I have just been involved in a lengthy exchange on Facebook over my criticism of the claim that warming on the scale projected by the IPCC for 2100 can be expected to have large net negative consequences. The response I got was that the person I was arguing with was not interested in my arguments. He does not know enough to judge for himself whether the conclusion is true, so prefers to believe what the experts say.



Accepting the views of experts on a question you are not competent to answer for yourself, assuming that you can figure out who they are and what they believe, is often a sensible policy, but one can sometimes do better. Sometimes one can look at arguments and evaluate them not on the basis of the science but of internal evidence, what they themselves say. Here are three examples:



The widely cited 97% figure is based mostly on Cook et. al. 2013, which is webbed. It is often reported as the percentage of climate scientists who believe that humans are the main cause of warming and that warming will have very bad effects. Simply reading the article tells you that the second half is false. The article is about causes of warming and offers no evidence on consequences. Anyone who says it does is either ignorant or dishonest, and other things he says can be evaluated on that basis.



a* cause of warming (“contribute to” in the language of one example). The corresponding figure for humans as the principal cause, which is not given in the article but can be calculated from its webbed data, is 1.6%. That tells you that anyone who reports the 97% figure as the number of articles holding that humans are the main cause of warming is either ignorant or dishonest. One person who has done so, in print, is John Cook, the lead author of the article. John Cook runs skepticalscience.com, which is a major source for arguments for one side of the global warming dispute, so knowing that he is willing to lie in print about his own work is a reason not to believe things on that site without checking them. [ My old blog post giving details If you read the article carefully you discover that the 97% figure, which is a count of article abstracts not scientists, is the percentage of abstracts which say or imply that humans are *





New York Review of Books attacking a Wall Street Journal piece that argued that climate was not a catastrophic threat that required an immediate response. In it, he gave his figure for the cost of waiting fifty years instead of taking the optimal steps now—$4.1 trillion dollars—and commented that “Wars have been started over smaller sums.” What he One of the economists who has been active in estimating consequences of warming is William Nordhaus. He is, among other things, the original source for the 2° limit. A few years ago, he published an article in theattacking apiece that argued that climate was not a catastrophic threat that required an immediate response. In it, he gave his figure for the cost of waiting fifty years instead of taking the optimal steps now—$4.1 trillion dollars—and commented that “Wars have been started over smaller sums.” What he did not mention was that that sum, spread out over the rest of the century and the entire world, came to about one twentieth of one percent of world GNP. He was attacking the WSJ authors for an argument which his own research, as he reported it, supported.





In a recent Facebook exchange on the consequences of AGW for agriculture, someone linked to an EPA piece on the subject. Reading it carefully, I noticed that the positive effects of warming and CO2 fertilization were facts, with numbers: “ The yields for some crops, like wheat and soybeans, could increase by 30% or more under a doubling of CO 2 concentrations. The yields for other crops, such as corn, exhibit a much smaller response (less than 10% increase).” The negative effects were vague and speculative: “some factors may counteract these potential increases in yield. For example, if temperature exceeds a crop's optimal level or if sufficient water and nutrients are not available, yield increases may be reduced or reversed.” The same pattern held through the article.





A careful reader might also notice that the piece referred to the negative effects of extreme weather without any attempt to distinguish between extreme weather that AGW made more likely (hot summers), less likely (cold winters), or would have an uncertain effect on (droughts, floods, hurricanes). It was reasonably clear that the article was designed to make it sound as though the effects of AGW would be negative without offering any good reason to believe it was true. One telling sentence: “Overall, climate change could make it more difficult to grow crops, raise animals, and catch fish in the same ways and same places as we have done in the past.” With most of a century to adjust, it is quite unlikely that farmers will continue to do everything in the same ways and the same places as in the past.





These are three examples of arguments for one side of the climate controversy by a source taken seriously by supporters of that side. Each can be evaluated on internal evidence, what it itself says, without requiring any expert knowledge of the subject. In each case, doing so gives you good reasons not to trust either the source or the conclusion.



