Dr Jim Haywood, Aerosol Research Manager
Atmospheric aerosols are microscopic particles suspended in the Earth's atmosphere, which generally act to cool the climate by reflecting sunlight back to Space and also by affecting clouds. The net impact of human activities, including greenhouse gases and aerosols, has been to warm the world's climate.
The Earth's atmosphere is made up of a number of components. These include gases such as nitrogen, oxygen and water vapour, and also atmospheric aerosols. Atmospheric aerosols are microscopic particles that are emitted from human and natural sources. The aerosols become suspended in the atmosphere. Human sources of aerosols include industrial aerosols from emissions of gases such as sulphur and nitrogen oxides, as well as direct emissions of smoke and soot from fossil-fuel and biomass burning.
Once in the atmosphere, aerosols have two effects upon climate, both of which lead to a cooling of the Earth:
Fig 2. The warming associated with greenhouse gases, the cooling associated with aerosols, and their effect on climate They can directly reflect sunlight back away from the Earth (left hand panel of Fig 1)
They can interact with clouds in complex ways leading to changes in cloud reflectivity, cloud lifetime, cloud height and cloud precipitation (Fig 1).
Human activities have increased concentrations of atmospheric aerosols, which have led to an associated cooling of climate. This cooling acts to counterbalance some of the warming due to increased concentrations of greenhouse gases which are also caused by human activities.
Just how much of a cooling effect these aerosols have on the climate is still uncertain owing to the complexity of the problem (Fig 1); but the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC, 2007) went some way to assessing this. It concluded that, although these microscopic particles do act to cool the climate, they do not offset the global warming effect of greenhouse gases (Fig 2).
A common misconception about aerosols is that they come from spray canisters, used for products such as deodorant, and that they damage the ozone layer. In the past the gases used as propellants in spray cans were damaging to the ozone layer, but not the aerosol particles themselves. Under the Montreal Protocol, these propellants have been replaced by non-ozone depleting substitutes. However, these gas replacements are greenhouse gases and add a small component to the global warming problem (from IPCC, 2007).