434 GREENHOUSE GASES EFFECTS
FIGURE 5 Predictions of globally—averaged warming caused by increasing the concentration of carbon dioxide by 1%
p.a. compound over 75 years showing the year-to-year changes. The changes for the northern and southern hemispheres
are shown separately.0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
YEAR–1.0–0.50.00.51.01.52.02.53.0TEMPERATURE DIFFERENCE (K)GLOBAL MEAN
MEAN OVER S HEMISPHEREMEAN OVER N HEMISPHERE(c)(a)(b)more realistic simulations in which the carbon-dioxide,
instead of being doubled in one step, is increased gradu-
ally at 1% p.a. compound to double after 70 years. On this
time-scale, the atmospheric response will be influenced by
changes occurring at depth in the oceans, and especially in
the top 1 km.
The first results of such an experiment were published by
Manabe et al. (1990) from GDFL. The globally and annually
averaged increase in surface air temperature was 2.3 K, lower
than in earlier models with a shallow ocean. The reduced
warming was especially marked in the Southern Hemisphere,
which showed little amplification in the Antarctic compared
with the Arctic. This is explained by the ocean circulation
in the southern oceans having a downward branch at about
65 S, which carries much of the additional ‘greenhouse’ flux
of heat from the surface to depth of 3 km, where it remains
for many decades.
Very similar results were produced with the earlier ver-
sion of the UKMO model by Murphy (1990), Murphy and
Mitchell (1995). The annually averaged response in global
mean surface temperature to CO 2 increasing 1% p.a. over
75 years is shown in Figure 4, and also in Figure 5, which also
shows the results for the hemispheres separately. Averaged
over the years ’66 −’77, the global mean warming was 1.7K.
The corresponding increase for the Northern Hemisphere
was 2.6K, with warming of 4 K over large areas of theArctic. The UKMO model, like the GDFL model, shows that
the much smaller response of the Southern Hemisphere is
due to the transport of heat from the surface to depth in a
strong down-welling circulation near 60S. A similar vertical
circulation, caused by melting ice, and penetrating to about
1.5 km depth, occurs at about 60N in the North Atlantic
(see Figure 6). After a slow start, the enhanced global warm-
ing settles down at about 0.3 K/decade. Moreover, the model
exhibits variability on inter-annual and decadal time-scales;
the peak-to-peak variation on the decadal scale being about
0.3K—of the same magnitude as the predicted signal due to
‘greenhouse’ warming.
A similar long-term run with a coupled atmosphere—
deep ocean model has been carried out at the Max Planck
Institute in Hamburg by Cusbasch et al. (1992). CO 2 is allowed
to increase rather more rapidly to double after 60 years and
produces a global mean warming of 1.3K, the lowest value
so far reported.
The transient responses to the doubling of CO 2 by all three
models, ranging from 1.3 to 2.3 K, correspond to about 60%
of the expected equilibrium response. This implies a lag of
about 30 years due largely to the delaying effect of oceans.
The predicted changes in precipitation, though small
on average, are far from uniformly distributed. The UKMO
model indicates increases in high latitudes of the Northern
Hemisphere throughout the year, in middle latitudesC007_002_r03.indd 434C007_002_r03.indd 434 11/18/2005 10:28:01 AM11/18/2005 10:28:01 AM