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.
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).