COMMENT NEUROSCIENCE Dispatch from PALAEONTOLOGY Sex, power and SUSTAINABILITY How ARCHAEOLOGY The the dark past of deep brain DNA — tracking our ancestors can India manage its Iceman cometh to a stimulation p.306 around the world p.307 mountain of waste? p.308 cinema near you p.310 JOE RAEDLE/GETTY The village of Ilulissat in western Greenland is surrounded by icebergs that have calved from the Jakobshavn Glacier. Geoengineer polar glaciers to slow sea-level rise Stalling the fastest flows of ice into the oceans would buy us a few centuries to deal with climate change and protect coasts, argue John C. Moore and colleagues. he ice sheets of Greenland and reach US$50 trillion a year. Sea walls and global warming. In our view, this is plausible Antarctica will contribute more to flood defences cost tens of billions of dollars because about 90% of ice flowing to the sea sea-level rise this century than any a year to construct and maintain2. from the Antarctic ice sheet3,4, and about half Tother source. By mid-century, a 2 °C increase At this price, geoengineering is competi- of that lost from Greenland travels in narrow, is predicted1 to swell the global oceans by tive. For example, building an artificial island fast ice streams. These streams measure tens around 20 centimetres, on average. By 2100, to host Hong Kong’s international airport, of kilometres or less across. Fast glaciers slide most large coastal cities will face sea levels that which added 1% to the city’s land area, cost on a film of water or wet sediment5. Stemming are more than a metre higher than currently. more than $20 billion. China’s Three Gorges the largest flows would allow the ice sheets If nothing is done, 0.5–5% of the world’s Dam, which spans the Yangtze River to con- to thicken, slowing or even reversing their population will be flooded each year after trol floods and generate power, is thought to contribution to sea-level rise. 2100 (ref. 2). For example, a 0.5-metre rise have cost about $33 billion. Geoengineering of glaciers has received in Guangzhou, China, would displace more We think that geoengineering of glaciers on little attention in journals. Most people than 1 million people; a 2-metre rise would a similar scale could delay much of Greenland assume that it is unfeasible and environ- affect more than 2 million1. Without coastal and Antarctica’s grounded ice from reaching mentally undesirable. We disagree. We protection, the global cost of damages could the sea for centuries, buying time to address understand the hesitancy to interfere with ©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri gh15ts r eMARCHserved. 2018 | VOL 555 | NATURE | 303 COMMENT glaciers — as glaciologists, we know the construction of the berm and by the loss of warmer waters from entering. pristine beauty of these places. But we have sediment once the glacier was slowed. Whether such engineering feats would also stood on ice shelves that are now open Building such a berm would tell us whether successfully delay sea-level rise, and for how ocean. If the world does nothing, ice sheets glacial geoengineering is feasible, or if there long, requires a better understanding of will keep shrinking and the losses will accel- would be unanticipated consequences. But many factors. These include how the ocean erate. Even if greenhouse-gas emissions are the project would have only a small impact on circulates below ice shelves; how floating slashed, which looks unlikely, it would take 2100 global sea levels, given that Greenland’s ice fractures and calves icebergs; and how decades for the climate to stabilize. contribution is likely to be just 10–20 centi- glaciers slide and erode at their bases. A Is allowing a ‘pristine’ glacier to waste away metres1. Antarctica will be the largest contrib- thorough study would be needed to deter- worth forcing one million people from their utor, and geoengineering there will require mine the stresses that pinned ice shelves can homes? Ten million? One hundred million? larger and more challenging projects. sustain before they fracture. Models of ice Should we spend vast sums to wall off all the dynamics should determine the most effec- world’s coasts, or can we address the problem 2. SUPPORT ICE SHELVES tive locations for pinning. at its source? Geoengineering is a political Where Antarctica’s ice sheets reach the sea, ice Material could be shipped to Antarctica and societal choice, because people’s reactions flows out as floating shelves. Pinned by rocks from elsewhere in the world, or dredged or depend on how the issue is framed. Buttress- and islands, these platforms hold back the quarried locally. But it would be difficult in ing of glaciers needs a serious look. It should glaciers and limit how much ice reaches the practice for engineers to work around the ice have fewer global environmental impacts sea. As the air and ocean around Antarctica shelves, which grow and shrink as the glaciers, than other proposals being discussed for warm, some ice shelves are becoming thinner, sheets and conditions fluctuate. Sea ice would reducing sea-level rise, such as injecting aero- particularly those fringing the Amundsen also get in the way. Technologies might need to sols into the stratosphere to reflect sunlight Sea. In 2002, scientists were shocked at the be developed to operate beneath floating ice. and cool the planet. collapse of 3,200 square kilometres of the Major disturbances to local ecosystems would To stimulate discussion, we explore three Larsen B ice shelf, which is now only 30% of be expected and would require thorough ways to delay the loss of ice sheets. the size it was during the 1980s7. Half a dozen assessment before and after pinning. other shelves around the Antarctic Peninsula 1. BLOCK WARM WATER have shattered in the past 30 years. 3. DRY SUBGLACIAL STREAMS The Jakobshavn glacier in western Greenland Sheer cliffs are left behind when an ice Fast-sliding ice streams supply 90% of ice is one of the fastest-moving ice masses on sheet collapses. These crumble, accelerating entering the sea. As the ice slides over the gla- Earth. It contributes more to sea-level rise the glacier’s retreat8. The West Antarctic ice cier bed, frictional heat generates about 90% than any other glacier in the Northern sheet is especially vulnerable because its bed of the water at the base of the ice streams5. Hemisphere. Ice loss from Jakobshavn rock lies below sea level and is deeper inland9. This water acts as a lubricant, speeding up the explains around 4% of twentieth-century sea- Warm ocean currents in the Amundsen Sea flow, which in turn generates more heat, and level rise, or about 0.06 millimetres per year6. are melting the bottoms of floating parts of creates more water and slippage. Jakobshavn is retreating at its front. the glaciers, making the sheets more unstable. Glaciers in Greenland and at lower Relatively warm water from the Atlantic is The Pine Island3 and Thwaites4 glaciers latitudes are relatively wet because their sur- flowing over a shallow sill (300 metres deep) in West Antarctica are the largest potential faces melt in summer, and rivers flow beneath and eating away at the glacier’s base. Making sources of sea-level them. In Antarctica, by contrast, there is little the sill shallower would reduce the volume of rise over the next “Scientists were seasonal melting and much less water below warm water and slow the melting. More sea two centuries. Both shocked at the the ice sheet. For example, the base of Pine ice would form. Icebergs would lodge on the glaciers are losing collapse of the Island Glacier releases about 50 cubic metres sill and prop up the glacier. height and flow- Larsen B ice of water per second, which is only about A 100-metre-high wall with sloping sides of ing more quickly shelf.” 10 millimetres per year over the catchment 15–45° could be built across the 5-kilometre than two decades area5. Removing this thin layer of water will fjord in front of Jakobshavn glacier by dredg- ago. Pine Island Glacier reached a flow slow the glacier, reducing frictional heating. ing around 0.1 cubic kilometres of gravel rate of about 4 kilometres per year in 2009, The glacier will stall and its ice will thicken. and sand from Greenland’s continental shelf compared with 2.5 kilometres per year in It is difficult to access the glacier’s bed (see ‘Glacial geoengineering’). This artificial 1996 (ref. 10). Models predict that, by 2150, beneath one kilometre of ice, but there are embankment, or berm, could be clad in con- these two glaciers might disgorge ice ten precedents. The IceCube Neutrino Obser- crete to stop it being eroded. The scale of the times faster than current rates, contributing vatory at the South Pole has used jets of berm would be comparable with large civil- 4 centimetres a year to global sea-level rise8. hot water to drill 60 holes to depths of engineering projects. For example, ten times One solution is to artificially pin the 1,500–2,500 metres in the ice sheet. At Enga- more material — 1 cubic kilometre — was ice shelves in front of the two glaciers by breen, Norway, a network of 5-metre-wide excavated to build the Suez Canal. Hong constructing berms and islands, extended tunnels in the bedrock feeds 30–40 cubic Kong’s airport required around 0.3 cubic from outcrops or built on the sea floor.