The Milankovitch Theory of Earths Orbital Variations

The Milankovitch theory attempts to account for the existence of Ice Ages at various intervals (see Figure 3-9). This theory makes use of the dependence of the insolation, the total solar radiation impinging on the earth in different latitudes, on a number of factors involving the earth's revolution around the sun. There is quite a bit of controversy about the particular latitudes at which such variations in insolation are most important, but most workers feel that changes in insolation at high latitudes, closer to the poles than 60°N or 60°S, are the most important.

The first orbital variation, which affects all latitudes in a similar fashion, involves the change in the earth's orbit from almost circular to somewhat elliptical and back, with a somewhat variable period of 90,000-100,000 years. When the earth's orbit is at its highest ellipticity, the solar radiation intensity reaching the earth may vary up to 30% in the course of a year. Right now, the earth's orbit is not very elliptical and indeed is becoming more circular.

The second orbital influence on solar irradiation, which affects different latitudes differently, is the 21,000-year cycle in the precession of the earth's axis around the normal to the earth's orbit; this changes the season at which the Northern (or Southern) Hemisphere is closer to the sun. This influence is most important when the earth's orbit is even a little bit elliptical, as it is now. At present, for example, the earth is about 3 x 106 miles closer to the sun during winter in the Northern Hemisphere than during winter in the Southern Hemisphere. This means that there is about 7% less solar irradiation during summer in the Northern Hemisphere than during summer in the Southern Hemisphere.1

The third orbital influence on solar irradiation at different latitudes is the 40,000-year cycle in the tilt of the earth's axis with respect to the normal to the plane of the earth's orbit. This angle varies between 21.8 and 24.4 degrees; at present, it is 23.4 degrees.

These three types of orbital variation, with three different cycle lengths, have been used in various climate models to calculate average global temperatures in the absence of true solar constant variations and in the absence of changes in the greenhouse effect or earth's albedo for at least one million years before the present. Figure 3-12 is one of many comparisons between the calculations from the Milankovitch theory and actual measurements. (The "measurements'' are actually reasonable guesses from various geological data.) The calculations were based on the assumption that the variation in insolation at latitude 50°N could be used if one further assumed that whenever

1This cycle is also connected with the change in the polestar, presently Polaris (almost); in 2600 b.c. the polestar was Alpha Draconis, about 25 celestial degrees away from Polaris.

Measured

700,00 years ago

Present

Measured

700,00 years ago

Present

Cold

Warm

Cold

Predicted

FIGURE 3-12 Comparison of warm and cold periods in the earth's climate as measured from geological data and as predicted from the Milankovitch theory. Redrawn from N. Calder, The Weather Machine. Viking Press, New York. London: BBC. Copyright © 1974. Used by permission of the author.

Warm

Cold

Warm

Cold

Predicted

FIGURE 3-12 Comparison of warm and cold periods in the earth's climate as measured from geological data and as predicted from the Milankovitch theory. Redrawn from N. Calder, The Weather Machine. Viking Press, New York. London: BBC. Copyright © 1974. Used by permission of the author.

the summer sunshine at that latitude was at least 2% stronger than at present, glaciers tended to disappear, and that ice began to accumulate when the summer sunshine was less. Figure 3-12 shows reasonable agreement between the measured and calculated results. More complex assumptions have recently led to even better agreement between measured and calculated results. Therefore, we have a reasonable explanation for the existence of Ice Ages and interglacial periods. These orbital cycles exist and must interact with the other influences on the earth's climate.

Small changes in the greenhouse effect AI can have appreciable effects on climate. These changes can occur when variations occur in the concentration of carbon dioxide, methane, and trace gases, especially those that can absorb radiation in the atmospheric "windows" as mentioned in Section 3.1.2. Figure 3-13 shows the close parallel between mean global temperature and the atmospheric carbon dioxide and methane concentrations as obtained from air trapped in Antarctic ice cores over the past 160,000 years. Although increased burning of fossil fuels is held responsible for the global increase in carbon dioxide concentration in the atmosphere during historic times, it appears that a major increase in carbon dioxide in the atmosphere started almost 20,000 years ago. Although this could be attributed to human activities, many of the other variations in carbon dioxide, and, for that matter, methane concentration, must have another cause. Figure 3-14 shows the recent (i.e., since 1840) increase in methane concentration in the atmosphere. In

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