As in other areas of the globe, human activities are directly affecting the climate of the Antarctic region. The 2 key human influences on climate are emissions of greenhouse gases (principally carbon dioxide – CO2 – methane and nitrous oxide) and substances that deplete the stratospheric ozone layer (chlorofluorocarbons, halons and related gases) (IPCC 2013, WMO 2018, IPCC in press-a). The increasing atmospheric concentration of greenhouse gases is acting globally to warm the lower parts of the atmosphere and surface. Ozone-depleting substances are also greenhouse gases, contributing to the warming of the lower atmosphere. At the same time, the increasing atmospheric concentration of CO2 and depletion of stratospheric ozone are acting to cool the upper levels of the atmosphere, as well as the surface of the Antarctic interior (IPCC 2013, WMO 2018). These temperature changes are placing various pressures on fundamental aspects of the Antarctic region, including the structure of the atmosphere, the mass balance of the ice sheet, the formation of sea ice, the availability of liquid water at the surface of the ocean and land, and the physical characteristics of the surrounding ocean. The extent and rate of change of the climate effects depend on their duration in climate systems (which, in the case of CO2, amounts to many centuries), and also on the timescale of the long-term changes they cause to the heat content of the deep ocean (which is also expected to be many centuries) (Solomon et al. 2010). International efforts through the Paris Climate Accord are attempting to control greenhouse gas emissions and limit the amount of global warming. The Montreal Protocol has already provided tangible benefits in limiting warming from ozone-depleting substances (Neale et al. 2021). Air temperature Whereas the Antarctic interior experiences temperatures well below the freezing point of water throughout the year, coastal areas, particularly in the Antarctic Peninsula region, can experience conditions where ice can melt to provide run-off. Additionally, some areas can experience rainfall. Knowledge of future temperature variability and change is important in anticipating pressures on habitats, such as changes associated with extreme physical conditions, changes in the availability of ice-free areas and sea ice for breeding, and changes in the presence of water for plant growth (Convey & Peck 2019). Temperatures at the Antarctic surface have shown differing regional patterns of change, with warming in parts of the Antarctic Peninsula and West Antarctica, and generally no significant change in East Antarctica (see Atmosphere). The observed patterns and trends are consistent with influences from changes in the large modes of climate variability, particularly the Southern Annular Mode (SAM) (Turner et al. 2020a). While climate variability is showing the imprint of climate change (Cai et al. 2015, Fogt & Marshall 2020), these modes are also countering the effects of greenhouse warming in some parts of Antarctica. The latest state-of-the-art climate simulations indicate that global warming will have a more significant impact on the Antarctic region during coming decades (Bracegirdle et al. 2020), with the likelihood of increased surface precipitation over coastal areas. In the stratosphere, ozone depletion in spring influences temperature trends, with cooling observed from the 1980s to around 2000. Subsequently, temperatures have stabilised or slightly warmed (Randel et al. 2017, WMO 2018). Increases in greenhouse gases have also led to cooling in all seasons (Randel et al. 2017, French et al. 2020a), which is strongest in the upper levels (particularly in the mesosphere). In combination with trends near the surface, the changes in the lower stratosphere have altered the thermal structure of the atmosphere, primarily resulting in the strengthening of the westerly wind belt over the Southern Ocean (Polvani et al. 2011). Winds The alteration of the temperature structure of the lower atmosphere by climate change has led to a general strengthening of the westerly winds in summer over the Southern Ocean, and a shift in the zone of strongest winds towards Antarctica (Thompson & Solomon 2002, Polvani et al. 2011, Perren et al. 2020). These changes have been implicated in a range of physical effects, including warming in the Antarctic Peninsula region (Turner et al. 2020a); changes in the ability of the Southern Ocean to take up CO2 (Xue et al. 2018, Keppler & Landschützer 2019); cooling of the surface waters, and warming and freshening below the surface (Kostov et al. 2018, Swart et al. 2018); influences on sea ice (Doddridge & Marshall 2017) and ice-shelf disintegration (Hattermann et al. 2021); and alteration of the overturning circulation in the upper oceanic layers (Li et al. 2019). Increasing wind speeds appear to be promoting regional drying in the Windmill Islands of East Antarctica, and this is implicated in the degradation of moss communities (Robinson et al. 2018, Bergstrom et al. 2021). Over the next few decades, the wind changes over the Southern Ocean are likely to be held in place due to competing effects – long-term cooling of the stratosphere from increasing greenhouse gases and the warming associated with increasing ozone concentrations as the ozone hole recovers (Arblaster et al. 2011). Increasing greenhouse gas concentrations are expected to cause general strengthening of the westerly winds in all seasons (Morgenstern 2021). At the Antarctic coast, the easterly surface winds are expected to weaken over the coming decades in response to changes in the strength and position of the westerlies over the Southern Ocean (Bracegirdle et al. 2020). Additionally, the northward penetration of the katabatic winds away from the coast is likely to be reduced as a result of surface warming and strengthening of the SAM (Bintanja et al. 2014). Although changes in surface wind direction and speed have potential implications for coastal and marine habitats in the Antarctic region, it is not clear how expected future wind changes will affect certain species. For example, Fretwell & Trathan (2020) concluded that the emperor penguin is susceptible to changes in wind regime. On the other hand, Antarctic seabirds have shown some tolerance to variability in present wind conditions. Precipitation Precipitation, in the form of falling snow, ice crystals, hail and rain, is a potential stressor of animals and plants in the Antarctic region (Convey & Peck 2019) as it can influence heat loss, and the physical properties of nesting and breeding habitats. On the other hand, the cold Antarctic climate limits the availability of liquid water, and plants especially can benefit from rain and melting ice. Precipitation has shown mixed patterns of change in the Antarctic and Southern Ocean region, reflecting influences from large-scale modes of climate variability, including the El Niño–Southern Oscillation and the SAM (Turner et al. 2014, Turner et al. 2020a). Parts of the Antarctic Peninsula and West Antarctica have shown evidence of a positive trend in the observational period (Turner et al. 2005a), while trends in East Antarctica are generally not significant (Bromwich et al. 2011). Changes in local precipitation can affect biological communities. For example, regional drying in the Windmill Islands of East Antarctica is implicated in the degradation of moss communities (Robinson et al. 2018). In contrast, growth has increased in plant communities on the Antarctic Peninsula as a result of precipitation increases and warming (Amesbury et al. 2017). Altered wind and precipitation patterns are causing extensive change in the alpine tundra (fellfield) ecosystem on Macquarie Island, with the collapse of endemic keystone species, such as the Macquarie cushion plant and associated bryophytes (Bergstrom et al. 2015, Bergstrom et al. 2021). Future projections suggest generally increased precipitation in Antarctica (Bintanja et al. 2014, Bracegirdle et al. 2020), with possible benefits for microflora (Singh et al. 2018), but negative effects on some penguin species (Bricher et al. 2008, Trathan et al. 2015). For example, Adélie penguins at Pointe Géologie, East Antarctica, suffered major breeding failures in 2013–14 when rain flooded nests, and downy chicks whose plumage is not yet waterproof died of hypothermia and starvation (Ropert-Coudert et al. 2015). Ultraviolet radiation The stratospheric ozone layer protects the biosphere from harmful levels of solar ultraviolet radiation (UVR). The development of the Antarctic ozone hole in the late 1970s (WMO 2018) has increased the exposure of large regions of Antarctica and the Southern Ocean to UVR, primarily in spring. Several studies have identified biological impacts in the Antarctic region from increased radiation exposure, including cellular damage in algae (Davidson & Belbin 2002), sea urchins (Schröder et al. 2005) and plants, including mosses (Newsham & Robinson 2009, Turnbull & Robinson 2009); reduced survival of krill larvae (Ban et al. 2007); and effects on human health (Russell et al. 2015, Neale et al. 2021). Over the coming decades, the ozone hole will continue to recover as a result of international controls imposed on ozone-depleting substances by the Montreal Protocol (WMO 2018); full recovery (defined as a return to 1980 conditions) is expected by around the middle of the 21st century (Dhomse et al. 2018, Amos et al. 2020). Accompanying the recovery, the UVR-induced biological stress caused by ozone depletion in the Antarctic region will subside. Towards the end of the 21st century, the speedup of the equator-to-pole circulation driven by global climate change (WMO 2018) will additionally increase the total ozone column in the Antarctic region, and lead to further reduced UVR exposure compared with conditions during the ozone hole era. Sea ice Any substantial changes in Antarctic sea ice coverage, thickness and properties have wide-ranging consequences and may have major impacts on global climate, regional weather, ocean properties and processes, ecosystems, biogeochemical cycles, sea level rise and human activities (Massom & Stammerjohn 2010). For example, the krill fishery at the Antarctic Peninsula is now mainly active in autumn and winter rather than in midsummer (Meyer et al. 2020). Numerous plant and animal species depend on sea ice as a food source, shelter, refuge and breeding platform, and are highly adapted to its presence and seasonal rhythms (Massom & Stammerjohn 2010). These organisms range from microscopic primary producers (e.g. ice algae) that proliferate in high concentrations within the icy matrix (Arrigo et al. 2017) and support pelagic herbivores such as krill (a keystone species that also supports major fisheries (Bluhm et al. 2017)), through to fish, seals, seabirds (including penguins) and whales (Ainley et al. 2017, Bester et al. 2017). Sea ice also controls the amount of light that is available to photosynthesising organisms within the upper water column (Clark et al. 2015). Each late spring to summer, seasonal sea ice melt releases a pulse of algae, fresh water and key nutrients that have accumulated within the ice to drive intense algal blooms around the high-latitude Southern Ocean. Sea ice algae released into the water column also form a primary food source for biodiverse benthic (seabed) ecosystems in shallow nearshore Antarctic regions (Clark et al. 2017). There is low confidence in climate model projections of the trajectory of Antarctic sea ice in coming decades in a warming world (IPCC in press-a). However, there is general agreement across model projections that substantial decreases in Antarctic sea ice extent and volume will occur by 2100 in response to climate change. Estimates of predicted decreases in sea ice area range from 29% to 90% in February (depending on the emissions scenario) and from 15% to 50% in September (Roach et al. 2020). The implications of these changes for the food webs of the Southern Ocean and Antarctic coastal environments are currently not well understood (Newman et al. 2019). Ocean temperature The Southern Ocean has warmed more rapidly and to a greater depth than the global ocean average in recent decades (Böning et al. 2008, Roemmich et al. 2015, Swart et al. 2018). Increased ocean heat transport has caused Antarctic ice shelves and glaciers to thin and retreat (Jacobs et al. 2012, Stevens et al. 2020), particularly in West Antarctica (Joughin & Alley 2011, Rignot et al. 2014). Melting of grounded ice, as well as thermal expansion of the ocean due to warming, contributes to global sea level rise (IMBIE team 2018). As water temperature increases, its ability to absorb CO2 decreases. Thus, increased ocean temperatures reduce the important role of the ocean as a carbon sink, leaving more greenhouse gases in the atmosphere and driving further global warming (Menzel & Merlis 2019). Ocean acidification The world’s oceans have taken up around 25–30% of the anthropogenic (human) CO2 released to the atmosphere. Some 40% of these emissions have been taken up by cold Southern Ocean waters that lie south of 40°S (Rintoul 2018, IPCC in press-a), altering its ionic content. The uptake of CO2 varies with season and location, resulting in non-uniformity in the vertical structure of carbon storage in the Southern Ocean (Rae et al. 2018). Current atmospheric CO2 levels, at more than 400 parts per million by volume (NOAA 2021), are higher than they have been for the past 80,000 years (see Figure 4 in case study: Vulnerability of the Antarctic ice sheet to future climate change), and even the past 25 million years (Noble et al. 2020). Compared with pre-industrial times (before the 1700s), when CO2 levels were around 280 parts per million, the pH of the Southern Ocean has dropped from pH 8.2 to pH 8.1 as a direct result of CO2 uptake (ACE CRC 2010). Although the ocean is still alkaline, it is becoming more acidic in a process known as ocean acidification. Because the pH scale is logarithmic, the 0.1 change means that the acidity of the ocean has increased by 30%. Ocean acidification is likely to have profound impacts on Antarctic marine species and ecosystems if it continues to increase at current rates (Doney et al. 2009, Hancock et al. 2020). Human food security and economies may also be affected (Doney et al. 2020). Although some marine species may be resilient (Peck et al. 2018), organisms that have evolved under comparatively stable conditions are expected to be vulnerable. Ocean acidification is already affecting many ocean species, especially organisms such as oysters and corals that make hard shells and skeletons by combining calcium and carbonate from seawater (IPCC in press-a). As ocean acidification increases, fewer carbonate ions are available for calcifying organisms to build and maintain their shells, skeletons and other structures. If the pH becomes too low, shells and skeletons can begin to dissolve (Hancock et al. 2020). The physiology and energy requirements of shelled organisms, such as pteropods and molluscs, may also be negatively affected (Seibel et al. 2012). Changes in the carbon chemistry of the oceans can reduce the growth rate of the larvae of some fish species, and affect their respiration and behaviour. Larval fish and other marine organisms lack the ability to self-regulate their internal pH. Increases in environmental CO2 concentration can lead to decreases in internal pH, which can cause behavioural and developmental issues and even death (Hancock et al. 2020, Brasier et al. 2021).