Long-term Cooling Trend In Arctic Abruptly Reverses, Signaling Potential For Sea Rise

Warming from greenhouse gases has trumped the Arctic's millennia-long natural cooling cycle, suggests new research. Although the Arctic has been receiving less energy from the summer sun for the past 8,000 years, Arctic summer temperatures began climbing in 1900 and accelerated after 1950.

The decade from 1999 to 2008 was the warmest in the Arctic in two millennia, scientists report in the journal Science. Arctic temperatures are now 2.2 degrees F (1.2 degrees C) warmer than in 1900.

To track Arctic temperatures 2,000 years into the past, the research team analyzed natural signals recorded in lake sediments, tree rings and ice cores. The natural archives are so detailed the team was able to reconstruct past Arctic temperatures decade by decade.

As part of a 21,000-year cycle, the Arctic has been getting progressively less summertime energy from the sun for the last 8,000 years. That decline won't reverse for another 4,000 years.

The new research shows the Arctic was cooling from A.D. 1 until 1900, as expected. However, the Arctic began warming around 1900, according to both the natural archives and the instrumental records.

"The amount of energy we're getting from the sun in the 20th century continued to go down, but the temperature went up higher than anything we've seen in the last 2,000 years," said team member Nicholas P. McKay of The University of Arizona in Tucson.

"The 20th century is the first century for which how much energy we're getting from the sun is no longer the most important thing governing the temperature of the Arctic," said McKay, a UA doctoral candidate in geosciences.

Greenhouse gases are the most likely cause of the recent rise in Arctic temperatures, said McKay and his co-author Jonathan T. Overpeck, a UA professor of geosciences and director of UA's Institute of the Environment.

Overpeck said, "The Arctic should be very sensitive to human-caused climate change, and our results suggest that indeed it is."

As the Arctic warms, the warming accelerates, he said, because there is less snow and ice to reflect solar energy back into space. Instead, the newly exposed dark soil and dark ocean surfaces absorb solar energy and warm further.

McKay, Overpeck, lead author Darrell S. Kaufman of Northern Arizona University in Flagstaff and their colleagues will publish their findings in the September 4 issue of Science. The National Science Foundation funded the research.

Overpeck and his colleagues have been working in the Arctic for about 20 years to understand the region's ancient climate. Until recently, the group had been able to peer back in time only 400 years.

About five years ago, Kaufman, Overpeck and their colleagues began a multi-institution project to analyze sediment cores from more than two dozen Arctic lakes. Lake sediments are often laid down in distinct yearly layers, much like the rings of a tree.

As part of the research for his master's degree at NAU, McKay collected and analyzed sediment cores from Hallet Lake in south-central Alaska.

The annual sediment layers contain indicators of temperature and climate. The changes in the abundance of algae remnants reflect the length of the growing season. In addition, warmer summers result in a thicker annual sediment layer because as glacial meltwater increases, more sediment is deposited.

For the new climate reconstruction, the researchers compared the information from lake sediments with previously published climate reconstructions of the Arctic based on glacial ice cores and tree rings. The data from the natural archives were calibrated against the instrumental temperature record.

The analysis shows that summer temperatures in the Arctic, in step with reduced energy from the sun, cooled at an average rate of about 0.36 degrees F (0.2 degrees C) per thousand years -- until the 20th century.

"The data tell a remarkably clear and consistent story," McKay said.

The scientists also compared their new work with climate reconstructions from a computer model of global climate based at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

The model's estimate of the reduction of seasonal sunlight in the Arctic and the resulting cooling was consistent with the analysis from natural archives. The finding gives scientists more confidence in computer projections of future Arctic temperatures.

The new study follows previous work showing that temperatures over the last century warmed almost three times faster in the Arctic than elsewhere in the Northern Hemisphere.

The finding has implications far beyond the Arctic, McKay and Overpeck said.

Warming in the Arctic may affect sea level rise, primarily from the melting of the great ice sheets, Overpeck said.

A warming Arctic affects weather in the southwestern U.S., McKay said. "Winter storms in the western U.S. are going further north than they used to -- and these are the same storms that bring our rain and snowfall."

Kaufman, McKay and Overpeck's co-authors on the paper, "Recent Warming Reverses Long-Term Arctic Cooling," are David P. Schneider, Caspar M. Ammann and Bette L. Otto-Bliesner of NCAR in Boulder, Colo.; Raymond S. Bradley of the University of Massachusetts, Amherst; Keith R. Briffa of the University of East Anglia in Norwich, UK; Gifford H. Miller of the University of Colorado in Boulder; Bo M. Vinther of the University of Copenhagen in Denmark; and the NSF Arctic System Science Program in Fairbanks, Alaska.