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Science 24 March 2000:
Vol. 287. no. 5461, pp. 2246 - 2250
DOI: 10.1126/science.287.5461.2246
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Reports

Simulation of Early 20th Century Global Warming

Thomas L. Delworth, * Thomas R. Knutson

The observed global warming of the past century occurred primarily in two distinct 20-year periods, from 1925 to 1944 and from 1978 to the present. Although the latter warming is often attributed to a human-induced increase of greenhouse gases, causes of the earlier warming are less clear because this period precedes the time of strongest increases in human-induced greenhouse gas (radiative) forcing. Results from a set of six integrations of a coupled ocean-atmosphere climate model suggest that the warming of the early 20th century could have resulted from a combination of human-induced radiative forcing and an unusually large realization of internal multidecadal variability of the coupled ocean-atmosphere system. This conclusion is dependent on the model's climate sensitivity, internal variability, and the specification of the time-varying human-induced radiative forcing.

Geophysical Fluid Dynamics Laboratory (GFDL)/National Oceanic and Atmospheric Administration, Princeton, NJ 08542, USA.
*   To whom correspondence should be addressed. E-mail: td{at}gfdl.gov

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