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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L19709, doi:10.1029/2007GL030948, 2007

Contributions of natural and anthropogenic forcing to changes in temperature extremes over the United States

Gerald A. Meehl

National Center for Atmospheric Research, Boulder, Colorado, USA


Julie M. Arblaster

National Center for Atmospheric Research, Boulder, Colorado, USA


Claudia Tebaldi

National Center for Atmospheric Research, Boulder, Colorado, USA


Abstract

Observations averaged over the U.S. for the second half of the 20th century have shown a decrease of frost days, an increase in growing season length, an increase in the number of warm nights, and an increase in heat wave intensity. For the first three, a nine member multi-model ensemble shows similar changes over the U.S. in 20th century experiments that combine anthropogenic and natural forcings, though the relative contributions of each are unclear. Here we show results from two global coupled climate models run with anthropogenic and natural forcings separately. Averaged over the continental U.S., they show that the observed changes in the four temperature extremes are accounted for with anthropogenic forcings, but not with natural forcings (even though there are some differences in the details of the forcings). This indicates that most of the changes in temperature extremes over the U.S. are likely due to human activity.

Received 8 June 2007; accepted 30 August 2007; published 13 October 2007.

Keywords: extremes; climate change.

Index Terms: 1626 Global Change: Global climate models (3337, 4928); 1637 Global Change: Regional climate change; 1630 Global Change: Impacts of global change (1225); 3305 Atmospheric Processes: Climate change and variability (1616, 1635, 3309, 4215, 4513); 9350 Geographic Location: North America.

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Citation: Meehl, G. A., J. M. Arblaster, and C. Tebaldi (2007), Contributions of natural and anthropogenic forcing to changes in temperature extremes over the United States, Geophys. Res. Lett., 34, L19709, doi:10.1029/2007GL030948.