To do so, the researchers compared 20th-century proxy climate records from 14 locations with thermometer-based measurements from those same areasa comparison that isn't possible for proxy records from past centuries, for which thermometer records are scarce or nonexistent.
Once it was clear that the proxy records matched the thermometer records, the researchers assumed that the earlier portions of the records were in fact accurate.
The team limited its study to the Northern Hemisphere during the last 1,200 years, for which there are relatively rich proxy climate data.
"We found that between [A.D.] 890 and 1170, there was statistically significant widespread warmth corresponding approximately to the so-called Medieval Warm Period," Osborn said.
However, the most widespread warmth was found not in the Middle Ages but during the 20th century.
Proxy records indicate warm conditions in the mid- and late 20th century. But the thermometer measurements clearly show that the expanding area of warmer-than-normal conditions continued through to the present day.
By now almost the entire Northern Hemisphere is warmer than normal, Osborn said.
Other studies have shown similar results, but they typically focused on average temperatures of vast regions. Average temperatures can become skewed if a few spots in a large region are very hot.
Osborn and Briffa's study, by contrast, didn't rely on average temperatures. It looked at temperature readings from many locations throughout the Northern Hemisphere.
"The 20th century stands out as the only period in the past 1,200 years when the records all indicate warmth at the same time," Osborn said.
Such studies have been conducted in the past. But because this approach is independent from the others, the study's findings add weight to those previous findings, says Mann, the Penn State scientist.
"This latest paper might be the nail in the coffin for the small minority of very vocal climate change denialists who continue to challenge the conclusion that the recent warming of the Earth's surface is out of the ordinary," Mann said.
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