Human-caused climate change has ravaged sea ice at both ends of Earth in what may be a disturbing new normal. February this year saw global sea ice extent hit a record low as sluggish ice growth during the Arctic winter coincided with the fourth consecutive year of extremely low sea ice cover during the Antarctic summer.
“It’s like a missing piece of a continent,” says at the University of Tasmania in Australia.
The millions of square kilometres of lost sea ice spell disaster for people and ecosystems in these remote parts of the planet. Yet the extraordinary decline will also have global consequences ranging from further disruption to Earth’s energy balance and ocean currents to a boost in carbon dioxide released by ice-free waters.
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Sea ice in both the Arctic and Antarctica are now far below historical average levels. But the story is different at each pole.
In the Arctic, where the ice floats in an ocean surrounded by continents, there has been a near-steady decline since the satellite record began in 1979. In every year since 2007, its minimum sea ice extent has dropped far below the long-term average, suggesting the region – which is warming nearly four times faster than the rest of the planet – is experiencing “a new normal”, says at the US National Snow and Ice Data Center. During its low point in Arctic summers, the area of missing sea ice is roughly equivalent to lopping off all of the continental US east of the Mississippi river. The ice is also declining in winter: in March this year, the maximum Arctic sea ice extent set a , with more than 1.3 million square kilometres less ice than the long-term average.
In Antarctica, where sea ice rings a continent surrounded by ocean, the change has been more complex. Until recently, “Antarctic sea ice wasn’t playing along with the global warming theme”, says Meier. Other factors such as natural oscillations in ocean temperature and wind patterns drove changes in sea ice year-to-year. Its extent actually saw a slow increase since the beginning of the satellite record, until a precipitous decline in late 2016.
In early 2022, sea ice cover during the Antarctic summer crashed below the previous record low, with nearly 2 million square kilometres less than average – a loss equivalent to an area about the size of Saudi Arabia. Each of the next three summers got close to or surpassed that, prompting researchers to suggest we are seeing a permanent “regime shift” like that in the Arctic. A recent of Antarctic sea ice based on a longer-term record of changes in the atmosphere suggests the ice extent is now lower than at any point in the 20th century.
However, the short satellite record and imperfect models mean some uncertainty remains about whether a regime shift has occurred in this region and the extent to which global warming is behind the change, says at the British Antarctic Survey. But this streak of extreme lows “does imply that the Antarctic sea ice is responding to climate change in a way we hadn’t seen previously”, she says.
The fundamental shift at both poles is already impacting people and ecosystems. “It’s a whole system change,” says at the University of Colorado, Boulder. In Antarctica, for instance, emperor penguin colonies that rely on the ice to raise their chicks have seen mass die-offs. In the Arctic, Indigenous communities that use sea ice as a hunting platform have seen the length of their hunting season and size of their catch shrink. In regions such as Alaska’s northern coast, the loss of sea ice has accelerated erosion by exposing the land to waves. The structure and ecology of the Arctic Ocean in general has started to become more like that seen at lower latitudes in a process called “Atlantification”.
But the consequences of losing so much sea ice are hardly limited to the poles. One of the clearest global ramifications is a reduction in the amount of solar radiation reflected from Earth back into space. Ice reflects most of the sunlight that reaches it. When it melts, it exposes dark ocean, which absorbs most of the incoming solar energy. A recent on this found the low levels of global sea ice since 2016 mean the cooling effect of the ice is now about 14 per cent weaker than in the 1980s. “This impact is large relative to what we already know we’re doing to the climate system via greenhouse gases,” says Holmes.
If gauged for the Arctic alone, the change is even greater, with the ice there losing a quarter of its cooling effect over that period. This is fuelling the accelerated rate of warming in the north known as “Arctic amplification”. The warming in turn reduces the difference in temperature between Arctic air and warmer air to the south, which can disrupt the polar jet stream that controls weather across parts of the northern hemisphere. “That completely changes our weather systems over the US and Europe,” says at the Norwegian Research Centre, leading to more persistent heatwaves, torrential rain and frigid outbreaks of polar air.
The loss of sea ice in Antarctica may also be contributing to an observed slowdown in the overturning circulation of the world’s oceans. These powerful currents are driven by dense, salty water, which is left over from the formation of sea ice, sinking to the deep ocean. The currents driven by this so-called Antarctic bottom water enable the ocean to absorb more heat from the atmosphere and carry nutrient and oxygen-rich waters from the Southern Ocean to deep-water ecosystems across the planet.
Researchers have a slowdown in the formation of this bottom water since the 1990s, which they have mainly attributed to more freshwater pouring in from melting ice shelves. But a possible regime shift in the sea ice, along with more meltwater, adds to concerns that this could slow even further, with projecting a 50 per cent decline over the next three decades under a high-emissions scenario. This would reduce the amount of heat the oceans can take up, says Klocker.
A slowdown could also affect other vital currents, including the Atlantic Meridional Overturning Circulation that warms Europe and the powerful that isolates the continent from the rest of the world’s warming oceans. “There are lots of delicate balances here and the sea ice is right in the middle of all of them,” says Doddridge.
Other consequences of sea ice loss are even more uncertain, but no less concerning. For instance, it helps keep waves and warmer water away from Antarctica’s ice shelves – the floating coastal structures formed as inland ice sheets flow into the sea. Loss of this buffering may increase the rate of icebergs breaking off those shelves, which would in turn speed up the flow of ice sheets into the oceans, adding to sea level rise. “We’ve only seen a few ice shelves disintegrate, but they have generally disintegrated after a period of low sea ice in that area,” says Doddridge.
In winter, when carbon-rich deep water mixes vigorously with the surface, the Antarctic sea ice also acts as a lid on the Southern Ocean, preventing dissolved carbon dioxide from being released into the atmosphere. The overall effect is uncertain as it is challenging to measure the flux of gases during the Antarctic winter, but the loss of sea ice could open that lid, switching the ocean itself from a sink for planet-warming greenhouse gases to a major new source, says Doddridge. “That is an absolutely terrifying impact that we cannot nail down at all.”
