1977 - 1988 1989 - 1995 2000 - 2005
North Pacific Pattern Arctic Oscillation Arctic Warm
-2.5 -2 -1 0 1 2 2.5
Figure 3.4: Recent Northern Hemisphere surface temperature anomalies averaged over periods with different types of dominating
pattern of natural variability. The Northern Pacific pattern was dominant in the Arctic from 1977 to 1988, while the Arctic Oscilla-
tion dominated the region from 1989 to 1995. In spring 2000 to 2005 neither of these alternate states is evident – the recent warm
period in the Arctic represents a new and uncertain climate pattern.
Source: J.E. Overland; data from NOAA/ESRL 2007
9
Using climate models to examine the centres around the world, for the recent past and for the
20th century and to look ahead future. These models were run several times, starting
with slightly different conditions but with the same exter-
We can understand the relative influence of external nal forcing, to simulate the effects of natural variability.
forcing from greenhouse gases and internal natural vari-
ability of the climate system for the 20th and 21st cen- Arctic
turies through the use of atmosphere-ocean coupled
climate models. To enable comparisons of the climate Figure 3.5(a) shows model runs for a subset of models,
model runs for the Intergovernmental Panel on Climate recreating the Arctic winter land temperatures of the 20th
Change Fourth Assessment Report
1
, the World Climate century. By the end of the century virtually all model runs
Research Programme (WCRP) established an archive arrive at temperatures that are above the 20th century
containing model results from over 20 national climate mean, implying the importance of greenhouse gas forc-
34 GLOBAL OUTLOOK FOR ICE AND SNOW
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