Will all the reefs die?
The short answer is no. However, reefs will change from how we know them today. When considering reefs, the two major types of reef in Australia are tropical coral reefs and temperate rocky reefs, home to kelp forests. Both these reef systems are sensitive to ocean warming and marine heatwaves and both have experienced extreme impacts in the past decade. For example, almost 100,000 Hectares, equivalent to 2.3 million basketball courts of kelp forests were lost in Western Australia in 2011 following an extreme marine heatwave. In the tropics, marine heatwaves in 2016 and 2017 resulted in severe coral bleaching and death on the Great Barrier Reef.
As oceans warm, these extreme events will become more frequent and species that are already living close to their upper thermal limits are likely to suffer. In Australia, species on both coral reefs and kelp forests have evolved for millions of years under remarkably stable climate conditions. Unlike on land where temperatures can fluctuate by over 30ºC in a year, in the oceans around Australia, temperatures may only fluctuate by 5-10ºC. As a consequence, many species have limited scope to deal with changes in temperature. On top of this, south-eastern and south-western Australia are warming 2-4 times faster than average, making these regions particularly vulnerable to change.
In response to changing ocean conditions, species can either move to cooler habitats, adapt, or die. Many species are unable to move or adapt fast enough to escape the warming, or the impacts of a sudden marine heat wave. For the species that can move elsewhere, they may be able to survive in their new habitat, but in doing so may be lost from the reefs they have departed. As a result, there will be a change in the types of species that we see on reefs, with increasing numbers of warm water species in places like Tasmania, and a loss of the traditional cool-water species. Giant kelp is one example of this, which has decreased by 95% on Tasmanian reefs in recent decades. Even in the tropics, many heat tolerant species will survive, but it might not be all the colourful corals and fishes that we think of today.
