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Study Finds Recent Global Warming Unprecedented in 1,300 Years

Renee Schoof

An undated handout photo from the Center for Northern Studies shows the Ward Hunt Ice Shelf disintegrating. (REUTERS/Denis Sarrazin/Center for Northern Studies/Handout)

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.

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.

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.

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

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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