Plot: The world is getting warmer. Is it our fault? Is the rise in temperature a product of the Industrial Revolution, the burning of fossil fuels, and the modern propensity for driving down to the mini-mart in a car the size of a woolly mammoth? Or is it just part of a natural global cycle? Bill investigates with a visit to the National Ice Core Laboratory in Denver, where scientists identify and evaluate atmospheric gases from 400,000 years ago, and a demonstration of how increased levels of carbon dioxide affect temperatures on Earth