Emerging evidence of abrupt changes in the Antarctic environment

Abrupt changes are shifts in climate or environmental conditions that occur much faster than normal or expected based on previously observed changes in that system [1] (Box 1). Abrupt changes involve self-reinforcing processes (also referred to as a positive feedback) that amplify the response of a system to climate change or other pressures, and this means that they can be difficult or even impossible to reverse. Abrupt changes are often referred to as regime shifts and involve a system transitioning to a markedly different state. Regime shifts can cause substantial, widespread and often irreversible impacts when they are the result of crossing a tipping point in the Earth system [2].

Abrupt changes are particularly relevant for decision making around future climate risks. They can result in “surprises” that challenge effective adaptation to climate changes [3].

Over past decades, many parts of the Antarctic environment showed a much more muted response to human-caused climate warming than the Arctic. It was unclear if the abrupt changes and tipping points that are well known for the Northern Hemisphere [2] had equivalents in the Antarctic. But multiple abrupt changes in Antarctica and the Southern Ocean are now being observed and are expected to worsen [1].

After decades of seemingly defying global warming, the sea ice covering the ocean around Antarctica has declined precipitously since 2014 (Fig. 1). This abrupt regime shift [4] is unprecedented compared to natural variations in Antarctic sea ice over past centuries that have been reconstructed from historical climate observations and ice cores [1, 5]. In the last 10 years, Antarctica has abruptly lost the same amount of sea ice that the Arctic has progressively lost in over 40 years [1, 6], and there is growing evidence that Antarctic sea ice loss could be a tipping element in the Earth system [1]. Even after net-zero CO₂ emissions are reached and global mean temperature is stabilised, it is possible that Antarctic sea ice will continue to be lost [1, 7] causing sustained impacts on the Antarctic environment [8, 9].

The dramatic loss of Antarctic sea ice has several profound implications: It increases vulnerability for species that rely on ice for habitat and breeding, with knock-on consequences for Antarctic food webs [8, 10, 11]; it reduces the stability of ice shelves around the Antarctic coast [8]; it can contribute to slowing the deep ocean overturning circulation that is generated around the Antarctic coast [12]; and it changes operating conditions for national Antarctic programs and tourism [8]. Similar to the process that is causing rapid climate warming across the Arctic, the loss of Antarctic sea ice now has the potential to amplify climate warming in the region by reducing the amount of energy from the sun that is reflected back to space [1, 13].

The movement of water into and through the deep ocean is a key feature of the Earth’s climate system. In the North Atlantic, collapse of this overturning circulation is a known tipping point risk as Earth’s climate continues to warm [2]. A similar risk is now developing through the slowdown of the Antarctic branch of global ocean overturning [1]. This slowdown is evident in ocean measurements [12, 14], and climate models indicate it will continue to worsen [15]. The impacts of this include reducing the natural drawdown of carbon dioxide from the atmosphere into the deep ocean, which would intensify human-caused global warming.

The slowdown of the Antarctic overturning circulation has been linked primarily to meltwater being added to the ocean around Antarctica through the subsurface melting of ice shelves [15]. In Antarctica, the slowed dense, deep-water formation creates a reinforcing feedback in which marine-based parts of the Antarctic ice sheet (and their floating ice shelf extensions) become exposed to more subsurface ocean warming – which then generates more melting of the ice sheet and ice shelves. This feedback could perpetuate an abrupt slowdown in Antarctic deep ocean circulation and accelerate Antarctic ice-shelf and ice-sheet loss.

The West Antarctic Ice Sheet and marine-based parts of the East Antarctic Ice Sheet are known global tipping elements [2, 16]. Substantial amounts of ice are already being lost from the West Antarctic Ice Sheet, contributing to global sea level rise.

Antarctic Ice Sheet stability can be considered the wild card for future sea level rise estimates, holding the greatest potential for rapid increases, but also the greatest uncertainty [17, 18]. Unstoppable loss of the West Antarctic Ice Sheet could be triggered at global warming levels of less than 2°C [2, 16]. This could generate several metres of sea level rise with vast economic, societal and environmental impacts for centuries to come. Improving knowledge in regions where the Antarctic ice sheet is already losing ice, and where its shape and the underlying bedrock make it susceptible to abrupt ice loss, will be pivotal for improving predictions of where and how fast ice will be lost under continued ocean and atmospheric warming [1].

The floating ice shelves that fringe the Antarctic ice sheet, are critical for slowing the discharge of ice from the continent [19]. However, due to their contact with the surrounding ocean, many of these floating ice shelves will continue to melt and thin, even under best-case CO₂ emission reduction scenarios [20]. Ice-shelf collapse can happen very abruptly due to weakening from surface melting during heatwaves, or from newly emerging processes like increased wave-driven fracturing as was observed for the Conger-Glenzer ice shelf collapse in March 2022 [21, 22].

Antarctica and the Southern Ocean host unique ecosystems that are also showing signs of abrupt change. Habitat changes are associated with winners and losers. For example, some plant species expand as previously ice-covered land becomes exposed [23]; filter-feeder communities on the ocean floor are replaced with macroalgae after ice-shelf collapse [24]; and krill decline as their sea-ice habitat disappears [25].

Emperor penguins rely on stable land-fast ice for breeding, and episodes of early ice breakout prior to chick fledging have resulted in instances of complete colony breeding failures [9-11]. These events are compounding in time and space [1]; that is, they are affecting colonies around the whole Antarctic continent in the same breeding season, and many colonies have experienced multiple breeding failure events over the past decade (Fig. 2).

Climate change-related impacts on Antarctic and Southern Ocean life may be worsened by other non-climatic stressors, including human pressures and the current outbreak of avian influenza. Existing policy efforts through the Antarctic Treaty System to reduce non-climatic pressures and establish protected areas for Antarctic and Southern Ocean ecosystems are critical for building resilience for Antarctic ecosystems and their iconic species. To be effective in the long-term, these policy responses will also need aligned actions that limit the climate change pressures now, impacting the Antarctic environment (Fig. 3). 

Abrupt changes in Antarctica have direct consequences for the world. The most pressing concerns include how quickly ice will be lost from the Antarctic continent and how the resulting sea level rise will impact coastlines worldwide; how changes in Southern Ocean productivity and deep circulation could reduce the drawdown of CO₂ from the atmosphere and worsen climate warming; and how sustained sea-ice decline and ocean circulation changes will alter regional weather and global climate.

Proposals for geoengineering solutions to slow Antarctic warming and ice loss are gaining attention, but scientific assessments show that these are not feasible in coming decades, and could even be environmentally dangerous [27]. Avoiding further abrupt changes in the Antarctic environment would require limiting the multiple climate change pressures impacting this region, and can only be achieved through rapid and ambitious greenhouse gas emission reductions to stabilise Earth’s climate with as little overshoot of 1.5°C global warming as possible [1, 28] (Fig. 3). This would reduce, but not fully remove, the risks of further abrupt changes in the Antarctic environment.

Decision-makers will need to develop responses even though uncertainties remain about when and how quickly abrupt changes in the Antarctic environment will unfold. This uncertainty arises from unknowns in how quickly the world will reduce greenhouse gas emissions to limit global warming, and our incomplete understanding of vulnerabilities in the Antarctic environment to a warming climate. Options to help governments, businesses and communities build resilience to uncertain future climate risks include using storyline approaches [3] that incorporate the impacts and costs of abrupt Antarctic changes amongst the plausible future scenarios they consider in adaptation planning.

Research is continuing to improve the detection, understanding and predictability of abrupt changes in the Antarctic environment. This relies on sustaining Antarctic and Southern Ocean observing systems, better characterising areas of concern for abrupt change, developing long-term reconstructions of how the Antarctic system behaved before observations began and during previous warm periods in Earth’s history, and building improved modelling capabilities that can better capture abrupt change processes in Antarctica and the Southern Ocean. These priorities will contribute to impactful Antarctic and Southern Ocean science that supports decision-making. Continued international cooperation in Antarctic and Southern Ocean science, and in bringing this evidence into decision-making through the Antarctic Treaty System and United Nations Framework Convention on Climate Change, is critical to understanding and protecting the Antarctic environment.