High northern latitudes have experienced a long-term, slow cooling trend for several millennia, the result of a wobble in Earth's rotation that has been increasing the distance between the sun and Earth and decreasing Arctic summer sunshine. The research team assembled high-resolution records of climate for the past 2,000 years and found that the cooling trend reversed in the mid-1990s.
The decade from 1999 to 2008 was the warmest in the last 200 decades and corresponds with a continuing buildup of human-generated greenhouse gases in Earth's atmosphere, said lead author Darrell Kaufman of Northern Arizona University. "Scientists have known for a while that the current period of warming was preceded by a long-term cooling trend, said Kaufman. "But our reconstruction quantifies the cooling with greater certainty than ever before."
Since the Earth is still moving away from the sun -- it's about 0.6 million miles further during the Northern Hemisphere summer solstice than it was in 1 B.C. -- it appears greenhouse gases began "overriding" the natural cooling of Earth in the middle of the last century, said Professor Gifford Miller of CU-Boulder's Institute for Arctic and Alpine Research, a study co-author. "We expect the Arctic will continue to warm in the coming decades, increasing land-based ice loss and triggering global increases in sea-level rise," he said.
The study was published in the Sept. 4 issue of Science. Other institutions participating in the study included the National Center for Atmospheric Research in Boulder, the University of Arizona, the University of Massachusetts, the University of East Anglia in Norwich, England, and the University of Copenhagen in Denmark. The study was funded by the National Science Foundation.
The research team reconstructed past temperatures on a decade-by-decade basis during the past 2,000 years using information gleaned from ancient lake sediments, ice cores, tree rings and other samples. As part of the study, the decade-by-decade climate data reconstruction was compared with sophisticated climate model simulations run by NCAR researchers.
The NCAR climate simulations agreed closely with the ground-based Arctic data used in the study, said NCAR scientist David Schneider, a co-author on the study. "This result is particularly important because the Arctic, perhaps more than any other region on Earth, is facing dramatic impacts from climate change," Schneider said. "This study provides us with a long-term record that reveals how greenhouse gases from human activities are overwhelming the Arctic's natural climate system."
The new Science study dovetails with a report published earlier this year by the U.S. Climate Change Science Program on changes in the Arctic and at high latitudes. The CCSP study's five lead authors -- including Miller and CU-Boulder INSTAAR Director Jim White -- concluded climate warming in the Arctic and at high latitudes likely will continue at a rapid pace given human-caused changes in Earth's atmosphere.
Arctic temperatures have reached their highest level in the past decade, averaging 2.5 degrees Fahrenheit higher than would have been expected if the 2,000-year cooling trend had continued through the latter part of the 20th century and into the 21st century, said Kaufman. Kaufman received his doctorate from CU-Boulder in 1991 while studying under Miller at INSTAAR.
Previous research has shown that Arctic temperatures increased three times faster during the 20th century than temperatures in the rest of the Northern Hemisphere -- a phenomenon known as "Arctic amplification," said Miller, also a professor of geological sciences at CU-Boulder. The amplification is caused by decreased Arctic sea ice and an increased absorption of the sun's heat by exposed ocean as well as "darker" land areas caused by decreases of Arctic snow and ice, he said.
"With less sea ice in winter, the ocean returns the heat stored in summer to the atmosphere, resulting in warmer winters throughout the Arctic," said Miller.
"Because we know that the processes responsible for past Arctic amplification are still operating, we can anticipate that it will continue into the next century," said Miller. "The magnitude of change was surprising, and reinforces the conclusion that humans are significantly altering Earth's climate."
"As we are confronted with evidence of global warming, it is extremely helpful to be able to use paleoclimate data to provide context for today's climate relative to the range and trajectory of recent climate regimes," said Neil Swanberg, director of NSF's Arctic System Science Program.
Gifford Miller | EurekAlert!
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