Putting together the evidence on both sides in this disputed question, it appears that throughout most of geological time there is some evidence of a zonal arrangement of climate. The evidence takes the form of traces of cool climates, of seasons, and of deserts. Nevertheless, there is also strong evidence that these conditions were in general less intense than at present and that times of relatively warm, moist climate without great seasonal extremes have prevailed very widely during periods much longer than those when a zonal arrangement as
marked as that of today prevailed. As Schuchert[65] puts it: "Today the variation on land between the tropics and the poles is roughly between 110° and -60°F., in the oceans between 85° and 31°F. In the geologic past the temperature of the oceans for the greater parts of the periods probably was most often between 85° and 55°F., while on land it may have varied between 90° and 0°F. At rare intervals the extremes were undoubtedly as great as they are today. The conclusion is therefore that at all times the earth had temperature zones, varying between the present-day intensity and times which were almost without such belts, and at these latter times the greater part of the earth had an almost uniformly mild climate, without winters."
It is these mild climates which we must now attempt to explain. This leads us to inquire what would happen to the climate of the earth as a whole if the conditions which now prevail at times of few sunspots were to become intensified. That they could become greatly intensified seems highly probable, for there is good reason to think that aside from the sunspot cycle the sun's atmosphere is in a disturbed condition. The prominences which sometimes shoot out hundreds of thousands of miles seem to be good evidence of this. Suppose that the sun's atmosphere should become very quiet. This would apparently mean that cyclonic storms would be much less numerous and less severe than during the present times of sunspot minima. The storms would also apparently follow paths in middle latitudes somewhat as they do now when sunspots are fewest. The first effect of such a condition, if we can judge from what happens at present, would be a rise in the general temperature of the earth, because less heat would be carried aloft by storms.
Today, as is shown in Earth and Sun, a difference of perhaps 10 per cent in the average storminess during periods of sunspot maxima and minima is correlated with a difference of 3°C. in the temperature at the earth's surface. This includes not only an actual lowering of 0.6°C. at times of sunspot maxima, but the overcoming of the effect of increased insolation at such times, an effect which Abbot calculates as about 2.5°C. If the storminess were to be reduced to one-half or one-quarter its present amount at sunspot minima, not only would the loss of heat by upward convection in storms be diminished, but the area covered by clouds would diminish so that the sun would have more chance to warm the lower air. Hence the average rise of temperature might amount to as much at 5° or 10°C.
Another effect of the decrease in storminess would be to make the so-called westerly winds, which are chiefly southwesterly in the northern hemisphere and northwesterly in the southern hemisphere, more strong and steady than at present. They would not continually suffer interruption by cyclonic winds from other directions, as is now the case, and would have a regularity like that of the trades. This conclusion is strongly reënforced in a paper by Clayton[66] which came to hand after this chapter had been completed. From his studies of the solar constant and the temperature of the earth which are described in Earth and Sun, he reaches the following conclusion: "The results of these researches have led me to believe: 1. That if there were no variation in solar radiation the atmospheric motions would establish a stable system with exchanges of air between equator and pole and between ocean and land, in which the only variations
would be daily and annual changes set in operation by the relative motions of the earth and sun. 2. The existing abnormal changes, which we call weather, have their origins chiefly, if not entirely, in the variations of solar radiation."
If cyclonic storms and "weather" were largely eliminated and if the planetary system of winds with its steady trades and southwesterlies became everywhere dominant, the regularity and volume of the poleward-flowing currents, such as the Gulf Stream and the Atlantic Drift in one ocean, and the Japanese Current in another, would be greatly increased. How important this is may be judged from the work of Helland-Hansen and Nansen.[67] These authors find that with the passage of each cyclonic storm there is a change in the temperature of the surface water of the Atlantic Ocean. Winds at right angles to the course of the Drift drive the water first in one direction and then in the other but do not advance it in its course. Winds with an easterly component, on the other hand, not only check the Drift but reverse it, driving the warm water back toward the southwest and allowing cold water to well up in its stead. The driving force in the Atlantic Drift is merely the excess of the winds with a westerly component over those with an easterly component.
Suppose that the numbers in Fig. 8 represent the strength of the winds in a certain part of the North Atlantic or North Pacific, that is, the total number of miles moved by the air per year. In quadrant A of the left-hand part all the winds move from a more or less southwesterly direction and produce a total movement
of the air amounting to thirty units per year. Those coming from points between north and west move twenty-five units; those between north and east, twenty units; and those between east and south, twenty-five units. Since the movement of the winds in quadrants B and D is the same, these winds have no effect in producing currents. They merely move the water back and forth, and thus give it time to lose whatever heat it has brought from more southerly latitudes. On the other hand, since the easterly winds in quadrant C do not wholly check the currents caused by the westerly winds of quadrant A, the effective force of the westerly winds amounts to ten, or the difference between a force of thirty in quadrant A and of twenty in quadrant C. Hence the water is moved forward toward the northeast, as shown by the thick part of arrow A.
