from the sun would have to vary enormously. Twenty million years ago the sun would have extended nearly to the earth's orbit and would have been so tenuous that it would have emitted no more heat than some of the nebulæ in space. Some millions of years later, when the sun's radius was twice as great as at present, that body would have emitted only one-fourth as much heat as now, which would mean that on the earth's surface the theoretical temperature would have been 200° below the present level. This is utterly out of accord with the uniformity of climate shown by the geological record. In the future, if the sun's contraction is the only source of heat, the sun can supply the present amount for only ten million years, which would mean a change utterly unlike anything of which the geological record holds even the faintest hint.[9]
Altogether the problem of how the sun can have remained so uniform and how the earth's atmosphere and other conditions can also have remained so uniform throughout hundreds of millions of years is one of the most puzzling in the whole realm of nature. If appeal is taken to radioactivity and the breaking up of uranium into radium and helium, conditions can be postulated which will give the required amount of energy. Such is also the case if it be supposed that there is some unknown process which may induce an atomic change like radioactivity in bodies which are now supposed to be stable elements. In either case, however, there is as yet no satisfactory explanation of the uniformity of the earth's climate. A hundred million or a thousand million years ago the temperature of the earth's surface was very much the same as now. The earth had then presumably ceased to emit any great amount of heat, if we may judge
from the fact that its surface was cool enough so that great ice sheets could accumulate on low lands within 40° of the equator. The atmosphere was apparently almost like that of today, and was almost certainly not different enough to make up for any great divergence of the sun from its present condition. We cannot escape the stupendous fact that in those remote times the sun must have been essentially the same as now, or else that some utterly unknown factor is at work.
[CHAPTER II]
THE VARIABILITY OF CLIMATE
The variability of the earth's climate is almost as extraordinary as its uniformity. This variability is made up partly of a long, slow tendency in one direction and partly of innumerable cycles of every conceivable duration from days, or even hours, up to millions of years. Perhaps the easiest way to grasp the full complexity of the matter is to put the chief types of climatic sequence in the form of a table.
| [TABLE 2] | |||
|---|---|---|---|
| TYPES OF CLIMATIC SEQUENCE | |||
| 1. | Cosmic uniformity. | 7. | Brückner periods. |
| 2. | Secular progression. | 8. | Sunspot cycles. |
| 3. | Geologic oscillations. | 9. | Seasonal alternations. |
| 4. | Glacial fluctuations. | 10. | Pleionian migrations. |
| 5. | Orbital precessions. | 11. | Cyclonic vacillations. |
| 6. | Historical pulsations. | 12. | Daily vibrations. |
In assigning names to the various types an attempt has been made to indicate something of the nature of the sequence so far as duration, periodicity, and general tendencies are concerned. Not even the rich English language of the twentieth century, however, furnishes words with enough shades of meaning to express all that
is desired. Moreover, except in degree, there is no sharp distinction between some of the related types, such as glacial fluctuations and historic pulsations. Yet, taken as a whole, the table brings out the great contrast between two absolutely diverse extremes. At the one end lies well-nigh eternal uniformity, or an extremely slow progress in one direction throughout countless ages; at the other, rapid and regular vibrations from day to day, or else irregular and seemingly unsystematic vacillations due to cyclonic storms, both of which types are repeated millions of times during even a single glacial fluctuation.
The meaning of cosmic uniformity has been explained in the preceding chapter. Its relation to the other types of climatic sequences seems to be that it sets sharply defined limits beyond which no changes of any kind have ever gone since life, as we know it, first began. Secular progression, on the other hand, means that in spite of all manner of variations, now this way and then the other, the normal climate of the earth, if there is such a thing, has on the whole probably changed a little, perhaps becoming more complex. After each period of continental uplift and glaciation—for such are preëminently the times of complexity—it is doubtful whether the earth has ever returned to quite its former degree of monotony. Today the earth has swung away from the great diversity of the glacial period. Yet we still have contrasts of what seem to us great magnitude. In low depressions, such as Turfan in the central deserts of Eurasia, the thermometer sometimes ranges from 0°F. in the morning to 60° in the shade at noon. On a cloudy day in the Amazon forest close to the seashore, on the contrary, the temperature for months may rise to 85° by day and sink no lower than 75° at night.