Table 1.

Summary of solar geoengineering options, confidence in ability to produce radiative forcing (RF), key advantages and disadvantages relative to stratospheric sulfate. Any solar geoengineering approach will introduce additional concerns, e.g. [21].

method	confidence that substantial global ΔRF (e.g. >3 W m−2) is achievable	advantages	disadvantages relative to stratospheric sulfate
stratospheric sulfates	very high: current technologies can likely be adapted to loft materials and disperse SO2 at relevant scales	similarity to volcanic sulfate gives empirical basis for estimating efficacy and risks
other stratospheric aerosols	moderate: depends on aerosol, lofting similar to sulfate but aerosol dispersal much more uncertain	some solid aerosols may have less strat. heating and minimal ozone loss	harder to bound uncertainty since not naturally occurring in stratosphere
marine cloud brightening	uncertain: observations support wide range of CCN impact on albedo; substantial process uncertainty	ability to make local alterations of albedo; and modulate on short timescales.	only applicable on marine stratus covering approximately 10% of the Earth means RF inherently patchy
cirrus thinning	uncertain: deep uncertainty about fraction of cirrus strongly dependent on homogeneous nucleation; no studies examining diffusion of CCN	works on longwave radiation so could provide better compensation	maximum potential cooling limited; zonal distribution of RF constrained by distribution of cirrus
space based	low physical uncertainty, but deep technological uncertainties	possibility of near ‘perfect’ alteration of solar constant	substantially more expensive