Great Barrier Reef.

Two years later, Outside magazine published this ― it’s...

another obituary for the reef.

And more recently, we got this news:

BBC News: And also this hour, the Great Barrier Reef is at a terminal stage.

Have we really killed the Great Barrier Reef?

The answer is no… but we sure are trying.

It’d be hard to call the time of death for

the Great Barrier Reef because it’s actually some 3,000 reefs

spread over an area the size of Italy.

So there’s plenty of room right now for there to be widespread damage and lots of

relatively healthy reefs ― that’s something dive operators there really want you to know.

“The contrast in colors down there and the water ― when the light hits the different

colors down there and yeah it’s absolutely amazing.”

But the world’s coral reefs, including the Great Barrier Reef, have had a really hard

time the past few years.

After decades of degradation from local threats like pollution, and overfishing ―

coral reefs have now also undergone a record-breaking “global bleaching event.”

That’s when coral turns white, which puts them at a high risk of dying.

It started in 2014, during the Northern Hemisphere summer.

Abnormally warm water caused corals in Guam and the Northern Mariana Islands to start

bleaching en masse.

By the end of 2014, corals around Hawaii, Florida and the Marshall Islands

were bleaching too.

When summer came around in the Southern Hemisphere, bleaching spreads to coral reefs

in the South Pacific and the Indian Ocean.

And towards the end of 2015, corals throughout the Caribbean were bleached too.

Hawaii’s corals bleached for a second time.

By now El Nino was in full force, and when combined with global warming, it kept sea

temperatures high enough to continue the bleaching event into 2016, hitting corals in Asia,

the east coast of Africa, and ... the Great Barrier Reef too.

― the worst bleaching, in fact, that the Great Barrier Reef had ever seen.

Bleaching continued into 2017, when the Great Barrier Reef was hit, again.

ABC News: Australia’s Great Barrier Reef in grave condition tonight...

FOX News: For the second year in a row, this video showing bleaching…

BBC News: Two-thirds of the Great Barrier Reef has now been devastated by severe coral bleaching

which is caused by rising water temperatures.

Researchers first documented global coral bleaching in 1998 ― a record warm year ―

and it happened again in 2010.

But this third global bleaching event is by far the worst.

And looking at this temperature trend, you can get why people are starting to say goodbye

to coral reefs.

But to really get what makes them vulnerable, you have to understand how coral reefs work.

Corals are related to jellyfish and sea anemones, but you’ve probably noticed that they don’t

look quite as... squishy.

There's a couple reasons for that.

For one thing, they live in colonies.

Each coral structure is made of hundreds or thousands of individual

coral animals called polyps.

Each of these little bumps is where a polyp lives.

They’re easy to overlook if you’re snorkeling but if you look closely, you can see them,

especially at night when they’re less likely to be hiding.

Those polyps build a skeleton together.

Not all corals do this but the ones that build reefs do so by creating a calcium carbonate

skeleton underneath them, layer by layer.

So the living polyps sit in little cups on top of an ever-expanding skeleton structure

which, in turn, sits on top of the compacted skeletons of previous corals from thousands

of years ago, otherwise known as limestone.

By building these structures with all these nooks and crannies, corals provide homes for

hundreds of other animals and plants ― an estimated 25% of all marine species,

even though they take up less than 1% of the ocean floor.

Those reefs provide billions of dollars worth of economic value to people every year, through

fisheries, tourism, and protection from storm waves.

But here’s the thing: corals can’t build reefs on their own.

Coral reefs exist because of an incredible partnership between animal and plant.

That’s because reef-building corals get the majority of their energy and nutrients

from single-celled algae that live inside coral polyps.

It’s where they get their greenish-brown color too.

They’re called zooxanthellae, and like other plants, they make energy from sunlight, that’s

why coral reefs mostly grow near the surface of the ocean ― where the sun shines.

But this partnership breaks down under heat stress.

After multiple weeks of temperatures even just a couple degrees celsius hotter than

the maximum temperature that they’re used to, The photosynthetic system in the algae starts

to accumulate reactive oxygen molecules like hydrogen peroxide,

which leak into the coral polyp cells.

To protect themselves from damage, the coral polyps kick the algae out of their bodies,

leaving the pale skeleton showing through.

By warming the planet, we are, among many other things, breaking up the team that built

the ocean’s most diverse ecosystem.

But bleached corals don’t necessarily die.

What happens next depends on how severe and long-lasting the high temperatures are.

When researchers assessed the damage to the Great Barrier Reef in 2016,

they found that coral death was concentrated in the northern section of the reef, where

bleaching had been the most severe.

In the central section, 33% of the reefs bleached severely, but there was only around 6% mortality.

That’s because zooxanthellae can return to a coral colony within a few weeks if the

water cools back down fast enough.

If not, the coral dies from starvation or disease.

If enough coral colonies die, a reef can get taken over by fuzzy brown seaweed.

Some coral reefs have transformed into… this.

If there are enough fish and other grazers to eat up the seaweed, new coral larvae can

settle there and the reef can start building up again.

After the 1998 bleaching, the corals of the Great Barrier Reef eventually recovered.

But it takes a decade or more for even the fastest growing corals to build back up, and

that 10 year timeline assumes one very important condition:

That they don’t bleach all over again.

And they almost certainly will.

Climate models project that in the coming decades, the conditions now causing mass bleaching

will become increasingly frequent, until eventually they happen every summer.

How soon that happens depends a lot on whether we start cutting our greenhouse gas emissions.

If we don’t, annual bleaching conditions are projected for parts of

the Great Barrier Reef by mid-century.

At that point, few coral reefs could survive.

If we buy them more time by slowing down global warming, corals and their zooxanthellae may

be able to acclimate or eventually evolve to tolerate warmer weather.

Coral reefs would still change, and probably still shrink,

but we could give them a better chance.

That’s why it doesn’t make sense to pronounce them dead.

This ecosystem is dynamic.

There are forces building it up, that are battling the forces tearing it down,

and those factors vary by species & by location.

But with global warming, humans have sided against the world’s coral reefs

to an unprecedented degree.

Some of the damage is now unavoidable.

But the battle isn’t over, and it’s not too late for us switch sides.

If you’d like to learn more about what the climate change is doing to the biodiversity

of our planet ― go over to audible.com/vox.

Their massive collection of audio books includes a lot of titles about climate change, including

“The Sixth Extinction: An Unnatural History.”

This is a Pulitzer Prize-winning book by Elizabeth Kolbert, covering not just coral reefs ―

but animals and plants around the world that are struggling to keep up with environmental change.

You can sign up for a free 30-day trial at audible.com/vox, and if you decide not to

continue on with them ― you still get to keep your book.

So sign up at audible.com/vox

and start spending your commute, or your cooking time, or your cleaning time,

learning more about the role that our species is playing

in the history of life on Earth.