The sound of ice cracking underneath the hull of a 25,500-ton icebreaker is unmistakable. No matter where you are — shuffling along the lunch line in the galley or sitting on the observation deck with a pack of cards — the wincing of steel and crunching of ice can shriek through the ship. It’s almost ghostly; undeniably haunting. 

The sight of the ice? That’s mesmerizing. From the deck of Australia’s icebreaker, the RSV Nuyina, on which I sailed to Antarctica for more than five weeks at the start of 2022, it’s like looking out over a Martian landscape that’s been covered in a coat of stark white paint. In the distance, castles of ice rise from the vast, unbroken ice sheet. At the foot of one fortress, a battalion of King penguins lurks, unfazed by the freezing temperatures. Behind the ship, smaller Adelie penguins avoid a scrap with a leopard seal by climbing onto an island of ice and scurrying away. 

Sea ice is vital to the Antarctic ecosystem. It’s not just a refuge for penguins and other animals, but a fundamental facet of life for creatures further down the food chain too, like Antarctic krill. It means life. The ice is also critical for heat because it’s more reflective than water, bouncing back more sunlight than the ocean, and it can act as a physical barrier, impacting the exchange of gases between the ocean and the atmosphere and protecting the continent’s ice shelves. 

The Antarctic is currently experiencing the lowest level of sea ice since satellites began taking measurements in 1979. It’s an anomaly scientists are concerned about and monitoring closely. It was just a decade ago that sea ice in the Antarctic reached record highs, but generally low extents have been observed since 2016. It’s worrying, and could signal a shift in the sea ice dynamics down south, but the situation is more dire at the opposite end of the planet.

There, at the Earth’s northern extreme, the Arctic is experiencing an increase in temperatures two to four times higher than anywhere else in the world, and sea ice has decreased by about 12% per decade since the beginning of the satellite era. About 548,000 square miles of sea ice has been lost since 1979, equivalent to losing an area of ice roughly half the size of India. It’s seen a more rapid decline since 2000. 

It’s one of the most obvious signs that greenhouse gas emissions are shifting the planet’s equilibrium. Researchers say we can take steps to slow the changes, but we need to act with urgency.

The 4 million people who call the Arctic home rely on the Arctic Ocean for food and transportation. The Indigenous peoples of the Arctic, who make up about 10% of the population, have a vibrant and longstanding cultural connection to the region that is slowly dripping away as regions become free of sea ice for the first time in millennia. 

Meanwhile, the distribution of wildlife is shifting and behaviors are changing, altering the interactions between predators and prey. The Arctic’s famous polar bears rely on the ice to hunt and now have to travel further to eat, whereas the narwhal, a near-mythic, tusked whale, faces increased threats from killer whales lingering in exposed, warmer waters and disruptions to its migratory patterns.

Our best models currently predict the Arctic will be «sea ice free» within the next few decades, perhaps as soon as the 2030s. Antarctica’s sea ice is more of a mystery. But at both poles, sea ice is disappearing at an unprecedented rate. 

And when the ice ends it’s not just the ends of the Earth that will change. It’s the entire planet.

An already changed Arctic

The Arctic Ocean’s sea ice expands during the winter, peaking in March, before retreating toward the North Pole. It typically reaches its lowest extent in mid-September. It never completely melts away — the North Pole itself is typically surrounded, and up to a fifth of the ice in the Arctic is so-called multiyear ice, persisting for more than a year. 

Our understanding of this rhythmic pulse in the Northern Hemisphere stretches back for millennia. Indigenous peoples of the Arctic have passed down knowledge of the sea ice’s extent for thousands of years, particularly around coastal communities. Iceland’s government has been keeping detailed records since the 1600s, while log books and diaries kept during early exploration by ship provide a surprising amount of detail on where and when the Arctic Ocean froze over.

Our ability to understand the ice changed dramatically with the launch of the Nimbus-7 satellite in late 1978. The NASA and NOAA polar-orbiting satellite was fitted with an instrument that provided a way to observe the extent of the sea ice all year round, no matter the weather conditions, by studying the microwave energy bounced back from the surface. Continuous records have been taken since 1979, and the analysis has been deeply troubling. The extent of Arctic sea ice has been decreasing across those four decades, with each of the last 16 years the lowest on record.

Video: Changes in Arctic sea ice

Video credit: NASA’s Scientific Visualization Studio

For decades, scientists have tried to pinpoint when the total extent of Arctic sea ice will drop below 1 million square kilometers (or about 386,000 square miles) — the marker denoting a «sea ice-free» summer. In 2009, for instance, one study used climate models to determine that this mark would be hit by 2037. Other research has shown that the timing is unpredictable, with analyses suggesting we might still be decades away.

In June, a study in the journal Nature Communications analyzed 41 years of satellite data, from 1979 to 2019, reiterating that human greenhouse gas emissions are the dominant force in reduction of Arctic sea ice. It also generated a flurry of worrying headlines focused on the first ice-free summer, citing the near end of a range that it said had shifted to as early as the 2030s to 2050s. But those headlines gloss over a critical point: The current losses of summer sea ice are already having devastating effects.

«Although the first ice-free Arctic summer has constantly been a point of interest for understanding and communicating climate change, it’s more a symbolic threshold in some sense,» says Zachary Labe, a climate scientist at Princeton University and the National Oceanic and Atmospheric Administration. «Arctic climate change is already happening now and in all months of the year.»

An Antarctic paradox

From the beginning of the satellite era until 2010, Antarctic sea ice experienced a slight increase, with an acceleration in winter sea ice extent between 2012 and 2014. This was unexpected. Global temperatures have unequivocally risen in this time, largely due to human-induced climate change, raising ocean temperatures. Sea ice should’ve been melting. It didn’t.

The phenomenon was dubbed the Antarctic paradox.

Many climate models haven’t been able to reproduce these effects, though at least one high-resolution model has had success. Though explaining the paradox has been difficult, scientists have several hypotheses.

Natalie Robinson, a marine physicist at New Zealand’s National Institute of Water and Atmospheric Research, points out that changing wind patterns, release of freshwater from Antarctica, and ocean stratification could all have played a role over the last four decades, but she says that pointing to one variable as a driver of the increase is virtually impossible. «In reality, all of these processes act simultaneously and influence each other,» she notes. 

About seven years ago, the story began to change. Antarctic sea ice extent plummeted in 2016 and hasn’t totally recovered since. In 2023, winter sea ice extent is dramatically lower than we’ve ever seen in the satellite era. 

«Antarctic sea-ice extent has now adopted a downward trajectory as expected under warming and is congruent with observations of surface warming in the Southern Ocean,» says Petra Heil, a polar ice scientist with the Australian Antarctic Division. Graphs generated by Labe show the stark decline. 

The record low extent has scientists concerned. Understanding the paradoxical increase over the past four decades could help unlock the reasons behind this sudden change. Does it represent a shift to a worrying new normal? Or is it merely a blip that can be attributed to the normal range of variability? 

«There is certainly a fair bit of concern in the scientific community that it’s the former,» says Robinson. 

«And we are racing to find out.»

When the ice ends

The great white sheets at either end of the Earth are particularly good at reflecting sunlight. Sea ice covers about 15% of the world’s oceans across the year, and up to 70% of the heating energy is reflected back into space. Cover that ice with a dusting of snow and up to 90% can be reflected.  

When the sea ice disappears, the energy is absorbed by the ocean, raising its temperature. «In a positive feedback loop this ocean warming leads to even more ice loss and global warming,» says Heil. She suggests conceptualizing the impact of sea-ice loss by thinking about sea ice as the air conditioning unit of the Earth.

When the sea ice disappears, our planetary AC unit is being switched off. It becomes harder to reflect that heat into space and we lose the ability to «self-regulate» the Earth’s climate.

The change doesn’t affect just the ocean surface and the Earth’s air temperatures, though. Sea ice also plays one of the most critical roles on the planet in the ocean’s depths. As seawater freezes into ice, salt is expelled, making the surrounding water denser. This heavier, colder water sinks and gets whisked around the planet. Warmer waters are predominantly pushed by wind into the polar regions, then freeze up into ice. The cycle is known as thermohaline circulation. 

«This process can be regarded as the starting point/engine of the global oceans’ overturning circulation,» says Jan Lieser, a sea ice scientist with the Australian Bureau of Meteorology and University of Tasmania.

As the oceans continue to warm at both poles and sea ice extent decreases, this deep ocean current is likely to be disturbed. The knock-on effects could disrupt the polar ecosystems as nutrients and ocean biogeochemistry are altered, particularly in the Southern Ocean, where circulation is also heavily influenced by Antarctic meltwater and the currents already show signs of slowdown. 

The atmosphere and ocean systems are incredibly complex and intertwined. Though the focus has long been on the extent of the sea ice, thickness also plays a role. So does snow cover. These measurements are harder to include in models because they’ve traditionally been difficult to gather. There are also differences at either pole. The Arctic typically has had thicker sea ice lasting for years, whereas Antarctic sea ice freezes new each year. 

It now seems highly unlikely that the current declines can be stopped but Heil, and her colleague Melinda Webster from the University of Washington, say «it’s possible to slow and mitigate further detrimental effects of a warming climate by reducing greenhouse gas emissions and implementing ways to reduce existing atmospheric greenhouse-gas concentrations to levels that can sustain a habitable climate.» 

On June 16, Heil and Webster, and more than 60 other polar scientists responded to the changes to the poles by calling for «urgent intensification of national and international research and observational capabilities in view of rapid Arctic and Antarctic change.»

«Action is required now,» she says, «to give future generations a fighting chance to mitigate the negative consequences of a warming climate.»

The anomaly in Antarctica’s sea ice this year, as if sounding its own alarm and affirming Heil’s calls, has only continued its downward trajectory.