Almost 18,000 years ago the last Ice Age was approaching its end, but the planet was still almost as cold as ever. Yet large parts of the Southern Hemisphere experienced a sudden deglaciation, or loss of ice, something climate scientists have struggled to explain. Now the discovery of huge volcanic eruptions, lasting almost 200 years at just the right time, have provided a likely answer.
If a major volcano erupted as recently at 17,700 years ago in most of the world, we'd probably have become familiar with its legacy long ago. But Mount Takahe lies under the West Antarctic Ice Sheet, making its past doings something of a mystery.
Using ice cores taken from across Antarctica, Dr Joseph McConnell of the Desert Research Institute, Nevada, found evidence of very high rates of sulfur and rare earths, such as cerium, emitted over a period of 192 years. The locations of the highest concentrations point the finger at Mount Takahe, McConnell reported in the Proceedings of the National Academy of Sciences.
The consequence of these eruptions can be seen in the geological record across the Southern Hemisphere, with glaciers retreating in Patagonia and New Zealand, and increased summer rainfall in Brazil.
McConnell and his co-authors explained the connection by noting that the eruptions were rich in halogens – fluorine and chlorine – based on the legacy they left in the snow. The injection of so much of these gasses high into the atmosphere would have affected the stratosphere in a similar but even more extreme way, as the chlorofluorocarbons (chemicals containing carbon, chlorine, and fluorine) were released prior to their banning by the Montreal Protocol. That is, the ozone layer was badly damaged, particularly over the South Pole.
The loss of ozone allowed high energy ultraviolet (UV) light to reach the surface of Earth. As our own legacy of ozone depletion has shown, increased UV radiation speeds up the westerly winds that whirl around Antarctica, causing them to shift closer to the pole. This, in turn, has increased summer rainfall at certain southern latitudes, sparking the release of methane from wetlands and a reduction in the carbon dioxide absorbed by the Southern Ocean.
All this greatly accelerated the slow warming already occurring as cycles in the Earth's orbit moved us into an interglacial period. This led to a much faster loss of ice than would normally be seen at the end of an ice age, particularly in the Southern Hemisphere. Even after Mount Takahe stopped erupting, its legacy left the late Ice Age warmer than it would otherwise have been.
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