Wide-scale disruption from warming oceans is increasing, but they could change our understanding of the climate

First seabirds started falling out of the sky, washing up on beaches from California to Canada.

Then emaciated and dehydrated sea lion pups began showing up, stranded and on the brink of death.

A surge in dead whales was reported in the same region, and that was followed by the largest toxic algal bloom in history seen along the Californian coast. Mixed among all that there were population booms of several marine species that normally aren’t seen surging in the same year.

Plague, famine, pestilence and death was sweeping the northern Pacific Ocean between 2014 and 2015.

This chaos was caused by a single massive heatwave, unlike anything ever seen before. But it was not the sort of heatwave we are used to thinking about, where the air gets thick with warmth. This occurred in the ocean, where the effects are normally hidden from view.

Nicknamed “the blob”, it was arguably the biggest marine heatwave ever seen. It may have been the worst but wide-scale disruption from marine heatwaves is increasingly being seen all around the globe, with regions such as Australia seemingly being hit with more than their fair share.

It might seem strange given their huge impact but the concept of a marine heatwave is new to science. The term was only coined in 2011. Since then a growing body of work documenting their cause and impact has developed.

Facebook Twitter Pinterest Coral on reefs around Lizard Island, on the Great Barrier Reef in Australia in July 2016, after the worst mass bleaching event in recorded history. Photograph: Justin Marshall/University of Queensland

According to Emanuele Di Lorenzo from the Georgia Institute of Technology, that emerging field of study could not only reveal a hitherto underestimated source of climate-related chaos, it could change our very understanding of the climate.



The eye of the storm

On the other side of the Pacific from “the blob”, Australia has been buffeted by a string of extreme marine heatwaves. This year at least three parts of the coast have been devastated by extreme water temperatures.

Australia, it seems, could be smack in the middle of this global chaos. According to work published in 2014, both the south-east and south-west coasts are among the world’s fastest warming ocean waters.

“They have been identified as global warming hotspots,” says Eric Oliver, an oceanographer at the University of Tasmania. “The seas there are warming fast and so we might expect there to be an increased likelihood or increased intensity of the events that happen there.

“Certainly attention is being focused on ocean changes on the south-east and south-west of Australia.”

A field born in the death of a forest

It was in the study of a marine heatwave in south-west Australia that the term was coined just five years ago. In a report that still used the term “marine heatwave” in scare quotes, scientists from the West Australian department of fisheries found the heatwave off the state’s coast was “a major temperature anomaly superimposed on the underlying long-term ocean-warming trend”.

That year, the researchers found, Western Australia had an unprecedented surge of hot water along its coast. Surface temperatures were up to 5C higher than the usual seasonal temperature. The pool of warm water stretched more than 1,500km from Ningaloo to the southern tip of the continent at Cape Leeuwin, and it extended more than 200km offshore. Unlike a terrestrial heatwave that will normally last a couple of weeks at most, this persisted for more than 10 weeks.

But five years later the full impact of that marine heatwave have are beginning to be more fully understood.

Thomas Wernberg, an ecologist from the University of Western Australia, examined the impact on the gigantic kelp forests that line the western and southern coast of Australia, publishing his results in the prestigious journal Science.

“It got so hot that the kelp forests died,” Wernberg says. For hundreds of kilometres, magnificent kelp forests that line the coast and support one of the world’s most biodiverse marine environments simply died in the heat

But it wasn’t just their death that was the problem. While heatwaves on land can kill and destroy large sections of terrestrial forests – usually by allowing fires to spread – those trees normally grow back. What was disturbing about this marine heatwave was that many of the vast underwater forests never came back. The warming climate created changes that meant the kelp didn’t recover. About 100km of kelp forests just disappeared, probably forever.

Facebook Twitter Pinterest Aerial footage of ‘unprecedented’ mangrove die-off in the Gulf of Carpentaria in Australia. The die-off is thought to be a result of low rainfall and warm temperatures. Photograph: Professor Norm Duke/James Cook University

“At the same time, there was a range extension of tropical and subtropical fish that love eating seaweed. So that basically means that, even when the temperatures came down, the kelp couldn’t recover – there was a range extension of the herbivorous fishes that were eating the kelp.”

In the place of the kelp forest, Wernberg found coral was starting to emerge. It was as if the heatwave in 2011 bulldozed the area, making way for a shift in the ecosystem that climate change was already trying to impose.

“It is probably too early to say if this will eventually lead to new coral reefs,” Wernberg says. “However, this is how I imagine the process would start.”

Wernberg estimated those kelp forests were directly responsible for sustaining rock lobster and abalone fisheries, as well as a tourist industry, together worth $10bn. If they were lost, it would be a serious problem for Australia, not to mention for the animals that rely on them.

Australia's vast kelp forests devastated by marine heatwave, study reveals Read more

Wernberg says the kelp forests in Western Australia were likely to keep contracting. “I think the next big heatwave is just going to push what we see in the north ultimately further down and then it just depends on how bad that heatwave is, whether we go all the way down to Perth or whether we just go another 10km,” he told the Guardian when the study first came out.

2016: the year of marine heat

In 2015 Wernberg established a working group of biologists, oceanographers and climate scientists in Australia to examine marine heatwaves. He saw it as an exciting new field of study.

That was timely, as less than a year later Australia would find itself virtually surrounded by pools of warm water that caused widespread and unprecedented destruction.

They were spurred on by a large El Niño, which spreads warm water across the middle of the Pacific Ocean. But El Niños had been seen before and these marine heatwaves appeared to be unprecedented.

Perhaps most dramatically, 2016 saw the Great Barrier Reef blasted by a marine heatwave that killed 22% of the coral there in one fell swoop. In the pristine northern sections, about half the coral is thought to have died.

The hotter water that bathed the reef has now subsided but the full damage is still being tallied. The immediate death of the coral is one thing but the after effects are starting to be seen, with a decline in fish numbers being reported.

And, unusually, there is continued bleaching in parts of the reef, even now as the southern hemisphere moves past the middle of winter.



Justin Marshall, of the University of Queensland, has been studying the reef ecosystem around Lizard Island in the remote northern part of the Great Barrier Reef and warns that there appears to be “complete ecosystem collapse” there.

He doesn’t have the final numbers from the surveys he is conducting but he says there are easily half as many fish there after the bleaching as there were before, and there are some species that were common before that are completely missing now.

Marshall says that could be the beginning of a “regime shift” there – where the once magnificent and resilient coral is replaced permanently by a bed of seaweed.

But as if disappearing coral reefs and kelp forests aren’t enough for one country, a marine heatwave in Australia in 2016 was also responsible for an unprecedented mangrove die-off.

On the other side of Cape York from the Great Barrier Reef, a related heatwave in the Gulf of Carpentaria spurred along what one expert studying said was the worst mangrove die-off seen anywhere in the world.

After hearing reports of the mangrove die-off, Norm Duke, an expert in mangrove ecology from James Cook University, got a helicopter and flew over 700km of coastline, to see what was happening.

He says he was shocked by what he saw. He calculated dead mangroves now covered a combined area of 7,000 hectares. That was the worst mangrove mass die-off seen anywhere in the world, he says.

“We have seen smaller instances of this kind of moisture stress before but what is so unusual now is its extent, and that it occurred across the whole southern gulf in a single month.”

Duke is assessing the precise structure of the die-off to figure out what the exact drivers were. By examining exactly which mangroves died, and measuring how far they were from the high-tide line, Duke hopes to figure out how much of the die-off is attributable to hot water and air, and how much to the dry weather. But, for now, Duke thinks all are to blame. “This is all correlated, so it’s hard to separate,” Duke says.

NASA’s image of the algal bloom. Photograph: NASA images/SeaWiFS Project, NASA/Goddard Space Flight Centre and ORBIMAGE.

Greg Browning from the Australian Bureau of Meteorology says with all these changes in the water temperature and the rainfall, big changes in ecosystems would almost be expected. “In a nutshell, there have been significantly below-average rainfall totals in the last two wet seasons ... and very warm sea surface temperatures,” he told the Guardian in July. “When you have those departures from average conditions, it’s bound to affect the ecosystem in some way.”

Just like the kelp forests and the coral reef, there is a distinct possibility some of these mangroves will be lost forever. Duke says if the disruption is severe enough, the mangrove-dominated regions can become salt pans – flat, unvegetated regions covered in salt.

And he says the most recent satellite images show the mangroves still haven’t recovered their leaves, suggesting they really are dead.

And last, but not least, Tasmania has been virtually poached this year.

Tasmania was bathed in an unprecedented pool of warm water that was 4.5C higher than average, devastating lucrative oyster farms, causing a drop in salmon catches and killing swathes of abalone.

Are marine heatwaves on the rise?

With two of the world’s global warming hot spots sitting just off the coasts of Australia, the country is likely to continue seeing these marine heatwaves bring chaos and destruction.

But the big question facing researchers is if they are increasing in frequency or severity or both, as a result of global warming.

Wernberg says it’s the apparent increase in the effects of marine heatwaves that has driven him and others to study them in more detail than ever before.

“It’s not that they’ve been understudied in the past,” he says. “It’s that they didn’t occur to the extent they are now.

“It seems like there are more and more extreme impacts attributed to them.”

Wernberg says it’s difficult to say “because you have one, then you have another one and then eventually you realise you are having more than you used to”.

Di Lorenzo, an oceanographer at Georgia Institute of Technology in the US, conducted a major study of “the blob”, which, at least by some measures, was the worst marine heatwave ever seen.

He says his study suggested it was made about 16% more likely as a result of climate change – but he warns that while he’s confident that the results show it was made significantly more likely by climate change, he’s not very confident with the precise figure. “I would feel comfortable with the sign of the effect, not necessarily with number.”

But generally, Di Lorenzo says, looking at what is happening, he thinks climate change is increasing both the frequency and severity of marine heatwaves. So much so, he wonders if climate models are wrong, and underestimating the fluctuations in temperature that will occur as the globe warms.

“The real system – if you look at the observations, and this is a paper I will publish very soon – the increase in variance is much much stronger than what models are predicting,” he says. “Maybe our models are too conservative.”

Di Lorenzo says this sort of “variance” – including things like heatwaves – will always be stronger in the ocean, because the ocean has a kind of “memory” that means events build on top of each other, multiplying their effects.

That memory is a result of temperature changing much more slowly in the ocean, as well as the ocean being able to absorb more heat in general.

Oliver, from the University of Tasmania, would not discuss the results because they were under review at a journal but data he presented at a conference, he and colleagues including Wernberg, found “more, longer, and more intense” marine heatwaves over the past century.

The results have not yet undergone peer-review but they found the same trend in many parts of the world. Since 1920, they found some regions were seeing an increase in frequency of about one extra marine heatwave every 20 years. But the plots show most of that increase happened in the past 30 years.

They also found they’re becoming hotter, increasing by almost 0.4C per decade in some regions. And they’re lasting longer – an extra 0.4 days per decade.

Putting it all together, the results globally were even more significant. Around the world, marine heatwaves were increasing by two days every decade since 1900.

Over that time, he found the frequency and duration had doubled. As a result, the number of days in which there was a marine heatwave somewhere in the world had increased four-fold.

“On average, there are 20 more [marine heatwave] days per year in the early 21st century than in the early 20th century,” the presentation concluded.

Oliver and Wernberg declined to comment on the results, since some scientific journals refuse to publish results if the authors have already discussed them with the media.

But Di Lorenzo, who wasn’t involved in Oliver’s study, said the increasing frequency of these events is well outside of what anyone predicted, and he’s excited to see how it turns out.

“I personally, as a scientist, I’m curious to see what happens. I hope to live long enough – maybe 20 or 30 years – to see what this experiment is going to turn into.”

He said the situation is very grave for humanity but exciting for scientists. He compared the situation to a surgeon being faced with a sick patient. “If he has a very complicated surgery, of course he cares for the patient, but on the other hand he is very excited about trying a new surgery and potentially solving it.”