Sydney’s Waters Could Be Tropical in Decades

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Recent Research Points to a Widespread “Tropicalisation” of Temperate Coastlines such as Sydney within the Next Few Decades

Climate models suggest that ocean temperatures off Sydney are just decades away from becoming “tropical”. A “business as usual” scenario of increasing CO2 emissions suggests winter sea surface temperatures will consistently exceed 18C between 2020 and 2030. And summer sea surface temperatures will consistently exceed 25C between 2040 and 2060.

Eastern Australian waters represent a climate change hotspot, with warming rates occurring twice as fast as the global average. A key reason for this is a strengthening of the East Australian current, which pushes warm tropical water southwards.

Other oceanic hotspots around the world include southern Japan, south-east US, south-east Africa and eastern South America. All these regions have in common the influence of strong ocean currents running close to the shore bringing warm tropical water.

Heading south

So what will warmer ocean temperatures do to Sydney? Examining what’s happening within these other hotspots can help us to make some predictions.

In our study, recently published in Proceedings of the Royal Society B, we show how tropical fishes are becoming increasingly abundant in many warming hotspots. Indeed, tropical fishes are already a common feature in Sydney during the late summer months, and some have even started to survive over winter.

And fish are not the only organisms moving. Tropical corals have also started to make their way down the NSW coastline, and researchers are now on the look out for them in the waters off Sydney.

Ocean deforestation

Nobody would complain if the only consequence of climate change was more species of marine life — greater biodiversity. However with these gains also come substantial losses. And the biggest losers are underwater algal forests.

Canopy algae, or seaweeds, are the linchpin of temperate coastlines. They provide food and shelter to hundreds of species, and fulfil a role similar to trees in terrestrial forests. However algal forests need relatively cool water to survive, and this is becoming increasingly elusive in these regions.

A dramatic example of warmer water on algal forests occurred during the extreme ocean heat wave off Western Australia associated with the massive La Niña in early 2011. Extreme ocean warming lasted only a few weeks, caused an unprecedented decline of algal forests and the temperate marine life communities they support.

So why is warm water damaging to algal forests? There are a few reasons. High temperatures can directly stress algae by damaging the machinery that supports their survival. Warmer tropical water also carries fewer nutrients, which the algae need to grow.

Source: The Guardian

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