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)