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(a)
1910
1930
1950
1970
1990
(b)
1910
1930
1950
1970
1990
Figure 1: a) Air temperature variations averaged over Northern Hemisphere land and b)
Sea surface temperaure averaged over the Northern Hemisphere ocean. Warm season is
solid, cold season dashed. (From Wallace, Zhang, and Bajuk, 1995).
greater difficulties in measuring SST, the smaller warming of the northern
hemisphere ocean relative to the land since the 70's is robust and generally agreed to. Further, the large difference between cold and warm season
northern hemisphere temperatures so evident for the land is not apparent
for the ocean temperatures.
A confirmation of these trends over the last decade or so for wintertime
conditions is given in Fig. 2 where we see that the northern land masses
have warmed but the north Pacific and north Atlantic have cooled. We
see, therefore, why the SST shown in Fig. 1 is not rising as fast as the
land temperatures: the land is heating over the last two decades while the
northern Atlantic and Pacific are cooling. We will return to this in section
3 below.