Climate and Time in Their Geological Relations / A Theory of Secular Changes of the Earth's Climate

by James Croll

The change in the eccentricity of the earth’s orbit may affect the climate in two different ways; viz., by either increasing or diminishing the mean annual amount of heat received from the sun, or by increasing or diminishing the difference between summer and winter temperature.

Let us consider the former case first. The total quantity of heat received from the sun during one revolution is inversely proportional to the minor axis.

The difference of the minor axis of the orbit when at its maximum and its minimum state of eccentricity is as 997 to 1000. This small amount of difference cannot therefore sensibly affect the climate. Hence we must seek for our cause in the second case under consideration.

There is of course as yet some little uncertainty in regard to the exact mean distance of the sun. I shall, however, in the present volume assume it to be 91,400,000 miles. When the eccentricity is at its superior limit, the distance of the sun from the earth, when the latter is in the aphelion of its orbit, is no less than 98,506,350 miles; and when in the perihelion it is only 84,293,650 miles. The earth is therefore 14,212,700 miles further from the sun in the former position than in the latter. The direct heat of the sun being inversely as the square of the distance, it follows that the amount of heat received by the earth when in these two positions will be as 19 to 26. Taking the present eccentricity to be ·0168, the earth’s distance during winter, when nearest to the sun, is 89,864,480 miles. Suppose now that, according to the precession of the equinoxes, winter in our northern hemisphere should happen when the earth is in the aphelion of its orbit, at the time when the orbit is at its greatest eccentricity; the earth would then be 8,641,870 miles further from the sun in winter than at present. The direct heat of the sun would therefore be one-fifth less during that season than at present; and in summer one-fifth greater. This enormous difference would affect the climate to a very great extent. But if winter under these circumstances should happen when the earth is in the perihelion of its orbit, the earth would then be 14,212,700 miles nearer the sun in winter than in summer. In this case the difference between winter and summer in the latitude of this country would be almost annihilated. But as the winter in the one hemisphere corresponds with the summer in the other, it follows that while the one hemisphere would be enduring the greatest extremes of summer heat and winter cold, the other would be enjoying a perpetual summer.

It is quite true that whatever may be the eccentricity of the earth’s orbit, the two hemispheres must receive equal quantities of heat per annum; for proximity to the sun is exactly compensated by the effect of swifter motion—the total amount of heat received from the sun between the two equinoxes is the same in both halves of the year, whatever the eccentricity of the earth’s orbit may be. For example, whatever extra heat the southern hemisphere may at present receive from the sun during its summer months owing to greater proximity to the sun, is exactly compensated by a corresponding loss arising from the shortness of the season; and, on the other hand, whatever deficiency of heat we in the northern hemisphere may at present have during our summer half year in consequence of the earth’s distance from the sun, is also exactly compensated by a corresponding length of season.

It has been shown in the introductory chapter that a simple change in the sun’s distance would not alone produce a glacial epoch, and that those physicists who confined their attention to purely astronomical effects were perfectly correct in affirming that no increase of eccentricity of the earth’s orbit could account for that epoch. But the important fact was overlooked that although the glacial epoch could not result directly from an increase of eccentricity, it might nevertheless do so indirectly. The glacial epoch, as I hope to show, was not due directly to an increase in the eccentricity of the earth’s orbit, but to a number of physical agents that were brought into operation as a result of an increase.

I shall now proceed to give an outline of what these physical agents were, how they were brought into operation, and the way in which they led to the glacial epoch.

When the eccentricity is about its superior limit, the combined effect of all those causes to which I allude is to lower to a very great extent the temperature of the hemisphere whose winters occur in aphelion, and to raise to nearly as great an extent the temperature of the opposite hemisphere, where winter of course occurs in perihelion.

With the eccentricity at its superior limit and the winter occurring in the aphelion, the earth would be 8,641,870 miles further from the sun during that season than at present. The reduction in the amount of heat received from the sun owing to this increased distance would, upon the principle we have stated in [Chapter II.], lower the midwinter temperature to an enormous extent. In temperate regions the greater portion of the moisture of the air is at present precipitated in the form of rain, and the very small portion which falls as snow disappears in the course of a few weeks at most. But in the circumstances under consideration, the mean winter temperature would be lowered so much below the freezing-point that what now falls as rain during that season would then fall as snow. This is not all; the winters would then not only be colder than now, but they would also be much longer. At present the winters are nearly eight days shorter than the summers; but with the eccentricity at its superior limit and the winter solstice in aphelion, the length of the winters would exceed that of the summers by no fewer than thirty-six days. The lowering of the temperature and the lengthening of the winter would both tend to the same effect, viz., to increase the amount of snow accumulated during the winter; for, other things being equal, the larger the snow-accumulating period the greater the accumulation. I may remark, however, that the absolute quantity of heat received during winter is not affected by the decrease in the sun’s heat,[28] for the additional length of the season compensates for this decrease. As regards the absolute amount of heat received, increase of the sun’s distance and lengthening of the winter are compensatory, but not so in regard to the amount of snow accumulated.

The consequence of this state of things would be that, at the commencement of the short summer, the ground would be covered with the winter’s accumulation of snow.