Forecast for sea off south-east Australia spells danger for marine life

Australia’s Bureau of Meteorology has forecast a patch of the Tasman Sea to reach temperatures at least 2.5°C above average from September to February.

Oceanographers say that it may be even more as their modelling has predicted temperatures rises that have been described as “literally off the scale.” It is not known if this section of the Tasman Sea has heated up this much previously, but at least one oceanographer has told the Guardian that it is the first time he’s seen such a prediction.

“There are two main mechanisms that drive ocean heat waves,” Dr Edward Doddridge, an oceanographer from the University of Tasmania, says in an interview with Cosmos.

“One is that you bring warm water from closer to the equator to somewhere closer towards the pole. That water brings heat with it. The other way is that you have clear skies and sunny days, more than normal, and sunlight warms the water leading to a heatwave. If both of those things happen together, then you can end up with really strong heatwaves.”

Two maps of australia heat temperature seas ocean
A Bureau of Meteorology map showing predicted sea surface temperatures. Credit: BoM.

Doddridge explains that warm water is expected to reach the southern shoreline of Australia around Victoria and Tasmania via the East Australian Current (EAC) which flows from the waters east of Queensland’s Great Barrier Reef southward.

“Because of climate change, the EAC is flowing further south, carrying that warm water from the Coral Sea south,” Dr Benjamin Mos from the University of Queensland writes in an email to Cosmos. “The region in the Tasman highlighted as most at risk for marine heatwaves in the near future is the same area that the EAC is now extending into.”

Hotter waters can have a massive impact on marine life, Doddridge says.

“We’ve seen a huge decrease in the giant kelp off the coast of Tasmania because of ocean warming. In the summertime, you’re pushing up towards the thermal thresholds of things that live in the ocean anyway. A heatwave on top of that – they’re in trouble.”

“We live in the atmosphere and we’re quite tolerant of fairly large temperature differences day-to-day,” he adds. “Organisms living in the water aren’t used to that because the ocean temperature is much more consistent and much more stable. They have much narrower ranges of temperature that they’re able to survive in.”

According to the Institute for Marine & Antarctic Studies, Tasmania has already lost 95% of its once widespread kelp forests. These habitats support a variety of other marine life from fish and crustaceans to fur seals.

“Unfortunately every marine heatwave is like an experiment where we learn more about how marine ecosystems are going to cope with climate change. Overall the signs don’t look good for many of our most important ecosystems (e.g. coral reefs, kelp forests, polar benthic ecosystems),” Mos notes.

“We are seeing shifts in species ranges, tropical species are moving south along the coast and taking up residence,” oceanography professor at the University of New South Wales Moninya Roughan explains to Cosmos. “Cool water species (kelp) are dying out in shallow waters (pushing deeper) or dying altogether.”

Increased sea temperatures are also related to more extreme and unpredictable weather events. This has been seen in the northern hemisphere after heatwaves have increased ocean temperatures and unleashed cyclones on parts of North America that haven’t seen tropical storms in many years.

“Warmer oceans evaporate more water,” Doddridge says. “They put more water vapor into the atmosphere, which then leads to more rain. This is seen more in the tropics than in mid latitudes, where this heatwave is predicted. There is very strong coupling between the ocean surface temperature and atmospheric circulation.”

While not directly linked to the marine heatwaves in the northern hemisphere, Mos explains that they are a warning.

“The heatwaves we have seen in the northern hemisphere and the warming predicted for the Tasman are both signs of climate change. We know that rising average global temperatures increases the likelihood of heatwaves occurring both on land and in our oceans, and also increases the risk that heatwaves will last longer and be more severe (i.e., warmer).”

“Climate change drives the uptrend (like the stock market on an uptrend),” Roughan says. “Short term events (like the atmospheric heatwave right now) drives shorter term highs (the peaks on the uptrend). Short term events can also drive ‘lows’ and dips, but we are still on an uptrend driven by climate change.”

The Tasman Sea has already seen an increase in average temperature of about 0.8°C since the 1960s.

“Even if we can’t attribute this particular event to climate change, we know that a warming world is going to lead to more intense, more frequent ocean heatwaves,” Doddridge comments. “Regardless of whether this individual event is caused by climate change – we won’t know that for certain until the science has been done many months from now – it’s an insight into what the future looks like.”

“There’s only one way to avoid that scenario, and that is to reduce the take up and burning of fossil fuels.”

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