WASHINGTON — A new scientific 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. Then, 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. Now 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 scientific studies of the climate.
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, and some critics said it was not a reliable way to reconstruct past climate over a long period. Mann said that while it's not always true that tree rings aren't reliable, his team decided to conduct a new study that didn't depend on them.
They took data from other natural sources of clues about past climate — corals, ice cores and lake and cave sediments.
"We found we got more or less the same answer," Mann said. The recent temperature increase is an anomaly over 1,300 years without using tree rings, and for 1,700 years if the tree-ring data are used, the study found.
Scientists have observed a warming of about 0.8 degrees Celsius during the past century. Mann said there was a burst of about 0.3 degrees from about 1900 to 1950. Then, in the 1950s to 1970s, temperatures were flat or showed a slight cooling, because heavy particle pollution, which has a cooling effect, masked the heating effect of greenhouse gases, Mann said.
Another, larger increase of temperature has been recorded in the past 30 years, he said, due largely to the increase of greenhouse gases. Particle pollution was reduced as a result of clean-air laws in the U.S. and other countries.
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