Testing solar radiation management as a geoengineering technique
5 November 2011
Solar radiation management (SRM) is one of the geoengineering techniques proposed as a potential means of offsetting some of the anthropogenic radiative forcing of climate as a means to reduce climate change. (Earlier post.) However, the effectiveness and associated risks of SRM are uncertain.
A team of researchers from California Institute of Technology, the Carnegie Institution for Science and Harvard University has now examined the possibility of testing solar radiation management (SRM) through sub-scale deployment as a means to test models of climate response to SRM and explore risks prior to full-scale implementation. Contrary to some claims, they note, this could provide meaningful tests of the climate’s response to SRM within a decade.
The response at long time-scales would need to be extrapolated from results measured by a short-term test; this can help reduce the uncertainty associated with relatively rapid climate feedbacks, but uncertainties that only manifest at long time-scales can never be resolved by such a test...However, tests could require several decades or longer to obtain accurate response estimates, particularly to understand the response of regional hydrological fields which are critical uncertainties. Some fields, like precipitation over land, have as large a response to short period forcing as to slowly-varying changes. This implies that the ratio of the hydrological to the temperature response that results from a sustained SRM deployment will differ from that of either a short-duration test or that which has been observed to result from large volcanic eruptions.—MacMynowski et al.
Their study is published in the RSC journal Energy & Environmental Science.
Solar radiation management is a class of theoretical concepts for manipulating the climate in order to reduce the risks of global warming caused by greenhouse gasses; however, the potential effectiveness and risks of SRM are uncertain.
Ideas for solar radiation management include increasing the amount of aerosols in the stratosphere, which could scatter incoming solar heat away from Earth’s surface, or creating low-altitude marine clouds to reflect these same rays. The size of the scale and the intricacies of the many atmospheric and climate processes make testing these ideas difficult.
The researchers turned to the Hadley Centre Coupled Model, version 3 (HadCM3L), a general circulation model of the atmosphere-ocean, for their simulations of SRM’s possible effect on the planet.
To take geoengineering methods like solar radiation management seriously, we need to build realistic models. The aim of our study was to create a framework to better understand what might be learned through testing. I do not believe that large-scale testing makes sense now.
Our goal was to examine the all-or-nothing assumption common in studies of SRM, by using climate models to find out if a limited test of SRM could be detected in the face of natural climate variability. Our results suggest that it should be possible to turn SRM on slowly—looking carefully for unexpected side-effects—before committing to full-scale use.— David Keith
Using models the team was able to demonstrate that smaller-scale tests of solar radiation management could help inform decisions about larger scale deployments. Short-term tests would be particularly effective at understanding the effects of geoengineering on fast-acting climate dynamics. But testing would require several decades and, even then, would need to be extrapolated out to the centuries-long time scales relevant to studying climate change.
While it is clearly premature to consider testing solar radiation management at a scale large enough to measure the climate response, it is not premature to understand what we can learn from such tests. But we did not address other important questions such as the necessary testing technology and the social and political implications of such tests.—Doug MacMynowski, Caltech
Some scientists have theorized that volcanic eruptions could stand in for tests, as they would cause same types of atmospheric changes as aerosols. But they wouldn’t be as effective as a sustained test.
No test can tell us everything we might want to know, but tests could tell us some things we would like to know. Tests could improve our understanding of likely consequences of intentional interference in the climate system and could also improve our knowledge about the climate’s response to the interference caused by our carbon dioxide emissions. We conducted a scientific investigation into what might be learned by testing these proposals. We are not advocating that such tests should actually be undertaken.—Ken Caldeira
Douglas G. MacMynowski, David W. Keith, Ken Caldeira and Ho-Jeong Shin (2011) Can we test geoengineering? Energy Environ. Sci. DOI: 10.1039/C1EE01256H
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