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Science 4 September 2009:
Vol. 325. no. 5945, pp. 1236 - 1239
DOI: 10.1126/science.1173983
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Reports

Recent Warming Reverses Long-Term Arctic Cooling

Darrell S. Kaufman,1,* David P. Schneider,2 Nicholas P. McKay,3 Caspar M. Ammann,2 Raymond S. Bradley,4 Keith R. Briffa,5 Gifford H. Miller,6 Bette L. Otto-Bliesner,2 Jonathan T. Overpeck,3 Bo M. Vinther,7 Arctic Lakes 2k Project Members{dagger}

The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

1 School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011, USA.
2 Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80305, USA.
3 Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA.
4 Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA.
5 Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK.
6 Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA.
7 Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.

{dagger} These authors and their affiliations are presented at the end of this paper.

* To whom correspondence should be addressed. E-mail: darrell.kaufman{at}nau.edu

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