Model projections

Results that come from the most sophisticated coupled atmosphere-ocean models of the kind described in the last chapter provide fundamental information on which to base climate projections. However, because they are so

Table 6.1 Radiative forcing (W m-2) globally averaged, for greenhouse gases and aerosols from the year 1750 to 2005 and from SRES scenarios to 2050 and 2100

Greenhouse gas

Year

Radioactive forcing (W m-2)

A1B

A1T

A1FI

A2

B1

B2

IS 92a

CO2

2005

1.66

2050

3.36

3.08

3.70

3.36

2.92

2.83

3.12

2100

4.94

3.85

6.61

5.88

3.52

4.19

4.94

CH4

2005

0.48

2050

0.70

0.73

0.78

0.75

0.52

0.68

0.73

2100

0.56

0.62

0.99

1.07

0.41

0.87

0.91

N2O

2005

0.16

2050

0.25

0.23

0.33

0.32

0.27

0.23

0.29

2100

0.31

0.26

0.55

0.51

0.32

0.29

0.40

O3(trop)

2005

0.35

2050

0.59

0.72

1.01

0.78

0.39

0.63

0.67

2100

0.50

0.46

1.24

1.22

0.19

0.78

0.90

Halocarbons

2005

0.34

Total aerosols

2005

-1.2a

a Including both direct and indirect effects.

demanding on computer time only a limited number of results from such models are available. Many studies have also therefore been carried out with simpler models. Some of these, while possessing a full description of atmospheric processes, only include a simplified description of the ocean; these can be useful in exploring regional change. Others, sometimes called energy balance models (see box on page 144), drastically simplify the dynamics and physics of both atmosphere and ocean and are useful in exploring changes in the global average response with widely different emission scenarios. Results from simplified models need to be carefully compared with those from the best coupled atmosphere-ocean models and the simplified models 'tuned' so that, for the particular parameters for which they are being employed, agreement with the more complete models is as close as possible. The projections presented in the next sections depend on results from all these kinds of models.

In order to assist comparison between models, experiments with many models have been run with the atmospheric concentration of carbon dioxide doubled from its pre-industrial level of 280 ppm. The global average temperature rise under steady conditions of doubled carbon dioxide concentration is known as the climate sensitivity.9 The Intergovernmental Panel on Climate Change (IPCC) in its 1990 Report gave a range of 1.5 to 4.5 °C for the climate sensitivity with a 'best estimate' of 2.5 °C; the IPCC 1995 and 2001 Reports confirmed these values. The 2007 Report stated; 'it is likely to be in the range 2 to 4.5 °C with a best estimate of 3 °C, and is very unlikely to be less than 1.5 °C. Cloud feedbacks [see Chapter 5] remain the largest source of uncertainty.'10 The projections presented in this chapter follow the IPCC 2007 Assessment.11

An estimate of climate sensitivity can also be obtained from paleoclimate information over the last million years (see Chapter 4) that connects variations of global average temperature with variations of climate forcings arising from changes in ice cover, vegetation and greenhouse gas concentrations (Chapter 4, Figures 4.6 and 4.7). The estimate of 3 ± 0.5 °C obtained in this way reported by James Hansen12 agrees very well with the model estimates mentioned above.

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