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Fifty years of satellite data pin 84% of polar ice loss on faster-flowing glaciers
The IMBIE team merged 42 independent satellite surveys from 27 missions to account for 11.3 trillion tonnes of ice lost from Greenland and Antarctica since 1979, and it assigns 84% of that loss to glaciers accelerating into the ocean.
The Scientist · Science desk
What happened
- The Ice Sheet Mass Balance Inter-comparison Exercise combined 42 independent satellite surveys from 27 missions, reaching back to 1972 for Greenland and 1979 for Antarctica through the early Landsat archive.
- Greenland and Antarctica shed 11,309 billion tonnes of ice between 1979 and 2023, enough to raise global sea level by 31.4 millimetres.
- Glaciers speeding up and discharging ice into the ocean drove 84% of that combined loss, with the remaining 16% coming from melting at the surface.
- The last four years of the record brought a temporary slowdown, with record snowfall over East Antarctica and a run of milder Greenland summers that roughly halved Greenland's surface melting.
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Why it matters
- exposure The ice sheets now supply about a quarter of global sea level rise, so a growing share of coastal exposure is governed by how fast a small number of outlet glaciers move.
- constraint Sea-level projections cannot be tightened by better surface-melt physics alone: until ice dynamics are constrained, planners work with a high-end 2150 estimate 2.6 times higher than one that leaves ice-sheet instability out.
- decision The four most recent years are the ones a planner is most tempted to extrapolate, and the authors label them a short-term fluctuation against a clear long-term trend.
Melting at the surface and ice flowing off the edge respond to different things. Melt follows summer air temperature over the ice sheet. Discharge follows how fast outlet glaciers move, and in Antarctica the IMBIE team attributes all of the continent's net loss to ocean melting of its outlet glaciers [10]. Otosaka, who led the study, said the pattern shows "the long-term trend is being driven by the dynamic response of the ice sheets to a warming ocean" [7].
In absolute terms, the flow share works out at roughly 9,500 billion tonnes of ice pushed into the ocean, against about 1,800 billion tonnes lost as surface meltwater [2]. Greenland supplied the larger part of the sea-level total. Antarctica's contribution is 13.3 millimetres [3]; subtracting that leaves 18.1 millimetres from Greenland [1].
The acceleration is legible in the decadal rates. Greenland was close to balance in the 1970s, and its 2010s loss rate was about 4.4 times its 1980s rate [9][3]. Antarctica went from around 48 billion tonnes a year in the 1980s to 202 billion in the 2010s [10], roughly 4.2 times [4]. Losses climb steeply from the 1990s onward [8].
In West Antarctica the flow term has not paused at all. Discharge there, led by Pine Island and Thwaites glaciers, rose in every decade of the record [11].
The record measures mass change and partitions it. It cannot say when discharge stops accelerating. Shepherd, who founded IMBIE and heads the School of Geography and Natural Science at Northumbria University, said: "Three centimeters (1.2 inches) of sea level rise may sound small, but that puts another six to nine million people at risk of coastal flooding and erosion." [15]
What the length buys is a test bed. The combined budget covers 44 years [5], long enough to ask whether an ice-sheet model reproduces the 1980s rates as well as the recent ones. Sutterley, the IMBIE co-lead, said satellites "give us continent-wide, year-on-year evidence of what is happening now, and that is exactly what we need to test models and narrow the uncertainties in projections of future sea level rise" [17].
On the reliability of the numbers themselves, Sutterley said: "The added length of the record, the larger pool of participating teams and our improved ability to assess uncertainties make this the most robust assessment of ice sheet mass balance produced so far." [16] The assessment was led from Northumbria University and the University of Washington Applied Physics Laboratory, supported by the European Space Agency and NASA, and published in Scientific Data [6].
What to watch
- Whether the record East Antarctic snowfall of 2020-2023 persists in the next IMBIE update, or the surface offset disappears.
- Whether Pine Island and Thwaites discharge keeps rising through the 2020s, which would extend the one unbroken trend in the record.
- Whether the next round of sea-level projections narrows the 2.6x high-end factor now that models can be scored against 44 years of mass balance.