WASHINGTON — Human-generated greenhouse gas emissions have helped reverse a 2,000-year trend of cooling in the Arctic, prompting warmer average temperatures in the past decade that now rank higher than at any time since 1 B.C., according to a study published Thursday in the online version of the journal Science.
The analysis, based on more than a dozen lake-sediment cores as well as glacier ice and tree ring records from the Arctic, provides one of the broadest pictures to date of how industrial emissions have shifted the Arctic’s long-standing natural climate patterns. Coupled with a separate report on the region issued Wednesday by the World Wildlife Fund, the studies suggest human-induced changes could transform not only the Arctic but climate conditions across the globe.
“It’s basically saying the greenhouse gas emissions are overwhelming the system,” said David Schneider, a visiting scientist at the National Center for Atmospheric Research and one of the Science article’s co-authors.
The historical study involved 30 researchers from the United States, Britain, Denmark, Norway, Canada and Finland.
Some climate skeptics have argued that the fact that the Earth wobbles in its axis of rotation has helped determine recent warming, rather than human activities.
Northern Arizona University professor Darrell Kaufman, the study’s lead author, notes this rotation means Earth was 620,000 miles closer to the sun in midsummer 2,000 years ago and continues to move farther away. The cooling trend that resulted, he said, “should have continued through the 21st century.”
Instead, summer temperatures in the Arctic are now 2.5 degrees Fahrenheit higher than they would have been expected to be under the natural cycle.
Mark Serreze, director of the National Snow and Ice Data Center at the University of Colorado at Boulder, said the study was significant because it helps confirm scientists’ current understanding of how Earth’s climate has changed over millennia.
“It’s not that we don’t know how the climate works, it just we didn’t have anyone at that time measuring the climate forcing then,” referring to 2,000 years ago. “Climate doesn’t change all by itself for no good reason. Something has to force it.”
Fred Singer, a prominent climate change skeptic who heads the Science and Environmental Policy Project, questioned the Science study on the grounds it does not properly reflect the fact that other researchers have found the Medieval Warm Period — which lasted between A.D. 800 and 1300 — had “higher temperatures than even the past 30 years.”
But documentation of the Medieval Warm Period is primarily about Europe, and natural records indicate average Arctic temperatures during that time were not as high.
