Figure 1 - Comparison of aspects of five geo-engineering proposals.

The schemes (see Box 1) span both carbon storage and reductions of solar radiation, and have been prominent in both the popular^{2, 3, 20} and scientific^{4, 8} media. The figure highlights schematically some facets of the four criteria: efficacy, cost, risk and time. The assessment gives scores relative to other schemes. For each facet, more colour denotes a higher ranking. Efficacy is assessed in the first line according to the provenance of a scheme, with those based on historical precedents^{8, 10} rated higher than those derived from theory and/or models. The extent of testing is shown in the second line, with related observations from experiments^{10, 15} or pilot studies scoring higher than model simulations, which in turn rank above a proposal with accompanying technical details. The full degree of efficacy is too uncertain at present to depict as a facet in this inter-comparison and will need further research. Affordability is categorized as initial cost assessment from the designer of a scheme³ in the upper line and a more realistic cost assessment including additional costs that come with a scheme's risks in the lower line. Safety provides an assessment of risk, which is related to known side-effects, with unknown side-effects represented here by system complexity (biogeochemical complexity^{10, 11} is larger than geochemical¹³ complexity, which is larger than physicochemical⁸ complexity) and the verification of both efficacy and side-effects. Other important but very uncertain aspects of risk, such as geopolitical and economic changes, require further research. Relevant aspects of time include the rate of climate mitigation in the top line (higher rates are better¹⁸), and the rapidity with which to halt any unanticipated deleterious effects, based on residence time of the agent of perturbation in the environment (shortest residence scores highest).