Beneath the waves, a disaster in the making

Marine heatwaves are devastating our coastal ecosystems like nothing we’ve ever seen.

Coral damaged by marine warming. PHOTO The Ocean Agency / XL Catlin Seaview Survey, via ABC

Coral damaged by marine warming. PHOTO The Ocean Agency / XL Catlin Seaview Survey, via ABC

“The great mother of life” was how Rachel Carson, author of the 1962 environmental classic Silent Spring, described the sea. Today the great mother of life is ailing, with a high fever.

Silent Spring was about bird-killing pesticides on land, but Carson’s main scientific focus was actually the sea. If she were still alive and working now she would be writing a story on a far bigger scale, about how human excess has blighted marine environments from the coast to the deep ocean.

As Carson and many others have pointed out, what happens under the sea’s surface is a mystery to land-dwelling humans. But more sophisticated surveillance tools and more focused and finely-tuned scientific observations are giving us unprecedented insights into its role in the planetary ecosystem.

For the past 20 years or more we’ve been getting some unsettling signals about warming ocean waters and how a rising level of carbon dioxide in the air affects the ocean’s acid-alkaline balance.

Now those signals are so strong as to be undeniable. Marine scientists are uncovering something approaching a horror story, with outcomes that will make no-one happy.

Nowhere on the planet is the damage to our marine environment so painfully clear than off Australia’s shores. From the Pilbara coast to the Coral Sea and south to Tasmania, ocean heatwaves have laid waste to species and habitats, leaving behind dramatically altered assemblages.

One of the world’s most persistent ocean-warming “hot spots” is off south-eastern Australia and Tasmania. Over the past 100 years or so it has been warming at a rate four times the global average.

Redmap, a national citizen science website for unusual sightings of marine species operated by the Institute for Marine and Antarctic Studies in Hobart, is now regularly recording marine creatures previously found only rarely in Tasmanian waters, including tropical species like the snipefish.

Getting a new species on the end of your line can be pleasing, but there’s no pleasure to be got from what some of them are doing. For instance, warm water and an invading sea urchin are together steadily destroying Tasmania’s highly-productive giant kelp forests.

A similar scenario has been playing out off Western Australia, where a marine heatwave event in 2010 wiped out nearly half the kelp forests over more than 100 km of rocky reefs. Recovery has been very patchy, with most of the cold-water kelp being displaced by warm-water seaweed.

In the summer of 2015-16 tropical Australian seas from the North-West Shelf to the Coral Sea warmed by over 2C for nearly four months – the most intense marine heatwave in the region in nearly 40 years of records.

The devastating impact of that event and a weaker warming the following summer is still being felt. The booming tourist trade across tropical Australia, but especially in north-east Queensland, is suddenly feeling threatened by unprecedented coral bleaching caused by warm waters.

A study published in the science journal Nature in April found that the event had permanently transformed the ecology of the reef. Most tellingly, it was severe enough to “cook” some northern Reef corals, which will never recover.

Another bleaching event in 2016-17 saw yet more severe bleaching in the Cairns-Townsville region and as far south as the waters off Mackay. That spells more trouble ahead for the tourism trade.

While that poses a big problem for the Queensland and Australian economies, from an ecological perspective that’s the least of our worries. A Nature Communications paper published last month shows that marine heatwaves are becoming both more frequent and more extreme.

The study, led by Eric Oliver of Canada’s Dalhousie University, looked at ocean surface temperature records dating back to 1925. It found that since then, the average number of marine heatwave days in a year has increased by 54 per cent, and that the trend has accelerated since 1982.

That finding, said Oliver, means that a marine ecosystem which 90 years ago would average 30 days of extreme heat each year is now experiencing around 45 heatwave days per year.

Senior Tasmanian marine biologist Alistair Hobday points out that such exposure to heatwave conditions causes extensive damage to ecosystems, with impacts on both biodiversity and economic activities including fisheries and aquaculture.

It’s the speed of change that has Hobday and his colleagues worried. He is surprised and dismayed that changes which just 10 years ago scientists thought we would take the best part of a century to reach are already being observed.

The Turnbull government’s fix for the Great Barrier Reef, addressing agricultural run-off and developing resilient corals, is a side issue, and tour operators on the Reef and elsewhere should understand that.

We have a full-blown crisis on our hands which can only be stopped at its source, which means an all-out, sustained effort to eliminate fossil-fuel emissions, here and everywhere.

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