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Down to Earth

Earth’s inner heat helps control Greenland’s ice melt

It’s not all carbon dioxide’s fault. Earth’s mantle interferes too. Christine Zenino

It’s assumed knowledge that ice melting in the polar regions is governed by seasons. In summer, the Northern hemisphere’s polar ice cap loses tonnes of its ice due to the warmth of the sun and ocean currents. Increasingly, human activity is making this melting worse as the Arctic warms in response to an increase in carbon dioxide.

But now, in a new report in the journal Nature Geoscience, climate scientists have found that the Earth’s interior heat can also melt some ice, at least in those regions that are completely blanketed. Previously, this heat flow was thought to be insignificant in comparison to other factors.

Irina Rogozhina of the German Helmholtz Centre in Potsdam and colleagues looked at Greenland’s ice sheets. Because a large amount of water is held in these permanent ice sheets, climate scientists have been monitoring them carefully. One of the important measures of their dynamics is the amount of water that melts from the base of these glaciers.

Rogozhina found that the amount of basal melting cannot be fully explained by the current climate models. For any models to match with observed melting, she suggests that they need to account for Earth’s internal heat. The lithosophere, Earth’s rocky shell, varies in thickness beneath Greenland for reasons that are currently not well understood. Some parts of Greenland’s lithosphere are very thick and act as an insulator, protecting the ice from the interior heat. But other parts are thin, and the heat flow there affects the ice sheets.

One thin part occurs in central Greenland. And Rogozhina finds that this area strongly influences how geothermal heat is distributed to the rest of Greenland. But it is not just the heat flow that must be considered—the study also takes account of the variable melting at the base of the ice sheet, as the melt water influences how efficiently heat is extracted from the Earth’s crust, and where that heat gets distributed.

The researchers combined thermal and mechanical computer models of the ice sheet with models of the heat flow through Earth’s lithosphere, to find that the oldest and thickest part of Greenland’s ice sheet is actually the most strongly influenced by heat flow from deep inside Earth. But the model also suggested that relatively hot regions can form right next to cool areas.

It’s a complex system which we may not yet understood fully, but the dynamics appear to depend on interactions between the flowing ice and the variable hot and cold regions.

Rogozhina’s findings indicate that the Earth’s solid structure, even at depths of many tens of kilometres, can play an important part in influencing the dynamics of surface processes, especially where permanent ice sheets are involved. Thus, scientists who hope to understand the dynamics of the polar ice sheets — and thus the future of Earth’s ocean levels — cannot ignore the underlying geology.

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