Sulfuric acid deposition from stratospheric sulfate aerosols?

So one of the possible dangers associated with stratospheric sulfate aerosols is that upon injecting them into the stratosphere, they would combine with water to form sulfuric acid and fall to earth to burn us all/pollute the oceans etc.  A paper by Kravtiz et al. investigated this risk, in association with what effects this may also produce at the Earth's surface.  The paper is very readable and quite short, so worth having a look at if you're into this sort of thing..

The aim of their experiment was to quantify the amount of sulfur deposition arising from 2 different scenarios.  Negative effects of sulfur deposition include damage to freshwater fish populations, affecting plant-parasite interation, reduce lake bacertia and affect forest bird populations - so basically mess up whole ecosystems.  Sulfate deposition can be both wet and dry but in this case Kravitz assumed all sulfur deposition to be wet (ie. all sulfuric acid) in order to represent the 'worst-case scenario'.  Whether sulfate deposition is harmful is dependant on 3 factors:

1. Amount of hydrated sulfate
2. Ecosystem sensitivity
3. The amount of sulfur introduced in the first place.

The method basically involves modelling; using a general circulation model to examine potential effects in different regions, coupled with a chemical model denoting the sulfur cycle in the atmosphere, which hydrated the sulfur addition based on humidity values for different regions.  The two scenarios modelled were as follows:
1) Daily injections of SO₂ into the lower stratosphere in the tropics, 5Tg per year in total.
2) Daily injections of SO₂ into the lower Arctic stratosphere, 3Tg per year in total.
*both were modelled over the same time period of 20 years.

To give you a bit of an idea of the magnitude of these injections, in comparison, the 5Tg scenario mimics a Mt Pinatubo-sized eruption every 4 years.

Under the 3Tg scenario, global average temperatures reduced to 2000 levels immediately; and only warmed by 0.3 degrees by 2026.  In contrast, the 5Tg scenario reduced global temperatures to 1980 levels and produced a 0.3 degrees cooling by 2026!  So if Mt Pinatubo had a big eruption every 4 years, there would be no more global warming.

Figure 1 shows where sulfate deposition occurred:















































Where sulfur was injected over the tropics there was increased sulfate deposition over most of the globe, except the tropics due to poleward transport, and deposition in the polar regions is particularly apparent.  In the Arctic injection case sulfate deposition was mainly in the Northern Hemisphere.  Additionally the green areas (little deposition) are over areas with little rainfall, because as previously mentioned the deposition is all assumed to be wet.

Kravitz concluded shortly that for terrestrial ecosystems, sulfur injections of these magnitudes were not enough to cause any damage, except for maybe in poorly buffered areas.  He also reached the same conclusion for the oceans.

Conclusions

• Additional sulfate deposition resulting for sulfur injections into the stratosphere at this magnitude will not negatively impact ecosystems.
• Models are accurate but localised differences may occur due to individual rainfall events, weather patterns etc.
• The need for higher amounts of sulfur injections may be required in future due to GHGs altering atmospheric circulation and causing a shorter lifetime if aerosols in the stratosphere (more sulfur = more deposition).

My thoughts on this method remain unchanged; I think it's a very good technique for fixing climate because it does just that!  Climate was actually shown to cool immediately after the sulfur injections, and doesn't negatively impact ecosystems according to the model.  Obviously more work is required into examining other effects but I think this paired with a carbon removal technique would be effective.


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.