Can carbon cycle geoengineering be a useful complement to ambitious climate mitigation ?

Intentional climate modification at a global scale – geoengineering – would probably be very risky, and perhaps even dangerous. Many negative consequences of geoengineering have already been predicted [1,2], and it is very likely that there will be unforeseen environmental consequences that may only emerge after geoengineering is enacted [3]. Indeed, I have argued previously that the net impacts of human intervention are likely to increase in proportion with the degree of intervention, with the result that undesirable impacts of geoengineering may well exacerbate, rather than mitigate, the growing stress on environmental systems resulting from anthropogenic climate warming [3]. Nevertheless, I acknowledge here that there may be reason to consider some forms of geoengineering strategies as a complement to aggressive mitigation of greenhouse gas emissions, for the purpose of achieving otherwise u nattainable climate targets. Proposed ideas to intentionally modify climate can be divided into two categories [4]. ‘Solar radiation management’ refers to ideas that are aimed at decreasing the amount of absorbed solar radiation by increasing the reflectivity of the land surface or atmosphere. This class of geoengineering is also referred to as ‘climate engineering’ or ‘shortwave geoengineering’, since in all cases climate modification is proposed via alteration of the planetary shortwave radiation balance. By contrast, the second class of geoengineering proposals are referred to as ‘carbon management’, ‘longwave geoengineering’, ‘CO2 removal’ or ‘carbon cycle geo engineering.’ This class of ideas proposes to alter the longwave radiation balance by artificially removing, or attempting to enhance, the natural sequestration of anthropogenic CO 2 . All geoengineering ideas share the commonality of representing a large-scale, technological and intentional intervention in the climate system for the purpose of offsetting some amount of anthropogenic climate warming. However, different geoengineering ideas carry very different degrees of risk, effectiveness and cost. Figure 1 shows a summary of the effectiveness, affordability, timeliness and safety of different geoengineering proposals, compiled as part of a recent Royal Society report on geoengineering [4]. In this ana lysis, some proposals are classified as relatively Carbon Management (2010) 1(1), 135–144 Can carbon cycle geoengineering be a useful complement to ambitious climate mitigation?