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.
