You can see the tiny air bubbles in there? Those are what we study. This is a piece of ice – about 20,000 years
old – from Antarctica. And bubbles trap air from 20,000 years ago,
so we can find out what air was like back then. Can figure out if carbon dioxide has gone
up or down. And what we’ve learned from that is 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. [♩] Hey smart people. Joe here. Earth’s atmosphere and climate have changed
in a big way, and they are continuing to change. There’s no doubt about that, and we’ve
known it for decades. But Earth’s climate has always changed throughout
its history. So how do we know this time is different? 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 working to find the
oldest ice on Earth. 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,
is old bubbles of air that existed on this planet as old as that ice is. Yeah That’s the atmosphere of the planet, trapped
in those little bubbles. What happens in the polar regions is it’s
too cold to melt. So when snow falls it doesn’t melt, it just
piles up and piles up, and eventually turns into ice under its own weight. But if you think about what snow is like,
if you have a snowflake you have air in between the snowflake. As snow becomes more and more dense, it tends
to squeeze out the air between snowflakes, but it turns out it doesn’t squeeze out
all the air. 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? So some graduate student sits there and counts
50,000 annual layers Of course it has to be a graduate student! What a lot of work. 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? This is from a place called Taylor Glacier
in Antarctica 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 special because it’s one
of the few places on Earth where the ancient ice has risen to the surface. So, you only have to drill 5-10 meters to
get the ice. Which is much easier than drilling a deep
ice core which is 3,000 meters and costs 50 million dollars. It’s basically a cylinder that has little
tiny teeth on the bottom. And when you rotate the barrel it carves out
the ice, but only a little bit in a ring, and it 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 it. It’s a slow process, it takes like a month
for the ship to get here. 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
so many years ago. This is the freezer. We won’t be in there long, so don’t worry
about the cold So this is what a typical ice core sample
looks like. Now you’ll notice that there’s no bubbles. That’s because when you get down below 600-700
meters, the pressure is so high that the air turns into something called a clathrate which
is an ice-like substance. 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 There’s still gas molecules but they’re
not in a gas phase. Man the patterns are so cool, you must randomly
see such cool ice phenomena It’s cold in here! This cold! Funny how that works. Okay, but how do you get the ancient air out
of the ice to measure it? I mean, without contaminating it with… all
this air around us? So this is how we actually extract the ancient
air, if you will. We take a piece of ice and put it in a vacuum
flask, and pump out all of 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 close this valve, and
then you only have an ice cube and a little bit of water vapor, but no air. Then we melt the ice, and the melting of the
ice releases those little air bubbles of ancient air. So because you already let out the “now
air,” the only gasses that are coming out are the ones that are trapped inside the ice. Right, and then we can purify the gas a little
bit by freezing the water. So they pump out all the modern air, melt
the ice to let the ancient atmosphere vaporize, re-freeze the water, and pump that ancient
atmosphere out so it can be measured. This is a liquid helium tank, it’s cold
enough - it’s at 4 kelvin, 4 degrees about absolute zero. It’s cold enough that all the air actually
condenses and turns into ice - air ice. Every gas, will freeze Every gas except helium. So then we take it over here. This is the analysis part of it. This tube is actually a bottle, a long skinny
bottle that’s capable of dipping itself into the liquid helium. You wouldn’t want to be getting your own
hands too close to 4 kelvin. No. The frozen air gets put into this, a mass
spectrometer, which basically measures the masses of really tiny things. We measure the chemical composition of the
atmosphere using isotopes: they’re like different flavors of atomic elements. 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 at the time the snow was falling. Ordinary nitrogen has a mass of 14, but the
rare isotope nitrogen 15 has a mass of 15. It turns out that relative proportions of
N15 and N14 are sensitive to temperature. So, whatever the temperature is at a particular
time, it’s creating different mixes of different flavors of gasses in the atmosphere, like
a fingerprint for temperature. That’s right, and that’s trapped in air
bubbles for posterity. So the sample here starts out waiting its
turn and when its turn comes the sample opens and goes into this little tiny tube, which
leads into the mass spectrometer, here, and it gets accelerated by a 3,000 volt electrical
gradient, which 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 N15 try to
go straight, and lighter things like N14 get bent more. It’s like being in a car. 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? 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 changed,
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. The way we know, is just like we talked about
with nitrogen, the carbon in carbon dioxide also has two flavors. There’s carbon 12, which is ordinary carbon,
and then a very rare form of carbon, carbon 13. So, that’s how we know it’s human caused. The atmosphere, as it goes up in CO2 concentration,
the carbon 13 of the atmosphere 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. 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 farther. And this is what we see: CO2 was pretty flat for most of the past 1,000
years. All around 280 ppm. Now we’re going to add in the carbon 13
abundance, this gold line. And you can see that was also pretty constant
for most of the last thousand years. But then around 1850, right when carbon dioxide
concentration started to rise, the carbon 13 abundance started taking a nosedive. And this kind of unambiguously tells you that
humans did it. That’s why I call it the smoking gun of
human causation. There are lots of other ways we know, but
this is the simplest. We’re 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 the carbon dioxide into the atmosphere. So, the carbon dioxide shot up, we think it
nearly quadrupled, and the climate warmed by 6 degrees. The most important thing is right away to
solve this global warming problem. We don’t have much time left. We have to put aside all of our political
differences, The health and wellbeing of the planet is so much more important than everything
else. We can do this, I know we can. We can. But will we? I hope so. Stay curious.
