Although it has been known for over 2000 yr that the position and orientation of the Earth relative to the Sun has not been constant, it was not until the mid-nineteenth century that the significance of such variations for the Earth's climate was really appreciated. At that time, James Croll, a Scottish natural historian, developed a hypothesis in which the ultimate cause of glaciations in the past was considered to be changes in the Earth's orbital parameters (Croll, 1867a,b; 1875). The hypothesis was later elaborated by Milankovitch (1941) and more recently by A. Berger (1977a, 1978, 1979, 1988). An excellent account of the way in which this hypothesis developed into a crucial theory in paleoclimatology (the "astronomical theory") is given by Imbrie and Imbrie (1979).
The basic elements of the Earth's orbital motion around the Sun today are as follows: the Earth moves in a slightly elliptical path during its annual revolution around the Sun; because of the elliptical path, the Earth is closest to the Sun (perihelion) around January 3, and around July 5 it is farthest away from the Sun (aphelion). As a result, at perihelion the Earth receives -3.5% more solar radiation than the annual mean (outside the atmosphere) and ~3.5% less at aphelion. The Earth is also tilted on its rotational axis 23.4° from a plane perpendicular to the plane of the ecliptic (the apparent surface over which it moves during its revolution around the Sun). None of these factors has remained constant through time due to gravitational effects of the Sun, Moon, and the other planets on the Earth. Variations have occurred in the degree of orbital eccentricity around the Sun, in the axial tilt (obliquity) of the Earth from the plane of the ecliptic, and in the timing of the perihelion with respect to seasons on the Earth (precession of the equinoxes) (Fig. 2.16).
Variations in orbital eccentricity are quasi-periodic with an average period length of -95,800 yr over the past 5 million yr. The orbit has varied from almost circular (essentially no difference between perihelion and aphelion) to maximum eccentricity when solar radiation receipts (outside the atmosphere) varied by -30% between aphelion and perihelion (e.g., at -210,000 yr B.P.; Fig. 2.16). Eccentricity variations thus affect the relative intensities of the seasons, which implies an opposite effect in each hemisphere.
Changes in axial tilt are periodic with a mean period of 41,000 yr. The angle of inclination has varied from 21.8 to 24.4° with the most recent maximum occurring about 100,000 yr ago (see Fig. 2.16). The angle defines the latitudes of the polar circles (Arctic and Antarctic) and the tropics, which in turn delimit the area of daylong polar night in winter, and the maximum latitudes reached by the zenith sun in midsummer in each hemisphere. Changes in obliquity have relatively little effect on radiation receipts at low latitudes but the effect increases towards the poles. As obliquity increases, summer radiation receipts at high latitudes increase, but winter radiation totals decline. This is seen in the summer radiation variations over the last 250,000 yr for 65 and 80° N (see Fig. 2.16), which reflect mainly the periodic changes in axial tilt. As the tilt is the same in both hemispheres, changes in obliquity affect radiation receipts in the Southern and Northern Hemispheres equally.
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