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Ice offers insight into volcanic impacts on climate

July 16, 2015, by Tim Radford

Information about volcanic eruptions dating back to the early Roman period can be found in ice cores.
Image: Ludovic Brucker/NASA Goddard Space Flight Centre

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Analysis of ice cores from Greenland and Antarctica yields new information on ancient volcanic blasts – and on the current effects of fossil fuel emissions. LONDON, 16 July, 2015 − Fifteen of the 16 coldest summers recorded in ancient history followed violent volcanic blasts that darkened the skies between 1000BC and 500 BC − and four of the coldest happened shortly after the largest volcanic events on record, according to US and European scientists. Studies of this kind, which reveal an intimate connection between discharges into the atmosphere and the consequences for the natural world, are an important part of the greater mosaic of research into climate change and global warming as a consequence of the human use of fossil fuels. Michael Sigl, an environmental chemist with both the Desert Research Institute in Nevada, US, and the Paul Scherrer Institute in Switzerland, and 23 colleagues from 18 institutions report in Nature journal that they analysed Greenland and Antarctic ice cores to create a more accurate timetable of more than 300 volcanic events, dating back to the early Roman period.

Dramatic shifts

Eruptive blasts from the past have been implicated in dramatic shifts in human history. Among them was the devastating bout of harvest failure, epidemic and famine − known as the Plague of Justinian − in the eastern Roman Empire from 541 to 543 AD. And ice cores offer a reservoir of annual levels of atmospheric sulphate − evidence of volcanic eruptions. The scientists matched these records with evidence from tree rings from Germany, the Alps, the US Great Basin, Siberia and New Zealand’s kauri forest. They also combed historic chronicles and accounts − from China, Babylon and from ancient and medieval Europe − that recorded telltale atmospheric observations such as weak sunlight, a discoloured solar disc, and very red twilight skies. Such matching has been attempted before, but the scientists believe their latest research has dated events with greater precision.

“Large volcanic eruptions were responsible for numerous and widespread summer cooling extremes over the past 2,500 years”

Large quantities of sulphate particles high in the atmosphere tend to block incoming sunlight, significantly reducing the temperature. For most of human history, the only source of such atmospheric pollution was violent volcanic eruption. “We are able to show that large volcanic eruptions in the tropics and high latitudes were the dominant drivers of climate variability, responsible for numerous and widespread summer cooling extremes over the past 2,500 years,” Dr Sigl says. “These cooler temperatures were caused by large amounts of volcanic sulphate particles injected into the upper atmosphere, shielding the Earth’s surface from incoming solar radiation.” The researchers also pinpointed the beginning of a climate crisis in the Dark Ages. In 536 AD, a veil of dust began to mask the Mediterranean − evidence of a massive eruption in the high latitudes. Four years later, a second volcano intensified the cooling, and a pattern of crop failure and famine persisted for the next 15 years, along with the Plague of Justinian, one of the greatest pandemics in human history. Volcanoes have already been implicated in recent climate research, with two recent studies proposing that an increase in eruptive activity might account for the seeming “pause” in global warming as a consequence of increasing levels of atmospheric carbon dioxide from the burning of fossil fuels. But climate impact is only one aspect of volcanic hazard. A new position paper from the European Science Foundation warns that in just the last 300 years, volcanic eruptions have directly killed more than 250,000 people and devastated entire communities.

Directly at risk

At the turn of the century, the population known to be directly at risk from eruption stood at more than 500 million − “a figure certain to grow”, says the paper. The European scientists also calculated the hazard of another mega-eruption on the scale of one 75,000 years ago from what is now Lake Toba, in Indonesia. This explosion in the distant Palaeolithic era is thought to have accounted for the deaths of two-thirds of all humans then living. A second such eruption would be enough to devastate the global food supply by depositing a metre of ash over millions of square kilometres, destroying the food resources of two billion people, and then reducing yields by cooling the climate between 5°C and 15°C for up to a decade. The scale of mortality would be impossible to predict, but the scientists conclude that “it is likely that it would be the greatest catastrophe since the dawn of civilisation”. – Climate News Network

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