Inside the Hockey Stick

This curve, known as the hockey stick because of its shape, represents an estimate of how the temperature of the earth’s Northern Hemisphere has changed over the past millennium.

To reach back centuries before the invention of the thermometer, Michael E. Mann, an associate professor of meteorology at Pennsylvania State University, and his colleagues used so-called proxy records, derived from tree rings, corals, and ice cores. In a 1999 study, the scientists overlapped that evidence with actual temperature data from the past 150 years to calibrate the proxy records, making them essentially surrogate thermometers for earlier centuries. In a 2001 report, the United Nations’ Intergovernmental Panel on Climate Change highlighted the hockey-stick graph produced by Mr. Mann and his colleagues. In the panel’s version of the figure, the spiky blue line depicts year-to-year temperature variations. The black line represents a smoother version of the curve. Because there are so few sources of data before 1600, the uncertainty in the temperature estimates (shown in gray) is much greater in the first part of the graph.

A panel of the National Research Council recently looked at the 1999 study by Mr. Mann’s group and found problems with it. The panel questioned some specific conclusions of the paper but supported some of its general findings.

One criticism of the 1999 graph centers on the method used to produce it. Some researchers argue that the technique automatically generates hockey-stick shapes in data. The National Research Council found that criticism valid but said it did not seriously affect the results. Studies done with other techniques have produced similar findings.

For the earliest part of the 1999 analysis, Mr. Mann’s group relied heavily on bristlecone pines from western North America. The original study noted that there were some difficulties in using such trees because of peculiarities in their recent growth, but Mr. Mann and his group attempted to quantify those problems and to work around them. The National Research Council suggested that researchers avoid using trees that are the most difficult to interpret. More-recent studies have avoided those trees and reached similar conclusions.

The 1999 study was the first to estimate the uncertainty of temperature estimates based on proxy data from 1,000 years ago. The National Research Council concluded that both the original study and more recent ones have underestimated the uncertainty of their conclusions.

The Chart

http://chronicle.com Section: Research & Publishing Volume 53, Issue 3, Page A11