Climate Change 2001:
Working Group I: The Scientific Basis
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Box 7.1, Figure 1: Observed and modelled variations of annual averages of Northern Hemisphere sea-ice extent (106 km2 ). Observed data for 1901 to 1998 are denoted by open circles (Chapman and Walsh, 1993, revised and updated) and for 1978 to 1998 by open triangles (Parkinson et al., 1999, updated). The modelled sea-ice extents are from the GFDL and Hadley Centre climate model runs forced by observed CO2 and aerosols. Modelled data are smoothed by a polynomial fit. Sea-ice extent in these models was determined as the area which had a thickness exceeding 2 cm. This criterion was determined to yield the best agreement with the observed mean during 1953 to 1998; this choice also reproduces the seasonal cycle realistically. Figure from Vinnikov et al. (1999)

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