The L-shaped centerpiece of climate-change politics is getting warped. Since 2001, a temperature plot called “the hockey stick” has girded claims that humans have warmed Earth and therefore must change behavior. But the hockey stick has encountered problems.
The temperature pattern in question appeared in scientific papers published in 1998-99 by climatologist Michael Mann and colleagues. Its implications were huge. Prior temperature records, all but the last 150 years of which rely on proxies such as tree rings and ocean sediments, indicated a warm period during roughly 1000-1400 followed by a cool period in 1400-1900. The variation supported arguments that at least some of the observed warming of about 1º F. during the past century, although coincident with a build-up of greenhouse gases in the atmosphere, might have natural causes.
‘Warmest decade’
Mann’s work replaced the broad swings of the earlier proxy temperature record, which it attributed to regional phenomena, with near stability-the hockey-stick handle-until the jump of the past century-the blade. On the basis of the reshaped temperature pattern, the Intergovernmental Panel on Climate Change in 2001 called the 1990s the “warmest decade and 1998 the warmest year in the millennium.” IPCC’s work stokes arguments for aggressive climate-change precaution. Politically, therefore, the hockey stick was very important.
It also became controversial. Criticism focused on Mann’s statistical methods. Then complaints emerged that doubtful scientists couldn’t gain access to data for review. Because of the hockey stick’s political importance, Congress requested independent studies of the issue, results of which are now at hand.
At the end of June, the National Academy of Sciences (NAS) reported a study upholding the hockey stick’s blade but questioning most of the handle. It expressed “high confidence” that the last few decades of the 20th Century were the warmest in 400 years. But it had less confidence that recent warming was unprecedented in years before 1600. “Because of larger uncertainties in temperature reconstructions for decades and individual years,” the study said, “and because not all proxies record temperatures for such short timescales, even less confidence can be placed in the Mann team’s conclusions about the 1990s and 1998 in particular.” NAS alluded to other evidence that observed warming results from human activity. But it did hockey-stick disciples no favors.
Last week the congressional committee that requested the NAS study, the House Energy and Commerce Committee, released preliminary findings of an informal study into the Mann team’s statistical methods. Edward Wegman, a statistics professor at George Mason University, led a team that included David Scott of Rice University, Yasmin Said of Johns Hopkins University, and others.
The statisticians were not impressed by statistical methods used in the Mann research. They said the study can’t support claims that the 1990s were the hottest decade and 1998 the hottest year in 1,000 years.
They were even less impressed by practices of what they called “the small community of paleoclimate researchers,” faulting members of that group for reviewing each other’s work and for failing to work with “mainstream” statisticians. “The public policy implications of this debate are financially staggering, and yet apparently no independent statistical expertise was sought or used,” the statisticians said.
These and other criticisms by the Wegman team combine with the NAS study to give the hockey stick vital perspective it so far has lacked. A large question is whether perspective receives the welcome it deserves in the skewed politics of climate change.
Charges of bullying
When Energy and Commerce Committee Chairman Joe Barton (R-Tex.) sent questions to Mann and his colleagues last year, climate-change alarmists and their political friends accused him of bullying scientists. Their complaints typified responses from that side of the issue to doubts about the need for drastic remedies: They dodged the question and vilified the person who asked it. They also were out of line.
The hockey stick is a political lever in a global issue that has bred costly mistakes (OGJ, July 17, 2006, p. 17). Questions about it and other alarmist claims remain very much in order. The two new studies show why.
