Global Warming

Longer-term climatechange 135

Probability (%)

Thresholds (mm day−^1 )

<0.1

0.01

0.1

1

10

100

>10 >20 >30 >50

Observations

GCM

RCM

Figure 6.10Example of

simulations showing the

probability of winter days

over the Alps with different

daily rainfall thresholds, as

observed, simulated by a

300-km resolution GCM

and by a 50-km resolution

RCM. The RCM shows

much better agreement

with observations

especially for higher

thresholds.

Longer-term climate change

Most of the projections of future climate that have been published cover
the Twenty-first century. For instance, the curves plotted in Figure 6.2 ex-
tend to the year 2100. They illustrate what is likely to occur if fossil fuels
continue to provide most of the world’s energy needs during that period.
From the beginning of the industrial revolution until 2000 the burning
of fossil fuels released approximately 600 Gt of carbon in the form of
carbon dioxide into the atmosphere. Under the SRES A1B scenario it is
projected that a further 1500 Gt will be released by the year 2100. As
Chapter 11 will show, the reserves of fossil fuels in total are sufficient to
enable their rate of use to continue to grow well beyond the year 2100.
If that were to happen the global average temperature would continue
to rise and could, in the twenty-second century, reach very high levels,
perhaps up to 10◦C higher than today (see Chapter 9). The associated
changes in climate would be correspondingly large and could well be
irreversible.
A further longer-term effect that may become important during this
century is that of positive feedbacks on the carbon cycle due to climate
change. This was mentioned in Chapter 3 (see box on page 40) and the
+30% uncertainty in 2100 in the atmospheric concentrations of carbon
dioxide shown in Figure 6.2 was introduced to allow for it. Taking this
effect into account would add nearly a further degree Celsius to the
projected increase in global average temperature in 2100 at the top end
of the range shown in Figure 6.4. Some of the further implications of
this feedback will be considered in Chapter 10 on page 255–259.
Especially when considering the longer term, there is also the possi-
bility of surprises – changes in the climate system that are unexpected.