Ranking geo-engineering schemes

P ropelling 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 climate2. Because such schemes are planetary in scale1, they are newsworthy and have long received widespread media attention2,3. Increasingly, the merits and drawbacks of at least testing the proposals are also prompting scientific debate4. Despite the surge in interest in old and new schemes5, at present no geo-engineering proposal has been tested or even subjected to preliminary trials. Although some proposals have been publicized for many years, little progress is evident6 in ranking geo-engineering schemes since an initial comprehensive effort7 15 years ago. Appraising the relative merits of geo-engineering designs for a purposeful perturbation of the Earth system is essential: funds to investigate such proposals in detail are limited, and not all schemes can be put in place if we are to monitor the Earth system’s response to each scheme with any confidence. The suggestions for geo-engineering are as diverse as they are controversial1,7, and ranking them is not straightforward.