An overview of geoengineering of climate using stratospheric sulphate aerosols

Abstract

We provide an overview of geoengineering by stratospheric sulphate aerosols. The state of understanding about this topic as of early 2008 is reviewed, summarizing the past 30 years of work in the area, highlighting some very recent studies using climate models, and discussing methods used to deliver sulphur species to the stratosphere. The studies reviewed here suggest that sulphate aerosols can counteract the globally averaged temperature increase associated with increasing greenhouse gases, and reduce changes to some other components of the Earth system. There are likely to be remaining regional climate changes after geoengineering, with some regions experiencing significant changes in temperature or precipitation. The aerosols also serve as surfaces for heterogeneous chemistry resulting in increased ozone depletion. The delivery of sulphur species to the stratosphere in a way that will produce particles of the right size is shown to be a complex and potentially very difficult task. Two simple delivery scenarios are explored, but similar exercises will be needed for other suggested delivery mechanisms. While the introduction of the geoengineering source of sulphate aerosol will perturb the sulphur cycle of the stratosphere signicantly, it is a small perturbation to the total (stratosphere and troposphere) sulphur cycle. The geoengineering source would thus be a small contributor to the total global source of ‘acid rain’ that could be compensated for through improved pollution control of anthropogenic tropospheric sources. Some areas of research remain unexplored. Although ozone may be depleted, with a consequent increase to solar ultraviolet-B (UVB) energy reaching the surface and a potential impact on health and biological populations, the aerosols will also scatter and attenuate this part of the energy spectrum, and this may compensate the UVB enhancement associated with ozone depletion. The aerosol will also change the ratio of diffuse to direct energy reaching the surface, and this may influence ecosystems. The impact of geoengineering on these components of the Earth system has not yet been studied. Representations for the formation, evolution and removal of aerosol and distribution of particle size are still very crude, and more work will be needed to gain confidence in our understanding of the deliberate production of this class of aerosols and their role in the climate system.

Aerosol microphysics modules in the framework of the ECHAM5 climate model – intercomparison under stratospheric conditions

“…Pinatubo eruption (Niemeier et al, 2009), In this 3-D simulation 17 Mt of SO 2 was initialized according to satellite observations after the Pinatubo eruption (Read et al, 1993).…”

“…In particular during the first days of an eruption, insoluble compounds like volcanic ash are major constituents of volcanic plumes (see Niemeier et al, 2009). However, fine ash particles are large compared to aerosols and sediment out very quickly so that they are not relevant for long term climate effects (Schneider et al, 1998).…”

Climate tipping points and solar geoengineering

“…Crutzen (2006), Rasch et al (2008), and a recent study on the costs of deployment (McClellan et al (2012)). As in DICE, costs are expressed as a fraction of gross output.…”

Climate response to aerosol injection at different stratospheric locations

“…Therefore, humankind will be forced to apply geoengineering to counter the unwanted consequences of global warming. It seems that among the many suggested geoengineering methods the most promising solution is the artificial enhancement of sub-micron stratospheric sulfate aerosol (Teller et al, 2002;Izrael, 2005;Crutzen, 2006;Rasch et al, 2008). Model calculations have shown that counteracting a greenhouse warming of +2 K required between 2 and 10 Mt of aerosol particles (Wigley, 2006;Izrael et al, 2007;Robock et al, 2008).…”

“…The mass required depends on many factors, such as particle size, duration of release, place of injection in the stratosphere, region of desirable influence, and so on. An alternative idea is to limit the influence of injected particles to the Arctic, to prevent melting of the Greenland Ice Sheet and Arctic Ocean sea ice (Lane et al, 2007). Dependence of climatic effect on latitude of injection (tropics vs Arctic) was studied by Robock et al (2008).…”