2,000-year study shows dramatic Arctic climate reversal

A new reconstruction of Arctic climate compiled from temperature indicators over the past 2,000 years shows persuasive evidence that until about 1950, the Arctic had been naturally cooling over the previous 1,900 years. This trend reversed about 50 years ago, according to a research team that includes Raymond Bradley, director of the Climate System Research Center.

“Strong warming in the 20th century contrasts sharply with the preceding cooling trend,” he and co-authors state, pointing to human activities that produce greenhouse gases as a likely factor in the turn-around. They found that “four of the five warmest decades of our 2,000-year-long reconstruction” occurred between 1950 and 2000.

The study, published in the Sept. 3 issue of Science, was conducted over the past five years by a multi-institution team including Bradley and colleagues at Northern Arizona University and the National Center for Atmospheric Research (NCAR), Boulder, Colo., the University of East Anglia, U.K., and others in Denmark, Alaska and Arizona. The work was supported by the National Science Foundation.

The researchers used lake sediment samples to reconstruct Arctic summer temperatures (June, July and August) for lands north of latitude 60 degrees, which for the first time allowed them to extend the climate record 1,600 years further into the past than previous studies. Sixty degrees north latitude passes near Anchorage, Edmonton, Oslo and Moscow. The compilation included 23 sites where lake sediment, glacier ice and tree rings could be used as confirmatory evidence, and overall the sample “accurately represents the Arctic-wide mean” temperature, the authors state.

Bradley says the results are meaningful for our understanding of the human influence on climate change. “Among the most striking features of our composite temperature reconstruction is a cooling from 2,000 to 100 years ago, but this abruptly changed in the mid-20th century,” the geoscientist notes. “The increase in temperature from the 1970s to the 1980s appears to have been the largest decade-to-decade shift of the entire 200-decade reconstruction.”

This contrasts with the first 1,000 years of the reconstruction, which shows summer temperatures in the Arctic cooled at a rate of about 0.2 degrees Celsius per millennium, leading to the “Little Ice Age” which ended about 1850.

The paleoclimate researchers believe this long cooling trend was caused by reduced Arctic sunshine levels caused by a slight change in the position of the Earth in relation to the sun. Although this underlying natural trend has persisted into the present day, by about 1950 summer temperatures in the Arctic were about 0.7 degrees Celsius higher than expected with the natural cooling, providing evidence of human influences on climate change. At present, Arctic temperatures average about 1.4 degrees Celsius higher than expected if the natural cooling had continued, they add.

Furthermore, these results mimic model simulations of climate driven by the same factors and carried out by NCAR scientists in Colorado. Their study involved a 2,000-year computer simulation of climate change that incorporated both the reduced energy received from the Sun and resulting changes in snow and sea-ice in the Arctic. The model’s ability to reproduce the temperature reconstructions over the last 2,000 years increases confidence in the model’s ability to accurately simulate future changes in climate in the Arctic, says Bradley. The Arctic may be the most sensitive area on Earth to human factors that influence climate change, adds NCAR’s David Schneider.

Lead author Darrell Kaufman of Northern Arizona University says results of this international study also agree with previous work showing that Arctic temperatures increased during the 20th century almost three times faster than temperatures increased throughout the rest of the Northern Hemisphere because as more dark land area is exposed in the Arctic it absorbs more sun than the snow once did and accelerates warming, a phenomenon known as Arctic amplification. This Arctic warming is expected to continue, the authors point out, with global consequences.

For the National Science Foundation, Neil Swanberg, director of its Arctic System Science Program, says, “This reconstruction uses a variety of data sources to extend high resolution records back in time sufficiently long to compare reconstructed temperatures to those from models that include changes in insolation due to changes in the Earth’s orbital patterns. That the results appear to match so well increases our confidence in our understanding of the processes that are impacting the global Earth system.”

Besides Bradley, the campus research team included alumni Caspar Ammann, now at NCAR, Tim Cook and Scott Lamoureux, as well as former postdocs Pierre Francus and Mark Abbott.

More Information

Illustration of reversed Arctic cooling

September 8, 2009.

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