Climate Change 2001:
Working Group I: The Scientific Basis
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Figure 12.6: (a) Five-year running mean Northern Hemisphere temperature anomalies since 1850 (relative to the 1880 to 1920 mean) from an energy-balance model forced by Dust Veil volcanic index and Lean et al. (1995) solar index (see Free and Robock, 1999). Two values of climate sensitivity to doubling CO2 were used; 3.0°C (thin solid line), and 1.5°C (dashed line). Also shown are the instrumental record (thick red line) and a reconstruction of temperatures from proxy records (crosses, from Mann et al., 1998). The size of both the forcings and the proxy temperature variations are subject to large uncertainties. Note that the Mann temperatures do not include data after 1980 and do not show the large observed warming then. (b) As for (a) but for simulations with volcanic, solar and anthropogenic forcing (greenhouse gases and direct and indirect effects of tropospheric aerosols). The net anthropogenic forcing at 1990 relative to 1760 was 1.3 Wm-2 , including a net cooling of 1.3 Wm-2 due to aerosol effects.

Studies linking forcing and response through correlation techniques
A number of authors have correlated solar forcing and volcanic forcing with hemispheric and global mean temperature time-series from instrumental and palaeo-data (Lean et al., 1995; Briffa et al., 1998; Lean and Rind, 1998; Mann et al., 1998) and found statistically significant correlations. Others have compared the simulated response, rather than the forcing, with observations and found qualitative evidence for the influence of natural forcing on climate (e.g., Crowley and Kim, 1996; Overpeck et al., 1997; Wigley et al., 1997; Bertrand et al., 1999) or significant correlations (e.g., Schönwiese et al., 1997; Free and Robock, 1999; Grieser and Schönwiese, 2001). Such a comparison is preferable as the climate response may differ substantially from the forcing. The results suggest that global scale low-frequency temperature variations are influenced by variations in known natural forcings. However, these results show that the late 20th century surface warming cannot be well represented by natural forcing (solar and volcanic individually or in combination) alone (for example Figures 12.6, 12.7; Lean and Rind, 1998; Free and Robock, 1999; Crowley, 2000; Tett et al., 2000; Thejll and Lassen, 2000).

Mann et al. (1998, 2000) used a multi-correlation technique and found significant correlations with solar and, less so, with the volcanic forcing over parts of the palaeo-record. The authors concluded that natural forcings have been important on decadal-to-century time-scales, but that the dramatic warming of the 20th century correlates best and very significantly with greenhouse gas forcing. The use of multiple correlations avoids the possibility of spuriously high correlations due to the common trend in the solar and temperature time-series (Laut and Gunderman, 1998). Attempts to estimate the contributions of natural and anthropogenic forcing to 20th century temperature evolution simultaneously are discussed in Section 12.4.

We conclude that climate forcing by changes in solar irradiance and volcanism have likely caused fluctuations in global and hemispheric mean temperatures. Qualitative comparisons suggest that natural forcings produce too little warming to fully explain the 20th century warming (see Figure 12.7). The indication that the trend in net solar plus volcanic forcing has been negative in recent decades (see Chapter 6) makes it unlikely that natural forcing can explain the increased rate of global warming since the middle of the 20th century. This question will be revisited in a more quantitative manner in Section 12.4.

Figure 12.7: Global mean surface temperature anomalies relative to the 1880 to 1920 mean from the instrumental record compared with ensembles of four simulations with a coupled ocean-atmosphere climate model (from Stott et al., 2000b; Tett et al., 2000) forced (a) with solar and volcanic forcing only, (b) with anthropogenic forcing including well mixed greenhouse gases, changes in stratospheric and tropospheric ozone and the direct and indirect effects of sulphate aerosols, and (c) with all forcings, both natural and anthropogenic. The thick line shows the instrumental data while the thin lines show the individual model simulations in the ensemble of four members. Note that the data are annual mean values. The model data are only sampled at the locations where there are observations. The changes in sulphate aerosol are calculated interactively, and changes in tropospheric ozone were calculated offline using a chemical transport model. Changes in cloud brightness (the first indirect effect of sulphate aerosols) were calculated by an offline simulation (Jones et al., 1999) and included in the model. The changes in stratospheric ozone were based on observations. The volcanic forcing was based on the data of Sato et al. (1993) and the solar forcing on Lean et al. (1995), updated to 1997. The net anthropogenic forcing at 1990 was 1.0 Wm-2 including a net cooling of 1.0 Wm-2 due to sulphate aerosols. The net natural forcing for 1990 relative to 1860 was 0.5 Wm-2 , and for 1992 was a net cooling of 2.0 Wm-2 due to Mt. Pinatubo. Other models forced with anthropogenic forcing give similar results to those shown in b (see Chapter 8, Section 8.6.1, Figure 8.15; Hasselmann et al., 1995; Mitchell et al., 1995b; Haywood et al., 1997; Boer et al., 2000a; Knutson et al., 2000).

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