of the atmosphere. But the data went back only 40 years, while we've been burning fossil fuels for more than 150 years, all during the industrial revolution. Just as scientists had used natural recorders to get at past temperatures, they now wondered if the giant ice sheets in Greenland and Antarctica might have captured past CO2 levels during and before the industrial revolution. And what they found was remarkable. Trapped in the ice cores are bubbles of air, time capsules preserving the atmosphere of the past. With care, these bubbles of fossilized air can be analyzed for carbon dioxide and other gases. Sensitive analysis of the air bubbles reveals that before 1957, the atmosphere had less CO2. Indeed, CO2's rapid rise starts at the beginning of the industrial revolution. Subsequent research has extended the curve far back in time. It's now established that current levels of CO2 are higher than at any time for the past 450,000 years. MICHAEL B. McELROY, Harvard University: When you look at a 450,000-year record, and the last 100 years, 150 years, stands out like a sore thumb, there's not much question that we're involved in some way or another. And truly, that's not surprising. I mean, we are such an incredibly powerful force on this planet- our species. I mean, most of the land area of the Earth that is habitable, that is cultivable, we've cultivated or we've inhabited. We've moved mountains. We've redirected rivers. We've mined all this coal. We're a global force.