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What are the main drivers of climate change?
Mark Gee, London
Professor Mitchell: There are many factors which may contribute to climate change. For example, over the last million years, most of the long term changes in climate are probably due to small but well-known changes in the Earth's orbit around the Sun. Over much of the last millennium most of the variation in climate can probably be explained by cooling due to major volcanic eruptions and warming due to changes in the Sun's heating.
The situation in the 20th century is more complicated. There is some evidence that increases in solar heating may have led to some warming early in the 20th century, but direct satellite measurements show no appreciable change in solar heating in the last three decades. Three major volcanic eruptions in 1963, 1982 and 1991 have led to short periods of cooling. Carbon dioxide has been increasing steadily and has been shown to be very likely (according to the IPCC Fourth Assessment Report) to be responsible for most of the warming in second half of the century.
The final piece of the jigsaw is that as well as producing carbon dioxide, burning fossil fuels also produce small particles called aerosols which tend to cool climate through reflecting sunlight back to space. The steady increase in aerosol concentration over the 20th century has probably offset some of the warming due to increased carbon dioxide (and any solar contribution). This may have contributed to the lack of warming in the middle part of the twentieth century before we began to pursue a clean air policy. Only when all of these factors are included do we get a satisfactory explanation of the magnitude and patterns of change over the last century.
In the future, carbon dioxide concentrations will continue to increase and are likely to remain the main driver of climate change for the foreseeable future unless there are large reductions in emissions.
The final piece of the jigsaw is that as well as producing carbon dioxide, burning fossil fuels also produce small particles called aerosols which tend to cool climate through reflecting sunlight back to space. The steady increase in aerosol concentration over the 20th century has probably offset some of the warming due to increased carbon dioxide (and any solar contribution). This may have contributed to the lack of warming in the middle part of the twentieth century before we began to pursue a clean air policy. Only when all of these factors are included do we get a satisfactory explanation of the magnitude and patterns of change over the last century.
In the future, carbon dioxide concentrations will continue to increase and are likely to remain the main driver of climate change for the foreseeable future unless there are large reductions in emissions.
