of people around. So you can care about this issue because of people you know. MARTY HOFFERT, New York University: We're in the fossil-fuel era of human history. If humankind is going to have a future on this planet, it is absolutely inevitable that we are going to have to find another energy source. The thing that's happening to us now is that we have to make that decision in the 21st century, whereas we may have been able to postpone it to the 22nd century, if not for the greenhouse effect. HENRY JACOBY: I think this is a very difficult problem, and I wouldn't bet we can solve it. I think it is extremely, extremely difficult, but it's worth our effort to really work hard to try to find a way to solve it because the potential is that we're changing the face of the Earth for all future generations. ANNOUNCER: For more on the climate and where we might be headed, visit our Web site. Examine the debate over the new consensus on warming, tally your own carbon energy use, see how ice cores are a history book on key climatic events, check out radical ideas to combat global warming, access a teacher's guide and join the discussion at pbs.org or America On Line keyword PBS. Next time on FRONTLINE: CLIFFORD BOGGESS: Welcome to death row. ANNOUNCER: His name is Clifford Boggess. He lives on death row. He killed two people. CLIFFORD BOGGESS: I leaned forward and shot