Understand · 2 November 2026
Sea ice: why its melting speeds up global warming (the albedo effect)
Sea ice reflects much of the sunlight it receives back into space. When it melts, the dark ocean absorbs that energy and warming accelerates. Everything you need to know, with the figures to back it up.
Sea ice is the ice that forms when seawater freezes at the surface of the polar oceans. When it melts, it barely raises sea level, but it speeds up global warming: a white surface reflects sunlight back into space, while the dark ocean it leaves behind absorbs it. This is known as the albedo effect. According to the National Snow and Ice Data Center (NSIDC), the minimum extent of Arctic sea ice at the end of summer has been shrinking by about 12.5% per decade since 1979.
Fly over the Arctic Ocean in summer: where sea ice still floats, everything is white; where it has melted, the water looks almost black. This difference in colour is no detail. It largely explains why the polar regions are warming faster than the rest of the globe, and why every square metre of ice lost matters.
What is happening
What exactly is sea ice?
Three kinds of polar ice are often confused:
- Sea ice forms when seawater freezes. It is thin, from a few tens of centimetres to a few metres thick. It grows every winter and retreats every summer.
- Ice sheets (Greenland, Antarctica) are giant glaciers resting on land, built up by snow accumulating over tens of thousands of years. They can be more than three kilometres thick.
- Icebergs are chunks of freshwater ice that break off glaciers and ice shelves, then drift across the ocean. The icebergs in the Mon Iceberg catalogue are real examples, tracked by satellite.
Sea ice already floats on water. Like an ice cube in a glass, its melting hardly raises sea level at all. It is ice sheets and mountain glaciers, by pouring ice stored on land into the ocean, that drive sea-level rise. Sea ice nonetheless plays a major role in the climate, through another mechanism.
The albedo effect, or the power to reflect light
A light-coloured surface sends much of the sun's light back into space; a dark surface absorbs it and warms up. That is why you feel hotter in a black T-shirt than in a white one in summer. Scientists call this ability to reflect light albedo. It ranges from 0 (everything absorbed) to 1 (everything reflected).
Sea ice is an excellent mirror. According to the NSIDC, bare sea ice reflects about 50 to 70% of the solar energy it receives, and up to 90% when covered with fresh snow. Open ocean, by contrast, reflects only about 6% and absorbs the rest.
In other words, replacing one square kilometre of snow-covered sea ice with one square kilometre of ocean means going from a surface that reflects almost all the energy it receives to a surface that keeps more than 90% of it.
A loop that feeds itself
This contrast creates what climate scientists call a positive feedback, an effect that amplifies its own cause:
- The air and water warm, so sea ice melts earlier in spring and refreezes later in autumn.
- More dark ocean is exposed to the sun during the long days of the polar summer, and it absorbs more heat.
- The heat stored in the ocean delays refreezing and melts even more ice the following summer.
The loop does not stop at summer. In autumn, the ocean releases the heat it has accumulated, warming the Arctic air and thinning the ice that forms in winter. Thinner ice melts more easily the following year.
Why is the Arctic warming faster than the rest of the world?
This phenomenon has a name: Arctic amplification. A study published in 2022 in the journal Communications Earth & Environment (Rantanen and colleagues) showed that the Arctic warmed nearly four times faster than the global average between 1979 and 2021, and up to seven times faster in some areas such as the Barents Sea. The authors identify the loss of sea ice and the albedo effect as one of the main drivers of this gap.
Other mechanisms contribute too, such as water vapour and clouds, or the way the stable, shallow polar atmosphere keeps heat close to the surface. But the loss of sea ice remains at the heart of the explanation.
And Antarctica?
Sea ice in the Southern Ocean behaves differently. It surrounds a continent instead of filling an enclosed ocean, and most of it melts every summer before forming again the following winter. For a long time it remained stable, or even grew slightly. Since 2016, however, it has reached very low extents, with a record low in February 2023. Scientists are still studying how much of this change is due to ocean warming and how much to natural variability in the winds.
What the data say
The NSIDC has measured sea ice extent by satellite every day since late 1978. Extent is the area of ocean covered by at least 15% ice. Mon Iceberg uses these public data to calculate the minimum reached each year, at the end of summer.
Arctic sea ice
- The September 2026 minimum was about 4.6 million km², roughly 26% below the 1981–2010 average minimum.
- The record low dates from 2012.
- The trend: according to the NSIDC, the minimum extent declined by about 12.5% per decade between 1979 and 2023, relative to the 1981–2010 average.
Extent does not tell the whole story. The ice is also much younger and thinner than forty years ago. So-called multi-year ice, which has survived several summers and can be several metres thick, has largely disappeared since the 1980s according to satellite observations tracked by the NSIDC and NASA. It has been replaced by more fragile first-year ice.
Antarctic sea ice
Sea ice there varies more from year to year. But the four lowest minimums measured since 1979 were all recorded between 2022 and 2025, according to the same NSIDC data.
And tomorrow?
In its Sixth Assessment Report (2021), the IPCC concludes that the Arctic Ocean is likely to be practically sea-ice-free in September at least once before 2050 under all the emissions scenarios it assessed, and more and more often as warming increases. "Practically ice-free" means less than one million km² of sea ice, compared with more than seven million on average in the 1980s.
A single year can be an exception: the weather in a given summer can slow down or speed up melting. What matters is the trend over several decades, and it is clear in the Arctic. You can follow it on the charts in the Climate section of Mon Iceberg, updated with each new report.
Why it matters, even far from the poles
- For the global climate: by absorbing more solar energy, the Arctic adds heat to the climate system. That heat also speeds up the melting of the Greenland ice sheet, which does raise sea level.
- For ecosystems: polar bears, seals and walruses depend on sea ice to hunt, rest or give birth. The algae that grow under the ice are the foundation of the entire Arctic food chain.
- For the people of the Arctic: Indigenous communities use the ice to travel and hunt. Thinner, less predictable ice makes these journeys dangerous.
- For mid-latitude weather: researchers are studying possible links between Arctic warming and certain weather events in Europe and North America. This topic is still debated in the scientific community.
What everyone can do
Nobody can refreeze the sea ice on their own. But how much ice disappears depends directly on the total amount of greenhouse gases humanity emits in the coming decades. Every tonne avoided counts.
In France, the average carbon footprint is between 8 and 9 tonnes of CO₂ equivalent per person per year, according to the latest estimates from the French Ministry for the Ecological Transition (SDES) and the national statistics institute (Insee). Transport, food and housing account for the largest share. In our Everyday action section, we explain the actions that matter most, each on its own page:
- Calculate your carbon footprint to know where to start;
- Take the train instead of flying, the most effective action for frequent travellers;
- Eat less red meat, without giving up the pleasure of food;
- Heat your home to 19 °C, and insulate it well;
- Make short trips on foot or by bike;
- Make your electronic devices last.
These actions do not replace public policy or business efforts, but they go hand in hand with them and show that another way of life is possible.
Frequently asked questions
What is the albedo effect?
Albedo is the share of sunlight a surface reflects back into space, from 0 (everything absorbed) to 1 (everything reflected). According to the NSIDC, snow-covered sea ice reflects up to 90% of solar energy, open ocean about 6%.
What is the difference between sea ice and an iceberg?
Sea ice is frozen seawater, thin and salty, that forms on the ocean surface. An iceberg is a block of fresh water that has broken off a glacier or an ice shelf and drifts across the ocean.
Will Arctic sea ice disappear?
According to the IPCC, the Arctic is likely to be practically ice-free in September at least once before 2050. Sea ice will still form every winter, but thinner and for a shorter time.