Studying geoengineering with natural and anthropogenic analogs

Solar radiation management (SRM) has been proposed as a possible option for offsetting some anthropogenic radiative forcing, with the goal of reducing some of the associated climatic changes. There are clearly significant uncertainties associated with SRM, and even small-scale experiments that might reduce uncertainty would carry some risk. However, there are also natural and anthropogenic analogs to SRM, such as volcanic eruptions in the case of stratospheric aerosol injection and ship tracks in the case of marine cloud albedo modification. It is essential to understand what we can learn from these analogs in order to validate models, particularly because of the problematic nature of outdoor experiments. It is also important to understand what we cannot learn, as this might better focus attention on what risks would need to be solely examined by numerical models. Stratospheric conditions following a major volcanic eruption, for example, are not the same as those to be expected from intentional geoengineering, both because of confounding effects of volcanic ash and the differences between continuous and impulsive injection of material into the stratosphere. Nonetheless, better data would help validate models; we thus recommend an appropriate plan be developed to better monitor the next large volcanic eruption. Similarly, more could be learned about cloud albedo modification from careful study not only of ship tracks, but of ship and other aerosol emission sources in cloud regimes beyond the narrow conditions under which ship tracks form; this would benefit from improved satellite observing capabilities.