The Antarctic ice sheet is forming and reforming in a completely different way to what was previously believed, a new study has revealed.
Liquid water locked deep below the ice sheet regularly thaws and refreezes to the bottom, creating as much as half the thickness of the ice in places and actively modifying its structure, the team found.
"We usually think of ice sheets like cakes - one layer at a time added from the top. This is like someone injected a layer of frosting at the bottom - a really thick layer," says Robin Bell, a geophysicist at Columbia University.
"Water has always been known to be important to ice sheet dynamics, but mostly as a lubricant. As ice sheets change, we want to predict how they will change. Our results show that models must include water beneath."
The scientists found that refrozen ice made up a quarter of the ice sheet base around its main study area - as much as half in some places. They suggest that such refreezing has been going on since East Antarctica became encased in a large ice sheet some 32 million years ago.
Deeply buried ice may melt because of insulation by overlying layers. Refreezing can take place in several ways. If it collects along mountain ridges and heads of valleys, where the ice is thinner, low temperatures from the surface can refreeze it. If it gets squeezed up valley walls, pressure changes rapidly. Once moved up to an area of less pressure, supercooled water can freeze almost instantly.
"Understanding these interactions is critical for the search for the oldest ice and also to better comprehend subglacial environments and ice sheet dynamics," says Fausto Ferraccioli of the British Antarctic Survey.
"Incorporating these processes into models will enable more accurate predictions of ice sheet response to global warming and its impact on future sea-level rise."
The researchers now plan to look into how the refreezing process acts along the margins of ice sheets, where the most visible change is occurring in Antarctica.