First published August 14, 2020.
Could the Arctic Ocean really lose its summer sea ice by 2035? In August 2020 a study in Nature Climate Change, led by scientists at the British Antarctic Survey with the UK Met Office, concluded that its climate model points that way. The short answer is that the projection concerns the end of summer, when ice is at its lowest, and "ice-free" means almost no ice rather than none at all. Winter ice would still form every year.
Here is what the researchers actually did, why melt ponds sit at the centre of their argument, how the 2035 date compares with other projections, and which popular claims about melting sea ice do not hold up.
What the 2020 study found
The paper, titled Sea-ice-free Arctic during the Last Interglacial supports fast future loss, was published on 10 August 2020. Its joint lead authors were Maria-Vittoria Guarino and Louise Sime of the British Antarctic Survey in Cambridge, working with colleagues from the Met Office and universities in the UK, Canada and the United States.
The team used the newest version of the Met Office's coupled climate model, HadGEM3, to recreate the Arctic during the Last Interglacial, a warm stretch of Earth's history that ran from roughly 130,000 to 116,000 years ago (often summarised as "127,000 years ago", the period's peak warmth). In that simulation, the Arctic lost all of its sea ice in summer. When the same model looked forward under future warming, it projected a similar summer loss by around 2035.
The study's real contribution was to solve a long-standing puzzle. Geological records show that Arctic land summers were much warmer during the Last Interglacial than older climate models could reproduce. The new model matched those temperatures, and the reason was that it simulated the sea ice disappearing.
Why melt ponds matter so much
Each spring and summer, shallow pools of meltwater spread across the surface of Arctic sea ice. These melt ponds look harmless, yet they change how much sunlight the ice reflects.
Bare sea ice reflects roughly half to more than two thirds of incoming sunlight, and fresh snow on top of it can reflect as much as 90 percent, according to the US National Snow and Ice Data Center. Dark open water reflects only around 6 to 10 percent and absorbs the rest. Melt ponds sit between the two: they darken the surface, let more energy into the ice and the water beneath, and speed up melting.
This creates a self-reinforcing loop known as the ice-albedo feedback. Less ice means more dark water, which absorbs more heat, which melts more ice. HadGEM3 included a more detailed melt-pond scheme than earlier models, and that extra realism is what allowed it to reproduce the ice-free Last Interglacial.
What "ice-free" actually means
Scientists rarely use "ice-free" to mean zero ice. The common working definition is an Arctic Ocean with less than one million square kilometres of sea ice, sometimes called a "blue ocean event". Thick, old ice tends to linger along the northern coasts of Greenland and the Canadian Arctic Archipelago even in very warm years, so a literal zero is unlikely for a long time.
Timing also matters. Arctic sea ice normally reaches its maximum extent in March and its minimum in September. Projections like the 2035 figure describe the September minimum. In winter the ocean would still freeze over across most of its area.
For scale, the lowest extent on record so far came in September 2012, at about 3.4 million square kilometres. Reaching below one million would mean losing roughly another two thirds of that late-summer ice.
How 2035 compares with other projections
The 2035 date is on the early side of the scientific range, but it is not an outlier. Several studies using the best-performing models from the latest generation of climate simulations (CMIP6) have pointed to first ice-free Septembers around the mid-2030s under very high emissions. Other approaches that weight models differently put the first ice-free year somewhere between 2040 and the 2070s under intermediate scenarios.
The Intergovernmental Panel on Climate Change's Sixth Assessment Report (2021) concluded that the Arctic is likely to be practically ice-free in September at least once before 2050 under every emissions scenario it assessed. Natural year-to-year variability means the first such summer could arrive earlier or later than any single model says, and repeated ice-free summers would follow more often under higher emissions.
The fair way to read the 2035 date, then, is as a conditional projection from one well-regarded model, not a fixed appointment.
Myths and misunderstandings
"Melting Arctic sea ice will flood coastal cities"
This is a frequent mix-up. Sea ice is frozen seawater that is already floating, so when it melts it barely changes sea level, in the same way melting ice cubes do not make a glass overflow. Sea-level rise comes mainly from land ice, such as the Greenland and Antarctic ice sheets and mountain glaciers, and from the expansion of seawater as it warms.
"The whole Arctic will be ice-free all year"
No mainstream projection says this for the coming decades. The concern is the late-summer minimum.
"So losing sea ice doesn't matter"
It matters a great deal, for different reasons. A darker Arctic absorbs more heat, which accelerates regional warming. That warming in turn speeds the melting of Greenland's land ice, which does raise sea levels. Polar bears, ringed seals, walruses and many Indigenous communities depend on sea ice for hunting, travel and food. Researchers are also studying possible links between Arctic warming and weather patterns farther south, although that area remains actively debated.
What the researchers said and what it means for action
When the study was released, Louise Sime said the findings should push societies toward "achieving a low-carbon world as soon as humanly feasible." Guarino described the work as the first time scientists could see how the Arctic became ice-free during the last warm period.
The useful takeaway is that the pace of future ice loss depends heavily on emissions. Lower-emission pathways delay ice-free summers and make them rarer; higher ones bring them sooner and more often. Even a temporary ice-free September would be a striking signal of how far the planet has warmed, and every fraction of a degree avoided makes a measurable difference to how much summer ice survives.
Sources: Guarino, Sime et al., Nature Climate Change (2020); Cambridge Network: British Antarctic Survey release; NSIDC: Quick facts about sea ice; Wikipedia: Arctic sea ice decline; originally reported via ECO Magazine.
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