A Closer Look at the Environmental Impacts of Stratospheric Aerosol Injections

Stratospheric Geoengineering with Sulfate Aerosols

One of the possible impacts of stratospheric aerosol injections is sulfuric acid deposition. The acidic deposition would occur as injections of sulfuric acid would combine with water in the stratosphere forming sulfuric acid which would in turn fall to earth. The negative effects of sulfur deposition include damage to freshwater fish populations, reductions in lake bacteria, changes in plant parasite interactions and changes to forest bird populations.

In 2009, Kravitz et al. investigated the impacts of sulfuric acid deposition in association with the effects it could produce on the earth’s surface. The authors assumed all sulfur deposition to be wet in order to examine the worst possible scenario. The extent to which sulfate deposition is harmful depends on three things; the amount of sulfur introduced, the amount of hydrated sulfate and ecosystem sensitivity.

To examine impacts they used a general circulation model to look at the impacts in different regions and a chemical model which calculates the sulfur cycle in the stratosphere where the conversion of SO2 is based upon respective concentrations of SO2 and humidity values. Over the same time period of 20years, two scenarios were modelled:

•  Daily injections of SO2 into the lower stratosphere in the tropics - 5Tg per/year in total
• Daily injections of SO2 into the lower Arctic stratosphere – 3Tg per/year in total

Their results (see figure 1 below) showed that under the 5Tg scenario, global temperatures could be reduced to levels similar to 1980 and 0.3 degrees of cooling would occur by 2026. Not nearly as impressive, but still an marked result, changes under the 3Tg scenario would see global average temperatures immediately reduced to 2000 levels and only 0.3 degrees of warming would occur by 2026. Sulfur injected over the tropics increased sulfate deposition across the all of the globe except the tropic due to poleward transportation. Deposition in the polar regions would also be particularly apparent. Sulfur injected in the Arctic would lead to deposition in the Northern Hemisphere.

Figure 1: Annually averaged total sulphate deposition

The study concluded that sulfur injections of both 3Tg and 5Tg magnitudes were not significant enough to cause any damage to the both the regions of the sea and land. This initial study was however flawed in two ways and consequently in 2010, Kravitz et al. published a follow uppaper to their study correcting these errors.

The first error of the paper was the calculation for average global surface sulfur emission. When corrected, the calculation strengthens the conclusion that the additional amount of sulfur from geo-engineering would be much smaller that current sources. The second error was their incorrect application of a formula that converts the model output of sulfate deposition to a form used in critical loading studies.

The incorrect application of the conversion formula means that figure 2 (see below) their original study needs to be replaced with the figure below. As you can see the magnitude of the values change significantly. Despite the changes, the authors still conclude that acid deposition from geo-engineering would be smaller than the amount already being experienced in industrialised areas. Furthermore, they hold their conclusion that everywhere in the world has a significant buffering capacity to the additional sulfuric acid that would result from geo-engineering except the most sensitive and pristine areas of the world.

Figure 2: Original model for Tropical SO2 Injections 5 Tg

Figure 3 - Corrected model for Tropical SO2 Injection 5 Tg 

Even after correcting their errors, the conclusions of Kravitz et al., (2009; 2010) are still the same. Sulfur injections from geo-engineering are not enough to cause any damage except in sensitive, poorly buffered areas; they will not have negative ecosystem impacts. The studies model are accurate however local differences may occur and there may be a need for more significant sulfur injections in the future if GHGs alter atmospheric circulations as aerosol lifetime would be shorter – in which case the model would not serve as accurately. Despite these possibilities, stratospheric aerosol injections are nonetheless a promising geo-engineering technique. The climate cools following sulfur injections and ecosystems are not impacted negatively. As is the case with most geo-engineering techniques their reputation as a taboo topic means more work is often needed to further examine and develop the robustness of techniques. More work on stratospheric aerosols would in my opinion be a good move and a step in the right direction towards fixing climate change!

Thanks for reading! 

Kravitz, B., A. Robock, L. Oman, G. Stenchikov, and A. B. Marquardt (2009), 'Sulfuric acid deposition from stratospheric geoengineering with sulfate aerosols', Journal of Geophysical Research, 114, D14109.

Kravitz., BA. Robock, L. Oman, G. Stenchikov, and A. B. Marquardt (2010), Correction to “Sulfuric acid deposition from stratospheric geo-engineering with sulfate aerosols,” J. Geophys. Res., 115, D16119