120 Climate changein the twenty-firstcenturyand beyond
Table 6.1Radiative forcing (W m−^2 ), globally averaged, for
greenhouse gases from the year 1750 to 2000 and from SRES scenarios
to 2050 and 2100Greenhouse gas Year A1B A1T A1FI A2 B1 B2 IS 92a
CO 2 2000 1.46
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
CH 4 2000 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
N 2 O 2000 0.15
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
O 3 (trop) 2000 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 2000 0.34
2050 0.49
2100 0.57Data from Ramaswamy, V.et al.2001. Radiative forcing of climate change. In
Houghton,Climate Change 2001, Chapter 6. Data selected from Tables 6.1 and
6.4. For 2050 and 2100 for the halocarbons, all scenarios make the same as-
sumptions.concentration has become known as the climate sensitivity.^7 The Inter-
governmental Panel on Climate Change (IPCC) in its 1990 Report gave
a ‘best estimate’ of 2.5◦C for the climate sensitivity; it also considered
that it was unlikely to lie outside the range of 1.5◦C to 4.5◦C, a range
that encompasses the results of the best coupled atmosphere–ocean gen-
eral circulation models (AOGCMs). The IPCC 1995 and 2001 Reports
have confirmed these values. Reasons why there remains this range of
uncertainty in the estimatesfrom climate models were explained in Chap-
ter 5. The projections presented in this chapter follow the IPCC 2001
Assessment.^8Projections of global average temperature
When information of the kind illustrated in Figures 6.1, 6.2 and 6.4(a) is
incorporated into simple or more complex models, projections of climate
change can be made. As we have seen in earlier chapters, a useful proxy