would effects be the greatest? This paper presents a framework for evaluating geoengineering schemes, including atmospheric seeding, space mirrors, and ocean fertilization. The specific case of geologic sequestration - the pumping of carbon dioxide into saline aquifers and other underground strata - will be examined in detail using physical and economic models to evaluate storage potential, estimated cost, and potential environmental risks.  
Results/Conclusions

Geoengineering schemes could reduce Earth’s temperature in the short term but carry enormous risks and uncertainties. Of all of the proposed geoengineering schemes, geologic sequestration may prove to have the fewest environmental co-effects. Underground reservoirs in the U.S. have the potential to store more than a century’s worth of electric power emissions at the relatively low cost of a few dollars per ton CO2 (not including the considerable costs of capture at industrial plants and transport).  Some risks of geologic sequestration include leakage and the potential for groundwater interactions in some locations.