Less than 20 years ago, the world's largest living structure looked like this, and this.

But today, ongoing pressures from climate change put it at serious risk for being wiped out entirely.

In fact, in August, an official report by the Australian government downgraded the reef's future outlook from "poor" to "very poor."

So, how exactly did we get here?

And can it be reversed?

The Great Barrier Reef is big.

So big, in fact, that you can see it from space.

It's important for people to remember that the Great Barrier Reef is much more than just one coral reef, which is what I think a lot of people think.

So, it's actually 3,000 separate, individual coral reefs, and every one of them is in a different condition.

It's also the world's largest living structure, as well as a home to thousands of plant and animal species.

Like with trees in a forest, the corals are really the foundation of the habitat.

They provide the structure for other organisms to live there.

A normal, healthy coral reef is very colorful.

There's tons of fish swimming around.

It's often quite loud underwater when you're diving on a coral reef that's healthy.

So, you know, it's just amazing and beautiful.

But this vast ecosystem has been declining for a while.

The culprit? Mass coral bleaching.

As the name suggests, coral bleaching strips reefs of their color, leaving them completely white.

It happens when unnaturally warm ocean water causes coral to expel the symbiotic, colorful algae it needs to survive.

Without this algae, the coral can't get the nutrients it needs and will starve to death if water temperatures don't return to normal.

Even small increases in temperature, when they're sustained for a long period, can be very stressful for them.

It's just like a person, right?

Like, you can go out into 114-degree weather for a short time and you'll be fine, but if you were to stay out there all day, you would suffer badly.

In 2016 and 2017, the Great Barrier Reef was hit with a massive marine heat wave, leading to the longest coral-bleaching events ever recorded.

It devastated the reef, killing off nearly 50% of the coral.

Many corals can live for hundreds of years, so if those corals die, to get back a coral that was 400 years old and died is gonna take 400 years.

But here's the thing.

This rate of bleaching is not only abnormal but completely unprecedented in the Great Barrier Reef's history.

We have to remember that the Great Barrier Reef is now essentially a changed ecosystem.

We've never had events as severe as those of 2016 and '17 before.

The cumulative consequence of all of that is that we don't know how well the Great Barrier Reef is going to recover from those events.

Before the 1980s, bleaching events would occur periodically, but never on a mass scale.

But global warming has rapidly accelerated this process.

From those coral skeletal records, we know conclusively that coral bleaching is a new phenomenon.

The first mass-bleaching event that occurred on the Great Barrier Reef was in 1982.

As bleaching events are becoming more severe and more frequent, it stymies the recovery, right?

It's almost as if you have a hurricane, you start to rebuild, and then you get hit by another one.

In fact, a 2019 report by the United Nations warned that if global temperatures increased by just 0.9 degrees Celsius, which is expected to happen, coral reefs could decline by 70% to 90%.

And if it warms by 1.8 degrees, 99% of the world's coral could be at risk.

As we approach 2 degrees of warming and go beyond 2 degrees of warming, we are probably not going to be able to protect the Great Barrier Reef anymore.

And we're actually going to lose it as the valuable ecosystem that we have.

As if that wasn't enough, the reef is also endangered by local pollution, overfishing, and coastal development.

Local management of impacts, such as pollution in runoff, sedimentation in our reefs, overfishing, these things are critically important, and they need to happen in concert with combating climate change.

It's also important to note that the Great Barrier Reef is one of the best-managed reefs in the world.

But even with the best countermeasures, the Great Barrier Reef simply can't withstand the effects of climate change.

Unless drastic international action is taken to reverse or halt greenhouse gas emissions, we might be facing a world without reefs as early as 2050.

My ballpark is that if we don't do something dramatic in the next 10 years, that we will have passed the time after which there will be no return.

So, this all sounds pretty bad, but why should we care?

What makes coral reefs so important?

First of all, coral reefs are an incredible source of biodiversity.

In fact, they provide a home for 25% of all marine life even, though they take up just 1% of the ocean floor.

So, we have six of the world's seven species of turtles, dugongs, whales, dolphins, seabirds, one-and-a-half thousand species of fish.

They're also worth billions of dollars in economic value.

The Great Barrier Reef alone brings in over $6 billion in tourism each year, and reefs provide a natural buffer from violent storm waves, preventing property damage and loss of life.

Reefs are an increasingly important resource for breakthrough medical treatments, too.

Right now, plants and animals found in coral reefs are being used to develop treatments for diseases like cancer and Alzheimer's.

So, if we lose corals, we potentially lose, you know, the key to solving diseases that we care about.

Not to mention, about half a billion people depend on reefs for food and work.

Even though some of the damage is irreversible, not all hope is lost for the world's coral reefs.

We as a coral reef science community are becoming increasingly focused on human intervention and, you know, basically preventing the extinction of corals and restoring reefs.

The most important thing that we need to do is to stop and reverse climate change and global warming.