Ranking geo-engineering schemes

Geo-engineering proposals for mitigating climate change continue to proliferate without being tested. It is time to select and assess the most promising ideas according to efficacy, cost, all aspects of risk and, importantly, their rate of mitigation. Propelling aerosols into the upper atmosphere or pumping carbon dioxide into the deep ocean are just two schemes that have been proposed to repair the Earth's climate through geo-engineering1 (see Box 1). In the absence of adequate reductions in anthropogenic CO₂ emissions, geo-engineering has been put forward as the only remaining option that might fix our rapidly changing climate.