Were these two great oceans placed beside each other on one side of the globe, and the two great continents placed beside each other on the other side, the northern hemisphere would not then be suitable for the present order of things: the land on the central and on the eastern side of the united continent would be far too cold.
The foregoing Conclusions not affected by the Imperfection of the Data.—The general results at which we have arrived in reference to the influence of ocean-currents on the climatic condition of the globe are not affected by the imperfection of the data employed. It is perfectly true that considerable uncertainty prevails regarding some of the data; but, after making the fullest allowance for every possible error, the influence of currents is so enormous that the general conclusion cannot be materially affected. I can hardly imagine that any one familiar with the physics of the subject will be likely to think that, owing to possible errors in the data, the effects have probably been doubled. Even admitting, however, that this were proved to be the case, still that would not materially alter the general conclusion at which we have arrived. The influence of ocean-currents in the distribution of heat over the surface of the globe would still be admittedly enormous, whether we concluded that owing to them the present temperature of the equator is 55° or 27° colder than it would otherwise be, or the poles 83° or 41° hotter than they would be did no currents exist.
Nay, more, suppose we should again halve the result; even in that case we should have to admit that, owing to ocean-currents, the equator is about 14° colder and the poles about 21° hotter than they would otherwise be; in other words, we should have to admit that, were it not for ocean-currents, the mean temperature of the equator would be about 100° and the mean temperature of the poles about −21°.
If the influence of ocean-currents in reducing the difference between the temperature of the equator and poles amounted to only a few degrees, it would of course be needless to put much weight on any results arrived at by the method of calculation which I have adopted; but when it is a matter of two hundred degrees, it is not at all likely that the general results will be very much affected by any errors which may ever be found in the data.
Objections of a palæontological nature have frequently been urged against the opinion that our island is much indebted for its mild climate to the influence of the Gulf-stream; but, from what has already been stated, it must be apparent that all objections of that nature are of little avail. The palæontologist may detect, from the character of the flora and fauna brought up from the sea-bottom by dredging and other means, the presence of a warm or of a cold current; but this can never enable him to prove that the temperate and polar regions are not affected to an enormous extent by warm water conveyed from the equatorial regions. For anything that palæontology can show to the contrary, were ocean-currents to cease, the mean annual temperature of our island might sink below the present midwinter temperature of Siberia. What would be the thermal condition of our globe were there no ocean-currents is a question for the physicist; not for the naturalist.
CHAPTER IV.
OUTLINE OF THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR CHANGES OF CLIMATE.
Eccentricity of the Earth’s Orbit; its Effect on Climate.—Glacial Epoch not the direct Result of an Increase of Eccentricity.—An important Consideration overlooked.—Change of Eccentricity affects Climate only indirectly.—Agencies which are brought into Operation by an Increase of Eccentricity.—How an Accumulation of Snow is produced.—The Effect of Snow on the Summer Temperature.—Reason of the low Summer Temperature of Polar Regions.—Deflection of Ocean-currents the chief Cause of secular Changes of Climate.—How the foregoing Causes deflect Ocean-currents.—Nearness of the Sun in Perigee a Cause of the Accumulation of Ice.—A remarkable Circumstance regarding the Causes which lead to secular Changes of Climate.—The primary Cause an Increase of Eccentricity.—Mean Temperature of whole Earth should be greater in Aphelion than in Perihelion.—Professor Tyndall on the Glacial Epoch.—A general Reduction of Temperature will not produce a Glacial Epoch.—Objection from the present Condition of the Planet Mars.
Primary cause of Change of Eccentricity of the Earth’s Orbit.—There are two causes affecting the position of the earth in relation to the sun, which must, to a very large extent, influence the earth’s climate; viz., the precession of the equinoxes and the change in the eccentricity of the earth’s orbit. If we duly examine the combined influence of these two causes, we shall find that the northern and southern portions of the globe are subject to an excessively slow secular change of climate, consisting in a slow periodic change of alternate warmer and colder cycles.
According to the calculations of Leverrier, the superior limit of the earth’s eccentricity is 0·07775.[27] The eccentricity is at present diminishing, and will continue to do so during 23,980 years, from the year 1800 a.d., when its value will be then ·00314.