Warm water seeping under Antarctic ice sheets may accelerate melting

Antarctica’s melting ice sheets may retreat faster as warm seawater intrudes underneath them. Warming ocean temperatures could also lead to a “runaway” feedback effect that allows warm seawater to push further inland, leading to more melting and faster sea level rise.

As the climate warms, the future of Antarctica’s vast ice sheets remains uncertain, with projections ranging widely for how rapidly they will melt and therefore how much they will contribute to sea level rise. One dynamic that researchers have only recently come to view as an important factor are intrusions of warm seawater beneath the ice.

“The intrusion mechanism is a lot more powerful than we previously understood,” says Alexander Bradley at the British Antarctic Survey.

Such intrusions occur due to the difference in density between the fresh water flowing out from beneath the ice sheet and the relatively warm ocean water where the ice meets the seafloor – an area known as the grounding line. This is difficult to directly observe as it occurs beneath hundreds of metres of ice, but simulations suggest the warm water could extend inland for kilometres in some places.

One model by Alexander Robel at the Georgia Institute of Technology in Atlanta and his colleagues found extensive intrusions could more than double the amount of ice loss from an ice sheet by adding heat from below and lubricating the flow of ice along the bedrock.

Bradley and his colleague Ian Hewitt at the University of Oxford built on that model, accounting for how the changing shape of cavities in the ice as it melts would alter the flow of intruding seawater.

They found that when ocean water reaches a certain temperature threshold, it melts ice at the grounding line faster than can be replaced by the flowing ice. As this cavity grows, more seawater can flow beneath the ice sheet and intrude further inland in what amounts to a “runaway” positive feedback effect.

“A small change in ocean temperature leads to dramatic change in the distance that the warm water is able to intrude,” says Bradley. He says the ocean warming required to set off this effect is within the range of projections of what we might see this century, although the model is not yet able to make predictions about specific ice sheets and not all ice sheets are equally subject to such intrusions.

“That positive feedback can cause there to be much more intrusion than we thought possible,” says Robel. “Whether that will be a tipping point that will lead to unrestrained incursion of seawater under the ice sheet – that’s probably a stretch.”