Translator: Judith Matz Reviewer: Cristina Bufi-Pöcksteiner
Hi, everyone.
My name is Terry Hughes.
I'm a coral reef scientist,
and as you can see from the title of my talk today,
something of an optimist.
So, I'm going to talk to you about the Great Barrier Reef,
about reefs in general and also about climate change.
Firstly, let me emphasize to you
how important coral reefs are to people all around the world.
Here in Australia and in other wealthy countries like the United States,
we tend to value coral reefs because of their aesthetics.
They are, after all, incredibly beautiful.
We also value them for their high biodiversity
and for their conservation value.
We love to see turtles and dugongs and so on.
But in many poorer, smaller, mostly developing countries,
about 130 of them,
coral reefs are incredibly important for people's food security
and for their livelihoods,
mainly through reef fishing and reef tourism.
Here in Australia, reef tourism is a five-billion-dollar industry
and it supports the livelihoods of 70,000 people.
So reefs are important not just because they look beautiful,
but because of what they do for human societies and human economies.
Unfortunately, many coral reefs around the world
are now in quite poor condition
and many of them are declining.
You could make a very long list of the bad things people do to coral reefs
but just broadly, there are three broad stressors
that are impacting on reefs all around the world.
The first set has to do with pollution:
run-off of nutrients and sediment from land, from coastal development.
And here's an example of that from the Great Barrier Reef.
So, that top photograph was taken in 1883.
I hasten to say I didn't take that picture.
(Laughter)
And it shows a very healthy coastal coral reef.
If you look at the top of those two photographs,
you can see a mountain range.
And those are actually the same positions.
So, that reef today is very degraded.
It looks like a mudflat, it's almost unrecognizable.
And that's the case for many of the coastal fringing reefs
along the east coast of Queensland.
When I first saw that top photograph, I just about fell off my chair
because you can't see that abundance of corals
and that particular mix of species on an inshore reef, today.
Today, those reefs are much muddier because of run-off from land.
You can still see mixes of species like that today,
but only further offshore, about 20 kilometers further off-shore.
A second set of stressors on reefs relates to harvesting and overfishing.
So, most of us like to eat seafood.
And as a consequence of that,
you can make quite a long list of species that today are much less common in waters,
even highly protected waters like the Great Barrier Reef,
than they were, say, 50 or 100 years ago.
The third main threat to coral reefs, and this refers to coral reefs everywhere,
is global warming and climate change.
Even the most pristine, most remote, most highly protected coral reef
is vulnerable to global warming.
So this is an underwater video
taken on a reef about 50 kilometers from where I'm standing,
earlier this year in March.
And it shows extensive bleaching by corals.
About 70% of the corals on this reef are bleached.
So what is bleaching?
Bleaching is a stress response by a coral.
Corals, as I'm sure you know,
have tiny microscopic algae inside their tissue, called zooxanthellae.
Those algae give the coral most of its color
and they are important for the nutrition of the coral
because the algae photosynthesize.
When bleaching occurs,
the algae are depleted
and the coral itself becomes nutritionally compromised.
If the bleaching is prolonged, if the coral remains white,
then they will slowly die of starvation,
because of the absence of the algae.
So, this story that I'm telling you, today, is a personal story for me
because I was heavily involved in a project
to document this bleaching event.
It was a project that I hoped I would never have to do.
So, at the height of the bleaching event,
I decided to do aerial surveys of the entire Great Barrier Reef.
And the research question here is very fundamental, very basic:
What is the footprint of the 2016 bleaching event?
Where is it occurring throughout the Great Barrier Reef?
Where is it severe, and where is it not severe?
So, first, let me remind you of the scale of this map.
The Great Barrier Reef is enormous, it's about the size of Italy.
So, from north to south, it's about 2,300 km.
The red squiggles on this map show the flight tracks that I undertook
in a helicopter and a light plane -
and no, the pilot wasn't drunk.
The idea was to criss-cross as many reefs as possible
and to score them from the air
in terms of the intensity of the bleaching that we could see.
So, to show you what we saw, here's another short video,
this time an aerial video, taken from a helicopter.
And you can see very clearly from the air, at the height of the bleaching,
that the entire reef where severe bleaching is occurring
looks snow-white.
These corals are very, very sick.
In fact, for the northern stretch between Port Douglas up to New Guinea,
which is about 700 km of coastline,
80% of the reefs look like this at the peak of the bleaching event.
And when bleaching is this severe, it leads to extensive mortality of corals.
So, at the same time
as I was whizzing about the northern, central and southern part of the reef,
we were also doing extensive surveys underwater,
and I'll come back to that in a moment.
So this is the result of the aerial surveys that I did.
And you can see that the map of the Great Barrier Reef is color-coded.
A red region in the north where the bleaching was extreme.
An intermediate area, in the middle, which we colored orange.
And the southern third of the Great Barrier, colored green,
where no bleaching occurred this time.
The footprint of the 2016 bleaching event has a very strong north-south pattern.
That's quite different from the footprints of two earlier events in 1998 and 2002.
Each of those three footprints has been different.
If you superimpose the three on top of each other,
then the cumulative impact of all three of those bleaching events
has been such that virtually all of the Great Barrier Reef
has now suffered moderate or severe bleaching since 1998.
Each of the three distinct footprints can be explained
by where the hot water was in each of those three years.
So in 2016,
the southern Great Barrier Reef was saved by a cyclone,
by cyclone Winston.
You may recall: Cyclone Winston passed over Fiji on Feb 20th.
It came to the Australian coastline a week or so later
and it cooled down the southern half of the Great Barrier Reef by 3 °C.
Without that chance event,
which cooled down the southern half of the Great Barrier Reef,
the entire reef would have bleached severely.
So underwater, we also did a huge amount of work,
ground-truthing the accuracy of the aerial surveys.
And it turns out, they map directly onto the underwater bleaching scores
that we also undertook.
On a subset of those 148 reefs,
we also measured different responses by different species.
Importantly, bleaching is actually very selective.
Scientists refer to species that are winners
versus species that are losers.
The significance of that is that bleaching changes the mix of species,
and that's been happening, now, on the world's coral reefs
for the last 25 or 30 years.
So this is the final result I'll show you.
This is a map for 83 of the reefs we studied underwater most intensively.
And we were shocked to find in March and April, at the end of the summer,
that already 35% of these corals, up to more than 50% of these corals,
were already dead.
That flies in the face of convention of what our expectation was
for mortality of corals after bleaching.
They're supposed to die slowly of starvation,
unless the zooxanthellae in their tissues regrow quickly enough.
But this time round, as well as that slow starvation mortality,
we also saw much quicker, almost instantaneous mortality,
at the height of the marine heatwave.
These corals didn't starve to death in March - they actually cooked.
So in the period since the peak of bleaching,
the last six months,
two things have been happening with these corals.
The ones that have survived, have now mostly regained their color.
They've recovered their color.
But recovery of the lost corals is a much slower process.
So let me finish up my talk by talking about the recovery of the corals
which have died in the northern third of the Great Barrier Reef.
This time round, the severity of bleaching is off the scale.
And we've seen corals that we would normally expect to be winners
during a bleaching event
to actually have very high rates of bleaching and mortality.
So the coral on the top of that photograph
is about a hundred years old.
Normally, that kind of coral doesn't bleach.
But this time around, they did.
If you lose a hundred-year-old coral,
you cannot replace it in a few years.
It takes at least that length of time for it to be replaced.
The fastest-growing corals can recover,
in other words, replace dead corals in about 10 to 15 years.
But these longer-lived corals take much, much longer.
So the key issue is:
We've now seen three events, 1998, 2002 and 2016.
When will the fourth event be?
More than likely in the next warm El Niño period,
which might be five years from now,
or we might be lucky enough that it will be another 10 years.
But that's barely enough time
for the fastest-growing corals to partially recover.
Nevermind these very old, much slower growing corals.
So, it's a very good idea
to improve the water quality of the Great Barrier Reef,
and the bottom photograph here shows a muddy river
pushing sediment and nutrients
into the water on the inner part of the Great Barrier Reef.
So the Australian government's response to the worst bleaching we've ever seen
on the Great Barrier Reef,
in the northern third of the Great Barrier Reef,
was to improve water quality
in the southern two-thirds of the Great Barrier Reef.
Clearly, the elephant in the room is climate change,
which is the actual cause of the bleaching.
So, we have now a window of opportunity
to change the trajectory of global emissions and global warming.
If we insist on business as usual,
with growth in emissions of greenhouse gases,
then the future of coral reefs will be pretty bleak.
It's really up to us
and it's up to governments all around the world.
We've already seen extensive damage to the world's coral reefs
with just one degree of global warming.
Two degrees will not be a safe place for the world's coral reefs
and anything beyond that, the further we go beyond that,
the worse the scenario is for reefs.
So there's no time to lose.
We absolutely must deal with greenhouse gas emissions
in order to protect these fantastic treasures.
Thank you.
(Applause)