Hockey Stick Controversy

The "Hockey Stick" has played a very significant role in debate over recent global warming and its causes over the past decade. A seminal paper by Mann, Bradley & Hughes (1998) (MBH98) used temperature histories said to be contained in varying sources (proxies) to reconstruct the temperatures of the past millenium. In doing so, MBH98 concluded that temperatures were relatively constant until about 1900 when temperatures skyrocketed. The graph is called a "Hockey Stick" because the reconstructed temperature curve is shaped like a hockey stick.

"My own short-form summary of our views on MBH98 is this. MBH98 made 5 main warranties: statistical skill, robustness, careful proxy selection, appropriate methodology and relatively even geographical balance. These warranties were fundamental to its acceptance. (My background is in business and I think in contract terms.) All their warranties have been breached. Their reconstruction failed critical cross-validation tests (we have publicized the R2 failure, but it fails others as well); it is not robust [to] the presence/absence of bristlecone pines; the supposedly carefully chosen proxies included bristlecone growth, which specialists say is contaminated by 20th century fertilization; their methodology includes a wildly biased “principal components” methodology (which is not actually a principal components method). The hockey stick is an imprint of bristlecone growth rate and reflects a non-temperature proxy from an isolated geographic region of the U.S.A." —Steve McIntyre

The following are some of the major issues involved with MBH98 proxies, most of which are still problematic as they apply to recent studies as well (see article for specifics).

Tree Rings

Ice Cores

Statistical Analysis (multiproxy studies)

Hockey Stick Studies
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