Medieval Global Warming

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The disagreement is not political; most of it arises from valid issues involving physics and mathematics. First the physics. An accurate thermometer wasn't invented until 1724 (by Fahrenheit), and good worldwide records didn't exist prior to the 1900s. For earlier eras, we depend on indirect estimates called proxies. These include the widths of tree rings, the ratio of oxygen isotopes in glacial ice, variations in species of microscopic animals trapped in sediment (different kinds thrive at different temperatures), and even historical records of harbor closures from ice. Of course, these proxies also respond to other elements of weather, such as rainfall, cloud cover, and storm patterns. Moreover, most proxies are sensitive to local conditions, and extrapolating to global climate can be hazardous. Chose the wrong proxies and you'll get the wrong answer.

The math questions involve the procedures for combining data sets. Mann used a well-known approach called principle component analysis. This method extracts from a set of proxy records the behavior that they have in common. It can be more sensitive than simply averaging data, since it typically suppresses nonglobal variations that appear in only a few records. But to use it, the proxy records must be sampled at the same times and have the same length. The data available to Mann and his colleagues weren't, so they had to be averaged, interpolated, and extrapolated. That required subjective judgments which-unfortunately-could have biased the conclusions.

When I first read the Mann papers in 1998, I was disappointed that they did not discuss such systematic biases in much detail, particularly since their conclusions repealed the medieval warm period. In most fields of science, researchers who express the most self-doubt and who understate their conclusions are the ones that are most respected. Scientists regard with disdain those who play their conclusions to the press. I was worried about the hockey stick from the beginning. When I wrote my book on paleoclimate (published in 2000), I initially included the hockey stick graph in the introductory chapter. In the second draft, I cut the figure, although I left a reference. I didn't trust it enough.

Last month's article by McIntyre and McKitrick raised pertinent questions. They had been given access (by Mann) to details of the work that were not publicly available. Independent analysis and (when possible) independent data sets are ultimately the arbiter of truth. This is precisely the way that science should, and usually does, proceed. That's why Nobel Prizes are often awarded one to three decades after the work was completed-to avoid mistakes. Truth is not easy to find, but a slow process is the only one that works reliably.

It was unfortunate that many scientists endorsed the hockey stick before it could be subjected to the tedious review of time. Ironically, it appears that these scientists skipped the vetting precisely because the results were so important.

Let me be clear. My own reading of the literature and study of paleoclimate suggests strongly that carbon dioxide from burning of fossil fuels will prove to be the greatest pollutant of human history. It is likely to have severe and detrimental effects on global climate. I would love to believe that the results of Mann et al. are correct, and that the last few years have been the warmest in a millennium.

Love to believe? My own words make me shudder. They trigger my scientist's instinct for caution. When a conclusion is attractive, I am tempted to lower my standards, to do shoddy work. But that is not the way to truth. When the conclusions are attractive, we must be extra cautious.

The public debate does not make that easy. Political journalists have jumped in, with discussion not only of the science, but of the political backgrounds of the scientists and their potential biases from funding sources. Scientists themselves are also at fault. Some are finding fame and glory, and even a sense that they are important. (That's remarkably rare in science.) We drift into ad hominem counterattacks. Criticize the hockey stick and some colleagues seem to think you have a political agenda-I've discovered this myself. Accept the hockey stick, and others accuse you of uncritical thought.

There are also the valid concerns of politicians who have to make decisions in a timely way. In 1947, Harry Truman grew so annoyed at the prevarications of economists that he joked that he wanted a one-armed advisor-who could not hedge his conclusions with the phrase "on the other hand."

Some people think that science is served by open debate between left-handed and right-handed advocates, just as in politics. But the history of science shows it is best done by people who have two hands each. Present results with caution, and insist on equivocating. Leave it to the president and his advisors to make decisions based on uncertain conclusions. Don't exaggerate the results. Use both hands. We cannot afford to lower our standards merely because the problem is so urgent.