The Outline of History/Chapter 5

by Herbert George Wells

 

 

V

CHANGES IN THE WORLD'S CLIMATE

§ 1. Why Life Must Change Continually. § 2. The Sun a Steadfast Star. § 3. Changes from Within the Earth. § 4. Life May Control Change.

 

§ 1

THE Record of the Rocks is like a great book that has been carelessly misused. All its pages are torn, worn, and defaced, and many are altogether missing. The outline of the story that we sketch here has been pieced together slowly and painfully in an investigation that is still incomplete and still in progress. The Carboniferous Rocks, the "coal-measures," give us a vision of the first great expansion of life over the wet low-lands. Then come the torn pages known as the Permian Rocks (which count as the last of the Palæozoic), that preserve very little for us of the land vestiges of their age. Only after a long interval of time does the history spread out generously again.

It must be borne in mind that great changes of climate have always been in progress, that have sometimes stimulated and sometimes checked life. Every species of living thing is always adapting itself more and more closely to its conditions. And conditions are always changing. There is no finality in adaptation. There is a continuing urgency towards fresh change.

About these changes of climate some explanations are necessary here. They are not regular changes; they are slow fluctuations between heat and cold. The reader must not think that because the sun and earth were once incandescent, the climatic history of the world is a simple story of cooling down. The centre of the earth is certainly very hot to this day, but we feel nothing of that internal heat at the surface; the internal heat, except for volcanoes and hot springs, has not been perceptible at the surface since first the rocks grew solid. Even in the Azoic or Archæozoic Age there are traces in ice-worn rocks and the like of periods of intense cold. Such cold waves have always been going on everywhere, alternately with warmer conditions. And there have been periods of great wetness and periods of great dryness throughout the earth.

A complete account of the causes of these great climatic fluctuations has still to be worked out, but we may perhaps point out some of the chief of them.^[1] Prominent among them is the fact that the earth does not spin in a perfect circle round the sun. Its path or orbit is like a hoop that is distorted; it is, roughly speaking, elliptical (ovo-elliptical), and the sun is nearer to one end of the ellipse than the other. It is at a point which is a focus of the ellipse. And the shape of this orbit never remains the same. It is slowly distorted by the attractions of the other planets, for ages it may be nearly circular, for ages it is more or less elliptical. As the ellipse becomes most nearly circular, then the focus becomes most nearly the centre. When the orbit becomes most elliptical, then the position of the sun becomes most remote from the middle or, to use the astronomer's phrase, most eccentric. When the orbit is most nearly circular, then it must be manifest that all the year round the earth must be getting much the same amount of heat from the sun; when the orbit is most distorted, then there will be a season in each year when the earth is nearest the sun (this phase is called Perihelion) and getting a great deal of heat comparatively, and a season when it will be at its farthest from the sun (Aphelion) and getting very little warmth. A planet at aphelion is travelling its slowest, and its fastest at perihelion; so that the hot part of its year will last for a much less time than the cold part of its year. (Sir Robert Ball calculated that the greatest difference possible between the seasons was thirty-three days.) During ages when the orbit is most nearly circular there will therefore be least extremes of climate, and when the orbit is at its greatest eccentricity, there will be an age of cold with great extremes of seasonal temperature. These changes in the orbit of the earth are due to the varying pull of all the planets, and Sir Robert Ball declared himself unable to calculate any regular cycle of orbital change, but Professor G. H. Darwin maintained that it is possible to make out a kind of cycle between greatest and least eccentricity of about 200,000 years.

But this change in the shape of the orbit is only one cause of the change of the world's climate. There are many others that have to be considered with it. As most people know, the change in the seasons is due to the fact that the equator of the earth is inclined at an angle to the plane of its orbit. If the earth stood up straight in its orbit, so that its equator was in the plane of its orbit, there would be no change in the seasons at all. The sun would always be overhead at the equator, and the day and night would each be exactly twelve hours long throughout the year everywhere. It is this inclination which causes the difference in the seasons and the unequal length of the day in summer and winter. There is, according to Laplace, a possible variation of nearly three degrees (from 22° 6′ to 24° 50′) in this inclination of the equator to the orbit, and when this is at a maximum, the difference between summer and winter is at its greatest. Great importance has been attached to this variation in the inclination of the equator to the orbit by Dr. Croll in his book Climate and Time. At present the angle is 23° 27′. Manifestly when the angle is at its least, the world's climate, other things being equal, will be most equable.

And as a third important factor there is what is called the precession of the equinoxes. This is a slow wabble of the pole of the spinning earth that takes 25,000 odd years. Any one who watches a spinning top as it "sleeps," will see its axis making a slow circular movement, exactly after the fashion of this circling movement of the earth's axis. The north pole, therefore, does not always point to the same north point among the stars; its pointing traces out a circle in the heavens every 25,000 years.

Now, there will be times when the earth is at its extreme of aphelion or of perihelion, when one hemisphere will be most turned to the sun in its midsummer position and the other most turned away at its midwinter position. And as the precession of the equinoxes goes on, a time will come when the summer-winter position will come not at aphelion and perihelion, but at the half-way points between them. When the summer of one hemisphere happens at perihelion and the winter at aphelion, it will be clear that the summer of the other hemisphere will happen at aphelion and its winter at perihelion. One hemisphere will have a short hot summer and a very cold winter, and the other a long cold summer and a briefer warmish winter. But when the summer-winter positions come at the half-way point of the orbit, and it is the spring of one hemisphere and the autumn of the other that is at aphelion or perihelion, there will not be the same wide difference between the climate of the two hemispheres.

Here are three wavering systems of change all going on independently of each other; the precession of the equinoxes, the change in the obliquity of the equator to the orbit, and the changes in the eccentricity of the orbit. Each system tends by itself to produce periods of equability and periods of greater climatic contrast. And all these systems of change interplay with each other. When it happens that at the same time the orbit is most nearly circular, the equator is at its least inclination from the plane of the earth's orbit, and the spring and autumn are at perihelion and aphelion, then all these causes will be conspiring to make climate warm and uniform; there will be least difference of summer and winter. When, on the other hand, the orbit is in its most eccentric stage of deformation, when also the equator is most tilted up and when further the summer and winter are at aphelion and perihelion, then climates will be at their extremest and winter at its bitterest. There will be great accumulations of ice and snow in winter; the heat of the brief hot summer will be partly reflected back into space by the white snow, and it will be unequal to the task of melting all the winter's ice before the earth spins away once more towards its chilly aphelion. The earth will accumulate cold so long as this conspiracy of extreme conditions continues.

So our earth's climate changes and wavers perpetually as these three systems of influence come together with a common tendency

 

Diagram to Illustrate One Set of Causes, the Astronomical Variations, Which Make the Climate of the World Change Slowly but Continuously.

It does not change in regular periods. It fluctuates through vast ages. As the world's climate changes, life must change too or perish.

 

towards warmth or severity, or as they contradict and cancel each other.

We can trace in the Record of the Rocks an irregular series of changes due to the interplay of these influences; there have been great ages when the separate rhythms of these three systems kept them out of agreement and the atmosphere was temperate, ages of world-wide warmth, and other ages when they seemed to concentrate bitterly to their utmost extremity, to freeze out and inflict the utmost stresses and hardship upon life.

And in accordance we find from the record in the rocks that there have been long periods of expansion and multiplication when life flowed and abounded and varied, and harsh ages when there was a great weeding out and disappearance of species, genera, and classes, and the learning of stern lessons by all that survived. Such a propitious conjunction it must have been that gave the age of luxuriant low-grade growth of the coal-measures; such an adverse series of circumstances that chilled the closing æons of the Palæozoic time.

It is probable that the warm spells have been long relatively to the cold ages. Our world to-day seems to be emerging with fluctuations from a prolonged phase of adversity and extreme conditions. Half a million years ahead it may be a winterless world with trees and vegetation even in the polar circles. At present we have no certainty in such a forecast, but later on, as knowledge increases, it may be possible to reckon with more precision, so that our race will make its plans thousands of years ahead to meet the coming changes.

 

§ 2

Another entirely different cause of changes in the general climate of the earth may be due to variations in the heat of the sun. We do not yet understand what causes the heat of the sun or what sustains that undying fire. It is possible that in the past there have been periods of greater and lesser intensity. About that we know nothing; human experience has been too short; and so far we have been able to find no evidence on this matter in the geological record. On the whole, scientific men are inclined to believe that the sun has blazed with a general steadfastness throughout geological time. It may have been cooling slowly, but, speaking upon the scale of things astronomical, it has certainly not cooled very much.

 

§ 3

A third great group of causes influencing climate are to be found in the forces within the world itself. Throughout the long history of the earth there has been a continuous wearing down of the hills and mountains by frost and rain and a carrying out of their material to become sedimentary rocks under the seas. There has been a continuous process of wearing down the land and filling up the seas, by which the seas, as they became shallower, must have spread more and more over the land. The reverse process, a process of crumpling and upheaval, has also been in progress, but less regularly. The forces of upheaval have been spasmodic; the forces of wearing down continuous. For long ages there has been comparatively little volcanic upheaval, and then have come periods in which vast mountain chains have been thrust up and the whole outline of land and sea changed. Such a time was the opening stage of the Cainozoic period, in which the Alps, the Himalayas, and the Andes were all thrust up from the sea-level to far beyond their present elevations, and the main outlines of the existing geography of the world were drawn.

Now, a time of high mountains and deep seas would mean a larger dry land surface for the world, and a more restricted sea surface, and a time of low lands would mean a time of wider and shallower seas. High mountains precipitate moisture from the atmosphere and hold it out of circulation as snow and glaciers, while smaller oceans mean a lesser area for surface evaporation. Other things being equal, lowland stages of the world's history would be ages of more general atmospheric moisture than periods of relatively greater height of the mountains and greater depth of the seas. But even small increases in the amount of moisture in the air have a powerful influence upon the transmission of radiant heat through that air. The sun's heat will pass much more freely through dry air than through moist air, and so a greater amount of heat would reach the land surfaces of the globe under the conditions of extremes of elevation and depth, than during the periods of relative lowness and shallowness. Dry phases in the history of the earth mean, therefore, hot days. But they also mean cold nights, because for the same reason that the heat comes abundantly to the earth, it will be abundantly radiated away. Moist phases mean, on the other hand, cooler days and warmer nights. The same principle applies to the seasons, and so a phase of great elevations and depressions of the surface would also be another contributory factor on the side of extreme climatic conditions.

And a stage of greater elevation and depression would intensify its extreme conditions by the gradual accumulation of ice caps upon the polar regions and upon the more elevated mountain masses. This accumulation would be at the expense of the sea, whose surface would thus be further shrunken in comparison with the land.

Here, then, is another set of varying influences that will play in with and help or check the influence of the astronomical variations stated in § 1 and § 2. There are other more localized forces at work into which we cannot go in any detail here, but which will be familiar to the student of the elements of physical geography; the influence of great ocean currents in carrying warmth from equatorial to more temperate latitudes; the interference of mountain chains with the moisture borne by prevalent winds and the like. As in the slow processes of nature these currents are deflected or the mountain chains worn down or displaced by fresh upheavals, the climate over great areas will be changed and all the conditions of life changed with it. Under the incessant slow variations of these astronomical, telluric, and geographical influences life has no rest. As its conditions change it must change or perish.

 

§ 4

And while we are enumerating the forces that change climate and the conditions of terrestrial life, we may perhaps look ahead a little and add a fourth set of influences, at first unimportant in the history of the world so far as the land surface is concerned, but becoming more important after the age of Reptiles, to which we shall proceed in our next chapter. These are the effects produced upon climate by life itself. Particularly great is the influence of vegetation, and especially that of forests. Every tree is continually transpiring water vapour into the air; the amount of water evaporated in summer by a lake surface is far less than the amount evaporated by the same area of beech forest. As in the later Mesozoic and the Cainozoic Age, great forests spread over the world, their action in keeping the air moist and mitigating and stabilizing climate by keeping the summer cool and the winter mild must have become more and more important. Moreover, forests accumulate and protect soil and so prepare the possibility of agricultural life.

Water-weeds again may accumulate to choke and deflect rivers, flood and convert great areas into marshes, and so lead to the destruction of forests or the replacement of grass-lands by boggy wildernesses.

Finally, with the appearance of human communities, came what is perhaps the most powerful of all living influences upon climate. By fire and plough and axe man alters his world. By destroying forests and by irrigation man has already affected the climate of great regions of the world's surface. The destruction of forests makes the seasons more extreme; this has happened, for instance, in the northeastern states of the United States of America. Moreover, the soil is no longer protected from the scour of rain, and is washed away, leaving only barren rock beneath. This has happened in Spain and Dalmatia and, some thousands of years earlier, in South Arabia. By irrigation, on the other hand, man restores the desert to life and mitigates climate. This process is going on in Northwest India and Australia. In the future, by making such operations world-wide and systematic, man may be able to control climate to an extent at which as yet we can only guess.

1. See Sir R. Ball's Causes of the Great Ice Age, and Dr. Croll's Climate and Time. These are sound books to read still, but the reader will find many of their conclusions modified in Wright's The Quaternary Ice Age, which is a quarter of a century more recent.