9.2.1.3 Radiative Forcing of Pre-Industrial Climate Change

Here we briefly discuss the radiative forcing estimates used for understanding climate during the last millennium, the mid-Holocene and the Last Glacial Maximum (LGM) (Section 9.3) and in estimates of climate sensitivity based on palaeoclimatic records (Section 9.6.3).

Regular variation in the Earth’s orbital parameters has been identified as the pacemaker of climate change on the glacial to interglacial time scale (see Berger, 1988 for a review). These orbital variations, which can be calculated from astronomical laws (Berger, 1978), force climate variations by changing the seasonal and latitudinal distribution of solar radiation (Chapter 6).

Insolation at the time of the LGM (21 ka) was similar to today. Nonetheless, the LGM climate remained cold due to the presence of large ice sheets in the Northern Hemisphere (Peltier, 1994, 2004) and reduced atmospheric CO₂ concentration (185 ppm according to recent ice core estimates, see Monnin et al., 2001). Most modelling studies of this period do not treat ice sheet extent and elevation or CO₂ concentration prognostically, but specify them as boundary conditions. The LGM radiative forcing from the reduced atmospheric concentrations of well-mixed greenhouse gases is likely to have been about –2.8 W m^–2 (see Figure 6.5). Ice sheet albedo forcing is estimated to have caused a global mean forcing of about –3.2 W m^–2 (based on a range of several LGM simulations) and radiative forcing from increased atmospheric aerosols (primarily dust and vegetation) is estimated to have been about –1 W m^–2 each. Therefore, the total annual and global mean radiative forcing during the LGM is likely to have been approximately –8 W m^–2 relative to 1750, with large seasonal and geographical variations and significant uncertainties (see Section 6.4.1).

The major mid-Holocene forcing relative to the present was due to orbital perturbations that led to large changes in the seasonal cycle of insolation. The Northern Hemisphere (NH) seasonal cycle was about 27 W m^–2 greater, whereas there was only a negligible change in NH annual mean solar forcing. For the Southern Hemisphere (SH), the seasonal forcing was –6.5 W m^–2. In contrast, the global and annual mean net forcing was only 0.011 W m^–2.

Changes in the Earth’s orbit have had little impact on annual mean insolation over the past millennium. Summer insolation decreased by 0.33 W m^–2 at 45°N over the millennium, winter insolation increased by 0.83 W m^–2 (Goosse et al., 2005a) (Errata), and the magnitude of the mean seasonal cycle of insolation in the NH decreased by 0.4 W m^–2. Changes in insolation are also thought to have arisen from small variations in solar irradiance, although both timing and magnitude of past solar radiation fluctuations are highly uncertain (see Chapters 2 and 6; Lean et al., 2002; Gray et al., 2005; Foukal et al., 2006). For example, sunspots were generally missing from approximately 1675 to 1715 (the so-called Maunder Minimum) and thus solar irradiance is thought to have been reduced during this period. The estimated difference between the present-day solar irradiance cycle mean and the Maunder Minimum is 0.08% (see Section 2.7.1.2.2), which corresponds to a radiative forcing of about 0.2 W m^–2, which is substantially lower than estimates used in the TAR (Chapter 2).

Natural external forcing also results from explosive volcanism that introduces aerosols into the stratosphere (Section 2.7.2), leading to a global negative forcing during the year following the eruption. Several reconstructions are available for the last two millennia and have been used to force climate models (Section 6.6.3). There is close agreement on the timing of large eruptions in the various compilations of historic volcanic activity, but large uncertainty in the magnitude of individual eruptions (Figure 6.13). Different reconstructions identify similar periods when eruptions happened more frequently. The uncertainty in the overall amplitude of the reconstruction of volcanic forcing is also important for quantifying the influence of volcanism on temperature reconstructions over longer periods, but is difficult to quantify and may be a substantial fraction of the best estimate (e.g., Hegerl et al., 2006a).