‘Exceptional’ Antarctic melt drives months of record-low global sea ice cover

§ Record low global sea ice

Arctic sea ice has been melting for months, driven by long sunny days and warm temperatures. But, as winter approaches, the melt season has now come to an end. Arctic sea ice reached its minimum for the year on 19 September and is now growing towards its winter peak.

At the Earth’s other pole, the opposite is happening. Antarctic sea ice – which has been growing since February – reached its winter peak on 10 September. Its melt season has now begun.

Using satellite data, scientists track the seasonal growth and melt of sea ice, allowing them to determine the size of the annual minima and maxima. Recording the sea ice extent at each pole – the area of ocean with at least 15% sea ice coverage – is a key way to monitor the “health” of Antarctic sea ice.

The plots below show Arctic (left) and Antarctic (right) sea ice extent over June-October. Sea ice extent in 2023 is shown in blue, the 1981-2010 average in grey and other years in other colours.

In the Antarctic, sea ice extent has been tracking at record-low levels for months. Dr Ella Gilbert – a regional climate modeller at the British Antarctic Survey – tells Carbon Brief that Antarctic conditions this year have been “truly exceptional” and “completely outside the bounds of normality”.

Global sea ice extent – the sum of sea ice extent in the Arctic and Antarctic – has also been tracking at a record low for months. 

The plot below shows the combined sea ice extent for the Arctic and Antarctic. The red line indicates sea ice extent in 2023, yellow shows 2016 and other blue lines indicate other years dating back until 1979 when the satellite record began.

§ ‘Exceptional’ Antarctic melt

The Antarctic has attracted widespread media attention throughout this year. In February,  Antarctic sea ice extent reached its summer minimum extent of 1.79m km2, setting the record for a second straight year.

Commenting on the new record low minimum – the third record to be set in seven years – one study warned that the Antarctic had entered a “new state”, in which the underlying processes controlling Antarctic sea ice coverage “may have altered”.

As the weather cooled in March, Antarctic sea ice “expanded at a fairly typical pace”, according to the NSIDC. Nonetheless, March 2023 average sea ice extent was the second lowest on record.

Antarctic sea ice extent remained “sharply below average throughout” April, clocking in with the second lowest daily extent on record by the end of the month.

Gilbert tells Carbon Brief that Antarctic sea ice “never really recovered” from its record-low February minimum, thanks to a “slow freeze-up”. 

Antarctic sea ice grew only 2.87m km2 over May 2023 – considerably less than the average growth of 3.25m km2. Air temperatures were up to 4C above average over the Weddell sea during the month, but around 4C below average over the Amusden sea.

By 31 May, Antarctic sea ice extent was again at a record low extent, clocking in at around 700,000 km2 smaller than the previous daily record lows recorded in 1980, 2017 and 2019. Sea ice continued to track at “extreme record low levels” throughout June, according to the NSIDC. 

The graphic below shows Antarctic daily sea ice extent in 2023 (red) compared to previous years over 1979-2023.

Dr Zachary Labe is a postdoctoral researcher working at NOAA Geophysical Fluid Dynamics Laboratory and the Atmospheric and Oceanic Sciences Program at Princeton University, and a Carbon Brief contributing editor.

He tells Carbon Brief that the record lows are linked to “both oceanic and atmospheric factors”, especially related to the strength of the Amundsen Sea Low – a low pressure trough named after the sea off West Antarctica that it typically sits above.

“Exceptionally low” sea ice extent continued into July, with the NSIDC noting that Antarctic ice extent as of mid-July was “more than 2.6 km2 below the 1981-2010 average, an area nearly as large as Argentina”. Antarctic sea ice extent was particularly low in the northern Weddell Sea, western Ross Sea and southern Bellingshausen Sea.

Antarctic sea ice continued to grow slowly as the season progressed. Average extent was at a record low in July, clocking in at 1.50m km2 below the previous record low set in 2022, the NSIDC noted. It added at the time:

“There is speculation that the Antarctic sea ice system has entered a new regime, in which ocean heat is now playing a stronger role in limiting autumn and winter ice growth and enhancing spring and summer melt.”

The graphic below shows Antarctic sea ice thickness in July 2023 (left) and the difference in sea ice thickness between July 2023 the 1981-2010 July average (right). In the right-hand map, the areas of deepest red show where sea ice thickness in July this year was below average.

By the beginning of August, Antarctic sea ice growth began to level off. On 15 August, Antarctic sea ice extent was 1.73m km2 below the previous record low for the date, which was set in 1986.

Antarctic sea ice growth then picked up as the month progressed. It continued to track at a record low, but increased more than average in the Bellingshausen and Amundsen Seas as well as in the Pacific Ocean.

On 10 September, Antarctic sea ice likely reached its annual maximum extent of 16.96m km2. This is the lowest sea ice maximum in the 45-year by “a wide margin”, the NSIDC says. The previous record-low Antarctic minimum extent was 17.99m km2.

It adds that this year’s winter peak is one of the earliest on record – 13 days earlier than the 1981-2010 average date of 23 September. 

The plot below shows Antarctic sea ice extent on 10 September, with the median sea ice extent for 1981-2010 shown by the orange line.

Gilbert notes that scientists have “no evidence of comparably low winter extent in the satellite record, nor in reconstructions of the last century or so”, adding:

“Given how variable sea ice is, it’s hard to say for certain whether this is the beginning of a longer-term shift towards a new regime in Antarctic sea ice. However, climate models predict that Antarctic sea ice will decline, and I think it’s only a matter of time until we see the signature of climate warming in Antarctic sea ice trends.” 

The record low Antarctic sea ice levels have received widespread attention in the media in recent months, with BBC News, the Times and the Daily Telegraph calling the record-low sea ice levels “mind–blowing” and “dramatic”. 

The impact on wildlife has also caused alarm. At the end of August, multiple outlets reported on a new study that found thousands of emperor penguin chicks in Antarctica had died because of record-low sea ice levels in 2022. 

And new research warns that the Antarctic is also warming twice as fast as the global average. (It is already well established that the Arctic has warmed four times faster than the global average over the past four decades.)

§ Arctic minimum

At the Earth’s other pole, the season has been less eventful.

The Arctic reached its winter peak on 6 March 2023 with a sea ice extent of 14.62m km2 – the fifth smallest winter peak in the 45-year satellite record. This point marked the beginning of the melt season for the year.

Arctic sea ice extent declined in the week following the March peak, but cool weather nearly halted the ice melt during the second half of the month. Slow Arctic ice melt continued throughout April with “only” 20,600 km2 of ice lost per day on average, according to the NSIDC.

Slower-than-average sea ice melt persisted throughout much of May. Air temperatures over the Arctic ocean were around 1-4C below average throughout the month – except for over the Barents, Kara, and Beaufort Seas, where temperatures were 2-6C above average.

The map below shows the regional seas that make up the Arctic Ocean.

Sea ice melt sped up as the season progressed, resulting in faster-than-average melting throughout June. By the end of that month, average Arctic sea ice extent was 10.96m km2 – the 13th lowest for the time of year in the satellite record.

Arctic sea ice extent declined at a near-average pace throughout July, clocking in at the 12th lowest in the satellite record for the time of year.

However, record-breaking heat swept across the world in August, causing Arctic sea ice melt to accelerate, the NSIDC noted at the time.

Temperatures in the first half of August were “below average north of Greenland, above average in the Chukchi and East Siberian Seas, and considerably above average in the Kara and Barents Seas,” the NSIDC said.

The maps below shows absolute air temperatures in the Arctic in August 2023 (left) and the difference in air temperature between August 2023 the 1981-2010 August average (right). In the right-hand map, the areas of deepest red show where August this year was substantially hotter than average.

By the middle of August, sea ice extent was near average on the Atlantic side of the Arctic, but “well below average” in most other regions other than a tongue of ice extending toward the coast in the East Siberian Sea, the NSIDC said:

“​​The Northwest Passage appears to be on the verge of becoming nearly ice free, particularly the southern route, known as Amundsen’s route.”

As is typical of August, the pace of sea ice loss slowed during the second half of the month as cooler conditions set in.

The left-hand map below shows Arctic sea ice concentration – a measure of the amount of sea ice in a given area, usually described as a percentage – during August 2023. The areas shaded white indicate a high concentration.

The right-hand map shows the difference between this August and the 1981-2010 average, where red indicates a lower sea ice concentration in 2023 than the baseline.

Labe tells Carbon Brief that there have been some “particularly large regional extremes” around the Arctic this season.

For example, he highlights the “massive amount of open water on the Pacific side of the Arctic, which stretches from the Beaufort Sea to the East Siberian Sea”. Sea ice extent in this region dropped to the second lowest in the satellite-era, beaten only in the year 2012, he says.

Labe adds that within the main Arctic ice pack, there are “many areas of open water” this year, indicating that the ice is not very compacted. By mid September, sea-ice area – a measure of how compacted the ice is – reached its fourth lowest for the time of year, he says.

He also notes that sea ice was “significantly thinner than average in the Beaufort sea region” in August.

Arctic sea ice extent reached its minimum for the year at 4.23m square kilometres (km2), on 19 September, according to the NSIDC. This is the sixth lowest in a satellite record – 1.99m km2 smaller than the average minimum over 1981-2010 .

Days before the minimum was announced, the Marine Climate Change Impacts Partnership published a review paper on Arctic sea ice. It found that the September minimum Arctic sea ice extent has reduced by around 12% per decade compared to the 1981-2010 average.

It added:

“More than half the observed loss of Arctic sea ice can be directly attributed to warming caused by anthropogenic greenhouse gas emissions.”

Labe tells Carbon Brief that the Arctic today is “radically different” than it was only two or three decades ago. “This is due to human-caused climate change,” he says.

According to the NSIDC, the 17 lowest Arctic sea ice minima on record have occurred in the past 17 years.