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-4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Figure 6.5 Zonal means of change from 1990 in atmospheric (top) and oceanic (bottom) temperatures (°C) shown as cross-sections. Values are multi-model means for the A1B scenario for 2046-65 (a) and 2080-99 (b). Stippling denotes regions where the multi-model ensemble mean divided by the multi-model standard deviation exceeds 1.0 (in magnitude).

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give the same radiative forcing.14 Such equivalent carbon dioxide amounts are denoted by CO2e. The information in Table 6.1 enables the conversion to be carried out.15 For instance, the increases in greenhouse gases (including ozone) other than carbon dioxide to date produce a radiative forcing equivalent to about three-quarters of that from carbon dioxide to date (see Figure 3.11). This proportion will drop substantially during the next few decades as the growth in carbon dioxide becomes more dominant in nearly all scenarios.

Calculations of equivalent carbon dioxide (CO 2 e) have often been made including only the contributions from other greenhouse gases, sometimes only the long-lived greenhouse gases (i.e. not including ozone). However, as Figure 3.11 and Table 6.1 show, there are substantial contributions to radiative forcing from aerosols in the atmosphere that are predominantly negative. Unless otherwise stated, calculations of CO2e in this book include the aerosol contributions.

Noting that doubled carbon dioxide produces a radiative forcing of about 3.7 W m-2, it can be seen from Table 6.1 that doubling of the CO 2 e amount from pre-industrial times will occur for the SRES marker scenarios around 2050 or before. For scenario A1B, assuming halocarbons and the cooling effect of aerosols in 2050 remain as in 2005 (Table 6.1), radiative forcing in 2050 is approximately equivalent to that from doubled CO2e (i.e. 3.7 W m-2). Referring now to Figure 6.4a, note that in 2050 for scenario A1B the temperature rise from pre-industrial times is about 2.2 °C. This is only about 75% of the 3 °C (the best value for climate sensitivity for the models employed to provide the results presented in Figure 6.4) that would be expected for doubled CO2e under steady conditions. As was shown in Chapter 5, this difference occurs because of the slowing effect of the oceans on the temperature rise. This means that, as the CO2e concentration continues to increase, at any given time there exists a commitment to further significant temperature rise that has not been realised at that time. This is illustrated by the temperature profile (see Figure 6.4a and b) for a scenario for which the concentration of all greenhouse gases and aerosols is kept constant at year 2000 levels. For this profile, warming continues throughout the century beginning with about 0.1 °C per decade for the first few decades.

What about the value of CO2e now? If the contributions to radiative forcing for 2005 (Figure 3.11) from all greenhouse gases and aerosols are summed and turned into CO2e,16 a value of around 375 ppm results (note that without the aerosol and ozone contributions the value would be about 455 ppm). That is not far from the present concentration of carbon dioxide itself, because the negative forcing of aerosol in global average terms has approximately offset the positive forcing of the increased contributions from gases other than carbon dioxide. Note that this calculation is only approximate as there is substantial uncertainty surrounding the magnitude of aerosol forcing (Figure 3.11). Will this aerosol offset continue to apply in the future? All scenarios continue to include substantial if reducing aerosol contributions during the twenty-first century.17 These considerations will surface again in Chapter 10 when we are looking at possibilities for the stabilisation of CO2e.

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