Popular Science Monthly/Volume 1/October 1872/Has our Climate Changed?
By DANIEL DRAPER, Esq.,
DIRECTOR OF METEOROLOGICAL OBSERVATORY, CENTRAL PARK.
Does the Clearing of Land affect the Fall of Rain?
MUCH solicitude is publicly felt as regards the supposed diminished quantity of water which fell last year—a point of the highest concern. There is a general impression that this alleged deficiency was to such an amount as to endanger a due supply to New York for the current year. And not only this, it has also been asserted that for several years past there has been a steadily occurring diminution in the rainfall. While the quantity of water has thus been becoming less and less, the demand has been becoming greater. Not only has the population of the city increased, but also that of the suburban districts, which derive their supplies from the same water gathering grounds that we do.
I therefore, supposed, since the registering rain gauge furnishes very reliable measures, that it would be useful to examine this subject critically. But, since we have had these gauges in operation only about three years, and as the investigation proved to be full of interest, I was led to draw upon other additional sources of information, selecting such as seemed to be of the most trustworthy kind. By the aid of these the examination has been extended as far back as 1836, and with the following results:
1. As respects the indications given by our own instruments, which may be thoroughly relied on, for the years 1869, 1870, 1871.
For the first of these years, 1869, the total rainfall was 46.82 inches, distributed as follows:
For 1870 the total rainfall was 42.32 inches, distributed as follows:
So far as these years are concerned, there does not appear any evidence of a decrease; on the contrary, in the last there is a very considerable excess over either of the others.
Extending our examination to preceding years, as far bach as the beginning of 1836, and grouping those years into three periods, each of ten, and one of six years, the statement comes to this:
This would make the annual rainfall, throughout these thirty-six years, 47.62 inches. That of the last three years is 47.06 inches. These numbers being substantially the same, it may be concluded that, though there are large variations from year to year, these on the whole will neutralize one another, when very long periods of time are considered.
In the foregoing table the numbers from 1836 to 1854 inclusive are derived from the observations made by the military officers at Fort Columbus, New York Harbor; those for the next twelve years are from the records of Prof. Morris in New York City; and the remainder are from the registers of this observatory. It is, of course, assumed that the rainfall at Fort Columbus, that in New York City, and that in the Central Park, are the same—an assumption which, I suppose, is under the circumstances admissible.
The amount of rainfall not only influences in a predominant manner the growth of plants, and therefore agricultural pursuits, determining the profitable cultivation of many different crops, it also exerts an influence on several manufacturing operations. If, therefore, the above statement be correct, no apprehension need be entertained of a permanent disturbance in these particulars. Although in the last 38 years great changes have been made in all those portions of the United States intervening between the Mississippi and the Atlantic Ocean, large surfaces having been cleared of the primeval forest, and brought under cultivation, their physical character and aspect having therefore been essentially altered, no corresponding diminution can nevertheless be traced in the mean amount of water that has fallen. On the contrary, there has been an actual increase. It appears, therefore, that the wide-spread public impression, that the clearing of Ian diminishes the volume of rain, is not founded on fact, and in truth this is no more than might have been expected from a correct consideration of the meteorological circumstances under which rain is produced.
It is the belief of European meteorologists that the mean rainfall on the western portions of that continent varies little, if at all, when periods of many years are considered. In England there are rainfall records reaching back to 1677. Since 1725 these records are unbroken; at present there are more than 1,500 rain-gauge stations in that country. The Scotch observations extend to 1731, the Irish to 1791.
A discussion of the observations made at the Royal Observatory at Greenwich, in 1859, led to the conclusion that the annual fall of rain, as compared with that previous to 1815, was becoming smaller; but more extended observations, taken from gauges at stations widely separated, led to the opposite conclusion, viz., that there was a perfect compensation, the decrease at one place being compensated by the increase at another.
This conclusion was strikingly illustrated by the Continental observatories. The rainfall at Paris was found not to have altered in 130 years, and, though the observations of 50 years at Marseilles gave a decrease, those for 54 years at Milan gave an increase.
Even in the same locality this principle of compensation may be noticed. Thus the rainfall in England, in the ten years from 1850 to 1859, was found by Mr. Symons to be five per cent, less than during the previous 40 years, but during the following six years it was five per cent, above the mean of the preceding ten.
It may, however, be supposed, that conclusions which apply to the old settled countries of Europe, in which but few important topographical changes through agricultural or other operations have taken place for many years, will scarcely apply to America, wherein the clearing of land and agricultural surface changes have been occurring on a very extensive scale. The foregoing conclusions, however, show us how insignificant is the meteorological result which these variations produce.
The Available Supply of Water.
The actual supply of water does not depend on rainfall alone. It is diminished by evaporation and also by percolation. When the condition of the atmosphere is such that, either by reason of the heat, the prevalence of dry winds, and other such causes, the water that has fallen is exposed to rapid vaporization, the available supply necessarily becomes less.
As regards percolation, much depends on the rate at which the rain falls, and the contemporaneous condition of the surface of the ground. The supply may come so rapidly that there is not time for it to soak into the earth. In this manner the quantity that properly belongs to a whole month may fall in the course of a few hours, and, rushing over the surface, may be lost. Again, if the surface be frozen, it may be impossible for the water to percolate into the ground, and, though it may descend in a more moderate manner, it may, in this as in the preceding case, be lost. Obviously, there are many causes of the kind which might be referred to; these, however, are sufficient to indicate the principle involved.
We have shown that agricultural conditions do not perceptibly affect the rainfall; they do, however, very powerfully influence what may be designated as the rain-waste. Thus, a growing plant vaporizes from its leaves an immense amount of water which its roots have abstracted from the ground. A sunflower will thus remove twenty ounces of water in a single day. There is in this respect a waste which varies in the different months, being greatest in those during which general vegetation is most rapid, and less in those—the winter months—when it is torpid. For these and other such reasons the monthly distribution of rain influences the actual supply.
It is interesting to remark that the rainfall in New York greatly exceeds that of London. Here it is 47.62 inches, in London it is but 25 inches, and the mean for all England is estimated at 31.25 inches.
But these considerations of the amount of rainfall are only a portion of a far more general and most important problem, viz.:
Is the Climate of New York changing, or, more generally, is that of the Atlantic States undergoing Modification?
In this case, as in the preceding, there is a popular belief that clearing of land, drainage, and other agricultural operations, tend to produce such a result. Land that has been ploughed and exposes a dark surface to the sun, absorbs more heat, that is, becomes hotter, than land covered with forest-growth. It does not seem unreasonable, then, to suppose that, where thousands of square miles of surface have been submitted to such operations, the corresponding effect should be traceable, at least in the temperature of certain seasons of the year.
Moreover, there are some interesting facts which are matters of public observation and constant remark. Thus, as every one knows, in the city of New York itself, there are no longer the deep snows which characterized the winter seasons years ago. The large sleighs, often drawn by very many horses, used in those times as the public conveyances, have altogether disappeared from the streets. .It would seem, therefore, that the winters have become milder. In like manner, though in support of this conclusion we have less palpable evidence, there is a very general opinion that the intolerable and long-continued heats, which formerly made the summer months almost unbearable, have greatly moderated, and, that, though the thermometer may occasionally rise as high as it formerly did, the continuance of the hot weather is shorter.
This popular opinion of change of climate through agricultural operations is far from being restricted to America. In Western Europe there is a belief that a great amelioration has taken place in all the Baltic countries since the time of the Roman domination.
In many instances these popular impressions are contradicted by well-ascertained facts. Thus, as respects the Baltic, there are records of the time of the breaking of the ice in some of the great rivers, such as the Dwina and Neva, for several centuries. These show that, during the last 300 years, the variation amounts but to a fraction of a single day.
Such fragments as have been preserved of the observations of the first discoverers of North America—the Icelandic voyagers—have been supposed to prove a change in the climate of New England during the last 800 years, it being affirmed that the vine formerly flourished in regions where it cannot now exist. One of the first papers communicated to the American Philosophical Society, in Philadelphia, was by Dr. Williamson, offering proof that, during the previous 40 or 50 years, a very great climate change had taken place; he attributed it to cultivation. Soon afterward, Dr. Williams, of Harvard University, offered evidence that the climate of Boston had changed 10° or 12° in about 160 years. A close examination of the evidence by more recent authorities has, however, shaken these conclusions. Thus, as regards the Icelandic voyages, it is shown that the description they give of the forest-growth of New England is the same that might be given now. Humboldt, in his "Views of Nature," comes to the conclusion that there has not been any change in the climate of the United States since its first colonial settlement, and in this, Noah Webster, Forry, and other American writers agree.
It is evident, however, that in a rapidly-growing city there are several local causes which may be assigned as giving origin to an increase of temperature. The quantity of fuel burnt increases with increased population and with the number of houses, and this must exert a perceptible effect in ameliorating the rigor of winter. Moreover, on sunshiny days, the reflection and radiation of the sun's warmth from the vertical sides of the houses must tend in no inconsiderable degree to raise the temperature locally, and aid in producing a thaw. The facts observed in a city are hence not a complete guide in the discussion of general climate changes.
If our climate be gradually changing, if the heat of summer is becoming less excessive, and the cold of winter more moderate, there are impending over us modifications in our social habits, and in many of our business occupations. Not only is the settlement of this question interesting in a meteorological or scientific point of view—the sanitary, engineering, manufacturing, mercantile, and agricultural consequences are also of the utmost importance.
Impressed with these considerations, I was therefore led to extend my researches from the rainfall question to this more general problem; and, with the intention of not being misled by local observations made in the city itself, which, as we have just remarked, are not altogether to be trusted, I have resorted to data of a more general topographical kind, such, for instance, as the times of closing and opening of the Hudson River. Also, with a view of extending the conclusions, whatever they might prove to be, to the Atlantic coast generally, I have used such published records of the meteorology of Philadelphia, Boston, and Charleston, as I could find access to. These reach from 1738, with certain breaks, up to the present time.
TABLE SHOWING THE NUMBER OF DAYS THAT THE HUDSON RIVER HAS BEEN
CLOSED BY ICE.
The data connected with the Hudson River have been derived from the Annual Reports of the Regents of the University; those of temperature for the locality of New York itself, from the observations taken at Fort Columbus, and by Prof. Morris, for the Smithsonian Institution. The remainder are from the records of this observatory. In the case of other Atlantic cities, the data are chiefly derived from the reports of the United States Army officers to the Secretary of War.
It appears from this that, from 1816 to the present time, we have an unbroken register. Taking 1817 as our starting-point, we have to 1868 five periods of ten years each. The number of days during which the river was closed in each of these five periods is: For the first, 92 days; second, 92; third, 94; fourth, 90; fifth, 91.
The third period gives a greater number of days than any of the others; the general mean is about 91 days.
The conclusion at which we arrive from the evidence thus furnished by the Hudson River is, that during 50 years, that is to say, the whole period of trustworthy records, there has been no important change in the number of days that the river has remained frozen. In this respect the conclusion is the same as that which we have seen in the case of the Baltic rivers for a period of 300 years.
The evidence thus furnished from the closure of a river by ice differs from that of thermometric observations. The latter give merely the intensity of heat at the special moment, and in the special locality at which the observation is made. The former represents the quantity of heat over a long line, including many localities. It is, therefore, the better form, and furnishes more trustworthy results.
Turning now to the records of the city of New York, as obtained from the sources above specified, we find they are continuous from 1821 to the present time. It would extend this report unduly were we to enter on an examination of each of these years respectively. Making a selection, then, let us compare the following groups of five years—first, from 1821 to 1827; second, from 1831 to 1837; third, from 1841 to 1847; fourth, from 1866 to 1872. It will be understood that the months selected are January, February, and March.
TABLE, SHOWING THE MEAN TEMPERATURE IN NEW YORK, FOR THE FIRST THREE MONTHS OF THE YEAR, JANUARY, FEBRUARY, AND MARCH.
The mean for January, February, and March, for thirty-three years, is 32.90 degrees. The mean for the above selection is 33.06 degrees.
The evidence thus derived from thermometric observations corroborates that derived from the freezing of the river, and undeniably leads to the conclusion that, if there has been any change in the winter climate of New York, it cannot be demonstrated by the extant thermometrical records of the last fifty years. This, therefore, adds weight to Humboldt's conclusion that there has been no sensible change in the Atlantic States since the time of their first settlement.
Let us next see what is the evidence afforded by an examination of the Philadelphia records. As in the preceding case, a discussion of all these would be too lengthy. They go as far back as 1748, but present, however, a broken series. Selecting from this, here and there, periods of five years, we may thus group them: first, from 1766 to 1772; second, from 1797 to 1803; third, from 1821 to 1827; fourth, from 1831 to 1837; fifth, from 1851 to 1857.
TABLE, SHOWING THE MEAN TEMPERATURE OF PHILADELPHIA, FOR THE FIRST THREE MONTHS OF THE YEAR, JANUARY, FEBRUARY, AND MARCH.
The mean for January, February, and March, for fifty-six years, is 35.56 degrees. The mean for the above selection is 35.23 degrees.
From this, it would seem that the mean temperature of the first three months of the year at Philadelphia is 2.66 degrees above that of New York, and that the same conclusion arrived at in the preceding instances reappears in this, viz., that there has been no change in the winter climate.
The Boston records reach back to 1780. Taking the same periods as in the preceding instances, as far as these records will permit, they are—first, 1797 to 1803; second, 1821 to 1827; third, 1831 to 1837; fourth, 1850 to 1856. It is to be remarked that these observations are not all from the same station.
TABLE, SHOWING THE MEAN TEMPERATURE OF BOSTON FOR THE FIRST THREE MONTHS OF THE YEAR, JANUARY, FEBRUARY, AND MARCH.
The mean for January, February, and March, for eighty-six years, is 29.63 degrees. The mean for the above selection is 29.66 degrees.
The mean temperature for the first three months of the year at Boston is 3.27 degrees lower than that of New York. These records give no substantial reason for supposing that, during the period of time to which they refer, there has been any sensible change in the winter climate of that locality.
In like manner, making a selection from the Charleston records, first, from 1749 to 1755; second, from 1754 to 1760; third, from 1822 to 1829; fourth, from 1830 to 1836; fifth, from 1849 to 1855—which date from 1738.
TABLE, SHOWING THE MEAN TEMPERATURE OF CHARLESTON FOR THE FIRST THREE MONTHS OF THE YEAR, JANUARY, FEBRUARY, AND MARCH.
In this series, again, unfortunately the observations are from different stations. They exhibit greater divergences than any of the preceding cases; but notwithstanding that, so far from invalidating, they strongly confirm the conclusion arrived at in those cases. Thus the mean of the first series is substantially the same as that of the third, being 55.39 and 55.51 respectively, though there is between them an interval of seventy-three years. The mean of the second is substantially the same as that of the fourth, being 52.93 degrees and 52.48 respectively, their interval being seventy-six years; and it may be especially remarked that the mean of the fifth series is very nearly the mean of all the other four, theirs being 54.07, and its 53.34 degrees.
Thus, again, we reach the same conclusion in the case of the city of Charleston that we arrived at in the case of New York, Philadelphia, and Boston, that the winter climate has not undergone any change.
The general conclusion which this examination seems to warrant, both as regards rainfall and winter climate, is this, that there has been no change in the lapse of many years. None can be substantiated as having occurred within a century. This proves that surface changes through agriculture, drainage, etc., give rise to no appreciable meteorological effect, and that the public opinion which asserts such an influence is altogether erroneous.
Only recently have precise and correct views been entertained of the progress of atmospheric changes. It is now known that cloudy weather, or rains, or fluctuations of the barometer and of the thermometer, are not of restricted or local origin, but that they have a progress in a determinate direction, often of thousands of miles. This fact is at the basis of the duties in which the Storm-Signal Corps is so ably engaged. In many parts of the United States there are prairie or treeless regions several hundreds of square miles in extent, yet these are not rainless because they are treeless; clouds drop water upon them to the same amount that they do on the neighboring wooded regions. Considerations such as these may satisfy us that the surface modifications which the Atlantic States have undergone, since their first settlement, have produced no meteorological effect, and that the rainfall and winter probably remain the same, that they were many centuries ago.
I have restricted myself, in the foregoing climate examinations, to the winter season, and have said nothing as regards the summer. Had I done otherwise, it would have extended this report to an inconvenient length. Perhaps, however, what has here been substantiated, as to the permanency in the cold of the winter, will be held as affording strong presumptive evidence of a like permanency in the heats of summer, and that in these respects there is a mean degree which is maintained through indefinitely long periods of time.
While such is our final conclusion, we must bear in mind that these mean or average results exhibit only one phase of the problem. They do not show the fact that there are brief cycles of heat and cold, of moisture and dryness, following each other under the operation of some unknown law, a law perhaps not of a meteorological but of an astronomical origin.
We should remember, however, the imperfections and probable errors of the old tables. In former times due care was not taken in the construction and verification of the thermometers. Making every allowance for this, we may perhaps admit that the conclusion at which we have arrived cannot be very far from the truth.