Hockey stick climate study faces scrutiny
As temperatures soared past 100 degrees Fahrenheit from Sacramento to St. Louis to New York the third week in July, the debate heated up in the nation’s capital over climate change. With one report recently released that criticizes the statistical methods behind the hockey stick climate analysis of the past 1,000 years, and another recent report taking a broader look at all evidence for climate change, Congress is considering how past changes fit into the climate future.
Climatologist Gerald North (foreground) and statistician Edward Wegman testified in front of the House Energy and Commerce Subcommittee on Oversight and Investigations in July about the famed hockey stick climate analysis. Photograph is by Christine McCarty, House Committee on Energy and Commerce.
The hockey stick is a climate change graph and study published in two papers in 1998 and 1999 by climatologists Michael Mann, Raymond Bradley and Malcolm Hughes. It indicated that the 1990s was the warmest decade in the prior millennium, with 1998 being the warmest year in the millennium.
Last year, Rep. Joe Barton (R-Texas), chair of the House energy committee, requested that Edward Wegman, a statistician at George Mason University in Virginia, form an ad hoc committee to examine the statistical methods used to form the hockey stick. Following that request, Rep. Sherwood Boehlert (R-N.Y.), chair of the House science committee, requested that the National Academy of Sciences (NAS) look into the same issue. NAS released its report in June; Wegman’s committee released its report in July.
Wegman’s team — which looked only at the statistical methods involved in the hockey stick analysis to determine if the conclusions were well-founded — wrote that the “assessments that the decade of the 1990s was the hottest decade of the millennium and that 1998 was the hottest year of the millennium cannot be supported.” Wegman testified before the House Energy and Commerce Subcommittee on Oversight and Investigations on July 18 that Mann’s team’s methods were flawed and that the team did not adequately consult statisticians on the work.
Additionally, Wegman’s team reported that the peer review on the original papers was inadequate and took issue with how “interconnected” the paleoclimate community is, writing that the community has a “self-reinforcing feedback mechanism.” Wegman testified, however, that his team was not asked to look at whether or not the climate is actually warming, and thus the team made no conclusions on that facet of the discussion.
Wegman’s team’s findings “parrot” claims about Mann’s team’s statistical methods that have already been refuted in peer-reviewed literature, says Mann, who is now at Pennsylvania State University. “The whole fuss” about the methods is simply “a red herring,” designed to “undermine the public confidence in the science underlying human-caused global warming,” Mann says. Many lines of evidence support human influences on climate, as outlined in the NAS report.
The NAS committee that looked into the hockey stick graph, as well as the larger issue of the state of climate change science today, agreed with the Wegman report that it is difficult to quantify a single decade or year as hotter than others going back as far as 1,000 years, testified Gerald North, a climate scientist at Texas A&M University and chair of the NAS committee. However, North testified, “reservations about Mann et al.’s statistics should not undermine the fact that climate is warming.”
The committee reported “high confidence” that surface temperatures during the late 20th century were warmer than any comparable period over the last 400 years, says Kurt Cuffey, a climatologist at the University of California in Berkeley, and a member of the NAS committee. The committee reported less confidence that late 20th century temperatures were warmer than any period going back 1,000 years, he says, due to having fewer climate proxies — lines of evidence of past climates, such as tree rings, boreholes, ice cores and retreating glaciers. That lower level of confidence, however, does not mean that Mann’s basic conclusion is erroneous, North said.
North told Congress to keep in mind the surface temperature reconstructions by Mann and his colleagues are “only one piece in a long line of evidence of warming.” The primary evidence, North said, comes from the climate proxies themselves, which have, in many cases, indicated that the late 20th century was warmer than any comparable time period over the past 2,000 years, as noted in the NAS report.
Gavin Schmidt, a climatologist at NASA Goddard Institute for Space Studies in New York City (speaking not as a NASA representative), calls the focus on Mann’s team’s analysis “a distraction” from the larger scientific discussion. But, he adds, it has galvanized many members of Congress on both sides of the aisle “to get a whole load of much more serious science on the table.”
Without such an entrée, the House Committee on Government Reform may never have held its hearing on climate change, Schmidt says, which it held the day after the hearing on the hockey stick. From this meeting and legislation put forth by politicians in both parties, including Sens. Joe Lieberman (D-Conn.), John McCain (R-Ariz.), Jim Jeffords (I-N.H.) and Boehlert, among others, it is clear that some politicians “have moved on to the now very important subject of impacts and possible solutions” to the global warming problem, Mann says.
Climate change is happening — the temperatures are going up, said Rep. Jan Schakowsky (D-Ill.), at the energy committee hearing. Whether human influence or natural variation is the principle cause of the warming does not matter, she said. A prudent person would focus on the issue and work on managing the changes and ameliorating the human impacts rather than discredit a single report, she said.
In the end, it’s time for Congress to act on climate change, agreed Rep. Charles Bass (R-N.H.) at the hearing: “I think we need to recognize that there’s a problem here,” and then develop a policy to address that problem, he said.