136 Climate changein the twenty-firstcenturyand beyond
The discovery of the ‘ozone hole’ is an example of a change in the at-
mosphere due to human activities, which was a scientific ‘surprise’. By
their very nature such ‘surprises’ cannot, of course, be foreseen. How-
ever, there are various parts of the system which are, as yet, not well
understood, where such possibilities might be looked for^20 ; for instance,
in the deep ocean circulation (see box in Chapter 5, page 99) or in the
stability of the major ice sheets. In the next section we shall look in more
detail at the first of these possibilities; the second will be addressed
in the section of the next chapter entitled ‘How much will sea level
rise?’.Changes in the ocean thermohaline circulation
The ocean thermohaline circulation (THC) was introduced in the box
on page 99 in Chapter 5, where Figure 5.18 illustrates the deep ocean
currents that transport heat and freshwater between all the world’s oceans.
Also mentionedin the box was the influence on the THC in past epochs
of the input of large amounts of fresh water from ice melt to the region in
the north Atlantic between Greenland and Scandinavia where the main
source region for the THC is located.
With climate change due to increasing greenhouse gases we have
seen that the precipitation will increase substantially especially at high
latitudes (Figure 6.5(b)), leading to additional fresh water input to the
oceans. The dense salty water in the north Atlantic source region for the
THC will become less salty and therefore less dense. As a result the THC
will weaken and less heat will flow northward from tropical regions to the
north Atlantic. All coupled ocean–atmosphere GCMs show this occur-
ring; an example is shown in Figure 6.11(a), which indicates a weakening
of about twenty per cent by 2100. Although there is disagreement be-
tween the models as to the extent of weakening, all model projections
of the pattern of temperature change under increasing greenhouse gases
show less warming in the region of the north Atlantic (Figure 6.5(a)) –
but none show actual cooling occurring in this region during the twenty-
first century. In the longer term, some models show the THC actually
cutting off completely after two or three centuries of increasing green-
house gases. Figure 6.11(b) illustrates the effect that cut-off would have
on the pattern of surface temperature around the globe. Note the severe
cooling that would occur in the north Atlantic and north west Europe
and the small compensating warming in the Southern hemisphere. In-
tense research is being pursued – both observations and modelling – to
elucidate further likely changes in the thermohaline circulation and their
possible impact.