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Strict Paris-aligned cuts are at least 89% likely to mean less Antarctic ice loss by 2100
Researchers writing in Nature Geoscience give at least 89% odds that the strictest Paris path costs Antarctica less ice by 2100 than very high emissions. For coastal planners, that puts part of Antarctica's future contribution to sea level within reach of emissions policy.
The Scientist · Science desk
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What happened
- Even on the most ambitious path, with carbon dioxide emissions at net zero around 2050, the team puts the chance that Antarctica loses ice overall this century at 92% or more.
- Warmer ocean water thins Antarctica's floating ice shelves from below, and as the shelves weaken or break, the glaciers they hold back can flow faster into the sea.
- Under very high emissions, a deliberately high-end analysis gave a middle estimate of about 15 cm of sea-level rise from Antarctica by 2100 and an upper estimate of about 25 cm.
- The researchers found extra snowfall in a warmer climate very unlikely to replace all the ice Antarctica loses.
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Why it matters
- decision With net ice loss very likely even on the strictest path, coastal plans need some Antarctic rise built in regardless of emissions policy, with mitigation changing the amount.
- constraint The 89% settles which path loses less ice, but sizing a seawall needs centimetre estimates for each scenario, and the published account reports them only for a high-end case under very high emissions.
- exposure A planner who treats 25 cm as a worst-case total would underbuild, since that figure covers Antarctica alone and leaves out Greenland, mountain glaciers and the expansion of warming seawater.
The 89% is the probability of a yes-or-no outcome: that the strictest Paris-aligned path ends the century with less Antarctic ice loss than a very-high-emissions path [2]. It is a lower bound. For a comparison between opposite ends of the emissions range, I'd call that a firm answer on direction, with the chance that the ambitious path fails to come out ahead by 2100 at most 11% [1].
Antarctica has long been hard to pin down, Kopp said [11]. Models must connect global warming to what happens in the nearby ocean and air, and then work out how the ice reacts. Vary the assumptions at any one of those stages and the answers diverge widely [11]. Running enough detailed simulations to explore every combination would take enormous computing resources [12]. The team turned to machine learning to narrow the projections [12].
The centimetre figures come from a deliberately pessimistic search. The researchers picked the combinations of assumptions that gave the largest projected losses while still agreeing with satellite observations [7]. That second condition matters: it throws out high-end settings that disagree with what has already been measured from orbit [7]. The result is a possible high-impact outcome, not the study's central forecast, and the 25 cm upper estimate is not an absolute ceiling [8].
The high end depends on feedbacks. How easily the ice slides over the ground beneath it affects the pace of loss, and under some combinations of conditions the changes reinforce one another and speed the flow of ice into the ocean [6]. "What we still control is how much, and whether we set off the chain reaction that takes us to the high end," said Yucheng Lin of the City University of Hong Kong, a former postdoctoral associate in Kopp's department [13][16].
Antarctica also responds slowly, so the consequences of today's emissions persist long after the gases enter the atmosphere [10]. "Every ton of greenhouse gas we add makes it worse, and the effect lasts for hundreds of years," Lin said [14].
Kopp, a distinguished professor at Rutgers and an author of the study [1], put the planning case directly. "Coastal communities need to prepare for rising seas, and the amount of rise they will have to manage still depends on the choices we make today," he said [3]. "Better projections can help communities decide how to protect homes, roads and other essential infrastructure," he added [15].
What to watch
- Per-scenario central estimates in the full Nature Geoscience paper would put a centimetre size on what the strictest path saves by 2100.
- Independent checks of the machine-learning projections against full ice-sheet simulations, especially in the high-end combinations where feedbacks reinforce one another.
- Whether regional sea-level guidance adopts the satellite-constrained high-end range as a planning case for coastal infrastructure.