Popular Science Monthly/Volume 7/October 1875/Croll on Climate and Time
|CROLL ON CLIMATE AND TIME.|
THE distribution of temperatures upon the globe is a subject of profound popular and scientific interest. More than any other, it affects the distribution of living forms, not only in zones of climate, but in geological time. The contour of the earth's surface, and the relations of land and water upon it, may produce important local changes, or establish local faunas and floras, but these are scarcely more than modifications of grander and more general results. Arctic plants may, indeed, flourish under the equator, but only on mountains where an arctic climate prevails. Heat determines the limits equally of the vine, the palm of the tropics, the cereals of the temperate zone, and. the food of the reindeer. Whether or not tree-ferns grow in Pennsylvania, and forests of pine in the Arctic Circle, depends on climate. Nor is this a phenomenon of the present age only. Geology proves from its records that, while a torrid zone may have existed since climates began, yet polar and temperate regions have witnessed changes both of climate and of life in wonderful succession, and in periods of immense duration. The causes which have contributed to these results have been sought and studied by some of the most eminent scientists of our time. Humboldt, Sir John Herschel, Sir William Thomson, Lieutenant Maury, Sir Charles Lyell, and more recently Dr. Carpenter and others, have investigated the subject in many of its aspects, and their conclusions are before the world; but no one, we believe, has presented it from so many points of view, or attacked its complex problems with greater vigor, than Mr. James Croll, whose volume is now before us. His conclusion is, that not only great secular changes of climate, but the distribution of temperatures upon the earth's surface at the present time, are due to causes which alter the volume, intensity, and direction, of the trade-winds and other prevailing winds of the globe. For the question at issue is not the amount of heat received upon the earth's surface, but the means by which it is distributed. It is not claimed that the great heat of the equatorial regions is carried directly to the polar regions by winds. That such a result is impossible is shown by the fact that the heated air rises at the equator, and moves toward the poles at an elevation where the temperature is at freezing, and its heat is lost or radiated into space.
But the prevailing winds, and mainly the trades, give rise to great surface-movements of the ocean, chief of which is the Gulf Stream. Here, however, the author enters on disputed ground. We have not space to follow him in his criticisms on Lieutenant Maury and Dr. Carpenter, each of whom attributes oceanic circulation to difference of specific gravity of the water rather than to winds.
It is evident, however, that the value of this question depends upon a previous one, What influence have ocean-currents on climate, and what is their capacity for the transference of heat? For clearness and brevity we will confine our inquiry to the North Atlantic and its great current, the Gulf Stream. By one of the lowest estimates made of the volume and velocity of this stream, there is conveyed by it not less than 2,787,840,000,000 cubic feet of water every hour. This is about 1,200 times as much as the average hourly discharge of the Mississippi. "No droughts affect it;" the flow is incessant, and the volume continues unchanged, save by causes which we will presently mention.
The mean temperature of this water as it emerges from the Gulf is about 65° Fahr. As the stream spreads over the North Atlantic its heat is imparted to the atmosphere, and diffused by winds. It is certain that the entire volume of Gulf-Stream water loses in this way 25° of its heat; and this represents its warming capacity. To this we refer the mild climate of England, of Norway, and Iceland. The heat thus conveyed and distributed by each cubic foot of water is 1,158,000 foot-pounds, and by the whole volume of Gulf-Stream water there is transferred every day from the equatorial regions 77,479,650,000,000,000,000 foot-pounds of heat. But these figures convey no definite impression of the vastness of the results. It is equal to one-fourth of all the heat received from the sun by the whole Atlantic Ocean from the Tropic of Cancer to the Arctic Circle, raising the temperature of its waters one-fifth—it is equal to about one-half the solar heat which falls upon the entire arctic regions. Under the equator there comes from the sun on each square foot of surface about 83 foot-pounds of heat every second, when the sun is in the zenith. Twenty-two per cent, of this is absorbed by the atmosphere; the remainder falls as heating-power upon the ocean. Now, the quantity conveyed is equal to all that thus falls upon 1,560,935 square miles of surface. If this source of heat be destroyed or turned away, the area of arctic winter would rapidly extend southward, covering England, both Old and New, with ice which the heat of summer would not remove.
Nor would any conceivable movements of the atmosphere supply the deficiency of heat. For not only does heated air at the equator rise, and radiate its heat in the cold regions of the upper atmosphere, but the capacity of air for heat is much less than water. So great is the difference in this respect that the Gulf Stream conveys as much heat as a current of air would if 3,234 times as large, of the same temperature, and moving with the same velocity. The heated air, however, which rises at the equator is charged with vapor, and in the opinion of Sir John Herschel this vapor, conveyed by the upper or anti-trades, is condensed in the temperate and arctic regions, greatly modifying their temperature. To the extent that the vapors are thus conveyed, the conclusion of Sir John is correct; for, as Prof. Tyndall has shown, in the conversion of one pound of aqueous vapor into water there is given out as much heat as is sufficient to melt five pounds of cast-iron. But Mr. Croll proves that the greater part of the vapor raised in the equatorial regions falls there as rain; that the upper winds are dry, and gather moisture only when they again reach the earth and become surface-winds in the temperate and arctic zones.
If all currents of the ocean and atmosphere should cease, no heat could be transferred from the equatorial to the arctic regions, and temperatures would depend on the solar heat falling in the respective latitudes. The equatorial and arctic regions would become uninhabitable, the mean temperatures rising to 135° Fahr. in the one, and sinking to 83° below zero in the other. The present difference of 80° would be increased to 218°, and only a narrow zone of temperate climate would prevail.
Dr. Franklin suggested that the Gulf Stream has its origin in the trade-winds. It was a crude but sagacious remark. Lieutenant Maury, however, thought the cause wholly inadequate, and that the phenomenon is better explained by difference in the density and specific gravity of ocean-waters, arising mainly from differences of temperature. Dr. Carpenter, too, insists on difference in specific gravity as a cause of ocean circulation, but claims that the circulation is a diffused or general one of the ocean-waters between poles and equator, and attaches comparatively little importance to the Gulf Stream. Mr. Croll, however, revives the views of Dr. Franklin with surprising ability, and finds in the great wind-currents, and chiefly in the trade-winds, a cause adequate to the result. The wind and ocean currents coincide all over the globe. The waters move with the general set of the trade-winds—the direction of the one is a reliable exponent of the set of the other.
Now, it is obvious that any influence which changes the direction of the winds will also affect that of the currents, and in that way the climates of the globe.
At present the equatorial waters heated by the sun to a temperature of 83° move westward between the tropics at the rate of thirty miles in twenty-four hours. This is called the equatorial current. It impinges upon the coast of South America, a small portion going southward, the principal portion northward, and is discharged as the Gulf Stream, It is deflected every year by changes in the trade-winds; it is thrown northward when the southeast trade is at its maximum. We will confine our attention now to the northeast trades. Should these be increased in velocity and volume, they would also assume a somewhat more northerly direction at the equator, carrying the equatorial current southward beyond the median line, and increasing the volume of the southern at the expense of the northern flow. From this cause the temperature of the Northern Hemisphere would be greatly lowered, while the mildness of summer would prevail in the Southern Hemisphere.
Causes, therefore, which alter the force and direction of the trades are adequate to change the climates of the globe, and in the opinion of Mr. Croll these causes are found in variations in the earth's path around the sun, combined with the precession of the equinoxes. These affect not, indeed, the total volume of heat received by the earth in a year, but the distribution of it by the means already referred to. If it happen that during a vast period of time the winters of our Northern Hemisphere should occur when the earth is farthest from the sun, and its orbit at its greatest eccentricity, the result would be winters long and cold, with summers short but hot. The earth would in that case be 8,641,870 miles farther from the sun in winter than at present, and during that season would receive one-fifth less direct heat from it. At present the winters are eight days shorter than the summers, but in the conditions referred to the winters would exceed the summers by thirty-six days.
It is not claimed by Mr. Croll that a cold or glacial epoch is directly caused by the increased distance of the earth from the sun, but from physical agents thus brought into operation. Some of these we will proceed to mention.
As the winters increase in length, and the cold in intensity, the volume of snow-fall will become greater, and its area extended. The limit at which the summer sun melts it will move slowly southward. Behind it will be a gradual accumulation of snow forming into ice. Mountain-slopes will be covered with it, until it flows down into valleys and onward, a vast sheet of glacial ice, equally on lowlands and mountains.
Out of this condition will arise several results which powerfully react, increasing and intensifying the cold of the growing winter. A volume of snow and ice covering the ground chills the air by direct radiation, and by contact lowers its general temperature, thus delaying or arresting the process of melting by the summer's sun. It is a familiar fact that in snow-covered regions the direct rays of the sun may be intensely hot, melting pitch from timber, or heating rocks, while the temperature of the air is that of the ice upon the ground. The regions of Hudson Bay are sterile, not because the heat from the summer sun is not intense, but because they are covered with ice all the year. But for this the climate might be as genial as that of England. Ice and snow maintain steadily a temperature of 32°, no matter how hot the sun's rays may be, and a rock or piece of earth will become greatly heated, while a block of ice consumes the heat that falls upon it. The solar heat is, therefore, expended in breaking down the molecular structure of the ice, and must continue to do so until it disappears. By as much heat as is used over a region in this way is the heating effect of the sun's rays diminished, and a low atmospheric temperature is the result.
But the sun's rays falling on snow are to a considerable extent reflected back into space from its innumerable surfaces, greatly decreasing their heating effect.
The snow-sheet exerts another important influence on temperature by condensing the vapor of the air into fogs as the summers come on. In this way the solar rays are arrested, and their heating power dissipated. This occurs continually during the summers of arctic and antarctic regions. Dr. Scoresby observes in regard to the arctic regions that "the sun, when near the northern tropic, gives scarcely any sensible quantity of light from noon till midnight; it is frequently invisible for several successive days, and snow is so common that it may be boldly stated that it falls nine days out of ten from April to July." These are the conditions of climate in which glaciers grow and throw abroad their chilling influence. We are now to consider their relations to the atmosphere and ocean in respect to their movements. The trade-winds owe their existence to the difference of temperature between the equator and the poles. Whatever increases this difference increases the strength and volume of the winds, whether in the Northern or Southern Hemisphere. The coming on of a period of northern glacial cold would be concurrent with increasing violence of the northeast trade-wind. It would sweep at its maximum far beyond the equator, for the Southern Hemisphere would be heated and the line of greatest equatorial heat would be southward from its present position. A deflection of the great equatorial current of the ocean would occur corresponding with this, and its vast volume of heated waters would pour into the southern instead of into the northern ocean. The Gulf Stream would cease to flow, or flow only with greatly diminished volume. "In the severest droughts it never fails," said Prof. Maury, but it may fail from other causes, and leave half a hemisphere rigid from the austerity of cold. Depleted of the Gulf Stream, the surface-waters of the North Atlantic would be warmed only by the direct rays of the sun, and would rapidly approximate to the low temperatures which now prevail only a few fathoms beneath the surface. The mean temperature of Scotland for January is 28° higher than its normal, and 15° above its normal for the year. The loss of the Gulf Stream would change all this.
Theories which have made the Ice period depend wholly on cold are shown to be untenable by Prof. Tyndall, who calls attention to the fact that great heat is as necessary to the production and growth of glaciers as intense cold, the one being needed to produce vapor, the other to condense and freeze it. Any accumulation of snow and glacial ice is impossible without this combination of circumstances; but these constitute an integral part of Mr. Croll's theory, which assigns a mild climate to one hemisphere while the other is wrapped with ice.
Alternation of climates in geological time is as certain as diversity of climate at the present day. Evidence of it is found in the geological record, and eras of glaciation have succeeded each other, but each one has buried or erased many traces of preceding ones, and only the last one is before us, the monumental history of which reveals its startling and wonderful features.
It is held indeed by Mr. Croll that eras of cold and glaciers alternating with those of temperate climate are fully accounted for by the causes stated. His conclusions, however, are not accepted by many eminent geologists. Prof. Dana says that climatic changes effected by the Gulf Stream have been brought about, "not by diversions of the current from the ocean, and its restoration to it again, but by variations in the amount and height of arctic lands, in one case closing and the other opening the arctic regions to the tropical stream, and the same for the Pacific current." While this view may not call in question the warming influences of the stream, it assigns other reasons than those given by Mr. Croll why its effects may be greatly varied. Nor can there be any doubt that changes in the general level of the land, altering coast-lines and the currents of both the atmosphere and the ocean, are important agents in modifying temperatures. We cannot follow into details Mr. Croll's exhaustive inquiries, but our readers will be interested in his answer to the well-known theory of Sir Charles Lyell. According to that eminent authority, a period of polar cold will result from a great increase of elevation and extent of land in polar regions, and a warm period in the polar zones will occur by a great accession of land in the equatorial regions. But Mr. Croll shows that such changes in the distribution of land would be followed by opposite results—that a great accession of land in the equatorial zone would destroy the system of ocean-circulation by which the heat of the equator is made to do service in warming the ocean and the air of colder zones.
In Mr. Croll's theory it is impossible that both hemispheres should be glaciated at the same time. Not only must periods of heat alternate with those of cold in one hemisphere, but a glacial epoch in one is accompanied by a temperate epoch in the other.
It is evident that changes of climate such as are shown to have occurred must have arisen from general, not from any local cause or accidental combination of causes, and if this be so there may yet appear a reliable means of determining not only the amount and extent of the changes, but the periods of their duration and recurrence. The question of time in geological history is an important and certainly a most interesting one, and the interest in it has increased since the announcement by Dr. Tiddeman and others that human relics have been found in deposits of the warm inter-glacial periods.
Two methods have been adopted by Mr. Croll which are supposed to throw light on this subject. One is to fix the period and duration of the epochs of greatest heat and cold by computing the period and duration of the astronomical coincidences already noticed, by which according to his theory those epochs were brought about. The other method is applied to estimating the time since the close of the last glacial epoch by changes known to have taken place in the earth's surface, and the general lowering of the land by denudation. To do this, he says, "we have only to ascertain the quantity of sediment annually carried down by the river systems."
By this means it is found that the lowering may have been nearly a foot in 6,000 years. But when we consider how greatly the general result may have been interfered with by the alternate elevation and depression of the land, a work now going on, we realize that Mr. Croll's conclusions do little more than profoundly impress the mind with the vastness of time required in some of the most obvious of Nature's operations.
By the first-named method computations have been made extending back three millions and forward one million of years. But we believe that most geologists agree with the statement of Sir Charles Lyell that "an attempt to assign chronological value to any except the latest geological epochs must in the present state of science be hopeless."
It may be stated, however, that during the period covered by the computations, three epochs of greatest eccentricity of the earth's orbit occurred, and in each of these it is found that the eccentricity is not uniform, but rises and falls. Hence the coming on and departure of each period may have been continuous, but by no means regular. Sometimes the changes were at the comparatively short intervals of 10,000 or 12,000 years, and the close relation these changes may have had to the life of the time, possibly to extinction of species, is plausibly suggested by Mr. Croll.
The period of great eccentricity to which the last glacial epoch is referred began about 240,000 years ago, and extended over a period of about 160,000 years. The conclusion, therefore, is that it closed about 80,000 years ago. It is not easy to understand, however, what value to attach to the words "close of the glacial epoch." Shall we say that it closed when the ice ceased to exist as a glacier along the shores of New England, or when it extended no farther southward than the Canadian highlands? The gigantic fields of ice which now cover both arctic and antarctic lands prove that, within areas more limited indeed than in former time, the glacial epoch still exists in its stern and sublime reality.
Whether we consider the facts of geology or those of astronomical computation, it seems evident that the growth and decline of the ice-sheet, and of the causes which produced it, have been in no sense cataclysmic or accidental, but secular; only after ages have passed are we enabled to realize, from the altered aspects of Nature, that a great change is in progress. At present the eccentricity of the earth's orbit is diminishing. In a little less than 24,000 years it will be "as nearly circular as it can ever be;" and if Mr. Croll's tables are to be relied on, no cycles of extreme heat or cold will occur for the next 150,000 years. We are entering a period of comparatively equable climate, arising from a more uniform distribution of solar heat over the surface of the globe.
- Climate and Time in their Geological Relations: a Theory of Secular Changes of the Earth's Climate. By James Croll, of Her Majesty's Geological Survey of Scotland. New York: D. Appleton & Co. 1875
- A foot pound is the amount of heat-force expended in raising one pound one foot; 772 foot-pounds is the equivalent of the heat that will raise one pound of water one degree of temperature.