New research shows that the Arctic reversed a long-term cooling trend and began warming rapidly in recent decades. The blue line shows estimates of Arctic temperatures over the last 2,000 years, based on proxy records from lake sediments, ice cores and tree rings. The green line shows the long-term cooling trend. The red line shows the recent warming based on actual observations. A 2000-year transient climate simulation with NCAR’s Community Climate System Model shows the same overall temperature decrease as does the proxy temperature reconstruction, which gives scientists confidence that their estimates are accurate. Credit: Courtesy Science, modified by UCAR.
Arctic temperatures in the 1990s reached their warmest level of any decade in at least 2,000 years, new research indicates. The study, which incorporates geologic records and computer simulations, provides new evidence that the Arctic would be cooling if not for greenhouse gas emissions that are overpowering natural climate patterns.
The international study, led by Northern Arizona University and the National Center for Atmospheric Research (NCAR), will be published in the September 4 edition of Science. It was primarily funded by the National Science Foundation, NCAR’s sponsor.
The scientists reconstructed summer temperatures across the Arctic over the last 2,000 years by decade, extending a view of climate far beyond the 400 years of Arctic-wide records previously available at that level of detail. They found that thousands of years of gradual Arctic cooling, related to natural changes in Earth’s orbit, would continue today if not for emissions of carbon dioxide and other greenhouse gases.
“This result is particularly important because the Arctic, perhaps more than any other region on Earth, is facing dramatic impacts from climate change,” says NCAR scientist David Schneider, one of the co-authors. “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.”
Darrell Kaufman of Northern Arizona University, the lead author and head of the synthesis project, says the results indicate that recent warming is more anomalous than previously documented.
“Scientists have known for a while that the current period of warming was preceded by a long-term cooling trend,” says Kaufman. “But our reconstruction quantifies the cooling with greater certainty than before.”
Greenhouse gases overtake a natural cycle
The new study is the first to quantify a pervasive cooling across the Arctic on a decade-by-decade basis that is related to an approximately 21,000-year cyclical wobble in Earth’s tilt relative to the Sun. Over the last 7,000 years, the timing of Earth’s closest pass by the Sun has shifted from September to January. This has gradually reduced the intensity of sunlight reaching the Arctic in summertime, when Earth is farther from the Sun.