作者：David Friedman @ 2015-6-9

http://daviddfriedman.blogspot.com/2015/06/judging-outside-your-expertise.html

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

Accepting the views of experts on a question you are not competent toanswer for yourself, assuming that you can figure out who they are and whatthey 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 thescience but of internal evidence, what they themselves say. Here are threeexamples:

The widely cited 97% figure is based mostly on Cook et. al. 2013, which iswebbed. It is often reported as the percentage of climate scientists whobelieve that humans are the main cause of warming and that warming will havevery bad effects.

Simply reading the article tells you that the second half is false. Thearticle is about causes of warming and offers no evidence on consequences.Anyone who says it does is either ignorant or dishonest, and other things hesays can be evaluated on that basis.

If you read the article carefully you discover that the 97% figure, whichis a count of article abstracts not scientists, is the percentage of abstractswhich say or imply that humans are *a* cause of warming (“contribute to” in thelanguage of one example).

The corresponding figure for humans as the principal cause, which is notgiven in the article but can be calculated from its webbed data, is 1.6%. Thattells you that anyone who reports the 97% figure as the number of articlesholding that humans are the main cause of warming is either ignorant ordishonest.

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]

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 anarticleinthe New York Review of Books attacking a Wall StreetJournal piece that argued that climate was not a catastrophic threat thatrequired 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 hedid not mentionwas that that sum, spread out over the rest of the century and theentire world, came to about one twentieth of one percent of world GNP. He wasattacking the WSJ authors for an argument which his own research, as hereported it, supported.

In a recent Facebook exchange on the consequences of AGW for agriculture,

someone linked to an EPApieceon the subject. Reading it carefully, I noticed that the positiveeffects of warming and CO2 fertilization were facts, with numbers: “The yieldsfor some crops, like wheat and soybeans, could increase by 30% or more under adoubling of CO2 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 maycounteract these potential increases in yield. For example, if temperatureexceeds a crop’s optimal level or if sufficient water and nutrients are notavailable, yield increases may be reduced or reversed.” The same pattern heldthrough the article.

A careful reader might also notice that the piece referred to the negativeeffects of extreme weather without any attempt to distinguish between extremeweather that AGW made more likely (hot summers), less likely (cold winters), orwould have an uncertain effect on (droughts, floods, hurricanes). It wasreasonably clear that the article was designed to make it sound as though theeffects of AGW would be negative without offering any good reason to believe itwas true.

One telling sentence: “Overall, climate change could make it moredifficult to grow crops, raise animals, and catch fish in the same ways andsame places as we have done in the past.” With most of a century to adjust, itis quite unlikely that farmers will continue to do everything in the same waysand the same places as in the past.

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

Readers may reasonably suspect that I too am biased. But nothing I havesaid here depends on your trusting me. In each case, you can look at the evidenceand evaluate it for yourself. And all of it is evidence provided by the peoplewhose work I am criticizing.

-width��6H��