Suez Canal is said to have brought an increased rainfall; from the Peninsula of Sinai, from Syria and from Algeria, in all of which deforestation is said to have changed luxuriant and fertile districts into deserts. One other example, quoted by a recent writer, may, perhaps, be referred to:
In 1551 the Marquis of Northampton went from Orleans to Nantes (on the river Loire), with his suite, in "five large, many-cabined boats," whereas navigation is now impossible above Saumur, the distance of which from Nantes is less than half that of Orleans. This change is ascribed to the deforestation carried on extensively in the surrounding country in the seventeenth century, and the consequent diminution in the volume of water in the Loire due to diminished rainfall.
There is no need to multiply these examples. They show, clearly enough, why the historical method is unsafe, and why it has given but meager results.
An Essential Consideration: Why Should Forests Influence Climate?
It is a curious fact that so few of those who are firmly convinced that climate is affected by forests, ever seem to ask themselves: "Why should forests influence climate?" We seem to accept it as a fact without asking ourselves why it should be so. If we stop a moment to consider the reasons which come to mind, we shall probably sooner or later enumerate them about as follows:
(a) Because forests must retard and obstruct air movement, favoring calms, and causing the air to ascend slightly over the trees. Both of these effects may be favorable, in a small way, to rainfall. The barrier effect, by reducing the velocity of high winds, ought to moderate the extremes of winter cold.
(b) By means of their shade, trees ought to check the warming of the ground, and of the air, especially in summer.
(c) Because of the retention of moisture in the forest litter, and of the decreased evaporation which may be expected to result from the lessened air movement under the trees, it seems not unreasonab]e to expect that forest air will be somewhat damper than that outside. This, under proper conditions, may also favor rainfall.
(d) The diffusion of the water vapor transpired by and evaporated from the leaves may perhaps increase the opportunity for rainfall.
(e) We may expect the tree cover to diminish nocturnal radiation from the ground underneath, and thus to maintain a slightly higher temperature within the forest than outside of it at night.
(f) Also, there may be some effect from the increased radiating surface due to the presence of the leaves or needles. This must be chiefly effective at night.
(g) The heating of the leaves must be less than that of bare ground, because of the evaporation of much water from the leaves, and because
