Volcanic deception

clark Cross (Letters, 6 February) states that volcanoes, earthquakes, hurricanes and tornados are major emitters of greenhouse gases. In fact, with the exception of volcanoes I can think of no mechanism whereby any of these natural phenomena could possibly be “major” contributors to global greenhouse gas emissions.

On the contrary, the debate is over the impact which increased carbon dioxide levels contribute to more frequent and intense storms and enhanced seismic activity. It is true that volcanic eruptions result in the emission of large quantities of CO2 and some climate sceptics have argued that they emit far more CO2 than human activities. This, however, is generally regarded as false.

Recent research findings have concluded that human activities are in fact responsible for the emission of roughly 135 times more CO2 annually than those of all the world’s active volcanoes put together. That means the annual global volcanic output of CO2 is roughly equivalent to the annual CO2 emissions of Kazakhstan or one large US state.

Mr Cross claims that the eruption of Mount Pinatubo resulted in the emission of more greenhouse gases than that produced by the entire human race in history. In fact, Mount Pinatubo’s entire CO2 output was about the same as that which humans currently put into the atmosphere every 13 hours. Modern volcanoes do still play a major part in the Earth’s climate but through their cooling, not warming, effect.

Volcanoes emit sulphur dioxide into the stratosphere where they condense into sulphate aerosols that reflect solar radiation back into space, cooling the planet for a few years. This is exactly what happened with the eruption of Mount Pinatubo in 1991, which caused the largest aerosol disturbance in the stratosphere in the 20th century, cooling the planet by up to 0.5C, thereby actually temporarily counteracting the global warming trend rather than contributing to it, and also after the eruption of Mount Tambora in 1815, leading to “the year without a summer”.

Most recently, volcanoes have been cited as a possible factor in explaining the “pause” in global warming over the last 17 years.

In short, it is through their cooling effects rather than warming ones that volcanoes have had climate change impacts in modern times, including the last 30 years which have been generally marked by a warming trend, and they cannot be the culprit for the accumulation of greenhouse gases in the atmosphere, which most scientists including many often termed climate sceptics attribute primarily to human sources.

(Dr) Robbie Wemyss

Willowglen Road

Stornoway, Isle of Lewis