Antarctic sea ice has recorded its third-smallest winter peak extent since satellite records began 47 years ago, new data reveals.
Provisional data from the US National Snow and Ice Data Center (NSIDC) shows that Antarctic sea ice reached a winter maximum of 17.81m square kilometres (km2) on 17 September.
This is 900,000km2 below the 1981-2010 average maximum extent – the historical baseline against which more recent sea ice extent is typically compared.
According to one expert, the “lengthening trend of lower Antarctic sea ice poses real concerns regarding stability and melting of the ice sheet”.
Meanwhile, at the Earth’s other pole, Arctic sea ice reached its annual minimum on 10 September, ranking as the joint-10th lowest in the satellite record.
At 1.6m km2, the 2025 minimum shares the spot with 2008 and 2010. The NSIDC notes that all 19 of the lowest sea ice extents in the record have occurred in the past 19 years.
Antarctic peak
For decades, scientists have been using satellite data to track the annual cycle of sea ice growth and melt at the world’s poles. This is a key way to monitor the “health” of sea ice in both the Arctic and Antarctic.
The map below shows Antarctic sea ice on the day of its maximum extent for the year on 17 September 2025, where the yellow line shows the 1981-2010 average.
The NSIDC says that sea ice extent was “markedly below average” in the Indian Ocean and the Bellingshausen Sea, but “slightly above average” over the Ross Sea.
In an NSIDC press release announcing the Antarctic maximum, Dr Ted Scambos, a senior research scientist at the Cooperative Institute for Research In Environmental Sciences, said:
“The lengthening trend of lower Antarctic sea ice poses real concerns regarding stability and melting of the ice sheet. However, it may also be leading to greater snowfall over the continent, which would slow the progression of sea level rise.”
Antarctic sea ice growth
In its typical annual cycle, Antarctic sea ice grows during winter towards its annual maximum extent in September or October. It then melts throughout the spring and summer towards its March minimum.
Earlier this year, Antarctic sea ice recorded its second-smallest summer minimum on record.
At 1.98m m2, this was the fourth consecutive year that Antarctic sea ice had fallen below 2m km2, the NSIDC noted.
In its monthly sea ice updates, the NSIDC reported that sea ice then grew at a “near-average pace”. During this period, sea ice “expanded rapidly” in the last areas to lose ice, including the Ross Sea and eastern Weddell Sea, it said.
The NSIDC explained that sea ice rebounded quickly in the Ross Sea area because ice extent had retreated “slowly” there the month before – meaning that the upper ocean layer did not have time to accumulate heat which would slow the winter freeze.
In April, “the situation in the Antarctic remained fickle”, the NSIDC said. At the beginning of the month, sea ice extent neared “record-low” daily extents, but as the month progressed ice cover expanded “fairly quickly”, it said.
May had “below average growth” in Antarctic sea ice and saw the fifth lowest record for Antarctic sea extent.
As June began, the the Bellingshausen Sea and eastern Queen Maud Lord regions were “far behind” in ice re-growth, it said, adding that the Bellingshausen Sea was almost entirely ice-free as temperatures were 6-8C above average.
In June, Antarctic sea ice was 1.28m km2 below the 1981-2010 baseline, with “particularly low” sea ice extent in the Bellingshausen Sea and the Indian Ocean sector, according to the NSIDC. This was the third-lowest sea ice extent ever recorded for the month of June, it said.
Throughout July, Antarctic sea ice extent grew at a “slower-than-average” rate, according to the NSIDC. By the end of the month, Antarctic sea ice extent was 1.3m km2 below the baseline, it noted.
Arctic melt season
In the Arctic, sea ice cover typically reaches its high point in March, before dropping to its September minimum at the end of the northern-hemisphere summer.
The 2025 Arctic sea ice winter peak was the smallest since satellite records began. The peak, recorded on 22 March, was 1.31m km2 below the average maximum for the 1981-2010 historical baseline.
In March, Arctic sea ice extent averaged 14.14m km2 – the lowest in the satellite record, according to the NSIDC. It noted that, at the time, average air temperature was above the historical baseline across much of the Arctic region.
Arctic sea ice extent then “changed very little” throughout April, remaining “nearly constant” until the final days of the month, the NSIDC reported.
It added that the final days of April saw Arctic sea ice extent drop due to ice retreat along the coast of the Barents Sea.
According to data, the main reason why the April total extent remained largely flat was due to an increase of sea ice in the northeastern Barents Seas that “offset” losses elsewhere.
Below-average air temperatures over the northern Norwegian and Barents Seas was the most “notable feature” of April 2025, the NSIDC said.
May was marked by a decline in Arctic sea ice extent at a faster-than-average pace, the NSIDC noted, resulting in the seventh-lowest May extent on record.
It added that ice loss in May was “primarily” in the Barents Sea, Bering Sea and the Sea of Okhotsk.
In June, Arctic sea ice extent averaged 10.48m km2 – the second-lowest average on record for the month, the NSIDC said. It noted that sea ice hit record-low levels over 20 June and 26 June and tracked at “near-record” low levels through the month. The Barents and Kara Seas were both “nearly ice-free” by the end June.
Hudson Bay ice extent was also “considerably below average” throughout June and northern parts of Baffin Bay were nearly ice-free, it said.
By the end of July, daily sea ice extent in the Arctic had fallen to 7.66m km2 – the third lowest in the satellite record, the NSIDC reported. It noted that, for most of the month, Arctic sea ice extent tracked close to levels recorded for 2012 – the year in which Arctic sea ice extent reached its lowest-ever September minimum.
Throughout August, the NSIDC reported that sea ice “rapidly melted and compacted” north of Alaska in the Beaufort Sea, with sea ice extent averaging at 5.41m km2 – the seventh lowest on record.
Dr Zack Labe – a climate scientist at Climate Central – tells Carbon Brief that northern Siberia saw August air temperatures more than 5C above the 1981-2010 average, resulting in “a striking amount of open water along the Atlantic side of the Arctic that would normally be ice-covered”.
At an annual minimum of 1.6m km2, this year’s Arctic minimum is “pretty unremarkable”, Labe tells Carbon Brief, and “adds to the evidence of a clear slowdown in the rate of summer Arctic sea ice loss”.
However, Labe stresses that this is “not surprising” – referencing a recent study which “clearly shows how internal variability can temporarily drive periods of slower melt in a warming climate, as well as periods of rapid melt, such as in the early 2000s”. (For more on this research, read Carbon Brief’s guest post).
He adds:
“It is only a matter of time before summertime melt accelerates again. This is not a good news story, especially since in many other months we still see a clear downward trend…
“While the past decade of summers may give the appearance of a slowdown, regional extremes such as in the Kara Sea this year underscore that the Arctic is already radically different from past decades. The driver is clear – human-caused climate change.”
Satellite switch
For decades, NSIDC has tracked sea ice using data from weather satellites run by the US Navy. However, earlier this year, Mongabay reported that NSIDC scientists “noticed holes in the data they were receiving”.
The article explains:
“When scientists inquired with the Department of Defense (DoD), they were told not all data were being downloaded and access to the data had been deprioritised. Soon after, the DoD said it would stop sharing…data altogether, citing military cybersecurity risks in the old systems.”
NSIDC scientist Walt Meier told Science that while the US satellites “are up there and functioning…we’re not getting all the data anymore, at least regularly”.
The DoD then set a cut-off date to “cease distribution data from the Defense Meteorological Satellite Programme” on 31 July.
In June, the NSIDC announced that it would “explore switching to a different sensor” aboard a Japanese satellite that was launched in 2012.
The only other option available to NSIDC was a “series of Chinese weather satellites, which the country is already using to produce its own record of sea ice”, Science noted. It added that a new US DoD weather satellite, launched last year, is “also capable of collecting similar data, but its data have not yet been made public”.
The switch was completed by the July cut-off date and NSIDC reprocessed all data for 2025 to use the new data source to ensure “consistency through the year”.
The post Antarctic sea ice winter peak in 2025 is third smallest on record appeared first on Carbon Brief.
Antarctic sea ice winter peak in 2025 is third smallest on record
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The planet is heating up more quickly than ever before.
For decades, greenhouse gas emissions caused by human activity have been building up in the atmosphere and trapping ever-higher levels of heat.
The resulting asymmetry between incoming solar energy and energy radiated back out into space – known as “Earth’s energy imbalance” – provides a direct measure of the extent to which humans are disrupting the Earth’s climate system.
This imbalance is growing and in 2025 its 10-year average reached a record high, indicating that global temperatures could increase at even higher rates in the future.
This is among the headline findings of the latest “indicators of global climate change” (IGCC) report, published in the journal Earth System Science Data, which tracks changes in the climate system on an annual basis.
The report, now in its fourth iteration, has been produced by dozens of scientists from around the world.
Its findings are designed to fill the gap between Intergovernmental Panel on Climate Change (IPCC) science reports, which are published every 5-7 years.
In this article, we unpack the IGCC report, which explores how human activity is driving a growing energy imbalance and why monitoring systems to track global climate are so crucial.
(For more on previous IGCC reports, see Carbon Brief’s coverage in 2023, 2024 and 2025.)
Greenhouse gas emissions remain at an all-time high
Global greenhouse gas emissions are continuing to increase, mostly as a result of the use of fossil fuels. However, deforestation, agriculture and industrial processes also play an important role.
Over the most recent decade (2015-24), emissions stood at the equivalent of 54.6bn tonnes of carbon dioxide equivalent (GtCO2e) per year. In 2024, the most recent year for which we have complete data, emissions reached 56.8GtCO2e.
As the chart below shows, these emissions have pushed up atmospheric levels of CO2, methane and nitrous oxide. In 2025, concentrations of these gases reached 425.6 parts per million (ppm), 1936.3 parts per billion (ppb) and 339.4ppb, respectively.
This represents a rise of 3.8%, 3.8% and 2.2%, respectively, since the 2019 levels reported in the IPCC’s sixth assessment report (AR6).
At the same time, declines in emissions of aerosols such as sulphur dioxide, partly as a result of efforts to tackle air pollution, are increasing the Earth’s energy imbalance. This is because aerosols have a cooling effect on the Earth’s climate, counteracting warming from CO2 and other greenhouse gas emissions.
(Tackling sulphur dioxide, alongside other particulate emissions, remains critical because the immediate health and environmental damage they cause far outweighs their short-term cooling effect on the climate.)
The Earth’s energy imbalance is rising rapidly
The Earth’s energy imbalance has long been recognised as a key indicator of how the climate is being affected by human activities.
However, it is only in the last few decades that scientists have been able to record temperature changes deep enough in the ocean to accurately quantify it.
Earth’s energy imbalance measures how quickly excess heat is accumulating in every part of the Earth system, primarily in the ocean, but also in land, ice and atmosphere.
Through this accumulation of heat, the energy imbalance influences the rate of sea level rise and ice melt across the world, as well as increasing the frequency and intensity of extreme weather events, such as storms, floods and droughts.
Without human influence, the Earth’s energy imbalance would be close to zero.
But, as greenhouse gas emissions have built up in the atmosphere, the imbalance has been growing since the 1970s. Recent increases to Earth’s energy imbalance have outpaced those projections made by climate models — indicating the planet could see more warming than expected in the future.
As the right-hand chart below shows, the imbalance is now at a record high, having more than doubled over the past two decades.
It has increased by around 40% since 2019, from an average 0.79 watts per square metre (Wm2) over 2006-18, according to IPCC AR6, to 1.12Wm2 over 2013-25.
The left-hand chart shows how heat is accumulating in the ocean (blues), ice (grey), land (orange) and atmosphere (purple).
Global temperature rise
The excess heat building up in the climate system from the energy imbalance is pushing up global temperatures at a record rate of 0.27C per decade.
We estimate that human-induced warming – the amount of observed global surface
temperature increase attributable to both the direct and indirect effects of human activities – reached 1.37C in 2025. This has risen from 1.0C in 2017, as reported in IPCC AR6.
While natural variability in the climate system – such as El Niño or La Niña events – can also influence temperatures year-to-year, the upward temperature trend we are seeing is being driven by the persistent imbalance in energy.
We now expect global temperatures to exceed the Paris Agreement limit of 1.5C above pre-industrial levels around the year 2030.
This is significant because 1.5C has been identified as the critical dividing line between manageable climate risks and catastrophic, potentially irreversible damage to global ecosystems and human societies.
Heat accumulating throughout the Earth system
While heat is accumulating throughout the Earth system, it is not being distributed evenly around the globe.
Since the 1970s, around 90% of this heat has been taken up by the ocean, affecting marine ecosystems, ocean circulation patterns, sea level rise and climate extremes.
For example, the number of marine heatwave days – periods of unusually high sea surface temperatures – has more than tripled globally since the early 1990s. The year 2025 alone saw 65 days of marine heatwaves – meaning they occurred, on average, more than one day a week.
Meanwhile, the cryosphere – the portion of the Earth made up of frozen water, including glaciers, ice sheets and permafrost – is experiencing widespread ice loss and thawing in response to the growing energy imbalance. This affects ecosystems, sea level rise and infrastructure in polar and high-latitude regions.
Rapid warming has also resulted in record extreme temperatures over land, with average maximum temperatures for any single day over 2016-25 around 1.92C above pre-industrial levels). This is an increase of almost half a degree compared to the previous decade (2006-15).
Sea level rise and the energy imbalance
Sea level rise provides one of the clearest long-term signals of a changing planet.
It is closely linked to Earth’s energy imbalance. As heat accumulates in the ocean, water expands, raising sea levels. Meanwhile, a warming land and atmosphere means addition of water to the oceans through melting of glaciers and ice sheets, also adding to sea level rise.
Over the long-term, sea levels have been rising, on average, at a rate of around 1.8mm per year since 1901, totalling a record 23cm in 2025. This is increasing the risk of coastal flooding, erosion and habitat loss in many low-lying areas around the world.
This rise can be seen in the left-hand chart below, which shows observed global sea level changes from tide gauges (grey and blue dashed lines) and satellites (red dashed lines) since 1901. The solid lines indicate the average across multiple datasets.
Sea level rise is accelerating consistent with the observed increase in Earth’s energy imbalance. Over 2006-25, sea levels have risen at a rate of 3.67mm per year – more than double the rate of 1.69mm per year seen over 1976-95.
This increasing rate is shown in the right-hand figure below, which shows four successive overlapping 20-year periods and the most-recent decade.
(Last year’s transition from El Niño to weak La Niña conditions affected global rainfall patterns and led to a small and temporary fall in global average sea level in 2025. This explains the slight decrease in rate of sea level rise for the most recent decade, which is affected more than the 20-year period 2006-25.)
The bigger picture
Despite greenhouse gas emissions not increasing as rapidly as in the 2000s, this year’s IGCC findings continue to show how far and how fast the climate is changing due to human activity.
A significant increase in decarbonisation efforts in the second half of this decade is required to slow down the rate of human-caused warming and limit the escalation of climate risks and impacts.
These findings, like many others produced by scientists across the globe, rely on international expertise, partnership and the maintenance and availability of global climate datasets and the global observing programmes that underpin them.
This year’s edition of IGCC used more than 40 global datasets produced by research teams around the world, including the NASA satellite record of the Earth’s energy imbalance and the ARGO deep ocean float network.
However, a number of long-term monitoring programmes could be threatened by funding decisions made by governments around the world, most notably the Trump administration in the US.
Local meteorological data and weather balloon measurement programmes in many countries have declined in recent years, especially in Africa, the west Pacific and South America. This reduces scientists’ ability to monitor and understand key indicators of climate change.
This is not just an issue for climate science. Many of these observations are key to weather forecasts and systems that provide early warning for extreme weather. For example, media reports have suggested that recent reductions in weather balloon measurements in Alaska led to a lack of warnings for a recent winter storm.
The continuity and integrity of the climate observations that scientists use to understand how the climate is changing depends on effective and sustained coordination by international organisations, such as the Global Climate Observing System, the World Meteorological Organization and World Climate Research Programme.
Without this data and its coordination, future assessments will be much more difficult at a time when urgent climate action is needed.
The post Guest post: How a record-high ‘energy imbalance’ is driving global warming appeared first on Carbon Brief.
Guest post: How a record-high ‘energy imbalance’ is driving global warming
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