Climate change

2016

Anthropogenic ocean warming and ocean acidification, superimposed on natural climate variations—in particular, ENSO and decadal variability (Holbrook et al. 2012)—pose key risks to all of Australia’s marine environment, from shallow-water habitats to deep-ocean communities (Thresher et al 2015). These include Australia’s coral reef ecosystems (e.g. Gattuso et al. 2014) and giant kelp communities (e.g. Johnson et al. 2011). Changes in the marine environment associated with climate change provide a progressively changing baseline on top of which natural climate variations and their extremes are occurring. This poses challenges for the ability of marine organisms to adapt to changes that are occurring on much shorter timescales than those that have occurred in the past across evolutionary timescales.

In response to changes in the marine environment associated with climate change, significant shifts have occurred in the ranges of various invertebrates and fish (Last et al. 2011a, Bates et al. 2014, Sunday et al. 2015). On the Great Barrier Reef, rising summer ocean temperatures increase the risk of mass coral bleaching (GBRMPA 2014a).

Ocean temperature

Nationally, as the oceans absorb heat from the atmosphere, sea surface temperatures are continuing to increase. Waters in the South-east and South-west marine regions are increasing in temperature the most, at more than 0.4 ˚C per decade (Figure MAR5). This compares with an average global trend of 0.12 ˚C per decade across 1979–2012 (Hartmann D et al. 2013). The surface ocean has warmed across the 21st century at approximately 7 times the rate observed during the 20th century (Sen Gupta et al. 2015), and the frequency of extreme sea surface temperature events in association with ENSO and the IOD has increased (Figure MAR6). Regional and seasonal variations exist in the sea surface temperature trends, with winter months showing statistically significant cooling close to the north and north-west Australian coastline (Figure MAR5). Warming of tropical waters on the continental shelf between 10.5°S and 29.5°S has already resulted in the southwards shift of climate zones by 200 kilometres along the east coast of Australia, and by approximately 100 kilometres along the west coast (Lough 2008). Climate change impacts on ocean temperatures are therefore assessed as having a high impact with a deteriorating trend.

There is already evidence that some species, including temperate fish fauna (Last et al. 2011a), have extended their distribution towards the poles as waters have warmed (Poloczanska et al. 2013). The introduction of new species into regions because of expansion of, or shifts in, their distribution has the potential to alter marine communities. This is already happening in some regions, such as the South-east Marine Region (e.g. Johnson et al. 2011). It is likely that more marine communities will undergo major changes to their structure (Hughes et al. 2003). Conversely, it is likely that other species will reduce their ranges when the edge of their range becomes thermally unsuitable and the timescales at which changes are occurring exceed their ability to adapt (e.g. Smale & Wernberg 2013). Already climate extreme events, when particularly strong phases of natural climate variability such as that associated with ENSO are superimposed on rising temperature baselines, are having widespread impacts on coral reefs, kelp communities, species distributions and species life history dynamics (Pearce et al. 2011, Pearce & Feng 2013, Wernberg et al. 2012, 2016). (See Interannual and subdecadal variability and Quality of habitats and communities.)

Altered temperatures may decouple population processes of functional groups that are currently tightly linked. For example, the breeding processes of many marine species are timed to coincide with peaks in prey species populations, whose timing is often driven by temperature. If the timing of the 2 processes is altered so that they no longer match, this will likely affect larval survival and population recruitment (e.g. Philippart et al. 2003).

Ocean acidification

The uptake of atmospheric carbon dioxide (CO2) by the ocean changes the chemistry of sea water. As CO2 dissolves in sea water, it reacts, lowering the pH of the water and decreasing the amount of dissolved carbonate ions in the water. This process is known as ocean acidification. Since pre-industrial times, the pH of waters around Australia is estimated to have decreased by between 0.08 and 0.10, consistent with global estimates of pH change (Figure MAR7). Superimposed on the large-scale change is variability at seasonal and local scales associated with natural processes, which can be large enough to amplify or offset ocean acidification in a range of environments (Shaw et al. 2013, Waldbusser et al. 2015, Mongin et al. 2016).

Coral reefs and shellfish production are particularly susceptible to decreases in the amount of dissolved carbonate ions in the ocean (Cooley et al. 2012, Dove et al. 2013). Although there is some evidence that particular species, including some noncalcifying algae, may benefit from ocean acidification (Fabricius et al. 2011), many will not. Already in parts of the north Pacific Ocean, where seasonal upwelling of corrosive water occurs, adaptation and mitigation actions have been implemented to minimise impacts on shellfish aquaculture industries (Cooley et al. 2016).

The resilience or adaptability of marine species to ocean acidification is variable (e.g. Browman 2016). Numerous field and experimental studies conducted under conditions projected to occur under high CO2 emissions scenarios have documented (Munday et al. 2010, Fabricius et al. 2011, Doney et al. 2012):

  • decreased growth of reef-building corals and coralline algae, which are the foundation of coral reef ecosystems
  • shifts in species composition and distribution
  • changes in the neurological functioning of fish
  • altered reproductive health, growth and physiology of organisms
  • changes in food-web structure

The pH of, and concentration of dissolved carbonate ions in, ocean waters around Australia will continue to decrease as the ocean takes up more atmospheric CO2. The change in anthropogenic greenhouse gas emissions (either reduction, no change or increase) will determine the rate at which the ocean pH and dissolved carbonate ion concentration continue to decrease (Lenton et al. 2016). However, ocean acidification will persist even if emissions are reduced. Ocean acidification is anticipated to lead to changes in ecosystems, and is thus likely to affect regional economies that rely on healthy and sustainable marine ecosystems, such as tourism, aquaculture and fisheries. Climate change impacts associated with ocean acidification are therefore assessed as having an impact with a deteriorating trend.

Ocean currents and eddies

Australia’s marine environment is influenced by 3 major currents:

  • the EAC, a western boundary current system that flows southwards along the east coast of Australia, redistributing heat between the ocean and the atmosphere, and between the tropics and the mid-latitudes
  • the Indonesian Throughflow, a major component of the global ocean circulation that moves water between the Pacific and Indian oceans
  • the Leeuwin Current, an eastern boundary current that flows southwards off Western Australia, redistributing Indian Ocean heat to the mid-latitudes; this differs from the cooler, equatorwards-flowing currents found along other eastern ocean boundaries.

Australia’s ocean boundary currents are important for redistributing heat, fresh water and nutrients along the coastal boundary. Major drivers of variability in these currents are ENSO, the IOD and the SAM, which influence the mass, temperature and salinity transport of the currents, and circulation on the continental shelf (Ridgway 2007, Holbrook et al. 2012, Doi et al. 2013). This, in turn, influences open ocean–coastal exchange (including nutrient supply and larval dispersal), and the variability of wind-driven coastal currents and upwelling.

Under climate change projections, the polewards eddy transport of the EAC extension is expected to increase (Cetina-Heredia et al. 2015), whereas core transport in the EAC, and transport in the Indonesian Throughflow and the Leeuwin Current will decrease (Sun et al. 2012). This will affect the exchange of water between the open ocean and inshore regions. It will also influence nutrient supply and larval dispersal in inshore regions, affecting species that have pelagic larval phases (e.g. some lobsters) and rely on cross-shelf transport.

An increase in the polewards eddy transport of the EAC extension has already been observed from 1980 to 2010 (Cetina-Heredia et al. 2014). This is the result of the separation zone (where the EAC forms the Tasman Front and the EAC extension) occurring at its most southerly extent more often, rather than an increase in the strength of the EAC. This has been linked to decadal variability of the Pacific Ocean subtropical gyre, resulting in changes in the partitioning of the EAC between the Tasman Front and the EAC extension (Cetina-Heredia et al. 2014).

Climate change, coupled with phases of ENSO, has produced anomalously strong changes in both the Leeuwin Current and the EAC, resulting in the marine heatwave in the south-east Indian Ocean during 2010–11 and the marine heatwave off eastern Tasmania during the summer of 2015–16 (Pearce & Feng 2013; see also Climate and system variability and Quality of habitats and communities). Climate change impacts on ocean currents and eddies are therefore assessed as having a high impact with a deteriorating trend.

Nutrient supply

Concentrations of macronutrients (e.g. nitrate and phosphate) in the surface ocean play an important role in controlling the ocean’s primary productivity (the rate at which new organic matter is developed at the base of the food web). Surface ocean waters around Australia typically have low macronutrient concentrations. The supply of nutrients into the upper ocean is facilitated primarily by seasonal movement of the mixed layer and eddy-driven mixing (Falkowski et al. 1998, Doney 2006). Wind-induced upwelling is confined to a few localised regions (e.g. along the Bonney Coast in the South-east Marine Region). Although land-based sources of nutrients can be significant, they are largely seasonal as a result of climatic variability in rainfall and confined to localised, nearshore regions (e.g. the inner lagoon of the Great Barrier Reef; Revelante et al. 1982).

As the ocean warms around Australia, it is expected that the upper ocean will become more stratified, which could result in a decline in the vertical supply of nutrients to the surface, reducing primary productivity (Bopp et al. 2013, Lenton et al. 2015; see also Box MAR1). This will have flow-on impacts on marine productivity and fisheries, and, in turn, on higher-order marine animals such as turtles, sharks and seabirds (Brown et al. 2010). In inshore areas, changes in precipitation associated with climate change will influence the frequency and intensity of flooding events, which will have flow-on impacts on sediment and nutrient flows into estuarine and coastal regions (see the Coasts report for further details). Increased eddy activity because of the strengthening of EAC eddy transport (see Ocean currents and eddies) may compensate for a decline in the vertical nutrient supply in the Tasman Sea (Matear et al. 2013, 2015).

At present, observations of nutrients in the shelf and oceanic waters around Australia are only sufficient to document the mean state of the ocean, and insufficient data are available to quantify recent trends. It is therefore unclear whether nutrient supply is changing in Australian waters.

Dissolved oxygen

Oxygen is consumed in aerobic respiration, and most marine ecosystems comprise aerobic organisms that need oxygen to survive. The oxygen content of the ocean varies spatially and temporally, reflecting areas of varying oxygen production and consumption.

Because of the distribution of the highest abundances of aerobic organisms, and therefore the highest rate of oxygen consumption, the dissolved oxygen concentrations in the ocean are lowest in the intermediate water (300–1000 metres; Riser & Johnson 2008). In some inshore regions with limited circulation, and in several subsurface oceanic zones, biological consumption of oxygen can lower oxygen concentrations considerably further; they can reach ultralow values that can be up to 50 times lower (e.g. less than 20 micromoles per litre [μmol/L] and reaching 1 μmol/L at their core) than the oxygen minimum found in intermediate water (Paulmier & Ruiz-Pino 2009). These low oxygen concentrations can lead to ecosystem-wide changes, including loss of biomass of species and food-web complexity, and potentially diminished ecosystem services (Chu & Tunnicliffe 2015).

At the scale of ocean basins, deoxygenation has been observed during the past 50 years and is projected to continue to occur because of warming waters from climate change (Joos et al. 2003, Helm et al. 2011, Andrews et al. 2013). This will result in an overall reduction in dissolved oxygen and an expansion of areas with low oxygen, known as oxygen minimum zones.

Observations of dissolved oxygen in shelf and offshore regions identify Australian waters as being generally well oxygenated with little spatial and temporal variability (Figure MAR8). However, current observations are not sufficient to determine decadal trends on regional scales (CSIRO 2014). Comprehensive measurements from inshore regions outside estuarine and embayment habitats are largely lacking. The application of new oxygen sensor technology through observing platforms, such as the Integrated Marine Observing System (IMOS) National Reference Stations (Lynch et al. 2014) and autonomous profiling floats deployed as part of IMOS, has the potential to enable monitoring of trends in dissolved oxygen.

Box MAR1 Plankton and climate change

Plankton is the foundation of the marine food web and, ultimately, supports nearly all life in our oceans, including the seafood we eat. Plankton species are sensitive indicators of ecosystem health and climate change because they are abundant, short lived, not harvested, and sensitive to changes in temperature, acidity and nutrients (Richardson 2008). Different pressures affect plankton on different scales. Eutrophication (the enrichment of water with dissolved nutrients that stimulate the growth of aquatic plant life) is a major pressure at the local scale, introduced species and fisheries are pressures at the regional scale, and climate change is a pressure at the continental scale (Edwards et al. 2001, 2010).

There is growing evidence that plankton communities in Australian waters are changing in response to climate change.

Climate change—water temperature

Many plankton species are showing poleward shifts in distribution, with phytoplankton communities off the east coast of Australia observed to have moved 300 kilometres in 60 years (Coughlan 2013). Of note, the algae species Noctiluca scintillans, which forms harmful algal blooms, has expanded its distribution in south-eastern Australian waters from 1860 to 2015, blooming for the first time in the Southern Ocean in 2010 (Figure MAR9). This range expansion appears to have been facilitated by ocean warming and an increased frequency in the southerly extension of eddies associated with the East Australian Current (McLeod et al. 2012).

These changes to the distribution of species are also changing the composition of zooplankton communities. Recent analysis of zooplankton data from Port Hacking (off Sydney) has shown that the community temperature index (the temperature preference of species in the community) is 0.7 °C higher than in the 1930s, reflecting a higher abundance of warmer-water species (Clement 2015). Further south off Maria Island (eastern Tasmania), water temperatures have increased by 1.5 °C since 1944 (Ridgway 2007). This has been associated with a marked decline in copepod (tiny marine crustacean) species with preferences for colder water and an increase in those that prefer warmer water (Johnson et al. 2011, Richardson et al. 2015; Figure MAR9). Warm-water zooplankton communities generally comprise species of smaller sizes and consequently lower biomass.

Climate change—ocean acidification

Many plankton organisms—including coccolithophores, foraminifera, mollusc larvae, pteropods (sea butterflies) and echinoderm larvae—have calcium carbonate shells. Oceans with a lower amount of dissolved carbonate ions in the water (because of ocean acidification) can alter the shell-formation processes in calcifying plankton (Orr et al. 2005). Analysis of the abundance timeseries of calcareous organisms at the Integrated Marine Observing System National Reference Stations has shown no overall decline in abundance (Richardson et al. 2015; Figure MAR10). There is some evidence in northern Australia that the shells of 2 pteropods, Creseis acicula and Diacavolinia longirostris, have thinned and become increasingly porous in the past 50 years, potentially reflecting a reduced capacity of these organisms to produce their shells (Roger et al. 2011).

Outlook

As ocean waters warm further, it is expected that greater numbers of tropical plankton species will expand into temperate waters around Australia. Because of their smaller sizes and lower biomass, this will result in reduced food abundance for higher trophic (food-chain) levels (Beaugrand et al. 2003).

The harmful algal bloom species Noctiluca scintillans has been implicated in the decline of fisheries in the Indian Ocean (Thangaraja et al. 2007) and has negatively affected production of caged fish (Smayda 1997). Further range expansion of this species could have similar negative impacts on aquaculture and fisheries in Australian waters.

In future decades, ocean acidification is expected to cause a thinning of the shells of some calcifying species and alter the abundance of some plankton species, thereby influencing food webs, and impairing the larval development of commercially important species, including many shelled molluscs (Orr et al. 2005, Martin et al. 2008, Ross et al. 2011, Waldbusser et al. 2015).

Evans K, Bax NJ, Smith DC (2016). Marine environment: Climate change. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/marine-environment/topic/2016/climate-change, DOI 10.4226/94/58b657ea7c296