More and more scientists say global warming is happening faster than they expected, while the world's major polluters fail to take meaningful action. As a candidate in the 2000 campaign, George W. Bush promised reductions in carbon dioxide emissions. But as soon as he took office he backed away, and not until this month did he call for halting the growth of greenhouse gases, by 2025 and without any mandates. All three of the candidates to replace him have said that's not good enough. John McCain, Barack Obama and Hillary Clinton all promise to deal with the issue in the "first one-hundred days," but what are they planning to do? A so-called "cap and trade" bill is already on the Senate's agenda. What does that mean, and where do the candidates stand? Should renewable technologies, like solar and wind, get the kind of subsidies lavished on oil and gas? What about nuclear power?