I was recently informed that the fine folks over at Bureaucrash have started a social networking site, and so I set up an account. And almost immediately, I discovered a fun little climate scientist bash session. I figured I'd weigh in, and quickly found that my post had reached rather epic proportions. Given that I get questions about climate change all the time, I figured I'd post my response here for easy reference.
Kevin, it's critical to realize that in the broadest planetary or biological terms, there's nothing wrong with global warming. You're right to point out that the Earth has been much warmer than it is today, and life did just fine. In fact, it flourished. So the problem is not that a warmer Earth would destroy life. Rather, the problem is that human civilization has been built up in a relatively narrow range of normal climatic conditions, and permanent shifts in those normal conditions would force people to undergo extremely costly adaptation. As John Broome writes in his book, Counting the Cost of Global Warming, "Unless there is a great ecological catastrophe, however, most of the harms that one can foresee from global warming could be classified as adjustment costs. I can see no reason why, in equilibrium, a warmer world should not be able to sustain just as good human life as a cooler one. The problem is that, over thousands of years, human beings and nature have become adjusted to a cooler world" (15).
Some indviduals' property would likely be destroyed by changing global conditions, and many would be put at increased risk of damage from extreme climate events. Some groups of people would find that the ecological conditions on which their livelihoods were dependent had changed, and this would create many problems, particular for many of those who are already among the worst off. So while a warmer Earth could likely eventually support lifestyles of equal or greater quality than it currently does, the difficulty of adapting the entirety of humanity to new conditions should give us pause. And that's not even to mention the impacts on the Earth's other species which, whether or not we attribute them intrinsic value of their own, certainly help to enrich our lives. As I've discussed in a paper that I'd be glad to send to you if you'd like, some of these effects could reasonably be thought of as infringements on rights, while others simply as objectionable consequences. But insofar as rights are involved, it seems like the proper libertarian response if we knew that climate change were occurring would mean something more than simply allowing people to adapt; we certainly wouldn't advocate that sort of solution for the victims of theft.
As for the scientific case for climate change, I think Stephen Gardiner put it well in his essay "Ethics and Global Climate Change" when he pointed out that "The skeptics are right…when they assert that the observational temperature record is a weak data set and that the long-term history of the climate is such that even if the data were more robust, we would be rash to conclude that humans are causing it solely on this basis. Still, it would be a mistake to infer too much from the truth of these claims. For it would be equally rash to dismiss the possibility of warming on these grounds. For, even though it might be true that the empirical evidence is consistent with there being no anthropogenic warming, it is also true that it provides just the kind of record we would expect if there were a real global warming problem" (567).
You point to several lines of evidence which you take to contradict the mainstream hypothesis, and I think that those should be addressed. You first point to the time lag between historical shifts in temperature trends and increases in levels of CO2. It should be made clear that no one is alleging that amphibians were driving their SUV's around, warming the Earth in time for the age of the dinosaurs. Rather, as temperatures rise, we would expect CO2 levels to rise, as they [higher temperatures] stimulate biological activity among heterotrophic organisms and cause aqueous CO2 to bubble off from the oceans. But these increased CO2 levels would be expected to create a feedback effect; the equilibrium temperature should be higher in those periods than it would be expected to be without any effect from CO2 warming, and indeed paleoclimatologists cannot explain climatic conditions in those time periods without citing the effects of an amplifying CO2 feedback. The problem is that today, we are adding a temperature amplifying agent to the atmosphere; this should be expected to produce some warming (ceteris paribus), no matter how skeptical you are about the anthropogenic climate change hypothesis (there may be counteracting consequences of increased CO2 levels, of course, in our current time period; I don't mean to marginalize that possibility, but those mechanisms have not appeared to be present in the past, as far as we know). It's simply a scientific fact that CO2 absorbs long-wave radiation.
You then point to the lack of temperature increase in this century, in the face of continually rising levels of CO2, as counter-evidence. I'd point out that your counter-hypothesis is based on the idea that practically all of the temperature variation we saw in the 20th century can be attributed to natural variability, and yet you are apparently pointing to a point in time where temperatures are not rising with the implication that there could not possibly be any natural trends which are currently counteracting the warming influence of atmospheric CO2. This seems like a structural problem for your position, but you could still be correct if we had no way to even begin to understand why global temperatures might not be increasing right now. Perhaps unfortunately (since it would be very nice if you were right), I believe that many scientists are claiming that the sun (which is certainly important in driving climatic conditions) has been in a relatively quiet period, and that this should be expected to cool the Earth. Given that the Earth has not cooled, this is unfortunately somewhat legitimate as evidence in favor of the anthropogenic climate change hypothesis: if the Earth gets warmer during times when natural trends alone would be expected to warm it, but does not get cooler when those trends would be expected to cool it, then it suggests that natural trends might not be able to provide the whole story, and the CO2 forcing may be the missing component.
Finally, you point to the fact that water vapor is the most significant greenhouse gas in the atmosphere, and on this point you are absolutely correct. Without the greenhouse effect, scientists believe that the Earth would be almost 60 degrees Fahrenheit colder than it is today. We have water vapor to thank for most of that warming, without which life as we know it would be completely impossible. Current estimates of future climatic conditions tell us to expect a change about an order of magnitude less significant than the total warming produced by the natural greenhouse effect. And that makes sense, given that the overwhelming majority of the greenhouse effect is the result of atmospheric water vapor. I'd also point out that predictions about future temperatures are not solely based on the greenhouse effect from increased levels of CO2, but also on a number of feedback mechanisms, including decreasing planetary albedo and, incidentally, increasing water vapor.
I'm sure you have a large number of further objections to the mainstream hypothesis, likely including but not limited to claims about cosmic rays, the incapacity of models to properly account for cloud activity or other intracellular activity, the proscription of variables in GCMs, the uneven vertical distribution of warming between the lower and upper atmosphere, the inherently cyclical nature of the climate system, and the apparently strong relationship between global temperatures and solar activity in the mid-20th century. I'd only suggest that perhaps you take some time to study the IPCC report and a basic textbook on the scientific basis for the anthropogenic climate change hypothesis, rather than regurgitating talking points from climate skeptics, and also recognize that uncertainty about the scientific basis for concern about climate change does not free libertarians of the responsibility of explaining what they would say if they knew that climate change were occurring.