The past decade has seen a great awareness of increases in the size and frequency of extreme environmental events across a variety of global settings, and the associated consequential damage to lives and livelihoods (Fischer et al., 2021). Many of these events have been attributed primarily to the burning of fossil fuels and the loss of nature. For some time now the science of such attribution has been robust at the level that is needed beyond reasonable doubt (e.g., Otto et al., 2018), which has led to serious efforts to consider ‘loss and damage’ payments from rich developed fossil-fuel-based economies to parts of the world experiencing the effects of extreme events. While much attention has been given to weather-driven events such as heatwaves and rainfall elsewhere in the world, there is yet to be as great an appreciation of the occurrence and impact of extreme events in Antarctica. Here, we open a discussion of Antarctic Extreme Events, focusing on their records across a variety of realms (ocean, atmosphere, cryosphere, biosphere, etc.), indicating their likely causes and suggesting how they may change in future. We do not restrict ourselves solely to those derived from enhanced greenhouse gases, rather we aim to understand a range of ways in which Antarctica has and can experience extreme events and their consequences. In the Antarctic, extreme events are manifested in many ways, including the effects of: climatic extremes such as extreme weather events; catastrophic events such as ice shelf collapse; possible step changes in the environment such as recent sea ice loss; very rapid periods of environmental change and corresponding rapid changes in key biota; and sudden, human-induced direct perturbations, such as the effects of whaling and sealing. By taking an inclusive approach it allows us to understand how and why (relatively) rapid change can occur in Antarctica through high-magnitude low-frequency events of a variety of types.

The fact that Antarctica is able to maintain a gigantic ice sheet is a consequence of its very cold climate, driven by atmospheric processes. The Antarctic atmospheric system is influenced heavily by the topographic effect of the continent. Weather systems progress clockwise around the continent, with less longitudinal penetration toward the pole than occurs in the Arctic (see Siegert M. et al., 2020). However, that is not to say that weather systems do not move poleward, penetrating inland across Antarctica. These poleward moving cyclones are particularly important for disturbing the otherwise cold and dry weather with warmer and more moist conditions (Sinclair and Dacre, 2019).

Notably, the most extreme ‘heatwave’ ever recorded globally occurred over East Antarctica in March 2022 when surface temperature anomalies of up to 38.5°C were observed (Berkeley Earth, 2022). This event was associated with an ‘atmospheric river’; a long filament-shaped atmospheric structure that carries abundant moisture across large distances (many hundreds of kilometres), leading to extreme localised heat and precipitation. These atmospheric rivers transport heat and moisture from the subtropics into the heart of the Antarctic continent.

Although it was so extreme, a formal attribution of the March 2022 event to human factors has not yet been conducted. However, an attribution analysis of an earlier record-breaking heatwave, that affected the Antarctic Peninsula in February 2020 and led to the highest recorded temperature in the Antarctic mainland (18.3°C at Esperanza Station), concluded a likely significant contribution from fossil-fuel burning (González-Herrero et al., 2023). More research is required to quantify the anthropogenic contribution to the severity of the March 2022 event, and the extent to which we may expect similar events both in the near term and further into the future. Moreover, we must determine whether such large temperature extremes could occur over regions of potentially high impact, such as vulnerable ice shelves.

Other types of extreme weather events have had significant impacts in recent years. Extreme cyclones were implicated in a major iceberg calving event of the Brunt Ice Shelf in 2020 (Francis et al., 2022) and the rapid sea ice decline in the Weddell Sea in 2016/17 (Turner et al., 2020). Although atmospheric rivers and cyclones are key drivers of Antarctic extremes, there is a significant proportion of events that are not associated with either and require other explanations (Hepworth et al., 2022), such as persistent high pressure systems and mid-latitude weather fronts.

Possibly the most recognisable extreme event that occurred in the atmosphere was the loss of stratospheric ozone, discovered above Antarctica in the 1980s. This loss was caused largely by a particular class of chemicals: Chlorofluorocarbons (CFCs). Whilst this event catalysed rapid and effective policy action by the global community in the development of the Montreal Protocol (adopted in 1987), the effects of the “ozone hole” are being felt decades later, with massive increases in ultra-violet radiation still detectable in the Antarctic spring. Recovery of the Antarctic ozone layer later this century is predicted but there are still ongoing effects. These relate not only to biology (discussed below) but also to the climate of Antarctica, where ozone depletion has led to stratospheric cooling and resulting intensification and poleward shift of the belt of westerly winds that encircle Antarctica (Thompson and Solomon, 2002). This change in circulation patterns has led to warming over the Antarctic Peninsula and cooling in parts of East Antarctica (Thompson et al., 2011).

Sea ice is the frozen ocean surface and is an integral part of the Earth system. When present it limits the transfer of heat and gases between the ocean and atmosphere, and reduces ocean flow and mixing. Sea ice growth and melt processes influence ocean salinity, and therefore stratification and global ocean circulation, and the ice itself provides an important ecological niche for many species. In addition, changes in sea ice impact biological production, the transport of biogeochemical properties into the ocean interior and the marine uptake of anthropogenic carbon (Henley et al., 2020). Extreme low sea ice years can provide significant stresses to the marine environment (as discussed later). Sea ice is recognised by the World Meteorological Organisation as one of seven headline Global Climate Indicators which are key to describing the changing climate (WMO, 2023).

Antarctic sea ice extent varies annually between a low of 2–4 million km², usually in late February, and a September high of 18–20 million km² (NSIDC, 2023). Accurate measurements of sea ice extent are available from satellite observations dating back to October 1978. During much of the satellite period, total Antarctic sea-ice extent underwent ‘gradual but uneven’ increases (Parkinson, 2019), despite the warming global climate and, by contrast, persistent Arctic sea ice loss (Stroeve and Notz, 2018). Indeed, Antarctic sea ice extent hit a record high in winter 2014. However, it then retreated to a record low in summer 2017 (Turner et al., 2017; Parkinson and DiGirolamo, 2021), and the four lowest annual minimum sea ice extents of the satellite era have occurred since, with both 2022 and 2023 setting new records (Figure 1A; NSIDC, 2023). In 2022, the summer minimum extent dropped to below 2 million km² for the first time, and winter extent remained at near-record lows for the time of year (Figure 1A). Both the high sea ice extents of 2013–2015 and the low sea ice extents since 2017 lie far outside the observed variability of the baseline period 1981–2010 (Figure 1A), indicating them as ‘extreme’ based on our existing understanding of the system and highlighting crucial yet poorly understood recent increases in year-to-year variability.

Antarctic sea ice loss is not uniform; it varies regionally. In February 2023, the Pacific sector, and Ross and Amundsen-Bellingshausen Seas had near-record low extents with little ice remaining, while the Weddell Sea and Indian sector had less dramatic change, although still lower than the long-term mean (Figure 1B). To truly understand changes in sea ice and its possible vulnerability to further change requires knowledge of sea ice volume and therefore thickness. However, estimating sea ice thickness from satellite data remains challenging, so changes in the volume of sea ice remain poorly resolved. The situation has been stressed in the World Climate Research Program (WCRP) Scientific Committee on Antarctic Research (SCAR) summary report on the International Workshop on Antarctic Sea Ice Thickness: “Sea ice thickness remains arguably the largest single gap in our knowledge of the climate system.”

The low sea ice conditions of the past 7 years certainly begs the question as to whether we are entering a regime where extreme low sea ice extent events become more frequent. Indeed, as in the Arctic where sea-ice loss leads to further absorption of solar heat due to a reduction in surface reflectivity, so too may persistent Antarctic sea ice reductions lead to enhanced warming. Conclusions about the role of human activity in such extremes, and expected future change, rely on reliable climate model simulations. However, coupled climate models have to date almost universally simulated a decrease of Antarctic sea ice cover for the period since 1979, leading to a mismatch with the pre-2017 satellite observations (Roach et al., 2020). This, combined with model biases in the average sea ice extent, has contributed to low confidence in the ability of climate models to simulate sea ice change under various forcing scenarios. Interestingly, the recent loss of sea ice brings long-term trends more in line with climate model simulations, but more analysis is needed before we can better understand the drivers behind this dramatic recent change.

In the ocean, extreme temperature events are typically characterised as Marine Heatwaves (MHWs)—extended periods of anomalously high temperatures that can exert significant impacts on marine biodiversity and ecosystems, and the economics of fisheries and other marine industries (Hobday et al., 2016). Although sparser than in some other ocean basins, nineteen such events were documented between 2002 and 2018 in the Southern Ocean, with significant increases in chlorophyll-a concentrations noted in response (Montie et al., 2020). Separately, impacts of MHWs on Southern Ocean diatoms have been recorded, including increased mortality (Samuels et al., 2021). MHWs have increased in frequency globally, though there is strong regional structure and they are thought to have decreased in parts of the Southern Ocean (Oliver et al., 2018). Globally they are predicted to further increase in frequency and intensity in the coming decades, with their effects compounded in some areas by extreme acidity events (Burger et al., 2022), though regional variability and data sparsity challenge projections for the Southern Ocean. Below the sea surface there has been a steady accumulation of heat in the Southern Ocean. This body of heat has been identified as one of the processes responsible for influencing the timing and magnitude of the rapid decrease in sea ice extent subsequent to 2017 (see above; Zhang et al., 2022).

Large-scale horizontal circulation in the Southern Ocean is dominated by the Antarctic Circumpolar Current (ACC). Much energy from the ACC is dispersed in large eddies and can lead to strong local currents. Eddy energy around the Southern Ocean was enhanced at a circumpolar scale in 2000–2002 following a short period of strong winds around 2–3 years previously (Meredith and Hogg, 2006); however, attributing a rising trend in eddy current speeds to the strengthening of Southern Ocean winds is challenged by intrinsic variability in the eddies (Hogg et al., 2022). It was shown recently that these eddies exert a key influence on the intensity and frequency of MHWs via ocean heat flux convergence (Bian et al., 2023), thus potential future increases in eddy energy are likely to be a significant factor in determining extreme ocean temperatures in coming decades.

Changes in the vertical (overturning) circulation in the Southern Ocean are likely less limited by eddies than is the horizontal flow, with important implications for heat and carbon sequestration, both of which are significant for global climate. Consequently, strengthening winds and/or stronger fluctuations in their magnitude are likely to drive proportionately larger changes in overturning flows. Quantifying changes in this component of the global circulation, and characterising its episodic nature and response to extremes in forcing, have become high priorities (Meredith et al., 2019).

Of particular concern are intrusions of water from the mid-depths of the ACC onto the Antarctic continental shelves. Such waters can penetrate across the shelves, especially where glacially-scoured canyons offer a preferred route. This Circumpolar Deep Water (CDW) is markedly warmer than water beneath ice shelves and proximal to marine-terminating glaciers. Hence, an influx of CDW has been identified as critical for melting ice shelves and glaciers in West Antarctica, including the rapidly-retreating Pine Island Glacier, Thwaites Glacier and numerous smaller glaciers along the western coast of the Antarctic Peninsula (Rignot et al., 2014; Cook et al., 2016). Changing wind forcing of ocean flow across the shelf break, including a strengthening anthropogenically-driven component, has been noted as critical (Holland et al., 2019). Thus far, there is comparatively less evidence for changing ocean circulation having influenced the “cold” sector glaciers and ice shelves around Antarctica, such as in the Weddell and Ross Seas. Numerical simulations indicate that these can be susceptible to extreme changes in ocean flows, however, with pronounced sudden consequences for ice shelf and ice sheet stability (Hellmer et al., 2012; Ross et al., 2012), though recent analyses highlight their comparative insensitivity for the near-term future (Naughten et al., 2021).

Ice shelves fringe three-quarters of Antarctica’s coastline, providing buttressing support that stabilises the rate of ice flow from the grounded ice sheet and its contribution to global sea level (Siegert M. J. et al., 2020) (see Figure 2A for an overview of the ice shelf system). Over the past 5 decades, satellites have observed the retreat, thinning and disintegration of Antarctic ice shelves (Paolo et al., 2015), with change concentrated in two key sectors of the continent.

On the Antarctic Peninsula, ice shelves have retreated on average over the last 50 years, with large sections of the Larsen-A, Larsen-B, and Wilkins ice shelves collapsing catastrophically in 1995, 2002, and 2008, respectively (Figure 2B, C). Following a period of relative stability in the 1990s, the collapse of the Larsen-B Ice Shelf was triggered by widespread meltwater ponding on the ice shelf surface where crevasse hydrofractures caused pressure-driven disintegration of the shelf in a matter of days (Scambos and Hulbe, 2000). In the austral summer of 2019/20 high levels of surface melting were observed across the Antarctic Peninsula. If such events become more frequent in a warming climate further incidences of ice shelf disintegration may occur. In West Antarctica, dynamic imbalance is driven by incursions of warm modified CDW melting the floating ice shelves (see above), with the interannual and long-term variability of ocean temperatures linked to atmospheric forcing associated with the El Niño-Southern Oscillation (ENSO) (Jenkins et al., 2018).

Since 2009, major iceberg calving events have occurred across the continent on ten Antarctic ice shelves, including the Larsen-C, Wordie and Wilkins ice shelves on the Peninsula, Thwaites and Pine Island Glaciers in West Antarctica, and Nansen, Mertz, Brunt, Amery and Conger ice shelves in East Antarctica. Recent studies have suggested that a complex link between atmospheric and ocean processes may have a role to play in ice shelf stability and calving. Extreme atmospheric conditions drive strong winds that can affect ocean swell, which may have had a role in triggering recent iceberg calving (Francis et al., 2021) and historical ice shelf collapse (Massom et al., 2018). Whilst iceberg calving is part of the normal process of mass loss, if calving frequency changes over time it can require decades of regrowth to replace the lost ice, and therefore may be an indicator of longer-term change. It is now clear that ice shelves can respond to change over short timescales and that long data records are required to disentangle natural variability from longer term more permanent change.

New satellite remote sensing datasets have revealed meltwater to be present on the surface of the Antarctic Ice Sheet, far inland of the grounding line (Stokes et al., 2019). In addition to the supraglacial lakes that form because of surface melting, meltwater has also been observed ponding within firn aquifers, also upstream of the Antarctic grounding line (Bell et al., 2018). Supraglacial and firn stores of meltwater have been related to weather and climate-driven events, such as atmospheric rivers, foehn and katabatic winds, rainfall and cloud cover (Bell et al., 2018).

Surficial meltwater has potential to impact ice flow through hydrological connections with the ice-bed interface. In Greenland, these connections are well established; here, surface meltwater drainage events have been directly related to accelerations in surface ice flow speeds, due to a decrease in basal friction and an increase in sliding (Joughin et al., 2008; Moon et al., 2015). On the Antarctic Peninsula, short term accelerations in ice flow have been observed and related to surface melt events (Tuckett et al., 2019; Wallis et al., 2023), though ocean forcing has also been implicated (Boxall et al., 2022). Firn aquifers may act to dampen the signal and buffer the response (Wallis et al., 2023) but may also play an important role in transmitting surface melt to the ice-bed interface (Bell et al., 2018). Surface meltwater and firn aquifers can also drive cryohydrologic heating through the release of latent heat, changing ice rheology and impacting glacier mass balance (Bell et al., 2018).

The abrupt removal of ice shelves (see above) also impacts ice flow (Figure 2C). Ice shelves buttress, or restrain, their tributary grounded glaciers, through drag forces at their lateral margins and basal stresses where ice shelves rest on pinning points (Goldberg et al., 2009) (Figure 2A). Incremental reductions in ice-shelf thickness can reduce these stresses, resulting in a near-instantaneous rise in ice flow (Paulo et al., 2015; Gudmundsson et al., 2019). This effect is dramatically exacerbated if ice shelves disintegrate entirely. On the Antarctic Peninsula, the larger tributary glaciers of the Larsen A and B ice shelves have substantially accelerated, retreated and thinned following the respective ice-shelf disintegration events in 1995 and 2002, with impacts continuing for years following ice-shelf removal (Berthier et al., 2012).

Internally-forced extreme events impacting the speed and direction of Antarctic grounded ice flow are also known. The Siple Coast ice streams in West Antarctica are susceptible to internal instabilities (e.g., water piracy and flow switching, changes in basal thermal regimes) with high-magnitude, ice dynamical implications (Retzlaff and Bentley, 1993; Joughin and Alley, 2012). The Kamb Ice Stream was fast flowing until ∼1865 AD, when it became stagnant, causing its upper catchment to subsequently thicken by up to 0.65 ± 0.07 m yr^-1 (Pritchard et al., 2009). The shut-down of this ice stream has been debated in glaciology, with one idea being that water supplied to the bed was ‘pirated’ or rerouted to neighbouring Whillans ice stream (Anandakrishnan and Alley, 1997). It is therefore considered to be a feature of ‘natural variability’. However, the significance of this idea, coupled with the known sensitivity of basal water routes to small surface changes (Wright et al., 2008) and the upstream impact of grounding line loss of ice (Payne et al., 2004), is that mass loss at the ice sheet margin could well be the trigger for considerable glaciological change to the ice sheet interior, suggesting it too may be affected by future fossil-fuel burning.

While only around 0.2%–0.4% of the Antarctic continent is exposed above the ice (Burton-Johnston et al., 2016), this proportion is likely to increase with further warming (particularly in the Antarctic Peninsula; Lee et al., 2017) and, in some places such as the Dry Valleys in East Antarctica, the area of exposure could be substantial. Such ice-free areas may act as sources of dust falling on snow and ice, reducing surface reflectivity. Thus, as extreme winds form enhanced supraglacial dust deposits, so increased melting can occur. In the Dry Valleys this can lead to sudden alteration in lake ecosystems (Doran et al., 2008). Dust-driven melting has also been observed in James Ross Island (Kavan et al., 2020) and Seymour Island (Meinander et al., 2022).

To humans visiting Antarctica, perhaps the most obvious examples of the effect of extreme events are on land breeding predators. Periodic “low krill years” at South Georgia (e.g., 1983, 1994 and 2009) are manifested by widespread breeding failures of krill-reliant predators, for example, with many dead fur seal pups observed on beaches (AA pers. obs.) and coincident indices of high temperature anomalies, predator reproductive output and low fisheries catches (Trathan et al., 2021). Likewise extreme events (a combination of extensive fast ice, absence of polynya and highly unusual occurrence of rain saturating the chicks followed by rapid sub-zero temperatures) led to the complete breeding failure of a well-studied colony of 20,000 pairs of Adélie penguins on Ile des Pétrels, near Dumont D'Urville research station, in 2013/14 and again in 2016/17 (Ropert-Coudert et al., 2014; 2018). In these inshore waters large icebergs can also have massive and abrupt restructuring roles due to grounding and scour. In the Ross Sea they also impact sea ice dynamics, fostering abrupt shifts in open water and phytoplankton blooms (Dayton et al., 2019; Kim et al., 2019).

By contrast, in open-ocean systems and lower down the food web, the effects of extreme events such as MHWs are less clear-cut. MHW study in the Southern Ocean is in its infancy (see above), although multiple stressor extreme events of MHWs and increased ocean acidity have been suggested to pose a threat in some high latitude surface water regions (Gruber et al., 2021). However, phytoplankton intraspecific trait diversity is high (Samuels et al., 2021; Bishop et al., 2022), conferring ability to adapt and providing a buffer to help withstand both MHWs and 21st century warming. In addition to acute multi-stress experimentation, studies also need to consider inbuilt resilience of the biota. This can include their acclimation and adaptive potential, refuges such as in the more stable deeper waters and their typically longer, often multi-year generation times that can buffer the effect of an annual breeding failure. Between 2018 and 2022, 42% of emperor penguin colonies (28 of 66 known colonies) likely experienced total or partial breeding failure due to fast ice break up in at least 1 year.

The effects of sudden step-changes in environmental forcing and in the response of the biota are better documented in Southern Ocean ecosystems than those of MHWs or other reversible extremes. In continental shelf systems, sudden ice shelf collapse has major consequences both for open water, enhancing areas with export potential for primary production and allowing colonization of benthic habitats recently supplied with food from above. These dramatic events foster rapid transitions, with colonization by pioneer species (Gutt et al., 2013). For pelagic systems, an important finding has been that periods of extremely rapid change in the environment are not always followed by commensurate shifts in the biota. A good example here is the poleward range shifts of Antarctic biota in response to the rapid warming and ice loss from the Atlantic sector over the last 80 years. While species distribution models might project that their ranges move poleward to track the isotherms to maintain within the same thermal regime, mesozooplankton species distributions maintained the same position and withstood warming (Tarling and Thorpe, 2017). Likewise, the range of Antarctic krill was stable during this rapid warming period but “jumped” abruptly south during the ∼1995–2015 warming hiatus (Atkinson et al., 2022).

In addition to these climate-induced changes, human over-exploitation of marine resources during the 1800s and 1900s has had long-lasting and catastrophic effects on the relative abundance of higher trophic levels. About one million blue and fin whales were removed between 1904 and 1980—part of a massive, global scale depredation of marine mammals to roughly one-fifth of their original biomass (Bar-on et al., 2018). Whilst whaling is the most widely known direct human perturbation to the Southern Ocean food web, it forms part of a sequence of over-exploitation, first of fur seals, then of successive whale species and followed by major unregulated and illegal fishing around the 1960s, which severely perturbed the balance of benthic predators around islands such as South Georgia and Kerguelen, with fish stocks very slow to recover. We are still debating the effects of these massive, human-induced perturbations, particularly over how far these effects have cascaded down the food chain, even to the possibility on impacting the biogeochemical cycling of key nutrients such as iron (Smetacek and Nicol, 2005).

Extreme events affecting terrestrial (including freshwater) ecosystems and biodiversity in Antarctica, as elsewhere, take a range of forms both directly relating to elements of climate and integrating with the consequences of other human-induced and natural events. Although trends and magnitudes of changes yet experienced in Antarctica vary widely, in the Antarctic Peninsula/maritime Antarctic region the degree of warming and precipitation change (including widespread change in summer precipitation from snow to rain) experienced since the mid-20th Century is arguably already a combination of multiple extreme events (Siegert et al., 2019a). However, the potentially damaging biological consequences resulting from the simultaneous development of the ozone hole (see above) do not appear to have come to pass (reviewed by Convey and Peck, 2019).

Lakes in Antarctica may be particularly vulnerable to the impacts of climate warming, with magnified rates of response in terms of factors such as the extent and duration of winter ice cover, water temperature, mixing and eutrophication (Butler, 1999; Quayle et al., 2002). Lake and other freshwater ecosystems are particularly understudied and undocumented in Antarctica, with work in this field representing a high conservation priority (Hawes et al., 2023). Many are small, shallow and vulnerable to changes in inflow and outflow, and catastrophic drainage can occur in instances where glacial or permafrost ‘dams’ are lost with melting (e.g., see Rosa et al., 2022).

The inherent flexibility and well-developed stress tolerance of many Antarctic terrestrial biota generally allows them to cope with the magnitude of climatic changes seen to date. In the absence of other anthropogenic influences, such as invasive species introduction, these features even allow them to respond positively as the changed environments and increased availability of ice-free habitat reduce the major environmental limitations they face at present. A hypothesised negative consequence of this warming has been an apparent increase in the occurrence of phytopathogenic fungal diseases in mosses in the maritime Antarctic (Rosa et al., 2020), although this has yet to be rigorously assessed. Some changes, while yet to be formally documented in Antarctic terrestrial habitats, already have clear negative consequences in analogous and often better-studied, High Arctic terrestrial habitats, which give important warnings. These include winter thaws and increasing rain in winter leading to ‘rain on snow’ events, which can encase the soil surface and its vegetation cover in a layer of impenetrable ice, and lead to anoxia in the underlying soil ecosystem and mortality of its contained invertebrate communities (Coulson et al., 2000; Peeters et al., 2019). Changing snow cover and distribution patterns, resulting from changes in precipitation levels and patterns of wind distribution, also lead to altered patterns of vegetation and invertebrate distribution and survival, driven by the ability (or not) to tolerate increased or reduced periods of snow cover and associated changes to the thermal regimes experienced in microhabitats (reviewed by Pedersen et al., 2022; Coulson et al., 2023). Furthermore, it has recently been discovered that extreme snowfall events can lead to large-scale Antarctic seabird breeding failures (Descamps et al., 2023).

Marine organisms that breed or moult onshore, such as penguins and seals, provide an important transfer of nutrients onto land, which can be a major driver of terrestrial biodiversity (Bokhorst et al., 2019). Anthropogenically-driven changes to sea-ice or other controlling factors will not only change where these penguins and seals are distributed, but have direct impacts on terrestrial biota which rely on the nutrient supply. Major anthropogenic extreme events, as described earlier, predate the onset of climate warming, and continue to accelerate in their impacts on Antarctic terrestrial ecosystems today. The uncontrolled overexploitation of marine mammals (fur seals, then whales) completely destabilised the Southern Ocean marine ecosystem. This caused both short-term local peaks and long-term large-scale changes in nutrient inputs and degrees of ‘natural’ disturbance to terrestrial ecosystems. In synergy with subsequent anthropogenic climate-change-driven changes in land-ice extent, this has been followed by increasingly widespread damaging impacts to terrestrial ecosystems and biodiversity (habitat destruction through trampling and over fertilisation). For example, recovering seal populations are now able to spread ever further south into areas of the maritime Antarctic with no known history of their occurrence (Convey and Hughes, 2022).

The increasing footprint of human activity in Antarctica (by national operations and tourism) brings with it analogous threats of extensive terrestrial ecosystem and biodiversity damage (Lee et al., 2022). These are exacerbated by the very small proportion of Antarctica’s area that is ice-free and the clear and very strong preference of national operators to construct and expand their stations in those areas. Certain parts of Antarctica, such as the Fildes Peninsula area of King George Island where a concentration of multiple national operator stations and facilities are present, and the McMurdo area on Ross Island, are now de facto accepted to be ‘brownfield’ sites even if not generally described as such in official literature. This calls into serious question the effectiveness of current Antarctic environmental management protocols (Convey, 2020). Human activities in Antarctica also carry the very well documented risk of introduction of non-native species, and also the movement of native Antarctic species between different distinct biogeographic regions within Antarctica to which they are not native, some of which may then become invasive (Hughes et al., 2019). The central contribution of human activity to this threat, and particularly that of national operators in the region, should not be underestimated. In simple summary, in the entire history of human contact with the Antarctic regions, there are no known instances of a natural colonisation and establishment event in terrestrial Antarctica, and only two proposed cases in the core sub-Antarctic, while human-assisted introductions account for c. 15 and c. 250 instances, respectively (Frenot et al., 2005; Hughes et al., 2015). Studies have confirmed that all classes of visitors to Antarctica, both within national operators and the tourism industry, carry potentially viable biological propagules with them (Chown et al., 2012). Nevertheless, of all introductions since the mid-20th Century (the start of the “scientific era”), the vast majority if not all are most plausibly linked with national operations, which are the major source of cargo arriving on the continent. While the terrestrial groups and species involved are generally small and cryptic, and might therefore be viewed by some as insignificant, some such species can be true ‘ecosystem engineers’, leading to step changes in key ecosystem functions that are currently rate limiting, or even functionally not important, in native Antarctic ecosystems such as decomposition and nutrient cycling (Bartlett et al., 2023) or predation (Lebouvier et al., 2020).

Several of the extreme events discussed above do not occur in isolation and their impacts can be linked in “cascades”. For example, the atmospheric river in March 2022 that led to record temperatures in central East Antarctica also led to surface warming of land ice and ice shelves on the coast, break-up of sea-ice, and collapse of the Conger Ice Shelf (Lhermitte et al., 2023). Similarly, for the ocean, anthropogenically-driven changes in winds can drive changes in intrusion of relatively warm waters to the margins of the ice sheet and, thus, affect extreme events of land ice and ice shelf loss. Physical and biological impacts can also interact, such as warming and the loss of land ice combining to lead to greater probability of alien species establishment. We cannot rule out future cascades where extreme events may have wide-ranging linked impacts in multiple realms, some of them potentially hard to predict.