2.5.2.4. Milankovitch cycles and ice ages

The three components of the orbital variations together effect both the total flux of incoming solar radiation and also the temporal and spatial distribution of that energy. These variations have the potential to influence the energy budget of the climate system (Milankovitch, 1941; Berger 1978), and can therefore be regarded as possible causes of climate change over a 10⁴ to 10⁵ year time scale. Being external to the climate system, they may be classified as external forcing mechanisms.

Milankovitch (1941) considered the changing seasonal (precession) and latitudinal (obliquity) patterns of incoming radiation to be critical factors in the growth of continental ice sheets and in the initiation of ice ages. He hypothesised that when axial tilt was small (large latitudinal temperature gradient), eccentricity was large and perihelion occurred during the Northern Hemisphere winter (warmer winters and colder summers), such a configuration would allow the persistence of accumulated snow throughout the summer months in the Northern Hemisphere. Additionally, the warmer winters and stronger atmospheric general circulation due to the increased temperature gradient would increase the amount of water vapour at the high latitudes available for snowfall.

For long-term proxy temperature data, spectral analysis, which permits the identification of cycles, has shown the existence of periodicities of 100,000, 43,000, 24,000 and 19,000 year (see Figure 2.2), all of which correspond closely with the theoretical Milankovitch cycles (Hays et al., 1976; Imbrie & Imbrie, 1979).

Nevertheless, verification of a causal link between the orbital forcing factors and the climatic response is far from being proved, and significant problems remain. Firstly, Figure 2.2 shows that the strongest signal in the observational data is the 100,000 year cycle. This would be the result of eccentricity variations in the Earth’s orbit, which alone account for the smallest insolation changes. Secondly, it is not clear why changes in climate appear to be global. A priori reasoning indicates that the effects of precession would cause opposite responses in each hemisphere. In fact, climate change is synchronised between Southern and Northern Hemispheres, with a growth of ice sheets during glaciations occurring in the Arctic and Antarctic. It is now widely believed that the circulation of the oceans provides the forcing factor for synchronisation (Duplessy & Shackleton, 1985; Broecker, 1987; Broecker & Denton, 1990; Boyle & Keigwin, 1987). This is discussed further in section 2.6.4.

Most crucially of all, however, it seems that the orbital forcing mechanisms alone, could not account for the observed climatic variations over the past 2 million years (Hoyle, 1981). In order to explain the magnitude of the observed climatic changes, it seems necessary to invoke various feedback mechanisms. Indeed, Milankovitch himself had expected the direct effects of variations in insolation to be magnified by feedback processes, such as, at high latitudes, the ice albedo effect (section 2.7).