ON THIN ICE

For many years, while the Arctic lost sea ice, the Antarctic did not, with gains in some areas and losses in others. Then, in the spring of 2016, overall Antarctic sea-ice coverage dropped dramatically. Over two years, the Antarctic lost as much sea ice as the Arctic had lost in three decades. Since then, Antarctic sea ice has been below average most years.

In the summer of early 2023, Antarctic sea ice was the lowest on record. On 19 February 2023, Antarctic sea ice set a new record minimum, 36% below the 1979–2022 average for the summer minimum.

With the arrival of winter, the surface of the ocean has frozen into sea ice at about the same pace every year since satellite measurements began in the late 1970s, even following low sea-ice summers.

But this winter, at the coldest and darkest time of year, Antarctic sea-ice extent remained exceptionally low. During the winter growth period, large areas of sea ice failed to freeze. For most of winter there was approximately 2.5 million square kilometres of sea ice missing, an area the size of Western Australia. At the 2023 winter maximum on 10 September, there was still 1.5 million square kilometres less sea ice than normal.

There are strong indications that a regime shift is under way in the Southern Ocean. Recent research shows that since 2016, ocean warming due to global heating has pushed Antarctic sea ice into a new state of diminished coverage that it will struggle to recover from.⁶

Ocean warming is changing the processes that drive sea-ice formation and melt. The atmosphere is the dominant influence on sea ice over timescales of days to weeks, while the ocean around and underneath the sea ice contributes to longer-term variability.

Sea ice formation activates the Antarctic ‘pump’ that drives a global network of currents. Changes in the amount and seasonal timing of sea ice interferes with this circulation that ventilates the deep ocean with oxygen and fuels much of the world’s marine productivity with nutrients.

The shrinking of the frozen ocean domain will undoubtedly affect atmospheric processes, by altering the prevalence of clouds and drastically changing the Earth’s surface heat budget.  

Decreasing sea-ice coverage will increase the heat and light input to the ocean surface. This may increase phytoplankton production and growth, although the future reservoir of nutrients in surface waters is unknown. It is also not known how a reduction in Antarctic sea ice will impact the Southern Ocean’s capability to absorb heat and CO₂.  

The large-scale loss of Antarctic sea ice and the resulting increased exposure of ice shelves to ocean swell may trigger their rapid disintegration.⁴ They can calve more icebergs and even collapse when sea ice is removed.⁷ Ice shelves that decrease or even collapse can ‘open the floodgates’ to significant sea level rise.

Sea-ice loss will also translate into a loss of biodiversity. For example, iconic emperor penguins depend on sea ice to breed and raise their chicks. If emission rates are not reduced, emperor penguins will become quasi-extinct by 2100 as 98% of their colonies disappear with the loss of suitable sea-ice habitat.¹⁰

Predicted changes in sea ice and consequent ocean temperatures are expected to affect Antarctic krill, a foundational species in the Antarctic marine food web and targeted by a commercial fishery. This has serious implications for the distribution and health of the krill-based ecosystems of the Southern Ocean. 

Our measurements show us that the deep ocean around Antarctica is warming.¹³ Ultimately, the loss of sea ice is consistent with warming ocean waters. This warming is caused by our greenhouse gas emissions. The only practical way to preserve sea ice, to support the ecosystems that depend on this ice, and to continue taking advantage of the Southern Ocean’s ability to store carbon and heat, is to reduce our emissions of greenhouse gases.

Dramatic changes in a seasonal cycle as critical as Antarctic sea ice underscore the urgency to reduce fossil fuel emissions and stop global heating.

Currently it does not appear that emission targets are strong enough, or that emissions reductions are happening rapidly enough, to avoid continued disruption of the Antarctic sea-ice cycle.

We need to expand our knowledge so we can predict the future of the sea-ice cycle. We also need to better understand and plan for impacts of sea-ice change on Australian and regional fisheries and weather.

We must accelerate our understanding of sea-ice processes and interactions. Satellite data are tremendously useful, but alone they cannot provide the critical information we need to understand the future of sea ice and how it interacts with the ocean and atmosphere and the consequences for ecosystems.

Field research campaigns are urgently needed. Advancing knowledge will require sustained long-term observations; icebreaking vessels equipped with sophisticated sensor suites that measure key properties of the sea ice, ocean, atmosphere and biosphere; networks of smart buoys; and autonomous robotics operating above, within and below the sea ice.

Combining observations with cutting-edge simulations will allow us to explore what different emissions scenarios mean for the future of the Antarctic region.

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2. Clem, K., R. Massom, S. Stammerjohn, and P. Reid. 2022b. Antarctic Sea Ice #1: Physical Role and Function. SCAR Antarctic Environments Portal, refereed article, https://doi.org/10.48361/tqhw-c793

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4. Reid, P.A., Massom, R.A. Change and variability in Antarctic coastal exposure, 1979–2020. Nat Commun 13, 1164 (2022). https://doi.org/10.1038/s41467-022-28676-z

5. Christie F.D.W. et al, Antarctic ice-shelf advance driven by anomalous atmospheric and sea-ice circulation, Nature Geoscience (2022). https://doi.org/10.1038/s41561-022-00938-x

6. Purich, A., Doddridge, E.W. Record low Antarctic sea ice coverage indicates a new sea ice state. Commun Earth Environ 4, 314 (2023). https://doi.org/10.1038/s43247-023-00961-9

7. Massom, R.A., Scambos, T.A., Bennetts, L.G. et al. Antarctic ice shelf disintegration triggered by sea ice loss and ocean swell. Nature 558, 383–389 (2018). https://doi.org/10.1038/s41586-018-0212-1

8. Clem, K., R. Massom, S. Stammerjohn, and P. Reid. 2022b. Antarctic Sea Ice #2: Biological Importance. SCAR Antarctic Environments Portal, refereed article, https://doi.org/10.48361/8tky-2793

9. Clem, K., R. Massom, S. Stammerjohn, and P. Reid. 2022a. Antarctic Sea Ice #3: Trends and Future Projections. SCAR Antarctic Environments Portal, refereed article, https://doi.org/10.48361/4d9d-1g64 

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