ASOC
SEA ICE
A story of change
Researchers have been recording Antarctica’s sea ice cover and seasonal patterns of advance, retreat and duration by satellite since 1978.
While sea ice cover in the Arctic is in clear decline, the trends in Antarctica are less apparent. Since 1978, sea ice extent and behavior has varied dramatically between sectors. Over the past decade, Antarctic sea ice cover has become much more unpredictable overall, prompting researchers to take a closer look at why.
Five sectors of change
CHANGING SEA ICE
At its winter maximum, Antarctic sea ice covers an area of around 11 million square miles (18 million sq.km): almost twice the size of the United States of America, and larger than Antarctica itself.
Sea ice covers such a vast area of the Southern Ocean around Antarctica that it spans multiple oceanic and atmospheric zones. For this reason, scientists typically divide the sea ice area into five sectors to explore the nature and drivers of sea ice change and variability. These sectors are: the Weddell Sea; Bellingshausen and Amundsen Sea; Ross Sea; West Pacific Ocean and Indian Ocean sectors.
Between 1978 and 2014 scientists were surprised to find that although temperatures were rising globally and Arctic sea ice was in steep decline, Antarctic sea ice was gradually increasing across four of the five sectors (all but the Amundsen/Bellingshausen Sea sector).
Arctic and Antarctic sea ice cover
UNPREDICTABLE SEA ICE
In 2014, when Arctic sea ice cover was trending to dramatic lows, Antarctica experienced record high sea ice cover. Two years later, in 2016, it plummeted and by 2017 it had hit a record low. The pace of this change eclipsed any change in the Arctic. Within only three years, 35 years of gradual gains had been lost. Researchers are still investigating the causes of this extremely rapid decline.
Antarctic Peninsula Sea Ice Decline
UNPREDICTABLE SEA ICE
While much of Antarctica experienced an increase in sea ice cover before 2014, followed by a rapid decline, the Amundsen/Bellingshausen Sea sector told a very different story.
Sea ice here was in steady decline throughout. Upper ocean temperatures in the Bellingshausen Sea, west of the Antarctic Peninsula, have increased by over 2°F (1°C) since 1955. On the west Antarctic Peninsula between 1978 and 2014, the winter sea ice duration declined by almost 100 days. However, between 2017 and 2019 it increased slightly, illustrating the unpredictability and seasonal variability of Antarctic sea ice.
What’s driving the changes?
CHANGING SEA ICE
The relationship between sea ice, ocean temperatures and the atmosphere in Antarctica is complex. In some parts of Antarctica, including the Amundsen/Bellingshausen Sea region, warm ocean currents driven by the climate crisis are thought to be the primary cause of sea ice decline. However across the continent more broadly, Antarctic sea ice dynamics are being affected by many factors including warming oceans, local winds, cloud cover, meltwater from ice shelves, the ozone hole, and variable climate patterns such as the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode.
Researchers are continuing to monitor Antarctica’s changing sea ice, building upon existing data, carrying out research on the complex interactive processes involved, and working on improving the performance of climate models to better predict the changes to come. A big unknown at this stage is the thickness distributions of Antarctic sea ice and its snow cover – and whether or not these are changing. The hope is that this information will emerge from a new suite of satellite altimeter missions.
CHANGING SEA ICE
What are the impacts?
Circulation
Heavy, dense water produced under sea ice plays a vital role in global ocean circulation.
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Circulation
Heavy, dense water produced under sea ice plays a vital role in global ocean circulation.
‘Antarctic Bottom Water’ (AABW) is a by-product of sea ice formation, and is the heaviest, most dense water on the planet, much denser than the water around it.
In 2012, researchers found that the amount of AABW being produced around East Antarctica had reduced by around 60% since the record began in 1970. Over the next five years AABW became warmer, fresher and less dense at an increasing rate.
The exact cause of the freshening is unclear, but it could be linked to reduced sea ice cover. As sea ice cover declines, there may be less AABW produced as a by-product of sea ice formation. It’s also possible that AABW is being diluted by the accelerated melting of ice shelves and ice sheets, which introduce l freshwater, which is less dense, into the ocean. Both of these things may be occurring at the same time.
Why does this matter?
AABW helps power global ocean circulation, which in turn helps regulate the global climate. Due to its relative density, when AABW is produced it sinks from the surface of the ocean to the depths. This cascading action helps stimulate slow, deep water currents such as the Gulf Stream, that circulate around the globe and regulate our climate.
Fresher, lighter AABW may not sink as rapidly, reducing the strength of its current-pumping effect. This could cause global ocean circulation to slow down or even stop, resulting in chaotic changes to the climate.
There is already growing evidence that major global ocean currents are slowing down in response to the climate crisis, particularly in the northern hemisphere, so researchers are keeping a close eye on Antarctic Bottom Water.
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Related reading
Now that you’ve learned about how Antarctic sea ice is changing, read on to learn more about extraordinary Antarctica.
Antarctic Ice
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