Pressures resulting from climate change

2011

 

Australia's oceans and marine ecosystems are changing in response to changes in the global climate systems. A recent review of the Australian marine impacts of climate change found that significant changes were under way in 15 of the 17 environmental aspects considered, and that these changes could be linked to climate change factors with varying degrees of confidence.40 The review concluded that:

  • Australian ocean temperatures have warmed, with south-western and south-eastern waters warming fastest
  • the flow of the East Australian Current has strengthened, and is likely to strengthen by a further 20% by 2100
  • marine biodiversity is changing in south-east Australia in response to increasing temperatures and a stronger East Australian Current
  • observed declines of more than 10% in growth rates of massive corals on the Great Barrier Reef are likely to be due to ocean acidification and thermal stress.

The most important changes deriving from climate change that will affect marine ecosystems are gradually increasing water and air temperatures, sea level rises and acidification. Nearshore, the increased frequency of storms and associated run-off of fresh water, nutrients and suspended sediments will also be very important.

3.1.1 Temperature

Sea surface temperatures (SSTs) around Australia have significantly increased since the early 20th century (by 0.7 °C, comparing 1910–29 with 1989–2008). This rate of warming is similar to that for global average land and sea temperatures. All global and regional temperatures have accelerated their rate of warming since the middle of the 20th century (Figure 6.12)—for Australian SSTs, the rate of warming was 0.08 °C per decade from 1910 to 2008, and 0.11 °C per decade from 1950 to 2008. The warmest year for Australian average SSTs was 1998, and 6 of the 10 warmest years for SST have occurred in the last 10 years (based on data since 1910).41 The rate of warming of the ocean, although interrupted by volcanic eruptions and hence variable, has been steady since 1950, and is observable at all depths in the ocean.42 Although there are seasonal and spatial variations in the magnitude of SST increase around Australia, the greatest rates of warming have been observed off the south-west and south-east coasts.41

By the 2030s, SSTs are projected to be around 1 °C higher (relative to 1980–99) around Australia, with slightly less warming to the south of the continent. By the 2070s, SSTs are projected to be 1.5–3.0 °C higher, with slightly less warming to the south of the continent and the greatest warming to the east and north-east of Tasmania.41

This changing ocean temperature directly affects the distribution and abundance of many species and habitats, including seagrasses, macroalgae, phytoplankton, coral reefs, tropical and temperate fish, pelagic fish, marine reptiles and seabirds. The general trend is that species habitats and distributions are forced southward, consistent with the prevailing temperature regime. In the future, we are likely to see further declines in nearshore seagrass meadows and algal beds due to storms, turbidity and warmer water, and a loss of diversity in coral-dependent fish and other coral-dependent organisms.

For species that require shallow and cool coastal waters, such as for breeding or nursery grounds, this southward shift in temperatures will eventually result in major population reductions as the availability of habitat decreases and finally disappears south of the mainland and Tasmania. Temperature alone is likely to create the greatest set of ecological changes in shallow-water marine ecosystems in the coming decades.41 Increasing ocean temperatures play an important role in coral bleaching, and probably pose the most severe threat to Australia's coral reef systems (see Box 6.2).

Ocean acidification

The natural physical and biological processes of the ocean’s carbon cycle absorb carbon dioxide gas from the atmosphere. Human-derived carbon dioxide emissions have increased, mainly as a result of fossil-fuel combustion, land-use practices and concrete production during and since the industrial revolution. The end result is more carbon dioxide dissolved in the world's oceans.

The ocean is a weakly alkaline solution (with a pH of around 8.1), but the extra carbon dioxide changes the carbon chemistry of the surface waters of the ocean. The carbon dioxide forms a weak acid (carbonic acid) in water, making the ocean more acidic (lowering the ocean's pH). This process is referred to as ‘ocean acidification’.

The process of ocean acidification is already under way and has lowered the pH of the global oceans by about 0.1 pH units from their pre-industrial state. The concentration of atmospheric carbon dioxide is now higher than at any time in at least the past 650 000 years, and probably the past 20 million years. By the end of this century, the ocean’s pH is likely to drop to 0.2–0.3 units below pre-industrial levels.44

Carbon dioxide–driven acidification shifts the proportion of dissolved carbon dioxide away from carbonate ions and towards bicarbonate ions. Organisms that make their shells from calcium carbonate need carbonate ions for the biological calcification processes that create their shell. Ocean acidification poses a risk to marine food chains, potentially affecting fisheries and highly valued species by also affecting the primary production systems in the ocean. Observational data have now begun to detect changes in calcification in Southern Ocean zooplankton and Great Barrier Reef corals, indicating that acidification has already started to have detectable impacts on biological processes in our oceans.44

Box 6.2 Coral bleaching in Australia's waters

The world’s tropical coral reefs are increasingly threatened by climate change and ocean acidification. Ocean warming leads to increased risk of mass coral bleaching events, coral disease outbreaks and the formation of stronger storms. The ‘bleaching’ of corals occurs when the coral host expels its zooxanthellae (marine algae living in symbiosis with the coral) in response to increased water temperatures. This often results in the death of coral organisms, and the subsequent overgrowth of skeletal structures with algae, or erosion of the skeletal remains.

Ocean acidification reduces the availability of the carbonate ion that is needed to build aragonite (the chemical building block of corals), reducing the capacity of marine calcifying organisms, including corals, to build calcium carbonate skeletons and maintain reef structures.

Australia has some of the world’s most spectacular coral reefs: the Great Barrier Reef in the east and Ningaloo Reef in the west (added to the list of Australia’s World Heritage properties in 2011). Australia also has significant coral reefs at high latitudes, including Lord Howe Island in the south-east and the Houtman Abrolhos Islands in the south-west. Individually and collectively, these reef systems are an important part of Australia’s and the world’s natural heritage and add significant revenue to the national economy—the Great Barrier Reef alone contributes more than $5 billion per year.

Severe coral bleaching on Australian reefs has, in the past two decades, been confined mainly to the Great Barrier Reef and other reefs at low latitudes (e.g. Scott Reef in the north-west); however, the first extensive bleaching events have now also been recorded around Ningaloo Reef, and the high-latitude reefs of Lord Howe, Houtman Abrolhos and Rottnest islands.

The scientific evidence supporting a causal relationship between concentrations of greenhouse gases (mainly carbon dioxide) in the atmosphere and declining health of the world’s coral reef ecosystems is growing stronger. Since 1998, when more than 16% of the world’s coral reefs were devastated by coral bleaching, several extensive bleaching events of varying severity have occurred on Australia’s coral reefs. An expanding body of experimental research indicates that interactions of thermal stress with other stressors, such as ocean acidification and declining water quality, are likely to increase the risk to reef ecosystems. For example, the risk of mortality from thermal bleaching is higher under more acidic conditions, and potentially under conditions of high nutrient concentrations. Further, the increased fragility of coral skeletons and accelerated rates of reef erosion under more acidic ocean conditions will increase the susceptibility of reefs to storm damage. The decreased calcification rate of corals in a low-pH ocean will also reduce the speed at which corals and coral reefs can recover from events such as tropical cyclones and mass bleaching, further reducing the resilience of the ecosystem.

The recent history of extensive bleaching episodes, in conjunction with projections for ocean acidification, raises important questions about whether Australia’s high-latitude and low-latitude reefs could become refuges or high-risk sites in the world’s changing oceans.

Information provided by Ken Anthony, Research Team Leader—Climate Change and Ocean Acidification, Australian Institute of Marine Science (AIMS); Peter Harrison, Director, Coral Reef Research Centre, Southern Cross University; Janice Lough, Senior Principal Research Scientist, AIMS; Richard Brinkman, Lead Physical Oceanographer, AIMS; Jamie Oliver, Science Leader, Western Australia, AIMS; and David Wachenfeld, Chief Scientist, Director—Science Co-ordination, Great Barrier Reef Marine Park Authority; July 2011.

Ward T (2011). Marine environment: Pressures resulting from climate change. In: Australia state of the environment 2011, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/science/soe/2011-report/6-marine/3-pressures/3-1-climate-change, DOI 10.4226/94/58b657ea7c296