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an end by stepping out in front of truck, or getting into an altercation with your neighbor, you may, for all practical purposes, live forever.

So you may have a really different demographic out there at the end of this century than anybody is thinking about. It is a really important question.

Alan Wolsky, Argonne National Laboratory: This is a very simple question. You spoke in terms of CO₂ concentrations, but I know that if you paint the window with three coats of black paint, the second and third coats aren't nearly as efficacious as the first coat. In a simple theory, the effect might be an exponential relationship. What is the “first coat” of CO₂? What's the concentration of CO₂ beyond which it doesn't really matter whether you add more CO₂ to the atmosphere?

James Edmonds: Are you asking when the temperature would increase to a point that it wouldn't matter anymore, or is the question one of when does it no longer pay to control and you ought to just let it ride?

Alan Wolsky: It's the simple first question.

James Edmonds: The artistry of the FCCC goal was that it said just don't do anything dangerous, and you're asking, I think, “What is dangerous?” That question has not been answered.

Alan Wolsky: Let me illustrate my question by asking another and giving a speculative answer. We know water is a greenhouse gas. We know water is given off when you burn methane. So why aren't people upset by increasing the amount of water in the atmosphere? I speculate that the correct answer is that there is already so much water in the atmosphere, that the likely anthropogenic increment doesn't matter.

James Edmonds: It's a simple question. It doesn't have a simple answer.

Alan Wolsky: Does it have an order-of-magnitude answer?

James Edmonds: Yes, if you say I think that if we ran most of the crop models at concentrations of 1,200 parts per million, you would have a hard time keeping agriculture operating. Nobody has actually done that, as far as I know, and it sounds like dinner conversation.

Klaus Lackner, Los Alamos National Laboratory: I'm really very much in agreement with you on the issue that we have to go down to zero in emissions. I do want to point out, on the other hand, the scales of the other carbon reservoirs and how long it would take to flood them. This is something you didn't point out, even though you have the numbers there. Vegetation contains roughly 600 gigatons of carbon, which corresponds to only about 100 years of fossil energy output at the current rate of emission.

Similarly, the ocean contains about 1,000-1,500 gigatons of carbon in the form of CO₂, and doubling this would change the pH from top to bottom by roughly 0.3. So overall, these other reservoirs, (ocean, biomass, and soil), which naturally are sinks, cannot really take up all of the carbon emitted. The scale at which we are generating CO₂ is very large compared to the size of the natural reservoirs.

James Edmonds: That's exactly right, except for the ocean, of course.