NARRATOR: At the National Ice Core Lab, in Lakewood, Colorado, a giant freezer stores over 45,000 feet of ice, drilled from 34 sites around the cryosphere. Dating back hundreds of thousands of years, these ice cores are time capsules that allow scientists, like Jim White, to peer deep into the history of ice.

JAMES WHITE (University of Colorado at Boulder): This piece of ice is interesting, 'cause it has a couple of things you can see, right away. One is there are bubbles throughout here. These bubbles are little packets of air. It's these bubbles we can take out and measure CO2 and methane and nitrous oxide. It's the only medium that really collects the atmosphere itself.

The other thing you can see in here, quite clearly, is you can see the layers, and the thickness is going to tell you how much snow fell that year, so you get a couple of pieces of climate information and a dating scale, just out of visually looking at this ice core.

NARRATOR: Most importantly, scientists have identified a direct historical link between increases in greenhouse gases, like carbon dioxide, and steep rises in global temperatures.

At every peak, big rises in sea level followed as Greenland's ice sheet shrank.

The ice core records also reveal a particularly telling moment in Greenland's history. Roughly 125,000 years ago, temperatures rose by about seven degrees Fahrenheit; the entire southern portion of the ice sheet melted, and global sea levels rose by over 10 feet.

It was caused by a change in the Earth's orbit around the Sun, which increased temperatures and released carbon dioxide from the oceans.

The more recent ice core record shows the potential for a similar meltdown. Right now, greenhouse gas levels in the atmosphere are even higher than they were 125,000 years ago, higher than they've ever been in the last half-million years. Temperatures are already following suit.

The only explanation is the burning of fossil fuels.

JAMES WHITE: What we see in this ice core is very solid evidence that what's happening today in the atmosphere is different. It's not a normal part of the climate cycle. It's something caused by human beings.

NARRATOR: Rising temperatures are once again pushing Greenland towards a major meltdown, but what the ice cores can't tell us is how long it will take. The last time Greenland lost a significant portion of its ice, White suspects, it happened over thousands of years. But this time, it could happen much faster.