What is it about?

A ‘climate forcing’ is anything that affects our climate without being part of the climate system itself. Climate forcings can be both natural and human-made. For example, a volcanic eruption will throw out dust particles and gases that limit the amount of sunlight reaching the earth – which temporarily cools the climate. Particles and gases given out by human activities, for example carbon dioxide, will have the same effect. This study from the early 2000s looked at what happens when lots of different ‘climate forcings’ happen at once. They used computer simulations to recreate the earth’s temperature throughout the 20th century. Each time they ran the simulation, they added different combinations of ‘climate forcings’: • Solar radiation • Volcanic eruptions • Greenhouse gases such as carbon dioxide • Sulfate aerosols (particles released when fossil fuels are burned) • Ozone (released by car engines and power plants).

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Why is it important?

This study showed that the level of climate warming that occurred in the early 20th century was mainly caused by natural factors. But it also showed that the level of warming in the late 20th century could not have happened without human factors. KEY TAKEAWAY: This important study from the early 2000s provided further evidence that human activity was a major cause of global warming, and that natural factors alone could not have caused the level of global warming that occurred during the 20th century.

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This page is a summary of: Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate, Journal of Climate, October 2004, American Meteorological Society,
DOI: 10.1175/1520-0442(2004)0172.0.co;2.
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