Some of the main uncertainties in understanding climate change concern the role of aerosols.
Aerosol particles vary widely in size and chemical composition, and both are important in governing their effects on climate and human health. They have both natural and anthropogenic sources. Most atmospheric aerosol consists of a mixture of components: inorganic salts, organic carbon, black carbon, trace metals and water.
Aerosol particles affect the climate directly by scattering and absorbing of solar radiation and indirectly by modifying the properties of clouds. Black carbon causes warming, whereas all other aerosol causes cooling. The short lifetime of aerosols (days/weeks) means that their effects are more regional and less persistent into the future than those of the long lived greenhouse gases.
Climate can change as a consequence of many factors, some of which are natural, and some of which are anthropogenic. The global warming as the globe has experienced over the past ~150 years is due a combination of factors, the most important one being man-made greenhouse gases, which cause the earth to trap more heat. Aerosol cooling has masked some of this warming. The influence of natural factors, such as the sun and volcanoes, has been minor and episodic, respectively. Over the longer term (decades to centuries), CO2 will be the main driver of climate change because of its very long presence in the atmosphere (in sharp contrast to aerosols).
The strong increase in man-made aerosols during the middle of the 20th century has counteracted the increase in greenhouse gases during that same period, causing the global temperature to remain relatively steady. After the late 1970’s however, greenhouse forcing became dominant, and the globe warmed again.
Aerosol concentrations are expected to be reduced globally, but at a different pace in different regions. Reductions will be achieved first in developed countries, whereas in developing countries the aerosol burden may first increase before it starts to decline at a later stage. A consequence of reductions in aerosol concentrations will be a more pronounced warming, as the underlying greenhouse warming trend will be revealed. Trends in aerosol concentrations continue to have an important influence on the climate, especially regionally.
Many combustion processes result in the simultaneous emission of (warming) greenhouse gases and (cooling and warming) aerosols. In devising policies to curb health effects, simultaneous climate impacts should be considered, and vice versa. It is becoming more and more clear that regional air quality and global climate issues are intricately linked with one another.