Longerterm climate change

Most of the projections of future climate that have been published cover the twenty-first century. For instance, the curves plotted in Figures 6.1 to 6.6 extend 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 300 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 would almost certainly 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 48-9) and the +30% uncertainty in 2100 in the atmospheric concentrations of carbon dioxide shown in Figure 6.2 was introduced to allow for it. Similarly the upper part of the uncertainty bars on the right-hand side of Figure 6.4a for the various SRES marker scenarios makes allowance for this uncertainty - for the A2 scenario in 2100 it amounts to about one degree Celsius. Some of the further implications of this feedback will be considered in Chapter 10 on page 311.

Especially when considering the longer term, there is also the possibility of surprises - changes in the climate system that are unexpected. The discovery of the 'ozone hole' is an example of a change in the atmosphere due to human activities, which was a scientific 'surprise'. By their very nature such 'surprises' cannot, of course, be foreseen. However, there are various parts of the system which are, as yet, not well understood, where such possibilities might be looked for;31 for instance, in the deep ocean circulation (see box in Chapter 5, page 120)

Thresholds (mm day 1)

Figure 6.13 Example 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. Red bars observed, green bars, simulated by GCM; blue bars, simulated by the RCM. The RCM shows much better agreement with observations especially for higher thresholds.

Thresholds (mm day 1)

Figure 6.13 Example 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. Red bars observed, green bars, simulated by GCM; blue bars, simulated by the RCM. The RCM shows much better agreement with observations especially for higher thresholds.

or in the stability of the major ice sheets (see paragraph on climate response, Chapter 5, page 132). 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?'.

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