You can see the little tiny air bubbles in there.

Those are what we study.

Basically, this is a piece of ice about 20,000 years old from Antarctica.

And the bubbles trap air from 20,000 years ago.

So we can find out what the air was like back then and figure out if carbon dioxide had gone up or down.

And what we've learned from that is that carbon dioxide is higher now than it's been for at least the last million years, probably the last 20 million years, but that's less certain.

So it's really quite a dramatic thing that we humans have done to the carbon dioxide.

[MUSIC PLAYING] Hey, smart people.

Joe here.

So Earth's atmosphere and climate have changed in a big way, and they're continuing to change.

There is no doubt about that, and we've known about it for decades.

But Earth's climate has always changed throughout its history.

So how do we know that this time is different?

But we know, because at places like the Scripps Institution of Oceanography in Southern California, we have freezers full of ancient ice that let us look into the past thousands, even millions of years, and measure exactly what Earth's atmosphere and its climate were like throughout deep history.

I recently stopped by to visit Dr. Jeffrey Severinghaus, who studies ice cores.

He's part of a team that's working to find the oldest ice on Earth because each of these little blocks of frozen water can tell us something about our planet's past, long before we existed, and where it's heading now that we do.

And inside these tiny bubbles in this ice are old bubbles of air that existed on this planet as old as that ice is.

That was the atmosphere from the planet trapped in those little bubbles.

What happens is, in the polar regions, it's too cold to melt.

So when snow falls, it doesn't melt.

It just piles up and piles up and piles up and eventually turns into ice, just from under its own weight.

If you think about what snow is like, you've got a snowflake, but you've got air in between the snowflakes.

And so as that snow becomes more and more dense, it tends to squeeze out the air in between the snowflakes.

But it turns out, it doesn't squeeze out all the air.

JOE HANSON: As more layers of snow fall and condense, those tiny voids are literally frozen in time, layer upon layer.

And there are a lot of layers.

Some ice cores have annual layers, just like trees do.

You know how you can count tree rings to figure out how old it is?

So some graduate student sits there and counts 50,000 annual layers.

JOE HANSON: Of course it has to be a graduate student.

What a lot of work.

JOE HANSON: But to study ancient ice, first you have to find ancient ice.

Where are you doing this research?

Where are you collecting these ice cores?

So this is from a place called Taylor Glacier in Antarctica.

JOE HANSON: Taylor Glacier is a 54-kilometer stretch of ice and rock.

People like Dr. Severinghaus can read it like a book, full of stories about our ancient climate.

Taylor Glacier is a special place because it's one of the few spots on Earth where the ancient ice has risen to the surface.

So you only have to drill down maybe 5, 10 meters to get the ice, very much easier than drilling a deep ice core, which is 3,000 meters and costs $50 million.

JOE HANSON: It's basically a cylinder that has tiny little teeth on the bottom.

And when you rotate the barrel, it carves out the ice on the edges and leaves behind an ice core in the middle.

Once the core is pulled up, it's packed up and sent off, carrying a slice of history inside of it.

It's a slow process.

It takes like a month for the ship to get here.

JOE HANSON: And whether you're standing in the middle of the Amazon rainforest or at the North Pole, you're breathing roughly the same air.

Our atmosphere is pretty much the same everywhere, which means that a tiny air bubble from that one spot is enough to paint a picture of what the entire planet's atmosphere looked like.

This is the freezer.

And we won't be in there long, so don't worry about the cold.

This is what a typical ice core sample actually looks like.

Notice that there's no bubbles.

That's because, when you get below about 600, 700 meters, the pressure is so high that the air turns into something called a clathrate, which is a ice-like substance.

JOE HANSON: Clathrates are crystals, where instead of bubbles, the molecules are trapped in a cage made by the bonds between frozen water molecules.

There's still gas in there?

They're still gas molecules, but they're not in a gas phase.

JOE HANSON: Man, the patterns are so cool.

You must just see, randomly, such cool ice phenomena.

JEFFREY SEVERINGHAUS: Oh, yeah.

JOE HANSON: It's cold in here.

This cold.

JEFFREY SEVERINGHAUS: Funny how that works, yeah.

OK, but how do you get the ancient air out of the ice to measure it without contaminating it with all this air around us?

So this is how we actually extract the ancient air, if you will.

Take a piece of ice and put it in a vacuum flask like this and pump out all of the air, the modern air, the air we're breathing right now, using a vacuum line.

This is a vacuum pump here.

So we make a seal, and we pump out all the modern air.

Then we close this valve, and then you only have an ice cube and a little bit of water vapor, but no air.

And then you melt the ice.

And then the melting of the ice releases those little air bubbles of ancient air.

JOE HANSON: So because you're already on the now air, the only gases that are coming out are the ones that are trapped inside the ice.

Right.

And so then once we've done that, we can purify the gas a little bit by freezing the water.

JOE HANSON: So they pump out all the modern air, melt ice to let the ancient atmosphere vaporize, refreeze the water, and pump that ancient atmosphere out so it can be measured.

This is a liquid helium tank.

So it's cold enough at 4 Kelvin-- JOE HANSON: Wow.

--4 degrees above absolute 0.

It's cold enough that all the air actually condenses and turns into ice, air ice.

JOE HANSON: Every gas will-- Every gas except helium, yeah.

Then we take it over here, and this is the analysis part of it.

So this tube is actually a bottle.

It's a very long, skinny bottle that's capable of dipping itself into the liquid helium.

JOE HANSON: You wouldn't want to be getting your own hands too close to 4 Kelvin.

No.

[LAUGHS] Now, the frozen air gets put into this, a mass spectrometer, which is a machine that we use to measure the masses of really tiny things.

We measure the chemical composition of the atmosphere using isotopes.

They're like different flavors of atomic elements.

The isotopes, those flavors of elements, have unique masses.

And the mixture of them in the air bubbles can tell us all kinds of things about ancient Earth.

We use the isotopes of nitrogen to tell ancient temperature, basically at the time that the snow has fallen.

Ordinary nitrogen has a mass of 14.

And the rare isotope nitrogen-15, it has a mass of 15.

And so it turns out that relative proportions of N-15 and N-14 are sensitive to temperature.

JOE HANSON: So whatever the temperature is at particular time, it's creating different mixes of different flavors of gases in the atmosphere-- That's right.

--like a fingerprint for the temperature.

Exactly.

Yeah.

And that's trapped in air bubbles for posterity.

So the sample here starts out waiting its turn.

When its turn comes, this valve opens, and then the sample goes through this little tiny tube.

And it bleeds very slowly into the actual mass spectrometer, which is here, then gets accelerated by a 3,000-volt electrical gradient that makes the ions go really fast.

And then they hit this magnet, and they're forced to make a 90-degree right turn.

And in doing so, heavy things like N-15 try to go straight, and lighter things like N-14 get bent more.

JOE HANSON: It's like being in a car.

Exactly, yeah.

So you can't turn as fast in a big, heavy car.

So they swing out, and then the detector is seeing what swung out farther.

So you're getting resolution of things that differ by a single neutron when they're flying through that curve.

Yep.

That's pretty cool.

The same idea can be used to find out more than just temperature.

Labs all over the world use elements trapped in air, trapped in ice cores, to paint a map from our distant past to today.

Oxygen isotopes can tell us how oceans change.

Mineral dust tells us about how the atmosphere moved around.

There are chemical clues about early volcanoes.

But maybe most importantly, we can trace changing levels of carbon dioxide.

So the climate has changed before.

How do we know that this time, it's us?

So the way we know is that, just like we talked about for nitrogen, in turns out that carbon in carbon dioxide also has two flavors, carbon-12, which is ordinary carbon, and then a very rare form of carbon, carbon-13.

That's how we know that it's human-caused.

The atmosphere, as it goes up in concentration, CO2 concentration, the carbon-13 is taking a nosedive.

And that's not what would happen if it was natural CO2 because fossil fuel CO2 is very depleted in carbon-13.

JOE HANSON: This comes from the fact that plants prefer to eat CO2 made of carbon-12.

And when we burn fossil fuels made from those ancient plants, the fraction of carbon-12 in the atmosphere goes up, while carbon-13 goes down.

We've only been measuring carbon dioxide in the atmosphere since 1957.

But using the data from ice cores, we can trace levels back way, way farther.

And this is what we see.

CO2 was pretty flat for most of the past 1,000 years, all around 280 parts per million.

Now we're going to add in the carbon-13 abundance record.

That's this gold block here.

You can see that that was also pretty constant for most of the last 1,000 years.

But then around 1850, right when the carbon dioxide concentration started to rise, carbon-13 abundance started taking a nosedive.

So this really unambiguously tells you that humans did it.

That's why I call it the smoking gun of human causation.

There's lots of other ways we know, but this is the simplest to explain.

We are moving into uncharted territory.

The last time something like this shows up in the ice record is around 55 million years ago, when a volcano popped up under an oil field and cooked basically everything.

It sent all of the carbon dioxide into the atmosphere.

So the carbon dioxide in the atmosphere shot up.

We think it basically quadrupled.

And the climate warmed by six degrees.

Most important thing is just, right away, is solve this global warming problem.

We don't have much time left.

We've got to put aside all of our political differences.

The health and well-being of the planet is so much more important than anything else.

And we can do this.

I know we can.

We can, but will we?

I hope so.

Stay curious.

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