Enhancing Stratospheric Aerosols

During large volcanic eruptions sulphur is ejected into the stratosphere. The resulting aerosols reflect some of the shortwave radiation which therefore cools the Earth’s surface. Unfortunately it is not only the planetary albedo that is affected by enhanced aerosol loading in the atmosphere. The chemical reactions and the radiative heating budget of the aerosol layer are also affected. For example, it has been observed that during volcanic eruptions that ozone (O₃) is depleted (Heckendorn et al, 2009).

It is thought that a process of artificially enhancing aerosols could potentially mitigate global warming (Heckendorn et al, 2009). Past computer models of globally injecting sulphate into the stratosphere will result in the equator being over cooled and the poles will be under cooled (Ban-Weiss and Calderia, 2010). There would also be less precipitation. Ban-Weiss and Calderia (2010) ran a global climate model with varying sulphate concentrations at different latitudes. They found that with more sulphate aerosol loading at the poles than the equator region, temperatures would be like those of a low CO₂ climate. However, effects on the water cycle were diminished with a more equal distribution of sulphate loading (Ban-Weiss and Calderia, 2010).

One problem with sulphate aerosol loading is that the warming from CO₂ is being balanced by the cooling from stratospheric aerosols. However the two forces have very different climate response times because stratospheric aerosols have a lifetime in the atmosphere of a few years whereas CO₂ can remain in the atmosphere for centuries to millennia (Goes et al, 2011). Therefore continuous loading of the sulphate into the atmosphere needs to take place. If the aerosol loading is not maintained (for example, in the case of war or discovery of negative effects of aerosol forcing) there would be rapid warming which society would struggle to cope with (Goes et al, 2011).

Other issues with enhanced aerosol loading are the resulting polar ozone depletion which would damage ecosystems and potentially human health (Goes et al, 2011). Also aerosol geoengineering does not stop ocean acidification. Ocean acidification is the reaction of CO₂ with sea water and can negatively impact coral reefs and the populations that depend on coral (Goes et al, 2011). The aerosol loading into the stratosphere does not affect the concentration of CO₂ in the atmosphere and therefore does not reduce ocean acidification. The variations in the concentrations of stratospheric aerosol affects the properties of El Niño, precipitation and temperature, and the Asian and African summer monsoon (Goes et al, 2011). 

Enhancing stratospheric aerosols is considered a viable option because of its cost effectiveness (Goes et al, 2011). However, I think this option is extremely risky both economically and environmentally. It cannot be considered as a main method for mitigating global warming.

Heckendorn, P., D. Weisenstein, S. Fueglistaler, B. P. Luo, E. Rozanov, M. Schraner, L. W. Thomason, and T. Peter (2009) ‘The impact of geoengineering aerosols on stratospheric temperature and ozone’ Environmental Research Letters, 4, pp.1-12

doi:10.1088/1748-9326/4/4/045108

Ban-Weiss, G., K. Calderia (2010) ‘Geoengineering as an optimization problem’ Environmental Research Letters, 5, 3, pp.1-9

doi:10.1088/1748-9326/5/3/034009

Goes, M., N. Tuana, K. Keller (2011) ‘The economics (or lack thereof) of aerosol geoengineering’ Climatic Change, online first

DOI: 10.1007/s10584-010-9961-z