Study: Global warming is largest rise in 1,300 years

WASHINGTON — A new study adds evidence that temperatures in the Northern Hemisphere fluctuated a bit over time, but that the sharp increase during the past few decades is bigger than anything in at least 1,300 years.

The report was published Tuesday in the Proceedings of the National Academy of Sciences. Its conclusion is that temperature increased and decreased a little over the centuries, but the fluctuations were small enough that the line was roughly flat, like the shaft of a horizontal hockey stick. From about 1980 to now, temperature increased sharply, more than any increase before, like the blade of the hockey stick.

For the past 10 years, climate-change skeptics have been calling the hockey stick bogus. The scientists who studied the climate record and produced the original hockey-stick graph have done a new study using more data from more sources — and they got the same pattern.

The new study "establishes further evidence that the recent warming isn't just part of a typical cycle," said climatologist Michael Mann, director of the Earth System Science Center at Pennsylvania State University.

"Of course, this alone doesn't establish the cause of that warming, that it must be due to human influences," Mann said. That's left to other climate studies.

Forces of nature — changes in the output of the sun's energy and volcanic eruptions — and random variation explain the changes in climate before industrial times, Mann said. But only if human factors are taken into account — particularly the production of long-lasting, heat-trapping gases from burning fossil fuels — can scientists explain the unusually high recent temperature increase, he said.

Mann's group's study collected additional data for the centuries before the mid-19th century, when scientists began recording temperatures.

Their previous study depended on tree rings.

Mann's team took data from other natural sources of clues about past climate: coral, ice cores and lake and cave sediments.