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Show 150 ISLAND L 1FE. [l'AR'l' I. almost perpetual snow storms, 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 ?nu,st have been its state during the height of the glacial epoch), can we suppose that the mere change from the distant Run in winter and near sun in summer, to the reverse, could bring about any important alteration- the physical and geog1·aphical causes of glaciation remwining ~~nchanged? For, certainly, the less powerful sun of summer, even though lasting somewhat longer, could not do more than tho 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 favoura. ble geographical and physical causes (and without this coinbination it is doubtful whether extreme glaciation would ever occur), then the ice-sheet will not be removed during the alternate phases of procession,· 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 a.nd efficiency upon the compamtive 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. 1 1 In reply to an objection of a somewhat similar nature to this, Dr. Croll has recently stated (Geol. Mag., Oct., 1879) that he "has not assumed that the comparative disappearance of the ice on the warm hemisphere during the period of high excentricity is due to any additional heat derived from the snn in consequence of tho greater length of the summer," but that "the real and effective cause of the disappearance of the ice was the enormous transference of equatorial heat to temperate and polar regions by means of ocean currents.'' But this is surely arguing in a circle ; for the ocean currents are mainly due to the difference of temperature of the polar and equatorial areas combined witl1 the peculiar form and position of the continents, and some one or more of these factors must be altered befure the ocean currenli:i toward::~ the north pole CHAP. YilT.] 'filE CAUSES OF GLACIAL EPOCHS. 151 If this argument is valid, then it would follow that so loner as cxcentricity was high, whatever condition of climate ~as bro:crht abo~t by it in combination with geographical causes, wo~ld persist t~1rougb several phases of precession; but this would not necessanly be the case when the excentricity itself changed, and bec~me mor~ moder~te. It would then depend upon the pro~ ortwna~e ~ffect of chmatal and geogmphical causes in producmg g~ae1atwn as to what chango would be produced by the changmg phases of precession; and we can best examine this questio~ by co.nsidering the probable effect of the change in precessiOn durmg the next period of 10,500 years, with the present moderate degree of excentricity. PTobable effect of Winter in aphelion on the Olirnate of BTitain. -Let us then suppose the winters of the northern hemisphere t? 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 consid~rably, covering large areas of high land with snow during the wmter months, and extending all glaciers and ice-fields. 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 sunheat of summer would no doubt consiqerably 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 increa"Sed heat of summer could not be in any way stored up, but would be largely prevented can be increased. The only factor available is the Antarctic ice, and if this were largely increased, the northward-flowing currents might be so increased as to melt some of the Arctic ice. But the very same argument applies to both poles. Without some geographical change the Antarctic ice could not materially diminish during its winter in perihelion, nor increase to any important extent during the opposite phase. We therefore seem to have no available agency by which to get rid of the ice oYer a glaciated country, so long as the geographical conditions rernained "!nclw.nyed and the excentricity continued high. |
