As the planet gets warmer, various places are setting temperature records. A new analysis of climate history suggests that's not all. Higher temperatures are more widespread now than they have been in 1200 years.
The last time a considerable swath of Earth heated up was about 1000 years ago. A rise in temperature allowed forests to expand across Russia and thawed out Greenland enough for Vikings to settle. Scientists infer these kinds of climate changes from tree rings, isotope ratios in ice cores or seashells, and other records that depend on temperature. To calibrate these temperature "proxies," climatologists compare them to thermometer measurements that go back 150 years. They then combine proxy temperatures from several sites to infer an average global or regional temperature, giving some proxies more weight because they show a stronger relation to temperature.
This statistical weighting has its uncertainties. To avoid the difficulties in calculating average temperatures, climatologists Timothy Osborn and Keith Briffa of the University of East Anglia in the U.K. took proxy data from 14 locations in the Northern Hemisphere that spanned 800 and 1995. Then they simply determined when each spot appeared warmer or colder than usual. The researchers selected only proxies that had precise dating, so that warm or cold spells at different times were not lumped together.
Although not all the sites had data for the entire time, the team found that more than half of the sites showed evidence of significantly warmer temperatures throughout the 20th century, whereas less than 30% provided such evidence in the medieval warming spell. Specific sites, like Greenland, may have been warmer 1000 years ago, says Osborn, but the overall extent of warming in the Northern Hemisphere is greater now. The team presents its findings in the 10 February issue of Science.
"It is hard to find fault in this analysis because it is so straightforward," says climatologist Mike Mann from Penn State University, University Park. Because this approach is different than previous studies, it "adds robustness to the claim that the 20th century is anomalously warm," Mann says. But physicist Willie Soon of Harvard University argues that proxies in general are not reliable enough measures of temperature. He has gathered data from hundreds of proxies and finds some that support warmer temperatures in the medieval era.
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