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Summary: Contributions of natural and anthropogenic forcing to changes in
temperature extremes over the United States
Gerald A. Meehl,1
Julie M. Arblaster,1,2
and Claudia Tebaldi1
Received 8 June 2007; revised 16 August 2007; accepted 30 August 2007; published 13 October 2007.
[1] 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
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