What is an ice-free Arctic? | Polarjournal
An ice-free Arctic ocean would be a disaster for animals such as walruses, polar bears and other animals who rely on sea ice for their habitat, but it would also jeopardise important ecological processes (Photo: Michael Wenger)

Temperatures in the Arctic are rising four times faster than they are in other parts of the world. This means that the sea ice that is vital for the region and its inhabitants is disappearing at an accelerating rate. Research published last week showed beyond doubt that the first “ice-free” summer in the Arctic Ocean will take place around 2030 — 20 years earlier that previously projected. But what exactly is “an ice-free Arctic”?

The latest bad news about the Arctic last week was pretty clear: “Ice-free summers in the Arctic could happen sooner than expected” or “Arctic Ocean could be ice-free from 2030”. What was harder to find a good explanation of was what precisely “ice-free” means. Does that mean no more ice at all? No more solid surfaces for seals and polar bears to rest and mate on, and for others to hunt on? No more closed white surfaces that have to be broken through by massive icebreakers to keep important transport routes open? Though these visions might one day come to pass, in the start at least, saying the Arctic Ocean is “ice-free” won’t be the same as saying there is no ice at all.

Ice charts such as those from the Norwegian met office show where the sea ice is and how much of an area is covered by ice. On the colour-coded ice charts, blue signifies areas where less than 10% of the water is covered by ice. But this does not mean that there is no ice there at all (Illustration: Norsk Meteorologisk Institutt)

If you aren’t a lay person, you’ll know that the Arctic Ocean is considered ice-free when the total surface area of ice measures less than 1 million square kilometres. At the same time, ice charts produced as a result of increasingly accurate satellite imagery show how much of the water’s surface is covered by ice. This information is often displayed on colour-coded maps that make it easier to differentiate between different coverage areas. Blue, the area of least concentration, represents open water, but, even here, sea ice can cover as much as 10% of the surface area.

Since the long-term average minimum sea-ice extent in September is about 5 million square kilometres, ice-free would mean that at least 80% of the areas normally covered with ice in September are classified as open water, and any ice that is there would only be in the form of loose floes and chunks

Such open areas can be treacherous, especially for shipping, because large lumps of ice, known as “growlers”, still lurk directly near the surface and can damage ships if their hulls aren’t ice-hardened.

An Arctic Ocean that is “ice-free” is an Arctic Ocean whose sea ice forms increasingly later. Because water stores the energy from the sun, the longer it stays exposed to the sun, the later the autumn onset of ice formation will begin. The ultimate consequence of that is an Arctic Ocean that is literally ice-free (Photo: Michael Wenger)

The current definition of an ice-free Arctic takes into account sea ice that doesn’t melt due to climatic and geographic features, as is the case with the ice north of Greenland and along the Northwest Passage. Even in the event of an ice-free summer, a million square kilometres still sounds like a fairly large area, and new ice would form during the winter. This ice will be short-lived though; the open, darker, surfaces of the Arctic Ocean store the sun’s energy, while at the same time receiving heat from deeper layers of water from the south.

Storms and other extreme weather events, which can be expected to occur more frequently in a warmer climate, will accelerate this process by causing increased vertical transport of the heated water masses from the surface to the depths, adding even more heat. This, in turn melts more ice, causing further delays to ice-formation in the autumn.

Keep this up, and it won’t take very long until even people without advanced degrees will be able to see that the Arctic Ocean is ice free.

Dr. Michael Wenger, PolarJournal

Kim et al (2023) Nature Comm 14 (3139) Observationally-constrained projections of an ice-free Arctic even under a low emission scenario; https://doi.org/10.1038/s41467-023-38511-8

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