David Perlman, Chronicle Science Editor
The Arctic climate has been warmer over the past decade than during any 10-year period in 2,000 years, according to a study by an international research team that adds powerful new evidence that human-generated greenhouse gases have speeded the pace of the planet's recent warming.
The report from an international team of climate scientists concludes that climate change in the Arctic has accelerated since the Industrial Revolution, abruptly reversing a long-term worldwide cooling trend.
"The study provides a clear example of how increased greenhouse gases are now changing our climate," said Caspar Ammann of the National Center for Atmospheric Research in Boulder, Colo., a co-author of the report published Thursday in the journal Science.
To deduce the Arctic's decade-by-decade climate trend over the centuries, the leading scientists in the international study analyzed sediment cores in 14 Arctic lakes that revealed the varied growth rates of long-buried plants. They also studied Arctic tree rings to determine their growth rates and ages as well as ice cores from glaciers across the Arctic that showed patterns of relative warm and cold.
Researchers at other institutions, seeking to look for patterns of climate change even further back in time, used astronomical records to study the well-known wobble of the globe as it spins on its axis. They found that the Northern Hemisphere has long been moving away from the sun's warmth. During the summer solstice, the Northern Hemisphere is now a million kilometers - about 621,000 miles - farther away from the sun than it was 2,000 years ago, according to the scientist's computer models.
The result was a global period of relative cold that would have continued, the scientists found. But about 1850, at the beginning of the Industrial Age, the planet's climate began overcoming the cooling trend, and the Arctic climate has warmed decade by decade ever since as greenhouse gas emissions have increased, the scientists say.
Stephen Schneider, a Stanford climate expert and biologist who did not participate in the study, called the seven-year study, involving seven major research institutions in three nations, "a heroic effort."
The study, he said, "shows that nature has been, unfortunately, cooperating with theory and showing us on a long-time scale of millennia that the mainstream view is once again bolstered."
It is clear again, Schneider said, that anthropogenic influences - the increasing emission of greenhouse gases into the Earth's atmosphere - are the prime cause of global warming.
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