The astronomical Milankovitch theory of climate variation

During at least the last million years, climate has fluctuated in a distinctive way. In recent years there have been attempts to explain the causes of the long-term climatic fluctuation (e.g. Imbrie and Imbrie, 1979; Bradley, 1985). The astronomical theory or the Milankovitch theory has undoubtedly attracted the greatest attention. This theory was in fact developed by Croll about 100 years ago, but was later elaborated by the Serbian geophysicist Milutin Milankovitch. The theory is based on the assumption that changes in the Earth's orbit and axis cause surface temperature changes on the Earth. Due to the gravitational influence of other planets, the shape of the Earth's orbit changes from almost circular to elliptical and back again over a period of approximately 100,000 years, a process referred to as the eccentricity of the orbit. (Fig. 2.1).

The tilt of the Earth's axis varies from 21°39' to 24°36' and back over a period of 41,000 years, called the obliquity of the ecliptic. The third factor, caused by gravitational pull exerted by the sun and the moon which causes the Earth to wobble around its axis, is termed precession of the equinoxes or precession of the solstices. This means that the seasons when the Earth is nearest to the sun (perihelion) varies with cycles of 23,000 and 19,000 years. At present, the northern hemispheric perihelion occurs in winter, while aphelion (Earth on the farthest point on the orbit) is in summer.

The total amount of radiation to the Earth is mainly determined by the eccentricity of the Earth's orbit. The other astronomical variables, however, affect the latitudinal energy distribution. The regularity of the astronomical effects makes it possible to calculate changes through

Periodicity c. 21.000 years

(a) Almost elliptical Almost circular

Periodicity 96,000 years b

Theory Change Climate Policy

Periodicity 42,000 years (i) Now

$ Summer

Winter

Figure 2.1 The components of the astronomical theory of climate change: (a) eccentricity of the orbit; (b) obliquity of the ecliptic; and (c) precession of the equinoxes. (Adapted from Imbrie and Imbrie, 1979).

(a) Almost elliptical Almost circular

Winter

Periodicity 96,000 years b

Periodicity c. 21.000 years

Periodicity 42,000 years (i) Now

Winter

$ Summer time. Milankovitch was able to make radiation input estimates for different latitudes, and thereby temperature changes. Generally, solar radiation in the low and middle latitudes is mainly related to precession and eccentricity variations, while the effects of eccentricity are modified by obliquity changes in higher latitudes.

The astronomical theory was first published in 1924. Soon it became apparent, however, that the Late Quaternary glacial episodes, as reconstructed at that time, were not in accordance with the astronomical theory. In the mid-1950s the theory was more or less rejected. In the 1970s, however, studies of sea-level changes and deep ocean sediments led to increased interest in the Milankovitch theory. The ¿l80 variations in marine microfossils, which record long-term environmental variations, made it possible to test the astronomical

VARIATION IN ATMOSPHERIC GAS CONTENT AND CLIMATE CHANGE 11

theory against climatic data. Spectral analysis of ocean climatic records (oxygen isotope variations) revealed cycles of 100,000, 41,000 and 23,000/19,000 years (Hays et al., 1976). Evidence of the astronomical variables were subsequently found in various proxy records (coral reefs, pollen, loess, ice cores, lacustrine sediments). These data sets therefore confirmed the hypothesis that changes in the orbital variables are the primary forcing mechanism for Quaternary climate change (e.g. Imbrie et al, 1993b).

Although the astronomical theory explains the main Quaternary climatic fluctuations, recent research has shown that other factors have also influenced global climate variation. Quaternary climatic cycles have not been constant. Prior to approximately 800,000 years ago, a periodicity of 41,000 years prevailed. Subsequently, the 100,000 yr climatic cycle dominated (Ruddiman et al., 1986). During the last 700,000-800,000 years, northern hemispheric ice sheets grew larger than those attained during the previous 1.6-1.7 million years (Ruddiman and Raymo, 1988). Elements in the climatic system that may modify the orbital climate forcing include the location of landmasses, tectonic activity, oceanic circulation, ice cover, carbon dioxide, methane, and dust particles.

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