Arctic sea ice now remains in its melting phase for about one week longer than it did in the late 1970s.
An international team compiled several decades of observations to examine how sea ice has changed at both poles. Their analysis, published in Nature Reviews Earth & Environment, covers its extent, thickness, age, the snow covering it and its annual cycle. In the Arctic, the melting season has lengthened by about 7.5 days since 1979.
Extent of Arctic sea ice on September 10, 2025. The yellow line indicates the average minimum extent for the 1981–2010 period.
Credit: NASA Earth Observatory / Lauren Dauphin, NSIDC data.
The change is mainly due to autumn. The ice does not necessarily begin melting much earlier in spring, but it reforms later. During summer, a larger part of the ocean remains ice-free. This dark water then absorbs more solar energy and retains more heat.
To understand this effect, it is enough to compare ice and water. Bright sea ice reflects between 60 and 90% of the sunlight it receives. The dark ocean reflects only about 7%. When the ice disappears, the water therefore absorbs much more energy. In autumn, this heat must be released before the surface can freeze.
This delay has another consequence. The ice that eventually forms has less time to thicken before spring returns. It is therefore younger and thinner when the next season arrives. Thin ice can melt more easily, once again leaving more water exposed to the Sun.
The changes do not concern only the amount of ice. Researchers also find that the snow covering Arctic sea ice is becoming thinner and that the ice surface is becoming smoother. These transformations alter the light passing through the ice and the conditions encountered by animals living in this environment.
Under the sea ice, algae form the base of part of the food chain. They are adapted to low light levels. A thinner layer of snow and ice lets more light through, which may disrupt them. The changes can then affect the organisms that consume them, followed by fish and other animals.
The Antarctic situation is not completely similar. The Arctic is primarily an ocean surrounded by continents, whereas Antarctica is a continent surrounded by ocean. Southern sea ice did not experience the same steady decline for a long time. Recent years have nevertheless recorded substantial losses, which are still difficult to interpret with the same precision.
Scientists are now preparing the fifth International Polar Year, whose main observation period is scheduled for 2032–2033. It will make it possible to measure ice thickness, snow and their changes at both poles more precisely, using instruments that did not exist during the previous international campaign in 2007–2008.