Tasman Sea a “hotspot” for ocean warming

By Beau Gamble 1 February 2012
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The temperature of waters east of Tasmania are rising rapidly, as balmy ocean currents shift toward the poles.

THE GLOBAL OCEAN is warming, but some places are feeling the heat more than others. The Tasman Sea, east of Australia, has been identified in a new study as one of five global ocean “hotspots”.

Temperatures here have risen here by 2ºC over the past 60 years – three times the average rate of warming in the the world’s oceans. The warming has been triggered by strengthening wind systems – a result of climate change – which have driven warm ocean currents toward the poles, beyond their known boundaries.

The rising temperatures could have stark consequences not only for marine life, but for the ocean’s capacity to take up heat and carbon from the atmosphere.

“We have more warm water coming from [just outside the tropics] – that’s the major cause of this intense warming,” says co-author Dr Wenju Cai, a marine and atmospheric scientist with the CSIRO in Melbourne. “And there’s strong support that it’s due to climate change.”

The study was published this week in the journal, Nature Climate Change. The other four global ocean hotspots occupy the western edges of the North Atlantic, South Atlantic, Indian Ocean and North Pacific

One of the hottest hotspots

Wenju and the team combined computer simulations with long-term observations, including monthly measurements that have been taken by the CSIRO since 1945, to understand how ocean circulation patterns are changing all over the world.

The hotspots have formed where fast-flowing ocean currents carry warm water westward, to arrive at the eastern edge of the major continents. In Australia’s case, the East Australian Current has shifted around 350km further south, making the Tasman Sea “one of the hottest hotspots,” Wenju told Australian Geographic.

What’s most surprising about the results, he says, is that changes in the hotspots have been highly synchronised, which points to a global cause.

” All these major western boundary currents are in fact behaving in the same way,” he says. “It really points to a global increase in greenhouse gases as a cause.”

Adapt, migrate, or die

Ecosystems in the Tasman Sea are already seeing the effects of rising temperatures. As their environment warms, marine species must adapt, migrate, or face extinction.

“We are seeing a lot of sea urchins migrating south from NSW to Tasmanian water,” Wenju says. “There, they eat out all the Tasmanian kelp.” Because kelp forests provide food and shelter to a huge variety of marine species, their destruction can have severe knock-on effects.”

Warming waters have implications for aquaculture, too. “If you want to farm salmon (the source of a $350 million industry in Tasmania), they need a very particular range of temperatures in order to grow,” he says.

Professor Matthew England, joint director of the University of New South Wales’ Climate Change Research Centre, agrees that the rising temperatures could have major repercussions for marine life.

“Warming in the Tasman Sea has been particularly rapid,” Matthew says. “And with ecosystems sensitive to temperature change, this has implications for marine life in Australian waters.”

Hindering heat absorption

In addition to the ecological effects, elevated temperatures reduce the ability of the ocean to take up heat and carbon from the atmosphere.

When warm water flows into polar areas, it creates a stable layer at the surface – because warm water is lighter. With the surface layer less intergrated into the deeper ocean, less heat and carbon can be sucked downward. And this could lead to accelerated warming.

“This is an important study,” Matthew says. “It shows how global warming can profoundly alter ocean circulation patterns, which can in turn enhance the rate of warming in certain regions.”

The next step in the research, Wenju says, is to deploy a series of moored ocean sensors across the Eastern Australian Current, to monitor changes more accurately. “Because if you want to detect changes associated with global warming,” he says, “observing these hotspots will be very effective.”