Injecting light-reflecting particles into the atmosphere to counteract rising levels of greenhouse gas emissions cannot stabilise both temperatures and rainfall in all regions of the world at the same time, according to recent research. This raises serious questions about how such a process could be managed.
One proposal to reduce the effects of increasing levels of CO2 and other greenhouse gas (GHG) emissions is to use solar radiation management (SRM): that is, to send aerosols (fine particles or droplets), such as sulphate aerosols, into the stratosphere to reflect back some of the sun´s radiation, which would cause global temperatures to cool. It is argued that such types of interventions could be used under rising global temperatures to help prevent severe damage to people and ecosystems, for example, rapid sea level rise.
Previous studies suggest that, although modifying the climate through SRM could reduce temperatures to levels associated with a world having low CO2 emissions, there would be uncertain changes in precipitation (e.g. rainfall, snowfall) across different regions of the world. In addition, SRM would not counteract other severe impacts caused by high CO2 levels, such as acidification of the surface ocean.
This study modelled the impacts of 54 different SRM scenarios over a period of 80 years, from 2000 to 2080. All SRM scenarios were designed to stabilise average global temperatures to between approximately 14.6 and 15.7°C. These temperatures contrast with a predicted rise in average global temperature of about 2.5°C over the length of an 80 year simulation if GHG emissions continue to increase and there are no SRM interventions.
Results from the study suggest global precipitation would decrease under all SRM scenarios, compared with a predicted increase in global precipitation with no SRM. Thus, SRM interventions are unable to compensate for the effect of rising global levels of GHGs on both temperature and precipitation at the same time.
In addition, for all the SRM scenarios, the study modelled the regional changes in temperature and precipitation. For all regions, the SRM scenarios caused cooler temperatures compared with no SRM. In general, the greater the SRM intervention the more precipitation is reduced. But changes in precipitation across regions varied widely.
Early on in the projected time period, SRM could bring many regional climates back within one standard deviation of the late twentieth century conditions simultaneously. However, after 60 or 70 years, when increasing amounts of aerosols have been pumped into the atmosphere to counteract increasing levels of GHG emissions, there is often no scenario that could simultaneously return most regional climates close to their late twentieth century states. Different regions would require different levels of intervention to restore the regional climate.
For example, in the 2020s, SRM could return the climate to near baseline conditions for both China and India. But by the 2070s there would be no SRM that could restore temperature and precipitation conditions to baseline levels simultaneously in both these regions. Nevertheless, even under these circumstances, regions would be closer to baseline conditions than if there were no interventions.
There are, as yet, no well-developed practical suggestions of how to 'fine tune' the amount and distribution of SRM that could simultaneously achieve acceptable climatic conditions in all regions. The researchers also caution that international negotiations would probably become more complicated the longer such interventions were used.
Source: Ricke, K.L., Morgan, M.G. and Allen, M.R. (2010) Regional climate response to solar-radiation management. Nature Geoscience. 3: 537-541.