narrator: geologist paul hoffman's research centers on the question of what happened to create this planetary deep freeze a freeze that opens a lot of questions about how the earth maintains a habitable climate even today. hoffman: this only really became a question in the middle of the 20th century, when people realized that over the 5 billion years of our solar system, solar radiant energy has increased by almost 30%. so it's surprising, therefore, that the geological evidence indicates that the surface temperature of the earth hasn't changed very much over at least 3 1/2 billion years. so that means that there must be something internal to the earth that is self-adjusting so that the earth always maintains a stable temperature -- not unchanging, but limited in the amount of change despite this large increase in solar radiation. narrator: the earth's ability to self-adjust its temperature is linked to carbon dioxide, a greenhouse gas which captures heat that would normally radiate to outer space and sends it back to the surface. the more carbon dioxide in the atmosphere, the warmer the surf