The seas are warming. Collectively the oceans have absorbed more than 80% of the energy retained by the Earth through recent climate change.
However, actual warming of the water has been very uneven, with some seas heating up much more quickly than others. Temperate rises have been most extreme where there are strong currents flowing from hot tropical regions towards the poles.
And as warmer seas move further south, tropical wildlife is going with them, giving us a dramatic insight into how global warming is changing our oceans.
The East Australian current (the famous “EAC” used by migrating turtles in the movie Finding Nemo) brings warm water from off Queensland down the New South Wales coast to Tasmania. Similar currents also exist off southwestern Australia, Japan, the eastern United States, southeastern Africa and southern Brazil.
Many marine creatures have a wandering larval stage in their life cycle. These are often microscopic creatures that are transported by waves and currents far from their parents. Some larvae can travel for months or even years before settling down in suitable habitat and metamorphosing into the more recognisable crab, shell, sea-star or fish that we see along the coast.
This life history means that marine animals can respond rapidly to changing water temperatures and currents. Like Nemo they can be swept down the coast and survive in newly warming environments.
So let’s follow Nemo and find out what is happening along the eastern coast of Australia right now.
Heading South, Permanently
The Solitary Islands are off the New South Wales Coast, just north of Coffs Harbour. They are the front line in the tropicalisation of temperate oceans. Tropical herbivorous fish are settling in increasing numbers; parrotfish and surgeonfish scrape at rocks and coral to remove and eat seaweeds.
These fish demolish existing kelp beds and eat any young plants that attempt to grow. This in turn allows coral larvae brought down by the EAC to settle and thrive. Coral reefs are on the move.
Further south, the eastern coast of Tasmania is being invaded by animals that previously were only found in New South Wales. Recreational fishers, naturalists and scientists have recorded almost 50 newly arrived species, some in abundance.
No Room To Move
So does this matter? Who cares if everything just gets moved around? Coral reefs in New South Wales may sound attractive but there are a variety of reasons why warming of marine environments is bad news.
The long spined sea urchin, which creates ‘barrens’ by grazing seaweeds, has become established in warming seas off Tasmania. John Turnbull / Flickr
One big problem happens at the poleward end of large continents. Cooler species have nowhere to go. There is no suitable habitat to migrate to.
For example, there is a whole cluster of species that only occur in southern Tasmania, like the cute spotted handfish. Rising temperatures and invading species could easily drive this and other species off Tasmania to extinction.
The critically endangered spotted handfish lives only in southern Tasmania. Rick Stuart-Smith / Reef Life Survey
Not all species have a larval stage that can travel long distances, and such species could become marooned in areas with unsuitable temperatures. Many economically important animals such as rock lobsters, abalone and scallops like cool water and will become restricted in range and abundance.
It’s Not Just The Heat
Rising temperatures are not the only calamitous result of climate change.
Sea levels are rising as a result both of melting ice and of the expansion in volume that occurs when water warms. This will not only affect the hundreds of millions of humans that live next to the coastline, as their properties are inundated and they are forced to migrate, but also the specialised animal and plants that live along the coastline.
The additional carbon dioxide in the atmosphere is also a pollutant in its own right. Its effect on ocean chemistry is to make it more acidic. But this is perilous for animals like corals and molluscs that make their skeletons from calcium carbonate.
The deep sea will be affected as well. Coral beds living a kilometre or so below sea level on seamounts off southern Tasmania will get squeezed by rising temperatures and ocean acidification.
With rising seas, animals and plants that live on this wide Victorian rock platform will be pushed upwards to occupy a thin band on the cliff behind. Museum Victoria
We do not lack scientific data on these issues. Nothing has arisen that has shaken the scientific consensus that there will be lethal problems for marine animals and plants from a changing climate.
It is now a social and engineering problem. They key thing is that we move rapidly to decarbonise the global economy.