Aerosols and Global Warming

Fig. 1 - The Effects of Aerosols Over the Indian Ocean

The influence of atmospheric aerosols on global warming may not be as clear as scientists have previously believed.

Aerosols are classified as 'small particles suspended in the atmosphere'. Around 90% of atmospheric aerosols have natural origins, and include sea salt, dust, organic carbon from forest fires and sulfurous gases from volcanic activity. In addition, dimethysulfides from ocean algae can produce sulfate aerosols.

The remaining 10% of atmospheric aerosols are anthropogenic (human induced). Most of these consist of sulfates and carbon particles from the burning of biomass or fossil fuels. Sulfates are produced when sulfur dioxide from fossil fuels reacts with water vapour and other gases, while organic carbon and black carbon are produced when biomass is burned in land clearing activities. Although aerosols have a short lifetime in the atmosphere, they are quickly replaced by anthropogenic activities and wind erosion in dry areas.

The Significance of Aerosol Particle Size

When the size of aerosol particles is 2 micrometres or less (examples include sulfate particles and organic carbon), they tend to scatter, reflect and absorb incoming solar radiation. This tends to have a cooling effect, whereas particles larger than 2 micrometres (such as black carbon or soot) tend to absorb outgoing long wave radiation, creating a warming effect.

Aerosols Have a Net Negative Forcing Effect

Radiative forcing is described as the influence a substance has on atmospheric temperatures, and is usually quantified in watts per square metre (W/m²). As a whole, aerosols act to cool the atmosphere, and so have a negative forcing effect. Indeed, it is estimated that the sulfate aerosols from a single volcanic eruption can reduce global temperatures by around 0.3°C for up to 2 years.

The aerosols with negative forcing effects include sulphates ( -0.4 W/m²) and organic carbon from fossil fuels and biomass burning (- 0.15 -0.25 W/m²). Black carbon, however, has a positive forcing effect of +0.2 W/m².

Indirect Effects of Aerosols on Climate

In addition to their direct role in reducing global warming, aerosols (particularly sulfates), can affect cloud lifetime and droplet concentration by acting as cloud condensation nuclei. This tends to have a further cooling effect because of increased scattering of solar radiation.

As a consequence, some scientists believe global warming may well increase if measures to reduce future sulfate aerosol production are effective. Author Matthew Mc Dermott , however, points out that it is more likely that black carbon from Asian pollution will increase in the future. In either scenario, atmospheric temperatures are predicted to rise.

Another indirect effect of aerosols may be their influence on rainfall patterns: while they can increase droplet formation in clouds, recent studies have suggested that the nature of these droplets is such that actual rainfall in these clouds is delayed. As a result, when the cloud eventually forms rain later in its life cycle, the precipitation is more intense, causing flooding in some areas while other locations are subjected to drought conditions.

Pollution in China May be Affecting Rainfall in Australasia

Moreover, the variations in atmospheric temperatures caused by aerosols may be causing weather patterns to shift to other areas.The effects of this phenomenon over Australasia have been studied by CSIRO scientist Dr. Leo Rotsteyn, who suggests that increased anthropogenic haze over Asia (see figure 1) has increased rainfall in Australia between 1951 and 1996. This, he maintains, is caused by changes in temperature and pressure gradients over the Indian Ocean which have resulted in monsoonal winds moving further towards northwestern Australia.

The Intergovernmental Panel on Climate Change (IPCC) has recognised these examples of the more subtle effects of aerosols and acknowledged in 2008 that aerosols are the ‘dominant uncertaintly in radiative forcing’. NASA have responded to this statement by announcing the launch of their ‘Glory’ satellite in February. This craft will use state of the art equipment to monitor the impacts of aerosols on global temperatures in an attempt to clarify some of the controversy surrounding these tiny atmospheric particles.

References

CSIRO, 2006, ‘Aerosols and the Climate’, csiro.au

IPCC Third Assessment Report, 2001, 'Climate Change 2001, the Scientific Basis' , grida.no

Mc Dermott, M., 2009, ‘Aerosols More Important to Global Warming Than Acknowledged’, treehugger.com

Science Daily, 2008, ‘Role of Aerosols in Climate Change Examined’, sciencedaily.com