IMAGINARY CHANGES OF CLIMATE
Three consecutive years of drought, while they have stimulated the inventive resources of practical agriculturists, have had the natural effect of calling forth a plentiful crop of speculation from weather prophets and projectors, and half-instructed meteorologists, and all the philosophic tribe of Laputa in general, to whom the periodical press now affords such fatal facilities.
We have often noticed that in the tabular statements of those compilers of weather records who write to the Times, useful and welcome as their communications are, every season is sure to be “extraordinary,” almost every month one of the driest or wettest, or windiest, coldest or hottest, ever known.
Much observation, which ought to correct a tendency to exaggerate, seems in some minds to have rather a tendency to increase it. And many seem now to regard three dry hot years in succession as betokening some general change of climate, as if it was not perfectly certain, in the wide range of the table of what we call chances, that with our existing conditions of climate such a combination must every now and then recur.
We know an ingenious theorist who would fain persuade us that a cycle of six hundred unfavorable years has just reached its termination, and that English agriculturists, who left off making wine about A.D. 1250 because their grapes ceased to ripen, will soon be making it again, and continue to do so for an equivalent number of centuries. Others speculating quite as conjecturally and even more absurdly, seem to attribute the impending change of climate – of which they assume the reality – to the operation of men.
They fancy that by extended drainage and the cutting down of trees the amount of rainfall may have been diminished. As to drainage, it may suffice to remember that our rain is derived from the evaporation of the Atlantic, and is not likely to be affected by the diminution of that which proceeds from a few swamps. As to the effects of cutting down trees, the subject has been made so much of in recent times that it requires to be treated more respectfully.
Yet there are two considerations which militate against the theory, the first relative to its application to England, the second to the doctrine in general. As to the first, it may be greatly questioned whether there has been for some generations past any diminution of our tree covered surface.
There is a balance of gain and loss. Some woods have been felled. Some districts, once rich in hedge-row timber, have been partially stripped for reasons of scientific agriculture. But, on the other hand, the Englishman’s passion for planting and for preserving timber has been steadily at work in the contrary direction.
Take the neighborhood of London for an instance; any of us who are old enough may remember how scarcely more than a generation ago, the spectator from any height commanding an extensive prospect would note great bare tracts of heath,of cultivated but unenclosed land, and of recently enclosed common with low treeless hedges. All this is altered now.
Go where you will for a view in the “home counties,” a sea of leaves seems to extend around on every side. And we believe that most people, if they consult their experience, will make the same report of the parts of the kingdom with which they are acquainted. The local authorities who have been lately bestirring themselves to prevent disforesting of the New Forest, on the ground of apprehending deficiency of rain, are certainly indulging in very imaginative apprehensions.
But in the next place, what real ground is there for connecting together, in temperate climates at all events, the presence of forests and the frequency of rain as cause and effect? The assertion is so commonly made that to doubt its truth seems almost like contradicting a received physical fact. And yet it will be found on examination that it really rests on no solid foundation either of reasoning or experience. The question is a procès jugé etpas plaidé.
As for the reason of the supposition, none whatever has been assigned, so far as we are aware, except that a surface shaded with woods is somewhat cooler than one denuded of it, and therefore likelier to attract the passing shower.
How far this may be true it would be difficult to verify, or whether a hundred acres covered with wood really present a cooler area than if covered with turnips or other vegetation ; but the effect of such a cause as this must be very limited at any time, and (which is more important) it can apply only at the warm season of the year. In winter, when a full half of our rainfall takes place, the “open”is notoriously cooler than the shaded surface, and therefore should, by the supposition, attract more moisture.
In cold countries-Canada, for instance – the woodman can ply his trade in the sheltered interior of the forest when out of door work is completely suspended in the cleared land by reason of extremity of cold. Supposing, therefore, the precipitation from the air to be in any degree affected by the presence or absence of forest covering, one- half of the year ought to balance the other.
In France the prevalence of the theory that the clearing of woodlands diminished rain was such, a few years ago, as almost to produce a panic among philosophers; and in France the question could be tried experimentally with greater ease than here, because the surface of that country is more markedly divided between thick wood and bare fields than is the case with ours.
Something meriting the name of observation was, therefore, applied to the problem, and we believe we are right in saying that the result hitherto arrived at is altogether adverse to any connection between the alleged cause and effect. Rainfall cannot be shown to have diminished generally in France through the removal of forests, which in the last hundred years has been very extensive, nor can it be shown locally that rain has diminished where forests have been cut down.
One experimentalist compared for some years the meteorology of two adjoining valleys in Burgundy, the one covered with wood, the other entirely bare, and found that less rain fell in the former. But observation of this kind must no doubt be far more extensive and more prolonged to be really valuable, and none such has been applied since this question began to be agitated.
Of course, this scepticism of ours does not in any degree touch the real and provable utility of forests in agricultural economy. It has not been shown that they increase the rainfall, nor any good reason given for supposing it, but they are of the greatest service in economising it. They preserve the fallen water, both by the percolation which takes place more readily on the soil shaded by them than on exposed surfaces, and also by obstructing evaporation. They are most useful accessories both against flood and against drought.
But under our English climate there is a contingency perhaps more injurious in the long run both to human health and the fruits of the earth than even flood or drought. This is the prevalence of damp, cold,sunless weather. Some suppose that an over-abundance of trees has a tendency to aggravate this particular evil.
Certain it is that in the vicinity of that very New Forest, of which the maintenance is now so strongly urged by some climatologers, a large number of the inhabitants who can afford it are apt to migrate in what they deem the unhealthy season -the fall of the leaf -and resort to a more bracing and invigorating air.
Not only the effect of forest but the effect of modern field drainage requires to be established on far more accurate evidence than has as yet been obtained. We do not mean its effects on climate in the general sense, which we believe to be none at all, but on the preservation and distribution of water, on the permanent productiveness of the soil, and on human and animal health.
These are questions of which we shall have to leave the scientific investigation to those who may come after us; for the present we seem likely to content ourselves with deductions from very narrow experience with very large conjecture and very confident assertion.
H/T Steven Goddard