These were a fine set of comments, and let me reply briefly to some of the major points made. One interesting suggestion is that a tax limited to net emissions would create a private market (contrary to what I suggested in my post) for devices for removing carbon dioxide from the atmosphere. A power plant, say, would willingly pay a license fee for such a device if the fee were less than the tax savings from being able to offset the plant's emissions by carbon dioxide that the device removed from the atmosphere. Another commenter points out, correctly but misleadingly, that water vapor in the atmosphere blocks more heat from the earth's surface than carbon dioxide does. That is true. But an increase in atmospheric temperatures brought about by an increased atmospheric concentration of carbon dioxide increases the amount of water vapor in the atmosphere (because warmer air holds more water), and thus has a multiplier effect. It is also true but, to me at least, not reassuring that climate has fluctuated a great deal in the course of the earth's activity quite apart from any human influences on climate. I am not reassured because a human-engendered increase in global temperatures could coincide with and reinforce a natural increase, rather than offsetting it. In this connection, I emphasize the possibility of a runaway greenhouse effect. Becker discusses the underlying problem in terms of multiple equilibria. A less technical way to describe it is in terms of tipping, feedback, or a "vicious cycle." For example, higher atmospheric temperatures cause melting of the permafrost in Alaska and Siberia, which releases methane (stored in great abundance in permafrost), which causes a further temperature rise, which causes more melting and so more release of methane. The fact that severe climatic effects might unfold quite rapidly is a reason not to be comforted by the argument that, after all, the earth might be better off if it were warmer--Greenland would once again be green, Siberia would support agriculture, and so on. If abrupt global warming destroyed tropical agriculture, it would take decades to relocate that agriculture to formerly frigid areas; the transition costs would be immense. It is thus the possible rapidity of extreme climate changes that worries me; and it also makes me doubt that we can count on technology to bail us out. In the long run, yes; but we are probably decades away from the development of economical technologies for arresting human-engendered global warming. We are thus in a vulnerable period in which technology-forcing taxes (or quotas, as discussed by Becker) may be a prudent response. Finally, the fact that abrupt global warming is less probable than gradual global warming is small comfort, because a proper cost-benefit analysis of safety measures takes into account not only the probability of a harm but also the magnitude of the harm should it occur.