At the end of the last Ice Age there was a major increase in volcanic activity. However, a paper in Geophysical Research Letters explains this story doesn't go the way you might be expecting. Not only did the greenhouse gases from the volcanoes warm the planet, the melting ice triggered the volcanic eruptions, and in more ways than we previously realized.
We know long-term alterations to Earth's orbit ended the last Ice Age by shifting sunlight to when and where it has more impact, causing some melting of the ice caps. This triggered positive feedback mechanisms that released carbon dioxide, which warmed the planet further, leading to more melting. Nevertheless, we still don't fully understanding what those feedback mechanisms were.
"It's been established that melting ice caps and volcanic activity are linked – but what we've found is that erosion also plays a key role in the cycle," said Dr. Pietro Sternai of Cambridge University in a statement.
Ice sheets kilometers thick are very heavy – heavy enough to depress the Earth beneath them. When ice melts and some of this weight is removed, pressure in the magma chamber decreases. This reduces the compression of the rock keeping the magma contained, thus making it more likely for it to escape in the form of an eruption.
This effect can be very powerful. Twelve thousand years ago, coinciding with maximum melting, Iceland's eruption rates may have been 100 times as high as they are today, let alone during the glacial peak.
As glaciers melt they erode the land beneath them. Erosion is known to have a powerful influence on the atmosphere, but Sternai has broken new ground in exploring its influence on volcanic activity. Previous models of the effect of ice reduction above volcanic chambers ignored what happens when the melting process takes huge amounts of rock and soil with it. “The density of rock exceeds that of ice by approximately three times,” the paper notes.
"Previous attempts to model the huge increase in atmospheric CO2 at the end of the last ice age failed to account for the role of erosion, meaning that CO2 levels may have been seriously underestimated," Sternai added.
Less rock than ice is removed, but the higher density makes it worthy of consideration. Sternai's estimates suggest its role could be considerable.
A 3D model of the effects of melting on the pressure on Villarrica Volcano, Chile. Pietro Sternai
“We know that much faster warming than cooling can't be caused solely by changes in the Earth's orbit,” Sternai said. “It must be, at least to some extent, related to something within the Earth system itself. Erosion, by contributing to unload the Earth's surface and enhance volcanic CO2 emissions, may be the missing factor required to explain such persistent climate asymmetry."
With ice melting at the fastest rate since the era Sternai is studying, this work may help predict our future. Polar volcanoes are likely increasing their activity in response to glacial reductions, but it is unclear whether the timescales will be such that this makes a major contribution to atmospheric carbon dioxide levels over the next century.
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