Caldeira Lab

Can We Test Geoengineering?

Douglas G. MacMynowski, David W. Keith, Ken Caldeira & Ho-Jeong Shin

Solar radiation management (SRM), a form of geoengineering, might reduce the risks of climate change, but its effectiveness and risks are uncertain. While it is premature to consider any testing at a scale large enough to measure climate response, it is not premature to understand what we can learn from such tests. Indeed, understanding how testing might be linked to gradual deployment is crucial to understanding our ability to manage SRM’s deep uncertainties. We show that by modulating the forcing, it would be possible to learn something about the climate’s response from a subscale test, but that these tests could take decades.

MacMynowski, D. G., D. W. Keith, K. Caldeira, and H.-J. Shin, 2011: Can we test geoengineering? Energy & Environmental Science, 4, 5044, doi:10.1039/c1ee01256h.


Doug MacMynowski & Ken Caldeira in discussion - click to read the video transcript

Doug MacMynowski - click to read the video transcript



Solar radiation management (SRM), a form of geoengineering, might be used to offset some fraction of the anthropogenic radiative forcing of climate as a means to reduce climate change, but the risks and effectiveness of SRM are uncertain. We examine the possibility of testing 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, this could provide meaningful tests of the climate’s response to SRM within a decade. We use idealized simulations with the HadCM3L general circulation model (GCM) to estimate the response to SRM and signal-to-noise ratio for global-scale SRM forcing tests, and quantify the trade-offs between duration and intensity of the test and it’s ability to make quantitative measurements of the climate’s response to SRM forcing. 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. With this important caveat, the transient climate response may be bounded with 90% confidence to be no more than 1.5 C higher than it’s estimated value, in a single decade test that used roughly 1/10th the radiative forcing perturbation of a CO2- doubling. 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 slowlyvarying 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.


Figure: Linearity of response to forcing: surface air temperature response (C per W m-2, left) and precipitation response (change relative to baseline, per W m-2, right), estimated for solar perturbations of 0.5%, 1%, and 2% forcing amplitude at an 8-year period. The difference in response per W m-2 SRM between tests conducted at 1% (2.4 W m-2) and 2% (4.8 W m-2) change in radiative forcing is relatively small, but the difference is higher in a few regions between 1% and 0.5% changes. While the global mean response is reasonably linear, local nonlinearities can be significant, particularly at small amplitudes.