Extract from The Guardian
Wide-scale disruption from warming oceans is
increasing, but they could change our understanding of the climate
The Pacific coast has
witnessed record numbers of dead Cassin’s auklets this winter.
Photograph: D. Derickson/COASST
Monday 15 August 2016 06.20 AEST
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 ever toxic algal bloom
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.
Coral on reefs around Lizard Island, on the Great
Barrier Reef in Australia in July 2016, following 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 hot
spots,” 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 southwest 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.
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.
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.”
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