124 Climate changein the twenty-firstcenturyand beyond
sometime in the second half of the twenty-first century, depending on
the scenario. But, other greenhouse gases are also increasing and con-
tributing to the radiative forcing. So as to achieve an overall picture more
easily, it is often convenient to convert other greenhouse gases to equiv-
alent amounts of carbon dioxide, in other words to amounts of carbon
dioxide that would give the same radiative forcing.^11 The information
in Table 6.1 enables the conversion to be carried out. For instance, the
increases in the greenhouse gases other than carbon dioxide (including
ozone) to date produce about eighty per cent of the radiative forcing due
to the increase in carbon dioxide to date (see Figure 3.8). This propor-
tion will drop substantially during the next few decades as the growth in
carbon dioxide becomes more dominant in nearly all scenarios. Refer-
ring to Figure 6.4(a), and noting that doubled carbon dioxide produces a
radiative forcing of about 3.7 W m−^2 , it can be seen that doubling of the
equivalent carbon dioxide amount from pre-industrial times will occur
between 2040 and 2070 depending on the scenario.
Now, in Figure 6.4(a), select one of the scenarios, say A1B, and
note that the radiative forcing for this scenario reaches that equivalent
to doubled carbon dioxide (i.e. 3.7 W m−^2 ) in about 2040. Then move to
Figure 6.4(b) and note that in 2040 the temperature rise is about 1.7◦C.
This is only just over half the 2.8◦C (the value for climate sensitivity
used for the results presented in Figure 6.4(b) see figure caption) that
would be expected for doubled carbon dioxide under steady conditions.
As was shown in Chapter 5, this difference occurs because of the slowing
effect of the oceans on the temperature rise. But this means that, as the
carbon dioxide concentration continues to increase, at any given time
there exists a commitment to further significant temperature rise which
has not been realised at that time.Regional patterns of climate change
So far we have been presenting global climate change in terms of likely
increases in global average surface temperature that provide a useful
overall indicator of the magnitude of climate change. In terms of regional
implications, however, a global average conveys rather little information.
What is required is spatial detail. It is in the regional or local changes
that the effects and impacts of global climate change will be felt.
With respect to regional change, it is important to realise that, be-
cause of the way the atmospheric circulation operates and the interactions
that govern the behaviour of the whole climate system, climate change
over the globe will not be at all uniform. We can, for instance, expect
substantial differences between the changes over large land masses and
over the ocean; land possesses a much smaller thermal capacity and