The planet is hotter now than it has been for nearly the past 2,000 years, researchers report.
The new study is led by Michael Mann, a climatologist who helped develop the famous 1998 "hockey stick" graph—a reconstruction of Northern Hemisphere temperatures over the past thousand years showing a sharp uptick beginning around 1900.
In their new work, Mann and colleagues back up the hockey stick graph by citing other temperature indicators in the natural record.
The researchers analyzed coral reef skeletons, cores from glaciers and ice sheets and sea floor sediments, and stalagmites and stalagtites formed in caves—all of which trap chemicals that reveal what the temperatures were across past centuries.
"Ten years ago the estimates for earlier centuries were really primarily reliant on just one sort of information: tree ring measurements," said Mann of Pennsylvania State University.
"To satisfy the critics, we now have enough other sources that we can achieve meaningful reconstructions back a thousand years without tree ring data, and we get more or less the same answer"—that global warming is not mainly due to natural variability.
(Related: "Current Warming Period Is Longest in 1,200 Years, Study Says" [February 9, 2006].)
Hot Anomaly
Measurements of the planet's temperature from reliable thermometers stretches back only about 150 years, and measuring temperatures of earlier centuries is quite a bit harder.
Taking the planet's temperature in, say, A.D. 1000, requires measuring tree rings, cores from ice sheets and glaciers, and other natural records that reveal, indirectly, how warm it was in a given year.
But in these reconstructions, "there was quite a bit of uncertainty," Mann said.
The climate has varied over the centuries, with warmer and cooler stretches, the study affirmed.
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