4.1 The Earth’s orbit around the Sun

If the Sun’s output were constant then the amount of solar radiation reaching the Earth would depend only on the distance between these two bodies. This distance, Re, varies during the year due to the ellipticity of the Earth’s orbit which is measured by the value of its eccentricity (e). The value of e itself, however, varies in time with periods of around 100,000 and 413,000 years due to the gravitational influence of the Moon and other planets. At any particular point on the Earth the amount of radiation striking the top of the atmosphere also depends on two other orbital parameters. One of these is the tilt (𝜃t) of the Earth’s axis to the plane of its orbit which varies cyclically with a period of about 41,000 years. The other is the longitudinal position of the vernal equinox relative to the perihelion of the orbit (p), which is determined by the precession of the Earth’s axis. This varies with periods of about 19,000 and 23,000 years. Figure 20View Image shows values of e, 𝜃t and p calculated over a period of several hundred thousand years.
View Image

Figure 20: The eccentricity, precession and tilt of the Earth’s orbit calculated to take place over 350,000 years. From Burroughs (1992).

Averaged over the globe and over a year the solar energy flux at the Earth depends only on e but seasonal and geographical variations of the irradiance depend on 𝜃t and e sin p. However, it is not just the temperatures of individual seasons that are at stake: the intensity of radiation received at high northern latitudes in summer determines whether the winter growth of the ice cap will recede or whether the climate will be precipitated into an ice age. Thus changes in seasonal irradiance can lead to much longer term shifts in climatic regime.

Figure 3View Image showed temperature variations associated with transitions between ice ages and inter-glacials but also presented the concentrations of methane and carbon dioxide preserved in the ice core showing a strong correlation between these and temperature. (NB neither concentration reaches anything like the present day values of about 1700 ppbv and 380 ppmv respectively). One theory (Petit et al., 1999) suggests that the warming of southern high latitudes caused by the orbital variations is amplified by the release of CO2 from the southern oceans and this warming is then further amplified through a reduction in albedo resulting from the melting of Northern Hemisphere ice sheets. Such positive feedback mechanisms might explain the sharp increases in temperature seen in the record.

Cyclical variations in climate records with periods of around 19, 23, 41, 100 and 413 kyr are generally referred to as Milankovitch cycles after the geophysicist who made the first detailed investigation of solar-climate links related to orbital variations.

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