Falling sea levels caused a surge in volcanic activity at the start of the last ice age.

As temperatures plummeted as the Earth entered an ice age 80,000 years ago, a lot of the water in the oceans froze to form glaciers and ice sheets. Less water in the oceans meant lower sea levels.

What had been the sea floor became dry land. The release of the pressure of the sea on this land allowed magma beneath the surface to surge up through mid-ocean ridges and isolated volcanic islands, finds a study in Nature Communications.

Around 80,000 years ago the sea level fell by between tens of metres and about a hundred meters within 5,000 to 15,000 years. This was felt at mid-ocean ridges, where magma rises up to fill the gap formed when oceanic plates of the Earth's crust are pulled apart from one another, and at volcanic islands across the world.

"The decreasing pressure at the seafloor could have induced increased lava and carbon dioxide emissions," said Lars Rüpke of the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany, in a statement.

Volcanoes that weren't affected by sea level rise – those quite far in-land – were generally not more active during the ice age than before or after it.

The finding also solves a long-standing puzzle about greenhouse gases. During colder periods, carbon dioxide levels tend to be low, and during warmer periods carbon dioxide levels tend to be high. This is because of the greenhouse gas effect of carbon dioxide, which acts to trap radiation within the Earth's atmosphere rather than allowing it to radiate back out into space.

But during the glaciation that started 80,000 years ago, carbon dioxide levels didn't drop particularly drastically as the global temperatures fell.

Devastating volcanic eruptions could have been much more frequent in past ice ages. Jonathan Lewis / Wikimedia Commons

"The enhanced volcanic carbon dioxide flux may have stabilised the atmospheric carbon dioxide concentrations during the climate system's descent into the last ice age."

This could explain unresolved mismatches between carbon dioxide levels and temperature throughout the last 800,000 years or so of the Earth's history, the authors say. It's also hoped that although the period studied was many tens of thousands of years ago, the results could shed light on the relationship between sea levels and volcanism today.

"Such interactions could provide a novel component for earth system research to better understand the climate evolution at times of glacial sea level changes," said Gregor Knorr of the Alfred-Wegener-Institute, also an author of the study.