**Caldeira Lab**

# Solar Geoengineering to Limit the Rate of Temperature Change

### Douglas G. MacMartin, Ken Caldeira & David W. Keith

MacMartin, D. G., K. Caldeira, and D. W. Keith, 2014: Solar geoengineering to limit the rate of temperature change. *Phil. Trans. R. Soc. A*, **372**, 20140134, doi:10.1098/rsta.2014.0134.

## Abstract

Solar geoengineering has been suggested as a tool that might reduce damage from anthropogenic climate change. Analysis often assumes that geoengineering would be used to maintain a constant global mean temperature. Under this scenario, geoengineering would be required either indefinitely (on societal time scales) or until atmospheric CO2 concentrations were sufficiently reduced. Impacts of climate change, however, are related to the rate of change as well as its magnitude. We thus describe an alternative scenario in which solar geoengineering is used only to constrain the rate of change of global mean temperature; this leads to a finite deployment period for any emissions pathway that stabilizes global mean temperature. The length of deployment and amount of geoengineering required depends on the emissions pathway and allowable rate of change, e.g. in our simulations, reducing the maximum approximately 0.3°C per decade rate of change in an RCP 4.5 pathway to 0.1°C per decade would require geoengineering for 160 years; under RCP 6.0, the required time nearly doubles. We demonstrate that feedback control can limit rates of change in a climate model. Finally, we note that a decision to terminate use of solar geoengineering does not automatically imply rapid temperature increases: feedback could be used to limit rates of change in a gradual phase-out.

**Figure 1. **Predicted global mean temperature (*a*) and rate of change (*b*) corresponding to different RCPs, computed using a box-diffusion model with a climate sensitivity of 3.2°C per CO2-doubling and time constants chosen to match the HadCM3L GCM behaviour. (The results scale linearly with climate sensitivity.) Rates of change are calculated over 30 year intervals, and expressed in degrees Celsius per decade. A 0.1°C per decade rate of change is also shown in (*a*), starting in 2020, and for RCP 4.5, the approximate time over which SRM would be required in order to maintain this rate is shaded.

**Figure 4.** Rate of temperature change at grid-scale in HadCM3L for RCP 4.5 over a 30 year period from 2020 to 2050 without
solar geoengineering (a) and with solar geoengineering either to maintain constant global mean temperature (b) or to limit
global mean temperature warming to 0.1◦C per decade (c). Slopes are estimated using a linear least-squares fit and averaged
over five ensemblemembers. The local rate of change of temperature can still be much larger than the globalmean rate, but for
the 0.1◦C per decade case, is reduced in most places with SRM when compared with no SRM. Using SRM tomaintain a constant
global mean temperature despite increasing greenhouse gas concentrations results in some regionswarming and some cooling,
so that the average rate of change is zero. The grid-scale rate of change is not statistically significant everywhere (see electronic
supplemental material, figure S2).