7/26/2019 Nature.com_Antarctic Model Raises Prospect of Unstoppable Ice Collapse (31mrt16)

1/1

B Y J E F F T O L L E F S O N

Choices that the world makes thiscentury could determine the fate of themassive Antarctic ice sheet. A study

published online this week in Nature1findsthat continued growth in greenhouse-gasemissions over the next several decades couldtrigger an unstoppable collapse of Antarcticasice raising sea levels by more than a metre by2100 and more than 15 metres by 2500.

That is literally remapping how the planetlooks from space, says study co-author RobDeConto, a geoscientist at the University of

Massachusetts Amherst. The good news, hesays, is that it projects little or no sea-level risefrom Antarctic melt if greenhouse-gas emis-sions are reduced quickly enough to limit theaverage global temperature rise to about 2 C.

The findings add to a growing body ofresearch that suggests that Antarctic ice is lessstable than once thought. In its 2013 report2, theIntergovernmental Panel on Climate Changeestimated that Antarctic melting would con-tribute just a few centimetres to sea-level riseby 2100. But as scientists develop a better under-standing of how the ocean and atmosphereaffect the ice sheet, their projections of the

continents future are growing more dire.

DeConto and co-author David Pollard,a palaeoclimatologist at Pennsylvania StateUniversity in University Park, developed aclimate model that accounts for ice loss causedby warming ocean currents which can eat atthe underside of the ice sheet and for risingatmospheric temperatures that melt it fromabove. Ponds of meltwater that form on the icesurface often drain through cracks; this can setoff a chain reaction that breaks up ice shelvesand causes newly exposed ice cliffs to collapseunder their own weight.

They found that by including all of these pro-cesses, they could better simulate key geological

periods that have long puzzled scientists. Beforethe last ice age began130,000115,000 yearsago, for instance, sealevels were 69 metreshigher than today yet atmosphericcarbon-dioxide levelswere about 30% lower. And 3 million years ago,when CO2levels roughly equalled todays, theoceans may have been 1030 metres higher.

Incorporating the physics of ice melt drivenby atmospheric warming, along with cliffcollapse, helped DeConto and Pollard to repro-

duce these key periods with their model. That

was sort of an epiphany that maybe we were onto something, DeConto says. Ultimately, heand Pollard tested how well different versionsof their model simulated the past, and thenused the ones that performed best to projectfuture sea-level rise. They found that over time,atmospheric warming would become the maindriver of ice loss.

I think their processes are still a bit specu-lative, but its good work, says Nick Golledge,an ice-sheet modeller at the Victoria Universityof Wellington in New Zealand. His research,published inNaturein October3, suggests thatAntarctic ice melt driven by rising greenhouse-gas emissions could boost global sea levels byup to 39 centimetres by 2100, and by as much as3 metres by 2300.

Still, Golledge cautions, scientists know littleabout how the atmosphere and ocean affectedancient glaciers. We dont really have a greathandle on what the climate was like in thepast, he says.

A thirdNature

study, published in Decem-ber4, suggested that Antarctic melting wasunlikely to produce more than 30 centimetres ofsea-level rise by 2100. But its authors noted thatnewly identified processes such as surface melt-ing and the collapse of ice cliffs could increaseice loss. As such, DeConto and Pollards projec-tions are consistent with our recent study, saysco-author Tamsin Edwards, a physicist at theOpen University in Milton Keynes, UK.

Glaciologists are already aware of the destruc-tive power of the atmospheric-warming andcliff-collapse mechanisms that DeConto andPollards model simulates. A string of unusu-ally warm summers caused the sudden collapseof Antarcticas Larsen B ice shelf in 2002. Andscientists have witnessed the basic physics ofice-cliff collapse during calving events on theJakobshavn and Helheim glaciers in Greenland.

On the observational side, I see the thingsthey are talking about, says David Holland,a physical-climate scientist at New YorkUniversity. Theres a lot of observation andmodelling to go, but they are adjusting peoplesthinking in a very scientific way.

For DeConto, the new model resultsunderscore the choice that humanity is facing.If he and Pollard have the physics correct, thisprocess of ice-shelf disintegration, followed by

ice-cliff collapse, will be nearly impossible tostop once it gets under way.

Once the ocean warms up, that ice will not beable to recover until the oceans cool back down,he says a process that could take thousandsof years. Its a really long-term commitment.

1. Pollard, D. & DeConto, R. M.Naturehttp://dx.doi.org/10.1038/nature17145 (2016).

2. Church, J. A. et al.inClimate Change 2013: ThePhysical Science Basis. Contribution of WorkingGroup I to the Fifth Assessment Report of theIntergovernmental Panel on Climate Change(edsStocker, T. F. et al.) 11371177 (Cambridge Univ.Press, 2013).

3. Golledge, N. R. et al. Nature526,421425 (2015).

4. Ritz, C. et al. Nature528,115118 (2015).

C L I M AT E

Trigger seen forAntarctic collapseContinued growth of greenhouse-gas emissions thiscentury could raise sea levels more than 15 metres by 2500.

That was sortof an epiphanythat maybewe were on tosomething.

Recent studies suggest that the Antarctic ice sheet is much less stable than scientists once thought.

PAULNICKLE

N/NATIONALGEOGRAPHIC

CREATIVE

5 6 2 | N A T U R E | V O L 5 3 1 | 3 1 M A R C H 2 0 1 6

IN FOCUSNEWS

2016 Macmillan Publishers Limited. All rights reserved