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This year has been unusual for Antarctica, the huge frozen landmass at the bottom of our planet that influences some of the world’s most important atmospheric and oceanic systems.
The 2023 winter season for the region ended with a record-low sea ice cover in August. This coincided with the early opening of the ozone hole above the continent. August also saw the release of a research paper that chronicles the frequent occurrence of extreme events on the continent, a trend likely to continue this year and beyond.
Antarctica, whose temperature can go as low as -98°C, sees the melting of the sea ice cover every summer season between December and February before returning to its original state during the winters.
This July, when the continent is in its peak winter, the sea ice extent was around 13.5 million sq km, the lowest since 1978, as per data released by the NASA Earth Observatory in the US. By August, sea ice equivalent to the size of Greenland had not grown back.
Some scientists have statistically called it a five-sigma event, which means that without changes in the climate, such low sea ice cover would have happened once in 7.5 million years.
Sea ice forms and exists in the polar regions due to an interaction between the ocean and the atmosphere, both of which are showing changes due to global warming.
“Sea-surface warming has occurred globally, including in the Southern Ocean, and a warmer lower atmosphere has reduced the freeze potential, which means, there is less thermodynamical ice growth,” says Petra Heil, a sea ice scientist at the Australian Antarctic Division.
Global warming has also altered the position of the polar vortex, a large area of low pressure and cold air surrounding both of the Earth’s poles, which now has an increased southerly flow over the Antarctic sea ice, pushing more sea ice to the south.
Heil admits the current sea ice characteristics are highly unlikely and warns the change might not be just an aberration. “This suggests that Antarctic sea ice has transitioned to a different state with less sea ice overall and a shorter presence (duration) of sea ice. Indications are that this transition is due to anthropogenic global warming,” she adds.
Prior to 2014, the ice surrounding the continent was increasing slightly over the long term (about 1 per cent a decade). But since then, it has taken a sharp decline, with record lows in 2017, then in 2022 and now in 2023, suggests NASA Earth Observatory.
If the current trend of reduced Antarctic sea ice cover persists, it can have a cascading impact on the rest of the oceanic and atmospheric systems. Low sea ice levels mean the oceans absorb more of the sun’s energy. This increases the rate at which sea ice melts, creating a feedback loop that keeps increasing the melt.
It will also impact the Antarctic bottom water, which is the coldest, densest water mass on the planet that plays a crucial role in regulating the ocean’s ability to store heat and capture carbon.
“Less sea ice cover means less production of Antarctic bottom water, which is a critical element driving the meridional overturning circulation, and with it the oceanic ‘conveyor belt’ which renews water masses across many depth levels in the global ocean,” says Heil.
The meridional overturning circulation is an underwater phenomenon where warm ocean water from the Equator travels to the North Atlantic and cold water flows back towards the Equator and then to Antarctica.
It completes the loop by travelling back to the tropics and becomes warm again. This circulation brings warmth to various parts of the globe and also carries nutrients necessary to sustain ocean life.
A study published on June 12, 2023, in the journal Nature Climate Change presents observational evidence from the Weddell Sea in Antarctica that the bottom waters have shrunk by 20 per cent over the past 30 years, while shallower waters warmed at a rate five times higher than the rest of the global ocean.
Disruption of the sea ice melt and growth cycle also impacts the exchange of gases and other materials between the ocean and atmosphere. For instance, ocean water absorbs more carbon dioxide than sea ice, which stores the compound in the form of a salty brine solution.
Aerosol production also gets affected by the low sea ice cover, leading to less cloud cover over the Southern Ocean, which, in turn, leads to more of the sun’s energy striking the Earth and further warming.
The low sea ice extent in 2022 showed impacts on the local ecology and the situation in 2023 may make it worse. A research paper, published in Nature Communications Earth and Environment, says that out of five emperor penguin colonies studied in Antarctica, four showed complete failure of breeding in the season.
“Emperor penguins spend time on fast ice, which is sea ice closer to the coastline, to incubate their eggs and raise their chicks till they are ready (grown and moulted) to enter the water,” says Heil.
Similarly, weddel seals also nurture their pups on fast ice and sea ice also provides a stable platform for penguins and seals to haul and rest upon, according to Heil.
Apart from this, the primary production in the sea, which is mainly krill, zooplankton and phytoplankton, also gets affected by low sea ice extent, which can have a knock-on effect on other organisms dependent on them.
Owing to a combination of sea ice cover and rising temperatures over the past few years, Antarctica is experiencing recurrent extreme events in the atmosphere, ocean, cryosphere and biosphere.
In March 2022, east Antarctica saw record-breaking extreme heat and a subsequent melting event. During the event, temperature over the region was 38.5°C above normal for that time of the year, according to a paper published in the Geophysical Research Letters on August 31, 2023.
The extreme heat event—the largest recorded heat anomaly anywhere on the planet—was associated with an atmospheric river that brought warm and moist air from Australia to the heart of the continent.
In February 2020, the mainland of Antarctica recorded its highest temperature of 18.3oC at the Esperanza station. In the same year, cyclonic events were held responsible for the calving of an iceberg on the Brunt Ice Shelf.
Apart from atmospheric rivers and cyclones, high-pressure systems are also responsible for such extreme weather events in Antarctica, according to a research paper published in the Frontiers in Environmental Science on August 8, 2023.
The paper documents and analyses extreme events in Antarctica in the atmosphere, southern ocean, cryosphere and biosphere across different timescales.
In the southern ocean it talks about marine heat waves, which are regions of the ocean with above normal sea surface temperatures for extended periods of time. They can impact marine biodiversity, fisheries and other marine industries. Nineteen such events were recorded in Antarctica between 2002 and 2018.
There is also the threat of warm mid-depth water from the Southern Ocean, known as circumpolar deep water, cutting through Antarctica’s ice shelves and glaciers.
Since 2009, major icebergs have been calved from 10 ice shelves across the continent. This process of melting of ice shelves and glaciers is accentuated by changing atmospheric factors such as strong winds.
The removal of ice shelves and other extreme internal events also affect the flow of ice on the land surface of Antarctica. This happened in the Siple Coast ice stream in West Antarctica. The ice flow is also affected by melt-water forming due to atmospheric rivers, strong winds, rainfall and cloud cover.
These extreme events are also impacting the Antarctic biosphere. “Between 2018 and 2022, 42 per cent of emperor penguin colonies (28 of 66 known colonies) likely experienced total or partial breeding failure due to fast ice break up in at least one year,” says the research paper in Frontiers in Environmental Science. Similarly, complete breeding failure among 20,000 pairs of Adelie Penguins in Ile des Pétrels, near Dumont D'Urville research station, was observed in 2013-14 and then again in 2016-17.
Every year, the ozone hole over Antarctica begins to form at the end of September, peaking in October before closing in November or December. This year, though, the hole opened in August itself, triggering a fear among scientists that the world could see a larger-than-average hole.
By the beginning of September, the ozone hole had developed to a size that is usually seen one or two weeks later, as per Copernicus Atmosphere Monitoring Service (CAMS). On September 1, the ozone hole over Antarctica was 19.67 million sq km, according to the NASA ozone hole webpage.
The Antarctic ozone hole is an annual thinning of the stratosphere’s ozone layer, which has a high concentration of ozone molecules that absorb ultraviolet radiation from the sun.
What sets this year apart is the fact that the ozone hole is usually smaller during El Nino years. While a scientific correlation is still awaited, CAMS analysis suggests the early opening up of the Antarctic ozone could be due to the eruption of the Hunga-Tonga volcano in January 2022.
The volcanic eruption injected unprecedented amounts of water vapour into the stratosphere— 50 million tonnes according to estimates—which reached the Antarctic stratosphere after the end of the 2022 ozone hole.
The increased water vapour in the stratosphere can lead to an enhanced formation of polar stratospheric clouds on which chloroflorocarbons can react to speed up ozone depletion, reads the CAMS statement released on August 31.
In addition, the water vapour could contribute to cooling the Antarctic stratosphere, enhancing the formation of these polar stratospheric clouds and resulting in a stronger polar vortex.
The hole in the ozone layer, which opens and closes seasonally every year, has been gradually healing since the 1987 Montreal Protocol, which triggered the phasing out of ozone-depleting chemicals like chlorofluorocarbons.
In October 2022, the World Meteorological Organization predicted that the entire world, barring the poles, would recover from the ozone hole by 2040. The poles should bounce back by 2066.
Despite the recovery, the previous three years have been characterised by strong and persistent Antarctic stratospheric polar vortices, and the ozone holes have been among the largest and longest-lasting on record.
As per CAMS ozone holes remained unusually persistent between 2020 and 2022. In these years, the ozone hole started to form in mid-September, which is fairly typical, but reached its maximum extent later than normal, around the end of September, and remained open until late in December, with 2020 setting a late closing record on December 28.
This was first published in the 16-30 September, 2023 print edition of Down To Earth