While global temperatures are still on the rise, the last ten years have seen a dramatic reduction in the rate of increase. Some have attempted to say that this is proof that global warming has stopped, but a new study by scientists at the University of Colorado Boulder is pointing towards the real reason — volcanic eruptions releasing tons of sulphur dioxide into the atmosphere.
Mixed in to the ash and smoke released by a volcanic eruption are several gases, including water vapour, carbon dioxide and sulphur dioxide. The sulphur dioxide can produce acid rain downwind of the volcano, but if it gets high enough in the atmosphere — into the stratosphere — chemical reactions there form extremely tiny particles of sulphur, called aerosols. These highly-reflective sulphur aerosols can damage the stratospheric ozone layer, and they absorb heat coming off the Earth, thus warming the stratosphere, but at the same time, they reflect incoming sunlight back into space, thus cooling the surface of the planet.
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Scientists have known about this cooling effect from sulphur dioxide for some time now, and it has even been suggested as a temporary 'treatment' for the planet's increasing surface temperatures (until we can get over our addiction to fossil fuels), however there has been an understandable reluctance to use it, due to its potential to completely undo all the recovery our ozone layer is seeing these days.
Ryan Neely, a scientist with NOAA's Cooperative Institute for Research in Environmental Science (CIRES) at the University of Colorado Boulder, led a team of researchers to investigate temperature and climate factors from 2000 to 2010, looking for a reason for the lower rate of temperature increase. While some research was pointing to increased sulphur dioxide emissions from countries such as China and India, who have increased their emissions by as much as 60 per cent over that time period (due to burning more coal), other studies suggested that volcanoes were mainly to blame.
"This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet," said Neely, who, according to Science Daily, also said that these stratospheric sulphur aerosols have countered as much as 25 per cent of the warming from greenhouse gases.
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In this study, Neely and his team examined long-term measurements of the 'optical thickness' of the stratospheric aerosol layer. 'Optical thickness' is a measure of how much these tiny particles prevent sunlight from passing through the air. An increase in optical thickness means that more sunlight is being scattered or absorbed. According to Neely, the optical thickness of the stratospheric aerosol layer increased by between 4 and 7 per cent between 2000 and 2010, thus becoming more opaque to sunlight. This allowed less sunlight to reach the surface, and therefore lowered the temperature.
"The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth's climate," said Brian Toon, a professor of atmospheric sciences at CU-Boulder. "But overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up."
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