Wednesday, March 12, 2008

Scientists leaned towards warming in the 1970s

The climate denier argument, that scientists predicted massive cooling in the 1970s and therefore can't be trusted now, contradicted actual scientific opinion of the time. The National Academy of Sciences said in 1975 that "we do not have a good quantitative understanding of our climate machine and what determines its course. Without the fundamental understanding, it does not seem possible to predict climate."

Despite some hysterical media reports to the contrary at the time, it turns out that scientists were actually leaning toward warming. RealClimate reports that a survey of abstracts discussing future climate change from 1965 to 1979 found 7 abstracts predicting cooling, 42 predicting warming,* and 20 that were neutral. The 20 that were neutral probably doesn't add much information unless they expressly stated something like "we cannot make an accurate prediction as to climate change." The key issue is that for every one scientific paper predicting cooling, there were six that predicted the exact opposite.

These results should encourage us, in that climatology headed in the right direction from its infancy, without the massive modern research investment, much of a pre-existing scientific base, or even an instrumental record showing warming (temperatures slightly cooled from the 1940s to the early 1970s). It would be useful to confirm that the warming predictions were right for the right reasons - that the cumulative increase in greenhouse gas emissions would overcome the transitory and declining effect of aerosol pollution. That was likely the case though (page 4): "By 1978, the question of the relative role of aerosol cooling and greenhouse warming had been sorted out. Greenhouse warming, the researchers concluded, was the dominant forcing."

Andrew Dessler has raised an objection to research based on reviewing large number of abstracts, however. He points to an abstract he helped write as an example of one that only an expert in that climate subfield could understand as supporting or opposing the climate change consensus. Dessler argues that the best way to understand the science is to read the IPCC, an "expert synthesis" of the scientific viewpoint.

So is Dessler right? While the 1975 National Academy of Sciences report doesn't reach anything like the IPCC's depth, it still was an "expert synthesis," and its studied neutrality differs somewhat from the conclusion reached by reviewing abstracts (even after acknowledging the abstract review also covers a post-1975 period).

To a limited extent, Dessler has a point. People active in the field of work who can synthesize expert opinion can go into far greater depth than a review of abstracts. Despite that fact, the abstract review adds value. To the extent that the NAS report attempts to synthesize a consensus, I don't think the abstract review necessarily disagrees with it. The abstract review shows how the science was leaning, not a consensus. Being able to know that is valuable.

Reviewing abstracts may not be as fraught with error as Dessler fears. As a non-scientist reading Dessler's abstract, I miss a lot of it but still understand it to validate existing models for simulating water vapor in the atmosphere, and therefore supporting the consensus. A climate scientist, even one not an expert in that field, could follow much of the rest.

Finally, an abstract will clearly lay out the major conclusion of the research. Research that provides the umpteenth support for the widely-held consensus view is not a major conclusion, but a finding that contradicts the consensus will be a major conclusion, spelled out clearly in the abstract. An honest review of abstracts will likely detect deviations from consensus (some of the work on current abstracts haven't been honest), as well as all but the most-arcane implications. I'd guess that this review of abstracts from 1965-1979 will stand the test of time.

What hasn't stood the test of time on this issue is Newsweek's reporting in 1975, or its reporting on itself in 2006. The magazine claimed in 1975 that scientists were "almost unanimous" in a continuing trend towards cooling, and then claimed in 2006 that the science of the time was wrong but its own depiction of the state of science was accurate. Time for Newsweek to do another update.

More info on the abstract review is available here.

*RealClimate misstates the number as 44.

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