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This article by Alex Kirby of Climate News Network was originally published on Climate Central

Climate scientists think they may have found at least part of the answer to a conundrum which has been puzzling them recently — why the atmosphere has not warmed as much as expected over the last decade or so.

A team led by the University of Colorado-Boulder (CU-Boulder) thinks the reason may be emissions of sulphur dioxide (SO2), a known inhibitor of atmospheric warming, from many of the world’s volcanoes.

The puzzle is why the global average temperature has not increased as expected in step with rising greenhouse gas emissions. This has led some to suggest that global warming itself is faltering, and with it the entire scientific justification for action to stabilize the climate.

Alaska’s Mount Redoubt. Credit: R Clucas via Climate News Network

The chair of the Intergovernmental Panel on Climate Change, Dr R K Pachauri, was reported in The Australian on February 22 as having acknowledged “a 17-year pause in global temperature rises, confirmed recently by Britain’s Met Office”, but “said it would need to last ’30 to 40 years at least’ to break the long-term global warming trend”.

Most SO2 emissions are from fossil fuel burning at power plants and in industry, with smaller amounts coming from railways, large vessels, and some industrial processes.

Volcanoes Downplayed

Small quantities of the emissions eventually rise into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulphuric acid and water particles that reflect sunlight back into space, cooling the planet.

Scientists have known for years that this cooling mechanism from a range of aerosols is helping to prevent global average temperatures rising as much as they otherwise would under the influence of greenhouse gases, but it appears they have underestimated the effect of volcanic SO2.

India and China are estimated to have increased their industrial SO2 emissions by about 60 percent between 2000 and 2010 through coal burning.

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