Erik said; "It is hard to understand why past warm periods disprove current warming causes." Don said: "They don't. But they do show that other factors have caused the earth to warm substantially in recent history. The real question is how much warming is due to natural causes and how much to man-made causes." Don, I think you have hit the nail on the head with this question, but I don't believe it will ever be possible to answer that it definitively. The atmospheric interactions are just too complex to know for certain. Our knowledge of greenhouse gases, however, is pretty solid. We know the effect they have in the atmosphere and we know that the possible outcomes of even small changes in global temperatures could be catastrophic (like a change in sea level of even a meter or two). I think there is plenty of information to suggest that we should take action to limit our emissions. I see no down side to acting responsibly to lower our inputs to the atmosphere. Sure there are costs, but they pale in comparison to the costs of the possible sea level changes. I think sometimes people get sidetracked with the blame game. I am not blaming anyone any more than I blame myself. I like to have fuel to put into my gas tank. But, sometimes it seems like those who question global warming are suggesting that we do nothing, which I believe is a huge mistake. PS - I don't mean you Don. You have clearly stated options that would reduce emissions.