This Cold, Cold World: Science Takes a Look at the Weather
I
THE abandoned terraces of entrapped seas; the hundreds of lakes that lie north of latitude 40°; the forsaken channels of the Columbia, Niagara, and other great rivers; the corals that abound in the rocks of Labrador; the fossil plants on the north shore of Greenland; the numerous glacial moraines and deep glacial scorings, thousands of miles from any existing sheet of ice — these and many other things of like import tell us unmistakably that the climates of the earth were not always as they now are, and very definitely warn us that other profound climatic changes are quite certain yet to come.
Just why the earth as a whole has now and again enjoyed long periods of mild to warm climates, with relatively gentle winds and little rain, no one knows with certainty. Neither is it known beyond a doubt why, through other periods of lesser length, precipitation has been abundant and the temperature everywhere so reduced that in places great sheets of ice spread more than halfway from the poles to the equator. Just so, however, have the climates of the world undergone fundamental changes, now to warm and dry, then to cold and humid, over and over again; nor have all, if indeed any, of these separate periods been uniform through their own duration, but more or less variable within themselves.
Many explanations of these changes have been offered, but, as an Irishman might put it, the best that can be said for most of them is that each is more absurd than any of the others. A few of the explanations, however, have some merit.
Clearly, for instance, if the solar system ever passed, as evidently it may have, through a nebula of appreciable density, the strength of the solar radiation at the distance of the earth must have been more or less decreased during the time of such passage; nor would the amount of this decrease have depended very much on whether the nebula was of the luminous or of the coalsack type. However, it is quite certain that there is no considerable amount of nebular dust between us and the sun at the present time, and, even if we have gone through several nebulæ, that fact alone could account only for the cool to cold periods, and not at all for the longer mild ones — not unless, under the present output of solar radiation, the earth is progressively growing warmer and warmer and will continue to do so until the snow fields and the ice sheets shall have shrunk to but small fractions of their present great extents, or wholly vanished.
The easiest ipse dixit way to account for the climatic changes of the past, and one much in use, is just to attribute all such changes to adequate fluctuations of the right kind in the output of solar radiation. But is this the correct explanation? We do not know any reason whatever why the output of sunshine should alternate, time and again, between periods of greater and less intensity, each hundreds of thousands to many millions of years in duration. Then, too, although we do not know just how long ago any one of these major climatic changes began or ended, we do know that the geologic evidence is to the effect that whenever the land areas were extensive, mountain ranges high, and, in general, volcanic activity abundant, the climates were cool to glacial, and that they were warm and dry when the continents were comparatively small and their mountains relatively few and low. This close relation between continental extent, mountain height, and volcanic activity on the one hand, and kind of climate on the other, comes as near as one could wish to positive proof that the age-long advances and retreats of glacial ice over millions of square miles of the earth’s surface were not at all owing to nebulæ of any kind, to variations in the heat of the sun, to changes in the orbit of the earth, or to any other astronomical cause whatsoever.
Presumably, then, the earth now must be, and always must have been, chiefly responsible for the alterations of its own climates. It was not so much change of climate that expanded and contracted continents, as changes in the sizes of the great land areas and the elevations of their plateaus and mountain ranges, together with the profound differences thus caused in the winds and ocean currents, that altered the climates.
II
Let us, therefore, consider the present condition of the earth as to land and water area, and infer, as best we may, what climatic changes probably would result from certain minor geologic happenings. Seemingly, the present land area is substantially as great as it was at the beginning of any one of two or three of the past glacial periods, and probably many mountains are as high now as most mountains then were. If these things be true, it is evident, therefore, that we must be just teetering on an ice age which some relatively mild geologic action would be sufficient to start going. But what action, we ask, could this be? Any one of several, hence all the more precarious is our teetering.
If, for instance, the Panama Canal had been constructed of the ‘sea level’ type, as many urged it should be, and widened a few hundred miles, thus establishing again the free connection between the Caribbean Sea and the Pacific Ocean that formerly obtained, evidently much of the mild tropical waters which, in the form of the Gulf Stream drift, now temper the climate of Northern Europe, the British Isles, Iceland, Spitsbergen, and other highlatitude regions, would then flow on into the Pacific, leaving all the places it now warms to increased inclemency and inevitable chilling, even, in some cases, to the extent of utterly destructive glaciation.
Again, if that submarine ridge or plateau of but little depth which, by way of Iceland and the Faroe Islands, connects Greenland with North Scotland should rise above the ocean surface, where, presumably, it has been in the not very distant past, all access of the warmer surface water of the North Atlantic to the Arctic Ocean would be cut off. The whole polar basin beyond this ridge, including the Norwegian Sea, doubtless would become ice-covered winter and summer, since the cold arctic water would be hemmed in and the warm tropical water barred out, and therefore the climates of the bordering regions would be appreciably, even profoundly, modified.
Not only is it clear what sort of changes in the climates of the regions in question would occur under the given circumstances, but it is even possible to compute how great these changes would be — not to within a degree of the exact temperature, of course, but to within a few degrees. The way of making this calculation is not entirely simple, for it involves taking into account several different things. One of the more important of these is the effect of the size of an area in determining its climatic influence on adjacent regions, as is evident from the following considerations.
A solitary square yard of snow covering, for instance, in the midst of an extensive warm territory produces no appreciable effect on the temperature of the air a hundred feet away. It reflects, of course, most of the sunshine that falls upon it, and thereby reduces the heating of its surface, and also brings the air that is in actual contact with it down to its own temperature. But the amount of the air thus cooled is so small in comparison with that of the warmer air round about that the result is like unto the adding of only a drop or two of cold water to a kettleful of boiling water — one cannot detect the difference. When, however, the snow field is a circular area a mile in diameter, its cooling effect on a warm day is felt more than a hundred feet away from its edge at any place toward which the air from over the snow happens to be moving. Again, when the diameter of the snow field is one hundred miles, the cooling it produces is perceptible a long way from the leeward edge; and when the diameter is a thousand to fifteen hundred miles, the effect is about as great as it can become. Furthermore, until the snow or ice field gets to be very large, the sum total of its cooling effect round about is roughly proportional to its area. This is evident from the fact that the amount of sunshine reflected and thus wholly lost, so far as heating the earth is concerned, is, on the average, about the same for one square yard of snow surface as for any other, so long as there is no great difference in the latitude, or amount of sunshine received per unit area. But this is not the whole of it. Wind blowing over the snow cover cools but little if the distance across is short, but more and more as that distance increases, and for some time nearly in proportion thereto. After a time, however, the cooling has become so great that further chilling, incident to increase of distance over the snow, slows down and eventually practically ceases altogether.
If we put these and other facts together, it appears from theory, and is confirmed by observations, that the chilling effect of a snow field or ice sheet round about its border is roughly proportional to its area until that area approaches a million square miles, after which it increases more and more slowly with the further extension of the ice cap. To put it another way, the cool - ing off of the leeward edge of a snow field increases with the fetch or distance traveled by the wind over the cold surface.
Hence, as first developed by F. Kerner, and later elaborated by C. E. P. Brooks in his Climate through the Ages, it seems that if all the oceans were free from ice a fall in temperature about the north pole to only a degree or so Fahrenheit below the freezing point of sea water probably would lead to the formation of an extensive ice field measuring about two thousand miles from centre to circumference. Evidently in this case the average movement of the cold surface winds everywhere would have a component outward toward the warmer border.
In this connection a most important point to keep clearly in mind is the fact that, so long as water is water, it can successfully keep up the fight against cooling; keep it up by means of its own convection — that is, by the sinking of the chilled and contracted surface water and the consequent forcing of warmer under water to the top. But, when water is ice, convection in it does not and cannot occur, and any air that gets cold must stay cold, for there is then no heat available from below, except the ineffectually small amount that comes through the ice by conduction. And even this little is rendered still less when, as is certain to happen, the ice becomes covered by snow, for snow is an exceedingly poor conductor of heat, — in fact, it is all but a perfect insulator, — and is besides, to make matters worse, a good reflector of sunlight. Land areas, of course, are always like ice sheets in respect to convection; the surface remains surface no matter how cold it gets. Ice and snow, therefore, accumulate on land areas sooner than on ocean surfaces, and produce the same general cooling on their borders that occurs at the edges of ocean polar ice.
III
From the foregoing it seems that if the earth had some way of automatically cooling off a little, say a degree or two or three at most, it could bring on its own extensive ice sheets without any help from the outside. Apparently, too, it could bring them on without the aid of any more favorable distribution of land and water than now exists, or higher elevation of mountain and plateau.
Now, surprising as it may seem, the earth does have within itself, and always has had, a very effective means of automatic cooling. This is its power, often exercised, especially during active mountain building, of filling the air far above the highest clouds with dust of the finest texture — dust hurled to great heights by violently exploding volcanoes and then spread all over the earth in the form of a dry nebulous veil that no rain can clear away or wind dispel. A selective radiation screen it is — a screen that far more effectively shuts out incoming solar radiation than it shuts in outgoing earth radiation, thereby producing an inverse greenhouse effect, and correspondingly lowering the temperature of the earth and its atmosphere.
The theory of this cooling effect of a veil of volcanic dust is very straightforward and conclusive. Besides, on several occasions in recent times volcanic dust has been thrown into the upper air, and in every case the intensity of the sunshine was diminished as by a thin cloud, and the temperature of the earth beneath more or less lowered. In 1783, for instance, the volcanoes Skaptar Jökull in Iceland and Asama in Japan violently exploded and so filled the high atmosphere with their powdered débris that, as Benjamin Franklin, then in Europe, put it, one scarce could kindle brown paper with a burning lens. Naturally the next two or three years were abnormally cool. Similarly, the famous ‘year without a summer,’ 1816, — or, as it was facetiously called, ‘eighteenhundred-and-froze-to-death,’ — followed quickly the great explosion of Tambora in the island of Sumbawa, off the east end of Java. Again, Krakatoa, in the Strait of Sunda, hurled vast quantities of dust far above the highest clouds on August 27, 1883, dust that for two or three years colored the twilight skies of every land and clime and measurably reduced the average temperature of the whole world. Finally, Katmai in Alaska, on June 6, 1912, likewise filled the air with dust that spread over at least all the Northern Hemisphere, reducing the intensity of the sunshine and lowering the average value of the temperature wherever it went.
Suppose now, and the supposition is not unreasonable, that volcanic explosions of the Asama and Krakatoa type should become far more frequent than usual, say two or three a year, for just one hundred years,—a mere moment, as it were, in geologic time, — or even half a hundred, or, to be exceedingly conservative, for only so short a period as but a single decade. What climatic effects would be at least temporarily produced thereby, and what others might, perhaps, be even permanently established?
The immediate and inevitable result would be a distinct lowering of the average temperature of every season and for all parts of the world. That in turn would extend the snow and ice covering, also for all seasons. This snow covering would increase the loss of sunshine by reflection — virtually decrease the output of solar radiation so far as heating the earth is concerned. Furthermore, the extension of the snow and ice sheet would increase the cooling along its border, as explained above.
Besides all this, the amount of water vapor, the earth’s chief and warmest blanket, would be somewhat thinned, because the amount of water vapor that can exist in a given volume rapidly decreases with decrease of temperature. The earth’s blanket of carbon dioxide would also probably be slightly thinned, owing to its greater absorption by cold water than by warm. The amount and distribution of cloudiness would be changed to some extent, as likewise would the strength of the winds, frequency and course of storms, and every other weather phenomenon. Just what all this would amount to would obviously depend on the drop in temperature which would be fixed by the density of the veil of dust, as determined by the frequency, magnitude, and intensity of the volcanic explosions.
IV
But, one may ask, if in ten years, or fifty years, or one hundred years, the volcanoes should so spend their violence as to drop back to their present quiescent state wherein they only occasionally manifest their potential might and power, would not the earth then quickly return to its previous milder climates?
Maybe so, but, on the other hand, maybe not. If we tip an object just a little way off its base and then let go, it generally drops right back into its former position, but if we tip it farther and farther there soon comes a time when letting go does no good — it is already beyond its point of equilibrium and topples over without being pushed. So it is, or seems to be, with the snow and ice sheets of the earth. Slight extensions of them would usually, no doubt, soon disappear on the removal of the initial cause of their growth. Considerable increases, however, might not so completely vanish with the disappearance of the circumstance that started them. Indeed, under certain conditions they might even still further expand for a time, owing to their waste by reflection of sunshine. and the chilling they cause of the surface air.
The point is this. The size of the continents, the heights of their mountains and plateaus, and the oceanic circulation may be such that any considerable increase of the ice and snow sheets beyond their normal extent would, of itself, lead to a still greater extent thereof up to a certain limiting value, even though the cause of the first cooling were wholly removed. These are circumstances under which volcanic explosions might be most effective in producing lower temperatures. They always push the climates to a cooler stage, and under favorable circumstances might force them beyond their point of equilibrium so that they would automatically topple over into a cooler state of greater or less permanence, from which frigid or semi-frigid condition the earth could rescue itself only by suitable changes in its crust, proper alterations of the oceanic circulations, or other drastic action. All of these changes have occurred over and over in the past, and beyond reasonable doubt will occur again and again in the ages to come.
It does not follow, of course, from any or all of these considerations, that another ice age, or recrudescence of the one that now is slowly growing milder, will be upon us in the immediate future, or indeed within a million years, or even many millions; but it is a scientific certainty that we are not wholly safe from such a world catastrophe. Even all but insignificant geologic changes could profoundly alter the world’s climates and again start deepening and widening the six million square miles of glaciers that still are with us from the culmination of the last ice age.
Perhaps nothing of this kind will happen for a long, long while, but sooner or later it surely will if the future can be inferred from the past. When this will be we have no sure means of knowing, but we do know that the climatic gait of this our world is insecure and unsteady, teetering, indeed, on an ice age, however near or distant the inevitable fall.