Human Origins

by S. Laing

Cold alone is insufficient to produce glaciers and ice-caps, as may be seen by the example of the coldest regions in the world, Siberia and the tundras of Northern Asia and of North America, where the earth is permanently frozen to a depth of many feet; but there are no glaciers, The reason obviously is, that there is no sufficient supply of moist air from warm oceans to furnish more snow in winter than is melted in summer. Heat is in a certain sense as necessary as cold to account for glacial periods, for snow and ice can no more than other things be made out of nothing, and every snowflake implies an equal amount of aqueous vapour raised somewhere else by evaporation. But if an abundant supply of liquid or gaseous water is combined with cold sufficient to condense it into the solid form, it becomes fixed, and if the summer heat is insufficient to melt the excess of snow, it necessarily accumulates. The growth of glaciers, follows as an inevitable consequence. The snow is converted into ice by pressure and by alternate freezing and melting, and this grows year by year, until an equilibrium is established by the ice pushing down glaciers into lower levels, where the melting is more rapid, or into the sea, where the front is floated off in icebergs, and drifts into lower latitudes. The process is the same as that by which the rainfall on high levels is drained off by rivers into the sea, so that an equilibrium is established between waste and supply. And it is to be remarked that the glacier, though composed of solid ice, behaves exactly like a river, or rather like a river of some viscous fluid like pitch or treacle. Its size depends on the magnitude of the reservoir or area drained by it; it conforms to the configuration of the valley by which it descends and the obstacles which it encounters; it flows rapidly, and with a broken current, through narrow gorges and down steep inclines; slowly and tranquilly over wide and level areas; its velocity is greatest at the surface and in the middle where friction is least, slowest at the bottom and sides where it is greatest. In short a glacier is simply a solid and slowly-flowing river, discharging an excess of solid ice to the lower level from which it came, just as a liquid river does with the rainfall of warmer regions. The cause of this tendency of solid and brittle ice to flow like a viscous fluid is not quite understood, though recent researches, especially those of Tyndall, have thrown a good deal of light upon it; but all glacialists are agreed on the fact that it does so, and we can argue from it with great confidence as to the conditions under which glaciation has acted in the past and is now acting.

Thus even if Namsen had never crossed Greenland, or Ross had never discovered Mounts Erebus and Terror, we might have inferred with certainty the existence of enormous ice-caps, implying continental masses of elevated land, in both the Arctic and Antarctic circles, from the number and size of the icebergs floated off into the Northern Atlantic and Southern Pacific Oceans. Icebergs are frequently met with in the latter down to 50° south latitude, or even lower, of a mile in length and 500 feet high above the sea; and in some instances icebergs three miles long and 1000 feet high have been recorded. As upwards of eight feet of ice must be under water for every foot that floats above it, some of these icebergs must be considerably over a mile in thickness, which implies that there must be land ice towards the south pole so thick that it is, in places, over 5000 feet in thickness at its outer margin. It has been estimated from the great size and abundance of these icebergs, that in the interior of the great Antarctic continent the ice may be twenty miles or more thick, and in Greenland the great interior ice-cap rises in a dome to at least 9000 or 10,000 feet above the sea-level, a great part of which is solid ice, while during the great glacial period it was certainly very much thicker.

As a first step therefore towards a solution of the problem of the glacial period we may start with the axiom that it requires abundant evaporation, combined with a temperature low enough to precipitate an excess of that evaporation in the solid form. This does not necessarily imply any great and permanent refrigeration of the whole earth, for although this would give the cold it would not give the evaporation, and would tend rather to extend the conditions of Siberia than those of Greenland. Longer and colder winters with shorter and hotter summers would seem more adapted to the growth of glaciers.

But for a more exact investigation our next step must be to inquire what are the causes which may have produced these postulates of a glacial period, lower temperature with larger evaporation. They may be classed under two heads.

1st. Geographical causes, arising from latitude, aërial and oceanic currents, and a different distribution of sea and land.

2nd. Cosmic causes, such as variations of solar and terrestrial heat, passage through colder regions of space, the position of the poles, precession, and the eccentricity of the earth's orbit.

All these have had supporters in their time, but the result of the latest science has been to leave only two seriously in the field—Lyell's theory of a different distribution and elevation of sea and land, carrying with it changes in aërial and oceanic currents; and Croll's theory of the effects of precession combined with high eccentricity of the earth's orbit.

Thus, of the geographical causes, latitude is no doubt an important factor in determining temperature, but it cannot of itself be the cause of the glacial periods, for it has remained unchanged through all the vicissitudes of heat and cold in geological times. The latitude of Greenland and Spitzbergen is presumably the same now as it was in the Miocene period, when they were the seat of a luxuriant temperate vegetation; and at the present day we have only to follow the isothermal lines to see to what a great extent climate in the same latitudes is modified by other influences, such as the Gulf Stream.

Of cosmic causes, the progressive cooling of the earth naturally presents itself, at the first blush, as sufficient to account for the glacial period. But although this has doubtless been an all-important factor in pregeological times, in fashioning our planet from glowing vapour into a habitable earth, it is no longer operative as an immediate cause of vicissitudes of temperature. It is enough to say that if it were, the cooling ought to be progressive, and having once got into a glacial period we never ought to have got out of it. But we clearly have recovered from the paroxysms of cold, both of the first and second great glaciations of the recent period; and according to most geologists, from the immensely earlier ones of the Permian and Carboniferous, and perhaps of the Cambrian ages. As far as it acts at all on surface temperature, the secular cooling of the earth only acts indirectly by causing elevations and depressions of the outer crust, and crumpling it into wrinkles, which originate mountain chains, as the nucleus contracts, and thus affecting geographical conditions.

The same objection applies with equal force to the theory that the glacial period was caused by the sun giving out less heat owing to its cooling by radiation. Here also it is obvious that if a glacial period were once established from such a cause it ought never to recover, but progress from bad to worse. We ought also, in this case, to have had a uniform progressive refrigeration from the beginning of geological time down to the present day, which has certainly not been the case. On the contrary, geologists are generally agreed that there are unmistakable traces of at least two glacial periods in the Carboniferous and Permian ages, and the earliest Eocene was certainly cooler than its later stages, as shown by their flora.