Global sea levels may rise faster than previously expected, a new study suggests. The reason is that warming oceans appear to be melting Antarctic ice shelves from below much more rapidly than expected.
Ice shelves, which are extensions of gigantic glaciers that float on the water surface, act like buttresses that slow the flow of gigatons of ice into the sea. Now, researchers in Norway have discovered that long, channel-like grooves on the underside of these ice shelves can trap relatively warm ocean water. This sharply increases local melting.
The study has global implications. If Antarctic ice shelves thin and weaken, the downhill journey of the ice behind them can accelerate, fast-forwarding the process in which huge amounts of ice cascade into the ocean, causing sea levels worldwide to rise far faster than currently projected.
This dynamic has already been observed elsewhere in Antarctica. The Intergovernmental Panel on Climate Change (IPCC) has flagged polar ice shelf instability as a major but poorly understood risk factor that could lead to sea level rise that is far more rapid and severe than most current models predict .
What the team found
Using Fimbulisen Ice Shelf in East Antarctica as a case study, the team found that the shape of the ice shelf base can strongly alter how ocean water moves beneath it. Where the underside is channelled, the circulation can create small overturning cells that hold warmer water in place beneath the ice instead of letting it pass through quickly.
In those channels, melt rates can rise by about an order of magnitude locally. In simple terms, the geometry of the ice shelf helps decide where ocean heat goes, and how destructive that heat becomes.
"We found that the shape of the ice shelf underside is not just a passive feature. It can actively trap ocean heat in exactly the places where extra melting matters most," lead author Tore Hattermann from the iC3 Polar Reseach Hub in Tromsø, Norway explains.
Fimbulisen Ice Shelf is located in East Antarctica, a region that is colder and therefore usually seen as less immediately threatened than the rest of the continent.
"We observed beneath the Fimbulisen Ice Shelf that even small amounts of warmer water can substantially increase melting within the channels," Tore Hatterman says. "As a result, the channels can grow and, in the worst case, weaken the stability of the entire ice shelf."
Qin Zhou, who co-led the study, adds that "What is striking is that even modest inflows of warmer deep water can have a large effect when the ice shelf base is channelled. That means some ice shelves that scientists usually think of as cold may be more fragile than expected."
How the team worked
To reach these conclusions, the researchers combined a detailed map of the ice shelf underside with a high-resolution model of the ocean cavity beneath Fimbulisen.
They compared cases with a smoother ice base and a more realistic channelled one, under both cooler and slightly warmer ocean conditions. This allowed them to isolate the effect of the channels on water flow, mixing and melt.
The study also drew on earlier field observations from the region, showing the value of combining long-term measurements with modelling that can resolve small features beneath the ice. Tore Hattermann himself has spent hundreds of days camped out on Antarctic ice shelves.
Why this matters
The wider implications are serious. Faster melting inside channels can make those channels grow deeper and wider, causing uneven thinning in the deeper part of the ice shelf. That can reduce the shelf's structural strength and weaken its ability to hold back the glaciers feeding it.
"Current climate models do not capture this effect," Tore Hattermann warns. "This means that they risk underestimating the sensitivity the 'cold' ice shelves along East Antarctica's coastline to small changes or warming in coastal waters. Such changes have already been observed, and are projected to increase in the future."
That is important for science, because ice sheet and climate models need to capture these small-scale features more realistically. It is important for policy too, because decisions about coastal planning and adaptation depend on credible sea level projections. And it is important ecologically, because changes in the delivery of meltwater can influence ocean circulation and marine ecosystems around Antarctica.
