Hey, smart people.

Joe here.

If we look into the past about 56 million years ago, we find a moment, a thin line in the geologic record where the course of life on Earth was altered.

Carbon in the atmosphere soared.

Global temperatures rapidly rose by 6 degrees Celsius.

Weather got more extreme, and huge numbers of our planet's life forms disappeared forever.

The Paleocene Eocene Thermal Maximum is one of the most radical climate events ever unearthed.

And the mass extinction that followed it was triggered, at least in part, by an injection of greenhouse gases into the air.

Humans weren't at risk of dying out during the PETM because we didn't exist.

But now we do.

Extinction is a fact of life.

But the crisis we currently face is of a scale never before seen since our species arrived on Earth.

And this time, it's because of us.

[MUSIC PLAYING] Today, one million species are threatened with extinction.

While one extinction is a tragedy, a million risks being just another statistic.

So let's break down what this really means.

It means 1 in 8 non-microbial life forms on Earth could disappear, many within decades.

More than 40% of amphibians, almost 33% of reforming corals and sharks, over 20% of mammals, and up to 10% of insects all at risk.

For mammals alone, it will take millions of years to recover from the losses they're predicted to endure over the next half century.

That future is hard to imagine, but we can see what's happening right now.

The human population has more than doubled since 1970, adding over 4 billion people in half a century, or roughly one Germany every year.

And 40% of them live within 100 kilometers of the coast.

In 2014, only 3% of the ocean was free from human pressure, and 66% of it has been severely altered by human activities.

Surface waters are 30% more acidic than before the Industrial Revolution.

Healthy coral reefs have shrunk by nearly half.

Coastal mangrove forests and seagrass meadows that guard shores from flooding and storms have been on the decline for decades.

About a third of ocean fish stocks are being harvested at unsustainable levels.

And nearly 2/3 are on the brink.

Since 1900, global average sea level has risen by at least 16 centimeters; 6 centimeters of that rise happened just in the past 20 years.

Things are changing on land too.

Cities have more than doubled in area since 1992.

We'll add 25 million kilometers of new road and cement in the next 30 years, mainly in developing countries.

And as countries get richer, their resource use accelerates.

We extract 60 billion tons of renewable and non-renewable resources from the planet every year, twice as much as in 1980.

More than a third of the world's land surface and nearly 3/4 of our fresh water are now devoted to crops or livestock.

In the U.S., long grass is our biggest crop, covering an area three times larger than any other, making our yards the most heavily irrigated cultivated plant in America.

As diverse as human populations have become, our food is becoming less so.

Around 75% of what we eat comes from just 12 plant and 5 animal sources.

Wheat, corn, and rice make up nearly 60% of the plant-based calories in most diets.

More than 550 breeds of mammals used for food and agriculture throughout history have already gone extinct.

And pollinator losses put up to $577 billion in annual global crops at risk.

Today, 25% of the world's ice-free land is used for grazing, and half of agricultural expansion has come at the expense of forests.

Between 1990 and 2015, nearly 3 million square kilometers of native forest was cut down, and only 68% of the world's forest area remains from pre-industrial times.

These land changes altogether have left half a million terrestrial species without enough habitat to survive long-term.

As trade and travel expands, we're moving plants and animals around too.

Some countries are seeing as much as 70% more invasive species than in 1970.

Because we unwittingly spread just one fungus around the world, nearly 400 amphibians are now threatened and 90 have gone extinct.

No known disease has damaged global biodiversity more.

We've also spread our garbage around the world.

Marine plastic has increased 10 times since 1980, and 300 to 400 million tons of toxic waste is dumped into the world's waters every year.

Fertilizers washing into coastal ecosystems have produced more than 400 ocean dead zones, where there isn't enough oxygen to sustain most life.

Along with fertilizers, land clearing and crop production account for a quarter of our greenhouse gas emissions.

Those emissions have doubled since 1980, raising average global temperatures by at least 0.7 degrees Celsius, which doesn't sound like much.

But at just two degrees of warming, 1 in 20 extinctions will be directly caused by climate change.

Plants and plankton sequester 5.6 billion tons of CO2 every year.

That's 60% of global fossil fuel emissions removed by species we currently threaten.

The long list of plants and animals that disappeared 56 million years ago during the PETM owe their demise almost solely to carbon and climate change.

But the threats today to natural ecosystems go far beyond climate.

Diseases, land use, and pollution also threaten our planet, plants, and animals.

We are animals, too, and our actions have already affected our own future.

Yet, if this were to all disappear, if one million species were to go extinct, if we were to change our planet to this extent in the scale of geologic time, should we look back on this moment, the billions of us would be a paper-thin memory in the long history of Earth.

It's up to us to decide if the world we know today gets to be more than that in the future.

[MUSIC PLAYING]