Rare good news: bedrock rise may slow ice melt
GPS data suggests that geological movement might prevent the collapse of the West Antarctic Ice Sheet. Richard A Lovett reports.
In what appears to be rare good news for sea level rise, bedrock underneath parts of West Antarctica is rising at a rate of 41 millimetres per year, or more than four metres per century, scientists say.
That’s fast enough, says Valentina Barletta, a geophysicist from the Technical University of Denmark, that it could help prevent or delay the collapse of the West Antarctic Ice Sheet, a collection of gigantic glaciers on the Pacific Ocean side of Antarctica. Currently one of the most rapidly melting regions in the world, the sheet contains enough ice to raise sea levels by several metres if it were all to liquidise.
The finding is contained in a paper written by Barletta and colleagues in the journal Science.
Scientists have long known that the weight of glaciers pressing onto the underlying bedrock causes those rocks to sink into the Earth’s mantle, often by hundreds of metres. At more distant locations, as other parts of the Earth’s mantle shift under the pressure, rock rises.
When the ice melts, the opposite occurs in a process known as isostatic, or glacial rebound.
Richard Alley, a geophysicist from Pennsylvania State University in the US, who was not a member of the study team, compares it to sitting on a waterbed. Not only do you sink into it, but water sloshes out to the side.
“You would not set your mug of coffee on a waterbed and then sit down next to [it],” he says.
When you stand back up, water quickly fills the spot where you’d been sitting and the waterbed returns to its normal condition. How quickly this occurs depends on what the waterbed is filled with. “If the waterbed were filled with, say, cold maple syrup, or maybe hair grease from the 1950s,” Alley says, the same process would occur, but would take longer.
To measure what lies beneath the bedrock of West Antarctica, Barletta’s team installed GPS stations on rocky outcrops in a region called the Amundsen Sea and watched how they shifted over time, up or down. They also used satellite measurements to see how the surface levels of nearby glaciers were dropping, as ice melted.
They found that the bedrock beneath the ice is rising unexpectedly quickly — and that as melting increases in the future, the rate of rise is likely to be even more rapid, possibly reaching eight-to-10 metres in the next 100 years.
One implication, Barletta says, is that satellite measurements of the rate at which the ice is thinning are too low by about 10%. That’s because the rising bedrock means that at any given surface elevation, the glacier isn’t quite as thick as previously anticipated.
But that’s a fairly minor discovery.
More importantly, she says, rising bedrock can help preserve the ice sheet. For one thing, it changes the slope down which the ice is sliding, reducing its rate of movement into the sea.
Also, the bedrock isn’t uniformly smooth. It has potential “pinning points” that can anchor the glacier and slow its advance. Raising their elevation can increase their effectiveness in doing so.
Finally, the seabed is also rising at the places where the ice meets the water. This reduces the degree to which the snout of a glacier tends to float rather than scrape against bedrock, again retarding its motion.
“This is the most important [effect],” Barletta says. And all told, she emphasises, “uplift can promote ice stability”.
Her result is also of interest to researchers trying to unravel the secrets of the Earth’s interior. “Antarctica is the least-known continent,” she says. Not only is it remote, but everything beneath the surface is hidden by kilometres of ice.
“This is the first time we can get accurate and precise information on what is below the ice and below the crust,” she notes.
Specifically, the rapid isostatic rebound her study found is only possible if there is a “very special structure” beneath the crust, she explains, a “very fluid, very soft mantle that deforms very quickly”.
From a geophysical perspective, Barletta says, “this is the main discovery”.
One possible explanation is that a plume of hot, low-viscosity rock is rising from deep in the Earth’s interior.
Or perhaps the squishy spot is a relic from a long-ago collision between continents, which shoved a slab of water-rich seabed hundreds of kilometres deep into the mantle beneath West Antarctica. Water trapped in that slab would make it “a little more fluid than average,” she says, thereby producing the squishy spot.
Whatever the cause, understanding how that part of the Earth reacts to changes in the ice helps us understand not only how our planet is responding to climate change today, but how it responded to the end of the last Ice Age, and possibly the one before that.
“It’s all sort of interconnected, but Antarctica is a very important part,” Barletta says.
Alley agrees. “[There] is lots of interesting science coming out [of this paper],” he says. “In coming years, estimates of sea level rise will be more accurate because of this work.”
But he cautions against excess optimism. Even if the new results do show that the ice sheet is “a little more stable than estimated,” he says, sufficiently rapid warming will nevertheless cause its rapid retreat to continue.
Ultimately, he warns, it will be human decisions that will determine our planet’s future.
“Decisions about our energy system are still the most important influence on future sea-level rise,” he says.