This past summer's big-budget disaster movie The Day After Tomorrow depicted a near-future in which human-caused global warming dramatically disrupted the earth's climate system, plunging the world into a new ice age. Although the scenario in the film is clearly an unrealistic fantasy, some scientists say that relatively sudden climate change is theoretically possible—but how likely it is depends on whether human activity really causes global warming. Does the evidence suggest that higher amounts of so-called greenhouse gases in the atmosphere due to fossil fuel consumption are, in fact, causing global warming? And if so, what should we do about it? Peter Robinson speaks with Carl Pope and Fred Smith Jr.