Human-caused climate change worsens with every increment of additional warming, although some impacts can develop abruptly. The potential for abrupt changes is far less understood in the Antarctic compared with the Arctic, but evidence is emerging for rapid, interacting and sometimes self-perpetuating changes in the Antarctic environment. A regime shift has reduced Antarctic sea-ice extent far below its natural variability of past centuries, and in some respects is more abrupt, non-linear and potentially irreversible than Arctic sea-ice loss.

A marked slowdown in Antarctic Overturning Circulation is expected to intensify this century and may be faster than then anticipated Atlantic Meridional Overturning Circulation slowdown.

The tipping point for unstoppable ice loss from the West Antarctic Ice Sheet could be exceeded even under best-case CO2 emission reduction pathways, potentially initiating global tipping cascades.

Regime shifts are occurring in Antarctic and Southern Ocean biological systems through habitat transformation or exceedance of physiological thresholds, and compounding breeding failures are increasing extinction risk.

Amplifying feedbacks are common between these abrupt changes in the Antarctic environment, and stabilizing Earth’s climate with minimal overshoot of 1.5 °C will be imperative alongside global adaptation measures to minimise and prepare for the far-reaching impacts of Antarctic and Southern Ocean abrupt changes.

Antarctic Overturning CirculationThe global ocean meridional overturning circulation is a large-scale system of ocean currents that have a crucial role in regulating Earth’s climate by transporting heat, carbon, nutrients and oxygen. Changes in meridional overturning circulation strength have driven past abrupt climate changes, and a strong overturning circulation has been key to Earth’s climate stability over the past 11,000 years49. The global over-turning circulation is often described as having two ‘cells’; an upper cell sourced in the Atlantic, overlying a lower cell originating from the Southern Ocean. The Atlantic Meridional Overturning Circulation (AMOC) cell forms cold and salty dense waters in the North Atlantic

The Antarctic Overturning Circulation cell involves the production of dense shelf waters around the Antarctic continental margin (Fig.3a), which then overflow down the continental slope into the abyssal ocean as Antarctic Bottom Water5

Melting of ice shelves and calving of icebergs in West Antarctica has resulted in sustained ocean freshening in the Ross Sea since the 1950s. This is thought to have caused the 30% decline in Antarctic Bottom Water transport in the Australian Antarctic Basin (downstream of the Ross Sea production region) observed since 1994.

Accelerating ice loss from West Antarctica could cause cessation of Ross Sea dense shelf water production by mid-century. This vulnerability is supported by interannual-forced simulations of a high-resolution model that accurately simulates dense shelf water production around coastal Antarctica, which suggest production can cease altogether during mild winters.

In this same model, the addition of meltwater to the surface ocean drives a projected 42% decline in Antarctic Bottom Water formation by 2050 in a high-warming scenario
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Similarly, high meltwater forcing in a coarser resolution ocean model generates a near complete shutdown of Antarctic Bottom Water production this century.

This evidence suggests that a rapid and substantial slowdown in Antarctic Overturning Circulation is already underway, with observations suggesting that modelled rates of future decline may be underestimated. . . .

Advances in observations, process-based understanding, modelling and palaeoclimate evidence, suggest the Antarctic Overturning Circulation is already undergoing rapid change at current warming levels and will continue to decline in the 21st century.

This decline may be even more abrupt than the equivalent Northern Hemisphere processes, with one study indicating the decline in Antarctic Bottom Water transport by 2050 could be more than double the decline in AMOC strength

Habitat transformation leading to abrupt biological regime shifts has been documented in Antarctic terrestrial and marine systems, with associated ‘winners’ and ‘losers’.

Abrupt physical changes, including ice-shelf collapse and sea-ice loss, have led to local regime shifts in benthic shallow marine communities where filter feeder, ascidian[sea squirt, JM]-dominated communities are rapidly replaced by macroalgae[seaweed, JM] dominated communities. This response is due to increases in light, changes in sedimentation and increased disturbance from iceberg scour.

Observed transformations in benthic systems have frequently resulted in a reduction in biodiversity. In terrestrial biological systems of the Antarctic Peninsula, warming has resulted in rapid and self-perpetuating greening through colonisation of previously ice-covered or bare ground by mosses, algae and flowering plants (including invasive species).

However, actual rates of greening may have been slower than interpreted from satellite imagery and highlight the need for ground-truthing changes inferred by remote monitoring. The spread of flowering plant species is expected to be further perpetuated by increasing reproductive capability of the plants themselves.

Antarctic browning is being observed in other regions through the declining health of moss beds due to extreme desiccation caused by reductions in moisture availability from precipitation, blowing snow or meltwater flow. In East Antarctica, local habitat drying is leading to replacement of an Antarctic specialist moss with a cosmopolitan species, and is an example of potentially irreversible change to ecosystem structure