‘They’re going to get worse and worse’: marine heat wave prevails off Sydney

In November 2021, an unusual marine heat wave materialised off the coast of Sydney; sea surface temperatures in the area have yet to go down.

The Sydney Opera House
The Sydney Opera House. There are still sea surface temperatures of about 25°C east of Sydney. Image: Hongsik park (Siki Park), Flickr

When conservation ecologist Rob Harcourt went surfing off the coast of Sydney, Australia, he immediately knew the water was warmer than usual.

“We had at least 23 [degrees Celsius, or 73° Fahrenheit] sea surface temperature,” Harcourt, a professor at Macquarie University, told Mongabay. “It’s normally about 18[°C, or 64°F] in November, December. So that’s 4 to 5 degrees higher than normal. That’s huge.”

Last month, this marine heat wave, which started in November 2021, garnered international attention. Moninya Roughan, an oceanographer at the University of New South Wales, told The Guardian that the event was “extreme,” with hot water covering an area of about 200 square kilometers (77 square miles). Other experts noted that the event could be quite impactful to a range of marine animals, including sharks that traveled further south in search of food.

While this marine heat wave has moved slightly away from the coastline, which means it will be less damaging to coastal ecosystems, there are still sea surface temperatures of about 25°C (77°F) east of Sydney.

This marine heat wave is just one of many playing out in the global seas, from the Arctic Ocean to the Southern Ocean. In the region around Australia, there are also marine heat waves simmering off the coast of the island state of Tasmania and New Zealand’s North Island. In fact, Alistair Hobday, research director and principal scientist for the Australian government’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), called the Sydney event “pretty small” in comparison to other marine heat waves happening around the world.

Marine heat waves, which can be defined as warming events in which water temperatures breach a certain threshold and last for at least five days, can be triggered by a range of atmospheric, oceanic and climatic drivers. Yet human-induced climate change has also been playing a leading role in making these heat waves more frequent and intense, Roughan told Mongabay in an email.

“Think of it like the stock market on an uptrend, along the way you have lots of highs (and lows) but those highs are getting higher because of climate change,” Roughan said.

In 1982, only 60 per cent of the ocean had experienced a marine heat wave. But by 2020, 80 per cent of the ocean had experienced these events, according to Hobday.

Marine heat waves can have a range of implications on the marine environment, from coral bleaching to the redistribution of fish to the destruction of kelp forests. One of the most notable events took place in the Pacific Northwest between 2014 and 2016, which led to toxic algal blooms and mass marine mortalities. In 2016, there was also a significant marine heat wave that took place in the Tasman Sea that led to wild mollusk mortalities and disease outbreaks at oyster farms.

Sightings of tiger sharks (Galeocerdo cuvier) and whale sharks (Rhincodon typus) off the coast of New South Wales are likely to do with the marine heat wave, says Vanessa Pirotta, a wildlife scientist at Macquarie University.

“A lot of these marine megafauna — the larger animals — their movements are primarily dictated by movements of prey and prey distributions,” Pirotta told Mongabay. “So any shifts in environmental factors likely will have a shift in their prey, which will … bring those animals down.”

As the ocean continues to heat up due to human-induced climate change, experts say marine heat waves will only continue to grow in intensity and frequency.

‘We’re just expecting marine [heat] waves to increase in the future,” Neil Holbrook, professor of ocean and climate dynamics at the University of Tasmania, told Mongabay. “We’ve shown that they’ve increased in intensity and duration in the past. And unfortunately, because of global warming, they are just going to get worse and worse.”

“These are fundamental changes and they’re not reversible,” Harcourt said. “So we have to adapt fast, and we’re not really thinking about it, particularly at the moment because we’re all caught up in the pandemic.”

This story was published with permission from Mongabay.com.

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