Sea ice scientist Petra Heil on consequences of low Antarctic sea ice extent & role of anthropogenic warming
Antarctic sea ice in the current winter season has been at a record low. Sea ice is frozen ocean water that forms and melts entirely in the ocean and has a natural pattern of melting during the summers (December to February) and growing back during the winters (June to August).
During the winter months in the southern hemisphere, sea ice usually grows back from its lowest extent during the summer season. This process has been monitored using satellites for the last 44 years.
The fact that sea ice over a region almost the size of Greenland has not grown back in the current winter season has alarmed scientists. They have called it a highly unlikely event.
This February, Antarctica had also recorded its lowest summer sea ice extent. Down To Earth interacted with Petra Heil, sea ice scientist at the Australian Antarctic Program, to understand the reasons behind this occurrence, its impacts on the local ecology and the spectre of Antarctic sea ice extent going past the point of no return.
Akshit Sangomla: Why is the Antarctic sea ice not getting replenished in winter?
Petra Heil: Sea ice forms and exists at the interface between the ocean and atmosphere in the polar regions. As such, its evolution is subject to conditions afforded by each — the surface ocean and the lower atmosphere.
Sea-surface warming has occurred globally, including in the Southern Ocean. A warmer lower atmosphere has also reduced the freezing potential, which means there is less thermodynamical ice growth.
In addition, the atmospheric circulation has changed under global warming. This impacts Antarctic sea ice via the changed position of the polar vortex, for example, which now has an increased southerly flow over the Antarctic sea ice, pushing more sea ice to the south.
AS: What could be the impact of the low sea ice extent on the marine ecosystem of Antarctica?
PH: There are several levels at which marine ecosystems are impacted. First, there is a strong link of marine primary production and its overall rate on the presence and quality of sea ice. Less sea ice means less marine primary production. The follow-on effects mean less food to sustain higher trophic levels.
Another impact of low sea-ice extent is less support for higher trophic levels. For example, sea ice provides a stable platform for penguins and seals to haul out from the water and rest upon.
Similarly, less fast ice (the sea ice near the coastline) or shorter presence of fast ice impacts the breeding success. This is seen in, for instance, Weddell seals and Emperor penguins who spend substantial parts of their breeding cycle on the fast ice.
Weddell seals pup on the fast ice and spend time on the fast ice till the pups are strong enough to enter the water. Emperor penguins spend time on the sea ice to incubate their eggs and raise their chicks till they are ready (grown and moulted) to enter the water.
AS: Does the current data show that the sea ice extent in Antarctica has gone past a point of no return or a climate tipping point? How can we assess and be sure about such an occurrence?
PH: That is a difficult question to answer. The 2023 observations show that the deficit of Antarctic sea ice is statistically highly unlikely if we assume the recent past seasonal cycle of Antarctic sea ice. This suggests that Antarctic sea ice has transitioned to a different state with less sea ice overall and shorter presence (duration) of sea ice.
Indications are that this transition is due to anthropogenic global warming. How can we assess? Via mapping of the impacts of changed forcing on the formation, evolution and melt of Antarctic sea ice. This requires observational data, including sustained in-situ measurements, to support analysis including so-called fingerprinting. (Fingerprinting is when we use what is happening over a small region or part of the system to approximate an impact on the larger system.)
AS: What could be the climatic / oceanic consequences of Antarctic sea ice extent crossing its tipping point?
PH: Sea ice feeds influences other climate components via a number of processes. For one, there is the so-called sea ice-albedo feedback loop. Less sea ice means less high albedo surface on the Southern Ocean. This, in turn, means increased penetration of radiative forcing into the upper ocean, hence directly warming the upper ocean (called the oceanic mixed layer).
This contributes to further reducing the freeze potential and increased melt rates, thereby leading to less sea ice.
Lower 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.
Less sea ice overall influences the gas and other exchange between ocean and atmosphere. This includes the aerosol production and with it the cloud formation over the Southern Ocean. Subsequently, the radiation budget is affected with dramatic consequences for the energy budget at the surface of Earth.
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