Astronomy Of The Ice Ages

Lieut.Cdr. Allan Comer, MBE RN (retd.), Member of The William Herschel Society, on 1 February 2002

The speaker has spent many years since his retirement studying the Ice Ages, which have critically influenced geography and recent evolution, including that of Mankind. The ice-sheets could certainly have been seen from other planets had there been anyone there to see them, yet we on Earth do not seem to have entirely clear ideas about their causes. The speaker believes that the reasons for their occurrence must be sought in astronomy rather than geology, and in this talk presented his own theories for comparison with current ideas.

The speaker began his talk by pointing out by means of a diagram to scale that even when glaciation during an ice age was at its maximum, there was a huge part of the Earth's surface that was not glaciated.

He also pointed out that the ice of the ice ages is not the same as the ice that arises on a water surface if the water is sufficiently cooled, (alpha-ice), which forms in long needles (see picture). The ice that accumulates on land results from more snow falling as flakes in the course of successive years than can be removed by natural processes, causing a snow-cap to develop and extend. The snowflakes in its core compress into another variety of ice (beta-ice), as air is expelled and its density rises.

Snow is not a product of cold surface conditions - it cools the surface on which it accumulates. Its production requires first of all that water is evaporated, predominantly from the sea. This requires solar energy to stimulate surface molecules to break free, whereupon they rise quickly to a height where adiabatic cooling reduces them to sub-zero temperature, the heat released by the process contributing to the energy of weather. The super-cooled molecules then become visible as clouds when they coalesce into tiny crystals of snow, and finally return to earth as rain where they can be melted by the air they fall through, or as snow where there is insufficient heat to melt them.

Thus the ice ages did not require global cooling for ice caps to develop. Warm, pluvial conditions were needed to extract sufficient water from the sea to lower sea level by 150 metres, to create high water tables and verdant forests where now we have deserts, and to provide the snow to form the ice caps in polar and sub-polar regions.

The paper showed with some confidence that a process that could change the tilt of the Earth's axis would provide all the necessary factors to account for the ice ages. Fig 1 shows the pattern of insolation over the surface of the earth that arises with the present axial tilt of 23½°, and Fig 2 shows the pattern that would emerge if axial tilt should change by a few degrees either side of this present figure. It will be seen that an increased angle would provide less radiation to evaporate water into the atmosphere and more to destroy polar ice caps, and a decrease would evaporate more water and destroy the ice caps more slowly.

Fig 1 Sum of insolation indices for one year at 23.5deg. tilt

Fig 2 Sum of insolation indices for 15deg. (dotted) and 30deg. tilts

It was also shown that the gravitational interaction between Venus and the Earth might well cause precessions that would effectively change the tilt of the Earth's axis in just the manner that would be required, as shown in Figs 3 and 4. Venus produces a measurable perturbation of the Earth's orbit every 19 months due to its gravitational attraction. Fig 3 illustrates the reason the Earth slows down in its orbit as Venus catches it up, and speeds up when Venus has passed it. The lower diagram in Fig.3 shows that there are small forces (e, f) acting to draw both planets into a common plane of the ecliptic, because the orbits are at 3.5o to each other, and that these forces will not be self-correcting when the conjunction is over, but will always be towards the common orbit. The two orbits swing each side of the common orbit. It seems that at present the position is that the orbits are approximately at their furthest apart. The effect of this swing of the orbits is to change the tilt of the Earth (see Fig 4).

These ideas do not prove anything without further investigation, but they are promising, and they do show that much better definitions of ice- age phenomena would remove a great deal of confusion. The following were suggested:

Ice Ages. Times when more snow falls during successive years than can be destroyed by natural processes

Interglacials. Times when too little snow falls in successive years to maintain existing icecaps against natural erosion.

Glacial Epochs. Periods in tectonic history when land existed round the poles on which ice-age phenomena could leave evidence of ice.

Alan Comer

The paper was followed by a very lively discussion of many of the points raised, ranging from whether a powerful computer could calculate the periodicity of the change in tilt of the earth's axis, to whether magnetic reversals could have any effect on the conclusions. His ideas were generally thought to merit further consideration.