But although the evidence of some alternations of climate seems indisputable, and no suggestion of any adequate cause for them other than the alternating phases of precession during high excentricity has been made, it by no means follows that these changes were always very great—that is to say, that the ice completely disappeared and a warm climate prevailed throughout the whole year. It is quite evident that during the height of the glacial epoch there was a combination of causes at work which led to a large portion of North-western Europe and Eastern America being buried in ice to a greater extent even than Greenland is now, since it certainly extended beyond the land and filled up all the shallow seas between our islands and Scandinavia. Among these causes we must reckon a diminution of the force of the Gulf Stream, or its being diverted from the north-western coasts of Europe; and what we have to consider is, whether the alteration from a long cold winter and short hot summer to a short mild winter and long cool summer would greatly affect the amount of ice if the ocean currents remained the same. The force of these currents is, it is true, by our hypothesis, modified by the increase or diminution of the ice in the two hemispheres alternately, and they then react upon climate; but they cannot be thus changed till after the ice-accumulation has been considerably affected by other causes. Their direction may, indeed, be greatly
changed by slight alterations in the outline of the land, while they may be barred out altogether by other alterations of not very great amount; but such changes as these have no relation to the alteration of climates caused by the changing phases of precession.
Now, the existence at the present time of an ice-clad Greenland is an anomaly in the northern hemisphere, only to be explained by the fact that cold currents from the polar area flow down both sides of it. In Eastern Asia we have the lofty Stanivoi Mountains in the same latitude as the southern part of Greenland, which, though their summits are covered with perpetual snow, give rise to no ice-sheet, and, apparently, even to no important glaciers;—a fact undoubtedly connected with the warm Japan current flowing partially into the Sea of Okhotsk. So in North-west America we have the lofty coast range, culminating in Mt. St. Elias, nearly 15,000 feet high, and an extensive tract of high land to the north and north-west, with glaciers comparable in size with those of New Zealand, although situated in Lat. 60° instead of in Lat. 45°. Here, too, we have the main body of the Japan current turning east and south, and thus producing a mild climate, little inferior to that of Norway, warmed by the Gulf Stream. We thus have it made clear that could the two Arctic currents be diverted from Greenland, that country would become free from ice, and might even be completely forest-clad and inhabitable; while, if the Japan current were to be diverted from the coast of North America and a cold current come out of Behring's Strait, the entire northwestern extremity of America would even now become buried in ice.
Now it is the opinion of the best American geologists that during the height of the glacial epoch North-eastern America was considerably elevated.[[55]] This elevation would bring the wide area of the banks of Newfoundland far above water, causing the American coast to stretch out in an immense curve to a point more than 600 miles east of Halifax; and this would certainly divert much of the greatly reduced Gulf Stream straight across to the coast of
Spain. The consequence of such a state of things would probably be that the southward flowing Arctic currents would be much reduced in velocity; and the enormous quantity of icebergs continually produced by the ice-sheets of all the lands bordering the North Atlantic would hang about their shores and the adjacent seas, filling them with a dense ice-pack, equalling that of the Antarctic regions, and chilling the atmosphere so as to produce constant clouds and fog with almost perpetual snowstorms, even at midsummer, such as now prevail in the worst portions of the Southern Ocean.
But when such was the state of the North Atlantic (and, however caused, such must have been its state during the height of the glacial epoch), can we suppose that the mere change from the distant sun in winter and near sun in summer, to the reverse, could bring about any important alteration—the physical and geographical causes of glaciation remaining unchanged? For, certainly, the less powerful sun of summer, even though lasting somewhat longer, could not do more than the much more powerful sun did during the phase of summer in perihelion, while during the less severe winters the sun would have far less power than when it was equally near and at a very much greater altitude in summer. It seems to me, therefore, quite certain that whenever extreme glaciation has been brought about by high excentricity combined with favourable geographical and physical causes (and without this combination it is doubtful whether extreme glaciation would ever occur), then the ice-sheet will not be removed during the alternate phases of precession, so long as these geographical and physical causes remain unaltered. It is true that the warm and cold oceanic currents, which are the most important agents in increasing or diminishing glaciation, depend for their strength and efficiency upon the comparative extents of the northern and southern ice-sheets; but these ice-sheets cannot, I believe, increase or diminish to any important extent unless some geographical or physical change first occurs.[[56]]
If this argument is valid, then it would follow that, so long as excentricity was high, whatever condition of climate was brought about by it in combination with geographical causes, would persist through several phases of precession; but this would not necessarily be the case when the excentricity itself changed, and became more moderate. It would then depend upon the proportionate effect of climatal and geographical causes in producing glaciation as to what change would be produced by the changing phases of precession; and we can best examine this question by considering the probable effect of the change in precession during the next period of 10,500 years, with the present moderate degree of excentricity.
Probable Effect of Winter in Aphelion on the Climate of Britain.—Let us then suppose the winters of the northern hemisphere to become longer and much colder, the
summers being proportionately shorter and hotter, without any other change whatever. The long cold winter would certainly bring down the snow-line considerably, covering large areas of high land with snow during the winter months, and causing all glaciers and ice-fields to become larger. This would chill the superincumbent atmosphere to such an extent that the warm sun and winds of spring and early summer would bring clouds and fog, so that the sun-heat would be cut off and much vapour be condensed as snow. The greater sun-heat of summer would no doubt considerably reduce the snow and ice; but it is, I think, quite certain that the extra accumulation would not be all melted, and that therefore the snow-line would be permanently lowered. This would be a necessary result, because the greater part of the increased cold of winter would be stored up in snow and ice, while the increased heat of summer could not be in any way stored up, but would be largely prevented from producing any effect, by reflection from the surface of the snow and by the intervention of clouds and fog which would carry much of the heat they received to other regions. It follows that 10,000 years hence, when our winter will occur in aphelion (instead of, as now, in perihelion), there will be produced a colder climate, independently of any change of land and sea, of heights of mountains, or in the force of oceanic currents.