Another Slapshot in Climate 'Hockey Stick' Faceoff
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Hockey stick chart from the 2001 IPCC Third Assessment Report, showing Northern Hemisphere surface temperatures (departures from the 1961-1990 average, in degrees Celsius) of the past 1,000 years. Courtesy IPCC.
An enduring dispute in the scientific community and the blogosphere over an iconic climate science graph, known as the "Hockey Stick," has boiled over yet again in the past two weeks, with climate skeptics touting a new analysis they say greatly weakens the evidence supporting the mainstream scientific view that recent warming of Earth's climate is highly unusual and largely due to human emissions of greenhouse gases.
Others say the latest twist in the controversy boils down to baseless accusations of a scientific cover-up.
The fact that this debate is playing out in the blogosphere, rather than peer-reviewed scientific journals, has raised questions about how scientific discourse is conducted in the age of the Internet.
The Hockey Stick chart reconstructs the planet's temperatures going back 1,000 years -- temperature estimates before approximately the mid-1800s are based largely on proxy data such as ice cores and tree rings -- and shows unprecedented warming in the latter half of the 20th century. The recent temperature spike resembles the blade of a hockey stick, which is how the chart earned its nickname. The chart became famous when it was featured in a 2001 U.N. Intergovernmental Panel on Climate Change (IPCC) report.
Subsequent studies have largely affirmed the main conclusion of the original hockey stick research -- that recent warming is extremely unusual at least in the past millenium -- but it continues to be a primary target of criticism from skeptics who question its methods.
The blog Climate Audit, which is a project of Canadian statistician Steve McIntyre, has been investigating the hockey stick and other climate studies for several years. Spurred by the acquisition of new data, in the past few weeks McIntyre has written extensively about temperature reconstructions by a British dendroclimatologist, Keith Briffa, of the University of East Anglia. Briffa specializes in using tree-ring records to decipher Earth's climate history, and some of his work has been used to validate the findings of the original hockey stick study.
The studies McIntyre has questioned are based on tree-ring measurements from the Yamal region of northern Russia, where Briffa has found the familiar hockey stick pattern, with a rapid rise in temperatures in the 20th century. According to his recent posts, McIntyre (after years of effort) obtained Briffa's raw data, and he found that Briffa used very few tree-ring chronologies when he reconstructed 20th-century temperatures. Suspecting that the small sample size influenced the results, McIntyre then performed his own analysis to determine whether the same hockey stick pattern would be evident if he replaced Briffa's tree-ring chronologies with others from the Yamal region.
World map of tree-ring sites. Courtesy NOAA.
Once he substituted the different tree-ring chronologies, McIntyre found that the rapid warming trend shown in Briffa's studies, and used to verify the broader Hockey Stick study, disappeared. A Climate Audit chart compares Briffa's and McIntyre's climate reconstructions, illustrating the dramatic difference.
Allegations of a Cover-Up
McIntyre's fans portray him as a sleuth, hot on the trail of climate science malfeasance. The data has been cooked, McIntyre and others allege, in favor of manmade climate change. The fact that it took about a decade for McIntyre to pry the original data from Briffa, which he needed in order to replicate and then modify his work, has heightened the sense among many climate skeptics that a conspiracy is afoot. Stories of disappearing climate data from other researchers have also raised eyebrows.
McIntyre wrote that Briffa's work has been influential in the climate science community because it fits climatologists' preconceived notions. "... the resulting Yamal chronology with its enormous Hockey Stick blade was like crack cocaine for paleocliomatologists and got used in virtually every subsequent study," McIntyre wrote.
On Sept. 30, Briffa issued a statement denying accusations of cherry-picking data. "The substantive implication of McIntyre's comment... is that the recent data that make up this chronology (i.e. the ring-width measurements from living trees) were purposely selected by me from among a larger available data set, specifically because they exhibited recent growth increases." (Such growth increases would indicate a warming climate).
"This is not the case."
Briffa also downplayed the importance of McIntyre's analysis, stating, "I do not believe that McIntyre's preliminary post provides sufficient evidence to doubt the reality of unusually high summer temperatures in the last decades of the 20th century."
Meanwhile, McIntyre's critics are accusing him of having a politically driven agenda, and challenging him to prove his claims in the peer-reviewed scientific literature.
Last week, the blog RealClimate, which is run by climate scientists including the lead author of the original hockey stick study, posted a rebuttal to McIntyre's work that satirized his emphasis on the Yamal temperature reconstruction.
"Apparently everything we've done in our entire careers is a 'MASSIVE lie' (sic) because all of radiative physics, climate history, the instrumental record, modeling and satellite observations turn out to be based on 12 trees in an obscure part of Siberia. Who knew?" RealClimate's scientists wrote in a group post.
The RealClimate scienitsts also question the validity of what they call "Blog Science" as compared with peer-reviewed science.
"There is nothing wrong with people putting together new chronologies of tree rings or testing the robustness of previous results to updated data or new methodologies. Or even thinking about what would happen if it was all wrong," RealClimate stated. "What is objectionable is the conflation of technical criticism with unsupported, unjustified and unverified accusations of scientific misconduct. Steve McIntyre keeps insisting that he should be treated like a professional. But how professional is it to continue to slander scientists with vague insinuations and spin made-up tales of perfidy out of the whole cloth instead of submitting his work for peer-review?
"Peer-review is nothing sinister and not part of some global conspiracy, but instead it is the process by which people are forced to match their rhetoric to their actual results."