The curtains would separate polar ice sheets from warm ocean waters — but like other geoengineering proposals, the idea divides scientists

Could giant underwater curtains slow ice-sheet melting?

The curtains would separate polar ice sheets from warm ocean waters — but like other geoengineering proposals, the idea divides scientists

With ice in polar regions disappearing at record rates, a group of researchers has proposed a drastic idea in the hope of slowing the melting: erecting giant underwater ‘curtains’ near glaciers to protect them from warm water. The idea has been met with scepticism and serious consideration alike, while scientists grapple with questions of whether pursuing such a radical proposal would help the world to address climate change.

“We absolutely don’t know if [the idea] is going to work or not,” says John Moore, a glaciologist at the University of Lapland in Finland, and the main proponent of the idea. But he thinks that it is “vitally important” for scientists to explore it in case the polar ice sheets start to “go unstable early”, despite the best global efforts to cut greenhouse-gas emissions.

Moore has been travelling the world to present the idea through talks and workshops, such as one at Stanford University, California, in December 2023. Experiments on the concept by collaborators at the University of Cambridge, UK, are expected to start next month.

But other scientists say that like other geoengineering proposals to address climate change — such as cooling the Earth by altering clouds — talk of iceberg cutains is an unwelcome distraction from the task of stopping the melting before it spirals out of control. They say that cost and feasibility of the idea would be prohibitive.

The world’s two ice sheets — in Greenland and Antarctica — are, effectively, giant glaciers sitting atop a landmass. The sheets lose ice mainly through a few outlet glaciers that protrude into the ocean. Their lower parts are eroded by warm, salty seawater in the deep ocean. Although these are the sites of the most melting, the outlet glaciers also act as a plug, preventing the ice sheet from more rapidly sliding into the ocean.

However, as climate change worsens, a few of these unstable outlet-glacier plugs are swiftly melting, including the Thwaites and Pine Island glaciers in West Antarctica. According to a paper from October 2023¹, the West Antarctic Ice Sheet — which contains enough ice to raise the global sea level by roughly five metres if melted — is projected to see “unavoidable” and “widespread increases” in ice loss over the course of this century.

Moore and a growing team of collaborators — ranging from engineers to social scientists — are trying to find out whether it would be feasible to slow down the thawing by using anchored curtains.

The idea is to use curtains to prevent warm water from lapping at the base of the ice shelves — a concept derived from a 2018 proposal of building under-sea berms.

Each curtain would be around 100 metres in height, be anchored to the sea floor by a foundation and be buoyant (see 'Curtain call'). The team initially assessed “very durable and slippery plastics” as possible materials. “But obviously, plastics are not a good thing to introduce anywhere,” particularly in Antarctica, says Moore. He and his collaborators are now looking into natural fibres, including canvas, hemp and sisal (Agave sisalana).

Moore foresees around a decade of study and experiments before conducting an on-site pilot test in Greenland, if the local people consent to and support the idea.

Researchers divided

Moore has support from some high-profile supporters. The Centre for Climate Repair at the University of Cambridge plans to start laboratory experiments in February to test mathematical models of the curtain that were developed by other researchers, says Shaun Fitzgerald, the centre’s director and a fluid-mechanics engineer.

The centre is also planning an outdoor experiment in the River Cam for later this year, to help researchers to understand some of the fluid-mechanics issues and flow properties of the curtain, according to Fitzgerald.

For him, the goal of the curtains is not to stop the flow of warm water, but to reduce the rate. “The curtains are not going to fix the climate problem. They’re like sticking plasters to keep the ice … whilst we get greenhouse-gas levels down.”

However, other scientists remain doubtful of the idea’s feasibility.

“Polar environments are extremely difficult to work in. Even the task of accessing these places, let alone undertaking major engineering activities there, I view as problematic,” says Twila Moon, a glaciologist at the National Snow and Ice Data Center at the University of Colorado Boulder.

Moon understands that the aim of the curtain is to prevent severe sea-level rise by protecting the ice sheets. But she says that she would be “deeply surprised” if the curtains would make any measurable difference to sea levels, when so many other factors contribute to the rise.

“To me, it's much wiser for us to invest our time and our resources and our energies into mitigation and into supporting successful adaptation” to sea-level rise, she says.

Some scientists worry that the idea could have negative side effects. Lars Smedsrud, a polar oceanographer at the University of Bergen in Norway, notes that the curtain might also block the flow of nutrients between the glacier and sea, potentially harming the surrounding marine ecosystem.

He adds that the idea would not prevent the heating of the ocean, and instead will prevent only localized heating at the outlet glaciers. “The ocean would heat up more elsewhere, and perhaps cause more damage there,” he says.

Cost is also a major point of debate. Moore and his co-authors have estimated a US$40–80 billion price tag, plus US$1–2 billion annually for maintenance, to install a curtain that is 80 kilometres long, at a depth of 600 metres, which they said could help to stabilize the Thwaites and Pine Island glaciers “over the next few centuries”.

The figure almost equals the total climate finance mobilized by high-income countries in 2021. Moon questions the financial ask when there is “no confidence” that the idea will work.

But, in Fitzgerald’s view, the curtains’ cost should be compared with what countries would otherwise have to pay to cope with rising sea levels. “We’re talking trillions of dollars,” he says.

Moral hazard

Perhaps the greatest concern of all over the curtain idea — which applies to other geoengineering propositions too — is that it might weaken the urgency to reduce greenhouse-gas emissions and provide a cover for business-as-usual energy use.

“We already have in front of us a variety of emissions pathways. We know that those strongest action pathways … make a difference for ice loss,” says Moon. She regards the idea as “a distraction” from pursuing mitigation and more-proven technologies that require development, such as carbon capture.

“Absolutely, we need to mitigate — actually more quickly than we’ve been doing,” says Moore. But he says that the research provides a complement, not competition, to emissions reduction. “The real moral hazard is to be quiet and not to inform people about what potential [tools] could be available down the line.”

Christian Schoof, a fluid dynamicist at the University of British Columbia in Vancouver, Canada, says that he has closely followed the sea-bed-curtain debate. In his view, geoengineering approaches, such as sea-bed curtains, are a “stop-gap measure” to buy humanity time to address the root causes of climate change.

“All geoengineering ideas are mad until you consider what might happen if we do nothing,” he says. To him, the ice-sheet curtain is “certainly not a concept I’d write off”.

doi: https://doi.org/10.1038/d41586-024-00119-3

This story originally appeared on: Nature - Author:Xiaoying You