6.15.2.1 Well-mixed greenhouse gases

Emissions from the SRES scenarios (Nakic´enovic´ et al., 2000) were used in Chapter 4 to simulate the concentrations of well-mixed greenhouse gases and O₃ in the atmosphere for the period 2000 to 2100. Here we compute the associated radiative forcing due to well-mixed greenhouse gases using the simplified expressions given in Table 6.2 (for CO₂, the expression in the first row of that Table is employed). The numbers displayed in Table 6.14 represent the radiative forcings in 2050 and 2100 with respect to the year 2000. For the gases relevant to radiative forcing only, the SRES scenarios give a wide range of radiative forcings for the various compounds. Note that the CO₂ forcing increases substantially in the future. All the SRES scenarios give positive radiative forcing values for the well-mixed greenhouse gases (except for the radiative forcing by CH₄ in the B1 scenario for 2100). The scenario for gases relevant to radiative forcing and ozone depletion yields negative values of the radiative forcing, reflecting the reductions in atmospheric concentrations of ozone depleting gases due to emission control.

From Table 6.11, the radiative forcing due to CO₂ is at present slightly larger than 50% of the total greenhouse gas forcing (including tropospheric O₃). In 2050 and 2100 the SRES scenarios indicate a stronger dominance of CO₂ on the total greenhouse gas forcing (including tropospheric O₃), at about 70 to 80% of the total forcing. The SRES scenarios indicate that HFC-134a will give the largest radiative forcing amongst the HFCs.

Table 6.14: Future (2050 and 2100) radiative forcings due to gases that are relevant for radiative forcing only (based on SRES scenarios), and due to gases that are relevant for radiative forcing and ozone depletion (based on WMO (1999) scenarios). For some gases the results for the IS92a emission scenario, with updated model calculations, are also shown (see Chapter 4 for further details). The radiative forcings presented here are with respect to the year 2000. They may be added to the approximately present day (1998) total radiative forcing given in Table 6.1 to obtain the approximate radiative forcings with respect to pre-industrial times (1750).
Gas	Radiative forcing (Wm-2)
	2050
	A1b	A1t	A1fi	A2	B1	B2	A1p	A2p	B1p	B2p	IS92a
Gases relevant to radiative forcing only
CO2	1.90	1.62	2.24	1.90	1.46	1.37	1.97	1.93	1.30	1.45	1.66
CH4	0.22	0.25	0.30	0.27	0.04	0.20	0.31	0.26	0.09	0.24	0.25
N2O	0.10	0.08	0.18	0.17	0.12	0.08	0.10	0.17	0.12	0.09	0.14
Tropospheric O3	0.24	0.37	0.66	0.43	0.04	0.28	0.32	0.42	0.15	0.28	0.32
HFC-23	0.003	0.003	0.003	0.003	0.003	0.003	0.003	0.003	0.003	0.003
HFC-125	0.013	0.013	0.013	0.008	0.008	0.008	0.013	0.007	0.008	0.009	0.020
HFC-32	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
HFC-134a	0.079	0.078	0.076	0.043	0.042	0.048	0.111	0.054	0.057	0.070	0.079
HFC-143a	0.009	0.009	0.009	0.006	0.006	0.006	0.010	0.006	0.006	0.007
HFC-152a	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.005
HFC-227ea	0.010	0.010	0.010	0.006	0.006	0.007	0.010	0.005	0.006	0.007
HFC-245ca	0.017	0.017	0.016	0.008	0.009	0.010	0.017	0.008	0.009	0.011
HFC-43-10mee	0.002	0.002	0.002	0.001	0.001	0.001	0.002	0.001	0.001	0.001
CF4	0.007	0.007	0.007	0.007	0.004	0.008	0.013	0.010	0.008	0.012
C2F6	0.002	0.002	0.002	0.002	0.001	0.002	0.003	0.002	0.002	0.002
C4 F10	0.000	0.000	0.000	0.000	0.000	0.000	0.007	0.005	0.005	0.005
SF6	0.010	0.010	0.010	0.011	0.007	0.009	0.011	0.011	0.008	0.011
Gases relevant to radiative forcing and ozone depletion
CFC-11						-0.04
CFC-12						-0.06
CFC-113						-0.01
CFC-114						-0.002
CFC-115						0.000
CCl4						-0.009
CH3 CCl3						-0.004
HCFC-22						-0.020
HCFC-141b						-0.001
HCFC-142b						0.000
HCFC-123						0.000
Halon-1211						-0.001
Halon-1301						0.000
Gas	Radiative forcing (Wm-2)
	2100
	A1b	A1t	A1fi	A2	B1	B2	A1p	A2p	B1p	B2p	IS92a
Gases relevant to radiative forcing only
CO2	3.48	2.39	5.15	4.42	2.06	2.73	3.46	4.42	2.34	2.79	3.48
CH4	0.08	0.14	0.51	0.59	-0.07	0.39	0.15	0.58	0.10	0.27	0.43
N2 O	0.16	0.11	0.40	0.36	0.17	0.14	0.13	0.36	0.21	0.12	0.25
Tropospheric O3	0.15	0.11	0.89	0.87	-0.16	0.43	0.21	0.87	0.05	0.37	0.55
HFC-23	0.003	0.003	0.003	0.003	0.003	0.003	0.002	0.002	0.002	0.002
HFC-125	0.031	0.032	0.031	0.025	0.013	0.023	0.030	0.023	0.013	0.023
HFC-32	0.002	0.002	0.002	0.002	0.001	0.002	0.002	0.002	0.001	0.001
HFC-134a	0.129	0.142	0.133	0.130	0.055	0.123	0.172	0.168	0.076	0.154
HFC-143a	0.026	0.027	0.026	0.020	0.012	0.018	0.026	0.020	0.012	0.019
HFC-152a	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.007
HFC-227ea	0.021	0.022	0.021	0.018	0.011	0.020	0.020	0.016	0.010	0.018
HFC-245ca	0.021	0.023	0.022	0.024	0.009	0.024	0.020	0.023	0.009	0.020
HFC-43-10mee	0.004	0.005	0.004	0.003	0.002	0.003	0.004	0.002	0.002	0.002
CF4	0.021	0.021	0.021	0.025	0.010	0.024	0.028	0.034	0.015	0.033
C2 F6	0.004	0.004	0.004	0.005	0.002	0.005	0.006	0.007	0.003	0.007
C4 F10	0.000	0.000	0.000	0.000	0.000	0.000	0.022	0.016	0.013	0.017
SF6	0.027	0.027	0.027	0.031	0.016	0.020	0.027	0.032	0.017	0.034
Gases relevant to radiative forcing and ozone depletion
CFC-11						-0.06
CFC-12						-0.10
CFC-113						-0.02
CFC-114						-0.003
CFC-115						0.000
CCl4						-0.010
CH3 CCl3						-0.004
HCFC-22						-0.030
HCFC-141b						-0.001
HCFC-142b						-0.002
HCFC-123						0.000
Halon-1211						-0.001
Halon-1301						-0.001

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