Global Warming

106 Modellingthe climate

into account, the conclusion reached in the IPCC 2001 Report^20 is as

follows.

In the light of new evidence and taking into account the remaining

uncertainties, most of the observed warming over the last 50 years is likely

to have been due to the increase in greenhouse gas concentrations.

Confidence having been established in climate models in the ways we

have outlined in the last two sections, these models can now be used

to generate projections of the likely climate change in the future due

to human activities. Details of such projections will be presented in the

next chapter.

Before leaving comparison with observations, I should mention

some recent work relating to the warming of the ocean that adds fur-

ther confirmation to the picture that has been presented. In Chapter 2,

the effect of an increase of greenhouse gases was expressed in terms

of radiative forcing or, in other words, a net input of heat energy into

the earth-atmosphere system. Most of this extra energy is stored in the

ocean. The amount of this extra energy has been estimated from measure-

ments of the temperature increasein the ocean at different locations and

depths down to 3 km. From 1957 to 1994 the estimate is (19±9)× 1022

joules, the uncertainty relating mostly to inadequate sampling of some

large ocean areas.^21 This amount is a little over 0.5% of the solar energy

received by the Earth over this period. Within the limits of uncertainty, it

agrees well with the estimates of radiative forcing presented earlier and

also with model estimates of ocean heat uptake.^22

Is the climate chaotic?

Throughout this chapter the implicit assumption has been made that

climate change is predictable and that models can be used to pro-

vide predictions of climate change due to human activities. Before

leaving this chapter I want to consider whether this assumption is

justified.

The capability of the models themselves has been demonstrated so

far as weather forecasting is concerned. They also possess some skill in

seasonal forecasting. They can provide a good description of the current

climate and its seasonal variations. Further, they provide predictions

which on the whole are reproducible and which are reasonably consistent

between different models. But, it might be argued, this consistency could

be a property of the models rather than of the climate. Is there any other

evidence to support the view that the climate is predictable?