IPCC (1990) showed time evolution of the radiative forcing due to the well-mixed greenhouse gases. For the other radiative forcing mechanisms, the previous IPCC reports (IPCC, 1990; SAR) did not consider the evolution of the radiative forcing, and assessed mainly the radiative fluxes in the pre-industrial and present epochs. However, more recent studies have considered the time evolution of several forcing mechanisms (Hansen et al., 1993; Wigley et al., 1997; Myhre et al., 2001). The information on the time evolution of the radiative forcing from pre-industrial times to present illustrates the differing importance of the various radiative forcing mechanisms over the various time periods, as well as the different start times of their perturbation of the radiative balance. It also gives useful information in the form of inputs to climate models viz., as driving mechanisms to investigate potential causes of climate changes. Studies of the evolution of various anthropogenic as well as natural forcing mechanisms may then be used to explain potential causes of past climate change, e.g. since pre-industrial times (Chapter 12).

The knowledge of the various forcing mechanisms varies substantially (see Section 6.13) and, for some, the knowledge of their time evolution is more problematic than for others. For example, well-mixed greenhouse gas concentrations are observed very accurately from about 1950, with observations even further back in time, whereas data for most aerosol components are much more scarce and uncertain. Uncertainties in the evolution of many of the radiative forcing mechanisms shown in Figure 6.8 have not been assessed yet, and the presented time history should be regarded as an example of a possible evolution.

The forcing mechanisms, considered, where information on their time evolution is available, are plotted in Figure 6.8, with the present forcing corresponding to the best estimates given in Section 6.13. The time evolution differs considerably among the forcing mechanisms as different processes controlling the emissions and lifetimes are involved. Concentrations of the well-mixed greenhouse gases are taken from Chapters 3 and 4 and the simplified expressions for the radiative forcing in Table 6.2 (first row for CO₂) are used. The evolution of the radiative forcing due to tropospheric O₃ is taken from Berntsen et al. (2000). For the radiative forcing due to stratospheric O₃, information on the evolution is taken from the SAR and scaled to the present forcing of -0.15 Wm^-2 (Section 6.4).

For sulphate, the time evolution for SO₂ emission (Schlesinger et al., 1992) is used, updated with values after 1990 (Stern and Kaufmann, 1996). For black and organic carbon aerosols, the fossil fuel component is scaled to coal, diesel, and oil use and fossil fuel emission (IPCC, 1996b), respectively. Altered emission coefficients as a result of improved technology have not been taken into account and is a substantial uncertainty for the time evolution of the black carbon emission. The biomass component is scaled to the gross deforestation (see SAR). In addition to different forcing mechanisms of anthropogenic origin, two natural forcings have also been considered: solar irradiance variations and stratospheric aerosols of volcanic origin. The Lean et al. (1995) and Hoyt and Schatten (1993) estimates of direct solar forcing due to variation in total solar irradiance are shown. Differences between the solar irradiance constructions are due to use of different proxy parameters for the solar irradiance variations (Section 6.11). The optical depth of the stratospheric aerosols of volcanic origin is taken from Sato et al. (1993) and Robock and Free (1996). The data from Sato et al. (1993) (updated from www.giss.nasa.gov) are given for the period 1850 to 1998, whereas those from Robock and Free (1996) are for the period 1750 to 1988. In Robock and Free (1996) the optical depth is given for the Northern Hemisphere. The relationship between optical depth and radiative forcing from Lacis et al. (1992) is used. The stratospheric aerosols yield a very strong forcing immediately after major eruptions (Section 6.9); however, the lifetime of the stratospheric aerosols is only a few years. Therefore, the transient response due to the forcing by stratospheric aerosols cannot readily be compared to that due to the more sustained or steadily increasing forcings.

The relative evolution of the strengths of the different forcing mechanisms presented above are seen to be very different. The forcing due to well-mixed greenhouse gases (consisting of the components listed in Table 6.11) is the dominant forcing mechanism over the century time period. For tropospheric O₃, the relative radiative forcing compared to the well-mixed greenhouse gases has slightly increased; it was 10% of the well-mixed greenhouse gas forcing in 1900 and is 15% at present. The stratospheric O₃ forcing is significant only over the last two decades. The aerosols have short lifetimes compared with the well-mixed greenhouse gases, and their forcings at any given time depend on the current emissions. This is not the case for the well-mixed greenhouse gases for which both current and previous emissions are relevant. Table 6.13 shows five year averages for the radiative forcings denoted in Figure 6.8 over the period 1960 to 1995. For some of the forcing mechanisms a large increase in the radiative forcing (computed since pre-industrial times) is estimated to have occurred over the period 1960 to 1995. This is the case for the well-mixed greenhouse gases, tropospheric O₃, biomass burning aerosols and organic carbon aerosols from fossil fuel consumption, whereas, for sulphate aerosols and black carbon aerosols from fossil fuel consumption, a smaller increase is estimated.

As seen from Figure 6.8d the radiative forcing due to strato-spheric aerosols of volcanic activity has very large year to year variations. The solar irradiance, according to the two reconstructions, generally increases and may have contributed in an important manner to the warming in the 20th century, particularly in the period from 1900 to 1950. Volcanic activity was particularly strong around 1900 and at different times since 1963. Table 6.13 shows a strong radiative forcing due to the temporal evolution of the stratospheric aerosols of volcanic origin during the period 1961 to 1965; however, the strongest forcing in the course of the past four decades has occurred over the period from 1991 to 1995. The temporal evolution of the stratospheric aerosol content together with the small solar irradiance variations during the last few (two to four) decades indicates that the natural forcing has been negative over the past two and possibly even the past four decades. In contrast, the positive forcing due to well-mixed greenhouse gases has increased rapidly over the past four decades.

Table 6.13: Radiative forcings (5-year averages) due to change in well-mixed greenhouse gases (WMGG), stratospheric O3 , tropospheric O3 , direct effect of sulphate aerosols, fossil fuel organic carbon aerosols (FF OC), fossil fuel black carbon aerosols (FF BC), aerosols from biomass burning (organic carbon and black carbon) (BB), stratospheric aerosols of volcanic origin, and changes in Total Solar Irradiance. All values are evaluated with respect to pre-industrial times (1750).
Time period	WMGG	Strat O3	Trop O3	Sulphate	FF OC	FF BC	BB	Volcanic	Solara	Solarb
1961 to 1965	1.14	0.00	0.17	-0.26	-0.05	0.14	-0.04	-1.00	0.11	-0.07
1966 to 1970	1.27	0.00	0.20	-0.29	-0.06	0.14	-0.04	-0.77	0.11	0.04
1971 to 1975	1.44	-0.01	0.22	-0.33	-0.07	0.15	-0.05	-0.28	0.06	0.03
1976 to 1980	1.64	-0.04	0.25	-0.35	-0.08	0.16	-0.09	-0.15	0.17	0.14
1981 to 1985	1.85	-0.07	0.28	-0.36	-0.09	0.19	-0.16	-0.88	0.17	0.20
1986 to 1990	2.07	-0.11	0.31	-0.40	-0.10	0.20	-0.20	-0.35	0.21	0.19
1991 to 1995	2.26	-0.14	0.34	-0.40	-0.10	0.20	-0.20	-1.42	0.18	0.18
aFrom Lean et al. (1995). bFrom Hoyt and Schatten (1993).

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