Setting the record straight | "Hockey stick" broken?

The scientific consensus that humans are heating the planet is propped up, in part, by a hockey stick.

Actually, it's a graph that resembles a hockey stick turned on its side.

Based on analysis of tree rings, ice cores, coral reefs and lake sediments dating back a thousand years, it's one of the most comprehensive reconstructions of ancient climate.

The graph shows a fairly straight line until the start of the industrial age, when it jogs skyward. The work of University of Virginia climatologist Michael Mann, the hockey stick suggests the planet is hotter now than any time in the past millennium.

But two Canadians argue the hockey stick underestimates past climate swings because of a computing flaw that fails to distinguish the effect of temperature from other environmental factors, such as nutrient levels, that influence tree-ring size or coral growth.

The result, say mining consultant Stephen McIntyre and economist Ross McKitrick, is that the analysis is dominated by tree rings from 1,000-year-old bristlecone pines. The trees experienced a mysterious growth spurt in the late 1800s and 1900s, apparently not caused by rising temperatures.

The brouhaha even reached Congress this summer, when Texas Republican Rep. Joe Barton — a global-warming doubter — launched an investigation.

The debate hinges on esoteric math. Researchers who have examined the calculations agree there is a statistical crack in the stick, but most say it doesn't change the conclusions. Six other climate studies also found current warming unprecedented in 1,000 years.

A new study pushed the time window back 2,000 years. Scientists found more ups and downs in the past, but nothing in the historical record matched the recent temperature spike.