A worrying absence of sea ice

On February 28th 1898, commander Adrien de Gerlache and captain Georges Lecointe decided to go south, deep into Antarctic pack ice. This was a major change of plans as they were supposed to continue west following their initial itinerary. The pack ice was not their initial objective, their initial objective was to reach the coast of Victoria Land, where they would overwinter on land, and subsequently attempt to reach the Geomagnetic South Pole. These plans were however abandoned due to a series of unfortunate events, leading to considerable delays.

Initially planned itinerary of the RV Belgica expedition.
Source: book “Adrien de Gerlache, a biography” by Jozef Verlinden.

Their deliberate move into the dense sea ice was not without risk, and this was unanimously recognized by crew and scientists onboard the RV Belgica, who opposed de Gerlache’s and Lecointe’s plan to head south. One of the main risks of such a southward journey was to get stuck in the sea ice, which, under high-pressure conditions, could puncture the ship’s hull and sink it. Even if the RV Belgica would not suffer this fate, an endless entrapment in Antarctic sea ice could empty their rations and fuel reserves, leaving the crew and scientists to die of starvation and cold. However, as the Geomagnetic South Pole was out of reach, de Gerlache’s and Lecointe’s pursuit of glory had to be fulfilled with breaking an alternative record, being the farthest-south record of 78°09’30” south, which James Clark Ross set in 1842. Eventually, without the crew and scientists knowing, de Gerlache and Lecointe navigated the RV Belgica in a southern direction, condemning the expedition to an icy imprisonment. After traveling slowly through dense sea ice for days, on March 5th 1898, they got trapped, frozen, into the sea ice of the Bellingshausen Sea, not knowing when, worse if, they would ever escape.

RV Belgica trapped in the pack ice of the Bellingshausen Sea.
Credits: Frederick A. Cook Society, Library of Congress (Washington, DC).

The crew and scientists onboard today’s PS134 expedition in that same Bellingshausen Sea will, fortunately, not suffer the same fate as the Belgica expedition, since we are onboard a 20,000 horsepower icebreaker ship equipped for sustained operations at temperatures down to -50°C and sea ice up to 3 m thick, but more importantly, Antarctic sea ice this year is projected to reach its record minimum since the satellite observations started 40 years ago.

RV Polarstern in this year’s sea ice free Bellingshausen Sea. Credits: Daniela Röhnert.

The sea-ice extent in the Southern Ocean around Antarctica will reach a worrying minimum this austral summer. The current extent of Antarctica’s sea ice is extremely low, and reflects a persistent, strong trend toward lower-than-average Antarctic sea-ice extent that started in 2016 (Parkinson et al., 2019 PNAS). The link between global warming and the decline of sea ice in the Southern Ocean is still unclear, but climate change can’t be ruled out as a driving factor.

Antarctic sea-ice extent since 1979 (Parkinson et al., 2019 PNAS).

The Antarctic sea ice reaches, in general, its yearly maximum extent in September or October, after which it starts to melt during austral summer, and reaches its minimum in February.  During the Southern Hemisphere winter, the cold water of the Southern Ocean drives the growth of sea ice around Antarctica. During average conditions, the maximum extent of Antarctic sea ice is about 18 to 20 million square kilometers. During summer, it generally recedes to roughly 3 million square kilometers. At the beginning of February 2023, however, only 2.2 million square kilometers remained (Sea Ice Portal; https://www.meereisportal.de/), with melting to continue until at least the end of February.

Seasonal variations in Antarctic sea-ice extent
(Raphael and Handcock, 2022 Nature Reviews Earth & Environment).

One of the causes of this intense melting could be the unusually high air temperatures around the Antarctic Peninsula, as the monthly average to the west and east of this peninsula is currently about 1.5°C above the long-term average. Another potential cause is the current strong positive phase in the so-called “Southern Annular Mode” (SAM), which influences the prevailing wind circulation in the Antarctic. In a positive SAM phase, a low-pressure anomaly forms over the Antarctic, whilst a high-pressure zone develops over the middle latitudes. Under such conditions, as we observe this year, the westerly wind belt intensifies and contracts toward the Antarctic. As a result, inflow of the relatively warm Circumpolar Deep Water on the continental shelf intensifies, promoting sea-ice retreat.

Although melting sea ice does not directly raise sea level, because the ice is already floating in the ocean, this year’s minimum is nonetheless concerning. Sea ice has a stabilizing effect on the Antarctic Ice Sheet and its surrounding floating ice shelves by providing a buffer to wave-induced flexure, a recent study in Nature shows (Massom et al., 2018). The increased seasonal absence of a protective sea-ice buffer causes floating ice shelves to be exposed to ocean swell waves, potentially weakening them to the point of calving. When these ice shelves calve, and ultimately disintegrate, the oceanwards flows of Antarctica’s glaciers and ice sheets accelerate, which eventually lead to an acceleration in global sea-level rise. 

Sketch showing the various types of ice occurring in Antarctica.

The increased melt of sea ice furthermore aggravates warming in the Antarctic. The white, shiny surface of the sea ice reflects incoming solar energy back into space. The more the sea ice melts, the more of that energy is absorbed by the Southern Ocean, which, in turn, can melt even more of the ice.

The extent, growth, and decay of sea ice also plays a vital ecological role. Sea ice influences the growth of bacteria, phytoplankton, and algae, which form the base of the marine food chain. The ice moreover provides resting and breeding locations for marine mammals and birds, including seals and penguins. Fluctuations in sea-ice extent can therefore cause a series of changes throughout the Antarctic ecosystem, from the smallest microorganisms to the largest whales.

Historic documents highlight this year’s enormous fall in Antarctic sea-ice extent. Whilst the RV Belgica was stuck in the pack ice of the Bellingshausen Sea 125 years ago, in exactly the same region, our RV Polarstern can now operate in virtually ice-free waters. The photographs and diaries of the Belgica’s expedition members offer a unique chronicle of the ice conditions in the Bellingshausen Sea at the dawn of the industrial age, which climate researchers often use as a benchmark for comparison with today’s climate change.

The current ice-free conditions in the Bellingshausen Sea considerably facilitate the scientific activities of our PS134 expedition, and furthermore enable us to reach previously unexplored regions, on the other hand, this sea-ice absence underscores the urgency of modern changes observed around Antarctica.

The track of the RV Belgica 125 years ago (red) vs. the track of the RV Polarstern this year (white).
Source bathymetric data: IBCSO International Bathymetric Chart of the Southern Ocean (Version 2, 2022)

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