Transcription
This frozen library, the National Ice Core Lab in Denver, Colorado, has ice from all over... kept at minus 35 degrees. The oldest core here goes back some 400,000 years. Here really ancient ice from Greenland in the north, and Antarctica in the south, reveals Earth's climate history. Let's see what cores like this can tell us.
First are those layers I mentioned in the New Zealand snow. They've turned to ice, and we can count them, summer, winter, summer, winter. Like tree-rings, we can date the core. Other cores tell other stories. Look at this... it's the ash of an Icelandic volcano that blew up to Greenland 50,000 years ago.
Cores hold other and even more important secrets... look at these bubbles. They formed as the snow turned to ice and trapped old air that's still in there. Scientists now are working with cores from Antarctica that go back even further. They tell us, with a very high degree of accuracy, how much carbon dioxide was in the air that far back.
Researchers break chunks of ice in vacuum chambers and carefully analyze the gases that come off. They're able to measure very precisely levels of carbon dioxide in that ancient air. Looking at the cores, we see a pattern that repeats... 280 parts per million of CO2, then 180, 280, 180, 280.
By analyzing the chemistry of the oxygen atoms in the ice you can also see the pattern of rising and falling temperature over time. Colder during the ice ages, warmer during the interglacial periods. Now put the two lines together... and you can see how closely temperature and carbon dioxide track each other. They're not exactly alike. At times the orbits caused a little temperature change before the feedback effects of CO2 joined in. But, just as we saw in New Zealand, we can't explain the large size of the changes in temperature without the effects of CO2.
This is the signature of natural variation, the cycle of the ice ages driven by changes in Earth's orbit, with no human involvement. But here's where we are today. In just 250 years, since the Industrial Revolution, we've blown past 380 with no sign of slowing down. It's a level not seen in more than 400,000 years, forty times longer than the oldest human civilization.