The physical characteristics of the ocean, such as temperature, salinity, oxygen content and pH, are clearly changing in Australia’s oceans as a result of climate change. Summaries from the Intergovernmental Panel on Climate Change and other publications show long-term trends that are clearly attributable to increased greenhouse gases. The south-east and south-west of Australia are global warming hotspots (Hobday & Pecl 2014), with rates of warming above the global average.
Climate and climate change processes are a direct and indirect pressure on Indigenous communities (Altman & Jordan 2008). Across the diverse range of communities, these pressures are being observed at different rates, with limitations for communities to use Indigenous adaptation pathways (Nursey-Bray et al. 2019).
Pressures associated with climate change and associated extremes have high to very high impact on the Australian marine environment and are generally worsening. No pressures are assessed as improving. Traditional Owners generally found climate change to be having high impacts on sea Country; however, ocean currents and eddies were seen as having very low impact, and frequency and severity of extreme weather events were seen as low impact. Note that the spatial scale of Indigenous and western science assessments may be different. Related to United Nations Sustainable Development Goal targets 13.2, 14.3
Because of anthropogenic climate change, overall higher extremes of impacts associated with climate variability are expected to increase (Evans & Hobday 2021). The Indigenous assessment regionally was high impact, with an unclear trend.
Climate change influences on sea surface temperature and salinity affect the Australian marine environment; high-impact, enduring changes to marine ecosystems have been documented in the past 5 years (Benthuysen 2021). The Indigenous assessment regionally was high impact, with a deteriorating trend.
Ocean acidification conditions have deteriorated and will continue to deteriorate; these are linked to CO2 emissions scenarios. Coral reefs and shellfish production are considered particularly susceptible to the changing conditions (Tilbrook & Lenton 2021). The Indigenous assessment regionally was high impact, with a stable trend.
Boundary current dynamics are strongly linked to large-scale major climate modes (El Niño–Southern Oscillation, Indian Ocean Dipole and Southern Annular Mode) and regional forcing, whose variability is predicted to increase with climate change (Sloyan et al. 2021). The Indigenous assessment regionally was very low impact, with a stable trend.
Climate change is expected to result in declines in nutrient supply to surface waters and potentially reduced oxygen availability in subsurface waters due to increased stratification of the upper ocean. However, observations are currently inadequate to quantify trends (Matear et al. 2021). The Indigenous assessment regionally was high impact, with an unclear trend.
Extreme weather events apply significant pressure to the Australian region, but their trends on 5-year timescales are unclear (Holbrook & Hobday 2021). The Indigenous assessment regionally was low impact, with a stable trend.
Climate and system variability
Weather, climate and the marine environment vary in space and time. This variability includes seasonal, interannual, decadal and longer changes in water temperature; rainfall patterns, which affect ocean salinity; and surface winds, oceanic currents and tidal regimes, which can influence the amount of vertical mixing through the water column (Evans & Hobday 2021). These changes also affect marine ecosystems and the food web.
Seasonal cycles in ocean physical processes have been relatively stable on evolutionary timescales, and species within the marine environment have evolved in response. Ocean primary productivity responds to seasonal cycles in ocean processes (Dunstan et al. 2018) (see Primary production (phytoplankton)). Secondary producers and higher-order marine organisms have also evolved to synchronise biological processes such as breeding or migration with these cycles (see Secondary production (zooplankton)).
Australia’s marine environment is also influenced by cycles in climate on interannual timescales associated with natural climate phenomena such as the El Niño–Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM) (see the Climate chapter) (Risbey et al. 2009). These climate cycles change the timing and magnitude of seasonal variability (Salinger et al. 2016, Holbrook et al. 2020a).
Of the climate phenomena that occur in the Southern Hemisphere, ENSO and the IOD have potentially the greatest overall influence on the Australian marine environment, habitats, communities and species groups; the influence of the SAM is mostly across the central and southern parts of Australia (Risbey et al. 2009). Variability in the phases of these climate phenomena change rainfall patterns, sea surface temperatures, surface winds and oceanic currents, which can influence the degree of vertical mixing through the water column and the relative location of cyclone events. El Niño and La Niña events also sometimes indirectly affect sea level through changes in the occurrence or intensity of storms, such as tropical cyclones in the Pacific Ocean, which can affect Australia’s east coast (Holbrook et al. 2020a).
Simultaneous fluctuations in the phases of climatic phenomena, or particularly strong phases of each, can result in extreme changes in ocean processes, resulting in events such as the marine heatwaves that have been observed around Australia (Oliver et al. 2017, Benthuysen et al. 2018, Sen Gupta et al. 2020). Bleaching and loss of mangroves have been associated with phases of the dominant climate drivers (Babcock et al. 2019, Babcock et al. 2020).
ENSO events can cause extremes in the ocean environment that have substantial impacts on marine ecosystems. In the shallow‐water coastal marine environment, ENSO‐related extremes in sea level and seawater temperature have been found to impact coral, kelp, seagrass and mangrove ecosystems (Holbrook et al. 2020a). Coastal impacts from sea level extremes include exposure of shallow‐water ecosystems and inundation in low‐lying areas. Ocean temperature extremes, including marine heatwaves, cause coral bleaching, and can impact kelp and seagrass density (Holbrook et al. 2020a).
Climate cycles also occur on longer timescales. The most important phenomenon influencing the Australian marine environment is the Pacific Decadal Oscillation (PDO; Mantua & Hare 2002). The PDO has been described as a similar pattern of climate variability to ENSO, with positive phases having similar effects on the climate to El Niño and negative phases similar effects to La Niña, but operating on timescales of 15–70 years (Salinger et al. 2016). Interactions between phases of the PDO and ENSO can either modulate or enhance the phases of ENSO (Cai & van Rensch 2012), with flow-on effects on ocean processes and the marine environment.
As impacts associated with climate change increase, the overall higher extremes of impacts associated with climate variability are also expected to increase (Oliver et al. 2019). Changes to interannual and decadal variability are less clear; however, projections identify that ENSO will likely remain the dominant mode of interannual variability in the future, and the SAM will likely weaken (Kushnir et al. 2019).
Changes to water temperature and salinity
Ocean temperature and salinity are fundamental water properties that shape Australia’s marine environment (Benthuysen 2021). Climate change trends in sea surface temperature and salinity can affect the physical marine environment and marine ecosystems.
Climate change is predicted to continue influencing global trends in sea surface temperature and salinity (Bindoff et al. 2019, Abram et al. in press), and marine heatwaves (Collins et al. 2019). For Australia, continued warming trends are predicted to have widespread and potentially irreversible impacts on the marine environment (BOM & CSIRO 2020, Australian Academy of Science 2021).
Monitoring of water temperature and salinity dating back to the 1940s–1950s has been conducted at a few coastal sites, including the Rottnest Island (Feng et al. 2015), Maria Island (Ridgway 2007) and Port Hacking (Kelly et al. 2015) stations, now maintained through the Integrated Marine Observing System. Continued long-term monitoring and integration with ocean models are essential for comprehensive evaluation of climate change influences on the current state and trends of sea surface temperature and salinity.
Ocean warming
Australia’s average sea surface temperature has warmed by more than 1 °C since 1900 (BOM & CSIRO 2020, Australian Academy of Science 2021), although trends vary around Australia (Wijffels et al. 2018, BOM & CSIRO 2020, Richardson & Pattiaratchi 2020, Yang et al. 2021). Warming rates have increased in the Tasman Sea’s East Australian Current and its extension (Richardson et al. 2020a, Malan et al. 2021), and in the south-eastern Indian Ocean (Bulgin et al. 2020), although there were cold spells off Western Australia from 2015 to 2019 (Feng et al. 2021). The South-east Marine Region is a global marine warming hotspot, having experienced warming over the past several decades at almost 4 times the global average rate (Ridgway 2007, Lough & Hobday 2011, Hobday & Pecl 2014).
Long-term ocean warming is associated with poleward range shifts as species move towards cooler waters (Johnson et al. 2011, Hyndes et al. 2016, Vergés et al. 2016, Gervais et al. 2021) (see Species redistribution). This can cause changes in marine communities, such as the expansion of sea urchins into new areas (Ling & Keane 2018) (see Changes to ocean circulation). Ocean warming can also affect the lifecycle and populations of marine species (e.g. see Sea turtles and sea snakes).
Ocean warming trends contribute to marine heatwaves (Oliver et al. 2018a, Oliver et al. 2021) (see case study: Marine heatwaves), including recent record-breaking marine heatwaves (Santoso et al. 2017).
Prolonged, extremely warm waters cause acute widespread ecosystem damage, with species mortality and community structure shifts (Hughes et al. 2018a, Stuart-Smith et al. 2021c), and threaten marine biodiversity and ecosystem services (Smale et al. 2019). Such events can have persistent impacts, with kelp forests collapsing (Wernberg 2020), coral recovery taking years (Hughes et al. 2018b) and important fishery species recovering slowly (Caputi et al. 2019).
On the Great Barrier Reef, marine heatwaves caused mass coral bleaching in 2016 (GBRMPA 2017, Benthuysen et al. 2018, Hughes et al. 2018a), 2017 (GBRMPA 2017, Hughes et al. 2019a) and 2020 (Hughes & Pratchett 2020). In 2015–16, the northern Australia marine heatwave had the longest duration on record in the south-east tropical Indian Ocean (Benthuysen et al. 2018), leading to coral bleaching off north-western Australia (Gilmour et al. 2019). The Tasman Sea experienced its longest and most intense marine heatwave off south-eastern Australia in 2015–16 (Oliver et al. 2017), followed by a more spatially extensive marine heatwave in 2017–18 (Salinger et al. 2019). During 2020, marine heatwaves occurred in most shelf areas around Australia, with many regions reaching category 2 (strong) (WMO 2021).
Near-surface warming and freshening trends increase ocean stratification (the separation of the water column into distinct layers) (Zika et al. 2018). This can limit nutrient supply into the photic zone (the upper layer that receives sunlight) and reduce primary production (Bindoff et al. 2019). Both salinity and temperature affect the density of sea water, and changes in density can affect ocean circulation. Ocean warming can cause deoxygenation because oxygen solubility is lower in warmer waters; the lack of oxygen in turn affects ecosystem functioning (Breitburg et al. 2018) (see Changes to ocean nutrients and dissolved oxygen).
Salinity
Climate change is affecting ocean salinity, with the effects varying across regions. From 2004 to 2017, data on upper ocean salinity showed freshening in the eastern Indian Ocean and increasing salinity in the western Coral Sea (Hu et al. 2019). More recent analyses show freshening off northern Australia and increasing salinity off south-eastern Australia (2010–19 compared with 1950–59; Gould & Cunningham 2021). Off Tasmania, long-term salinity trends at Maria Island show increasing salinity related to an intensifying East Australian Current Extension (Ridgway 2007, Kelly et al. 2015). Global sea surface salinity trends indicate a reinforcement of surface distributions – that is, areas of net evaporation are becoming more saline – consistent with an amplified global hydrological cycle (Durack & Wijffels 2010, Durack 2015), with feedbacks from ocean warming (Zika et al. 2018).
However, extreme salinity events through flooding are suggested to be a pressure to more Australian marine environments than climate change trends in salinity (Bergstrom et al. 2021); drought–flood cycles have become more intense in Australia with climate change (Steffen et al. 2018).
Ocean acidification
Ocean acidification describes the changes in chemistry resulting from the ocean taking up about 25% of annual carbon dioxide (CO2) emissions (Tilbrook et al. 2020, Tilbrook & Lenton 2021). As CO2 levels have increased, so has ocean acidification – that is, sea water, which is naturally alkaline, becomes more acidic and has a lower pH. The changes also lower the saturation state of aragonite, a major mineral form used by marine organisms to produce shells and skeletal material – this makes it more difficult for such species to form shells.
Ocean acidification is predicted to lead to shifts in ecosystems and biodiversity (Hennige et al. 2014, Hurd et al. 2020), and increase the risk to regional economies that rely on healthy and sustainable marine ecosystems, such as tourism and aquaculture.
Coral reefs and shellfish production are considered particularly susceptible to the changing conditions (Cooley et al. 2012, Dove et al. 2013, Fabricius et al. 2020). Studies have documented decreased growth of reef-building corals and coralline algae that are the foundation of coral reef ecosystems; shifts in species composition and distribution; altered reproductive health, organism growth and physiology; and changes in food webs (Fabry et al. 2008, Doney et al. 2012). Shellfish production in temperate waters may be affected (Cooley et al. 2012), and seasonal undersaturation of aragonite in surface waters of the Southern Ocean is predicted to occur by 2030, which will affect calcifying zooplankton (e.g. pteropods) (Bednaršek et al. 2019).
However, some species, including some noncalcifying algae, may benefit from ocean acidification (Diaz‐Pulido et al. 2011).
Because ocean acidification conditions are linked to CO2 emissions, current trends in ocean pH and aragonite saturation will continue (Lenton et al. 2015, Bindoff et al. 2019). Recovery towards pre-industrial conditions will take centuries to millennia, even with a decrease in atmospheric CO2 levels (Mathesius et al. 2015).
There is also evidence that acidification and marine anthropogenic noise pressures (see Anthropogenic marine noise) may act together, with negative consequences for marine mammal and fish populations (e.g. Hester et al. 2008, Rossi et al. 2018, Radford et al. 2021).
Rates of ocean acidification
When compared with historical rates of change, ocean acidification is occurring rapidly (Hönisch et al. 2012). Since the late 1800s, the pH of the waters around Australia is estimated to have decreased on average by about 0.09–0.13, making the water about 25% more acidic than before the Industrial Revolution. In the same period, a decrease of 0.5–0.8 is estimated in aragonite saturation state (Figure 16). The largest changes in pH have occurred in temperate waters (Figure 16a), whereas the largest changes in aragonite saturation have occurred in the subtropics (Figure 16b). Superimposed on these large-scale changes is increased variability in pH and aragonite saturation on seasonal and local scales, particularly in coastal regions (Shaw et al. 2013, Mongin et al. 2016).
Conditions on the inner–central Great Barrier Reef are approaching a tipping point, predicted to cause a decline in coral juveniles and increased macroalgal cover (Fabricius et al. 2011, Fabricius et al. 2020, Smith et al. 2020b).
Current assessments of the state and trends of ocean acidification rely on very sparse data coverage. Sustained timeseries measurements are needed to determine temporal and spatial patterns of ocean acidification, particularly in coastal and shelf waters. Monitoring efforts should be coordinated with biological impact studies, modelling, and assessments of the vulnerability of ecosystems and marine-based economies to acidification (Ekstrom et al. 2015).
Changes to ocean circulation
Australian boundary currents exert a major influence on the marine environment (Pattiaratchi & Siji 2020, Sloyan et al. 2020, Condie et al. 2021a, Sloyan et al. 2021). They include the East Australian Current (EAC), the Indonesian Throughflow (ITF), the Leeuwin Current (LC), the South Australian Current, the Holloway Current and the Zeehan Current. Because Australia’s continental shelf is relatively narrow, these currents and the eddies associated with them have an immediate impact on the circulation of water over the continental shelf. This has flow-on effects on the entire marine ecosystem, from planktonic production to pelagic fish distribution.
Of all boundary currents influencing Australian waters, the EAC, the ITF and the LC have the largest influence on the northern, eastern and western coastal environments. The EAC redistributes heat and carbon between ocean and atmosphere, and between the tropics and mid-latitudes. The ITF, a major component of global ocean circulation, moves water from the Pacific Ocean to the Indian Ocean (Sprintall et al. 2019). It strongly influences Australian climate and seas off Western Australia. The LC flows southward off Western Australia, redistributing Indian Ocean heat to the mid-latitudes.
Although currents around Australia are strongly influenced by ENSO cycles, there has been a long-term strengthening trend over the past few decades (Pattiaratchi & Siji 2020, Sloyan et al. 2020, Sloyan et al. 2021; Figure 17). Southward transport by the EAC has continued to increase (Sloyan & O’Kane 2015) and has been largely responsible for recent marine heatwaves in south-eastern Australia (Oliver et al. 2018b). The strengthening EAC has also contributed to southward migration of many marine species (Sunday et al. 2015), including sea urchins that have transformed highly productive kelp beds off south-eastern Australia into impoverished ‘barrens’ (Ling et al. 2019), with associated declines in the survival rates of juvenile southern rock lobsters (Hinojosa et al. 2015).
Along the eastern coastal boundary, the EAC jet, and the EAC separation zone and eddy field have varied strongly in 2016–21 (Figure 17). This variability has resulted in seasonal changes in the meandering and structure of the EAC jet north of the separation zone (Kerry & Roughan 2020, Sloyan et al. 2020).
After decadal strengthening of the ITF and the LC (Feng et al. 2016, Sprintall et al. 2019), both currents have weakened after the 2015–16 extreme El Niño (Feng et al. 2018, Mayer et al. 2018, Feng et al. 2021), resulting in marine cold spells off the coast of Western Australia lasting several years (Feng et al. 2021).
Key knowledge gaps limit our ability to predict the future state of the boundary currents and thus to assess impacts on the marine ecosystem. Sustained long-term monitoring of Australia’s ocean currents and further development of global and regional models are needed to understand the drivers of variability in strength of the currents, associated production of eddies and influences on marine ecosystems.
Changes to ocean nutrients and dissolved oxygen
Macronutrients (e.g. nitrate and phosphate) in the surface ocean play an essential role in controlling the ocean’s primary productivity (see Primary production (phytoplankton)). The key physical processes supplying nutrients to the upper ocean are the seasonal deepening of the surface ocean mixed layer, the upwelling and vertical mixing associated with ocean eddies, and wind-driven upwelling. All of these processes are affected by climate change (see Changes to ocean circulation).
Likewise, oxygen is essential to living aerobic organisms in the ocean. In the surface ocean, oxygen levels are high because air–sea exchange keeps oxygen near saturation levels. Oxygen levels decline in the ocean interior and are lowest in the intermediate water (300–1,000 metres). Climate change is also affecting levels of dissolved oxygen, because oxygen is less soluble in warmer waters.
Climate change is expected to lead to increased stratification of surface waters, which is expected to reduce nutrient availability in these waters. Global warming is projected to reduce oxygen and lead to the expansion of areas with low oxygen (Kwiatkowski et al. 2020). Decreases in oxygen can cause ecosystem-wide changes, including losses in biomass, food-web complexity and ecosystem services (Chu & Tunnicliffe 2015).
Insufficient observations have been made in the Australian region to assess changes in nutrients and dissolved oxygen in oceanic waters (Matear et al. 2021). The application of new oxygen sensor technology through observing platforms such as moorings and autonomous profiling floats has the potential to quantify trends in dissolved oxygen directly from measurements. Model simulations show that long-term trends should be small compared with natural seasonal and interannual variability.
Marine heatwaves can significantly impact the supply of nutrients to the upper ocean, and change phytoplankton biomass and primary production (Roberts et al. 2019, Hayashida et al. 2020), which can have both beneficial and detrimental effects on ecosystems (Chiswell & Sutton 2020). An emerging area of research is whether the magnitude and frequency of marine heatwaves will impact surface nutrients and subsurface oxygen levels, and the implications for marine ecosystems. A second issue is the impact of Australia’s rapidly growing aquaculture industry, and land-based activities that increase the supply of nutrients to the coastal and marine environments (see the Coasts chapter).
Extreme weather events
Extreme weather events, such as severe storms, extreme heat and rainfall extremes, can have profound impacts on Australia’s marine and coastal environments. Because many impacts are most apparent in coastal environments, the Coasts chapter provides a more in-depth consideration of extreme weather events as a pressure (see the Coasts and Extreme events chapters).
Introduced species, diseases, pests and algal blooms
Introduced species, diseases, outbreaks of pests and blooms of harmful algae can have substantial impacts on Australia’s marine species and habitats (Hallegraeff et al. 2021, Nowak & Hood 2021), as well as direct and indirect impacts on human health and wellbeing (DAWE 2021a). Australia could also experience new impacts from invasive species, diseases, pests and harmful algae as a result of our growing marine industries such as fisheries and transport (see case study: The blue economy).
Algal blooms and disease outbreaks are natural ecosystem processes in the marine environment that can also act as important pressures. Trends in algal blooms and diseases are considered to have been stable over the past 5 years (Hallegraeff et al. 2021, Nowak & Hood 2021). These pressures are considered in detail in the Coasts chapter, because nearshore and coastal environments are where impacts are most acute (see the Coasts chapter).
An additional emerging area that is relevant to the offshore environment will be developing strategies to minimise the risk of facilitating the establishment and spread of invasive species when decommissioning ageing oil and gas infrastructure in Australian waters (and worldwide) over the next decade (Fortune & Paterson 2018). This is a growing area of research attention and investment (e.g. NDRI 2021). Similarly, invasive species will be an important consideration in planning and designing offshore infrastructure in the developing blue economy (e.g. Blue Economy CRC 2021).