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Earth hotter now than in past 2,000 years
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.
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.
And yet, Mann said, "you can go back nearly 2,000 years and the conclusion still holds—the current warmth is anomalous."
"The burst of warming over the past one to two decades takes us out of the envelope of natural variability."
The study will appears today in the journal Proceedings of the National Academy of Science.
Hockey Stick Graph.
The hockey stick graph has become a lightning rod for criticism of the idea that the planet is warming mainly due to human-made greenhouse gases.
Many critics contend that tree rings are unreliable temperature gauges, because temperature is not the only factor that affects the rings.
The controversy led to hearings in the U.S. Congress over the methods Mann and colleagues used in the 1998 study.
However, a 2006 report from the National Research Council—a private, nonprofit scientific institution that advises the U.S. government—supported the hockey stick study while detailing the major uncertainties.
In centuries past, isolated regions have warmed up from time to time, such as during the so-called Medieval Warm Period, when Europe experienced warmer temperatures from about A.D. 900 to 1400.
"But what's unique about modern warming is that essentially the whole globe is warming up in tandem," Mann said.
"The so-called hockey stick … it's alive and well."
Climatologist Gabriele Hegerl of the University of Edinburgh in Scotland is "pleasantly surprised" by the new study.
"Being able to get essentially [the] same result without tree ring data shows that what we are seeing is not something specific to tree rings," Hegerl said, "but a real temperature response."
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