Hockey Sticks and Melting Ice

June 23, 2006

This year’s Stanley Cup playoff series between Edmonton and Carolina may be prescient about climate in an odd sort of way. What happens when you mix Oilers and hockey sticks? The Hurricanes win.

A “hockey stick” that illustrates global warming has been put to the test for the last five years in a political game that is far rougher than anything a body checking NHL bruiser could get away with.

With some caveats, the hockey stick has passed the test – making it clearer than ever that human-induced global warming is taking place.

The “hockey stick” is a line graph that charts Northern Hemisphere temperatures for the past thousand years. The graph shows that temperatures were generally stable until a sharp up tick in the past century. The line looks like a hockey stick lying horizontally with the blade turned upward – hence the shorthand term “hockey stick.”

Michael Mann and two other geophysicists who developed the graph in the late 1990s used both thermometer records for recent temperatures and “proxy” records to estimate temperatures before thermometers came into widespread use in the mid-19th century. Proxy records include analyses of ice cores, tree rings, lake sediments, even paintings depicting glaciers as they were centuries ago.

The hockey stick turned into a punching bag for climate contrarians, who tossed around accusations of sloppy methodology and even fraud. The name calling and harassment got to be wearying for the hockey stick scientists – whose conclusion about the late 20th century’s extraordinary warmth, incidentally, was validated by other science teams using both models and other proxy temperature reconstructions.

Things came to a head last year when Congressman Joe Barton (R-TX), chairman of the House Energy and Commerce Committee, fired off a boorish letter to Mann and his two colleagues complaining about the hockey stick study’s methodology and demanding box loads of data and financial records.

By then, Barton’s counterpart at the House Science Committee, Sherwood Boehlert (R-NY), had had enough. He wrote an unusually sharp letter to Barton denouncing the specter of Congress “putting its thumbs on the scale of scientific debate.”

To settle the matter and protect scientists from intimidation by abusive politicians, Boehlert asked the National Academy of Sciences to review scientific studies reconstructing temperatures for the past two millennia.

The review team's new report concluded that there is high confidence that the last 25 years of the 20th century were warmer than any comparable period since 1600. For the centuries between 900 and 1600, the evidence is less definitive, but proxy records indicate that temperatures at many, but not all, locations were higher in the past 25 years than in the past millennium. For the period before 900, the review panel concluded that the quality of the proxy evidence currently available is not sufficient to make confident judgments about first millennium temperatures.

The panel found no evidence that Mann and his colleagues massaged the evidence to report a predetermined conclusion. While the review panel had a few concerns about methodology, they figured that Mann largely got it right.

Even if Mann were flat wrong, however, the climate contrarians would still have a weak case. As the review panel noted: “Surface temperature reconstructions for periods prior to the industrial era are only one of multiple lines of evidence supporting the conclusion that climatic warming is occurring in response to human activities, and they are not the primary evidence."

Good science is like good police work. When different types of evidence gathered from different investigations point to the same conclusion, the prosecution has a solid case.

Good politics is like a good hockey game. Sherwood Boehlert body checked Joe Barton, made a shot on goal, and scored. Unfortunately, scientists will have one less champion in Congress when Boehlert retires at the end of this year.