Popular Science Monthly/Volume 20/March 1882/The Gulf Stream and the Panama Canal
|THE GULF STREAM AND THE PANAMA CANAL.|
By JAMES GEIKIE, F. R. S.
NONE of the great "rivers of the ocean" has been so frequently and carefully studied as the Gulf Stream of the North Atlantic. Its course, its depth and breadth, its temperature, etc., have all been laboriously investigated, while its influence on the climate of North-western Europe bas formed a very fruitful subject of discussion. The origin of the stream is well known. The Caribbean Sea and the Gulf of Mexico are full-charged with the heated waters of the great equatorial current, and it is from this broad and deep reservoir that the famous Gulf Stream issues by the Straits of Florida to flow in a north-easterly direction toward the coasts of Europe. At the narrowest place in the straits the stream measures 30 statute miles in breadth and 1,950 feet in depth, but it gradually widens as it passes north until it spreads out over an enormous area. It has been perfectly well ascertained that the superficial stratum of the ocean throughout the whole vast space between the great Bank of Newfoundland and the coasts of France has a higher temperature than the normal of those latitudes, and is flowing persistently in a northerly and northeasterly direction toward the coasts of Greenland, Iceland, and Spitzbergen. It is doubtful whether all this body of heated water has passed through the Straits of Florida, and some physicists have maintained that only a very insignificant portion indeed has actually streamed out of the Gulf of Mexico. These writers have, therefore, held that a stoppage of the Gulf Stream would have only an infinitesimal effect upon the general temperature of the North Atlantic and the climate of North-western Europe. This view, however, has been completely overset by Dr. Croll, whose estimate of the enormous heating power of the Gulf Stream is now very generally accepted. According to this eminent physicist and geologist, the total quantity of heat conveyed by that current is probably equal to that of a stream 50 miles broad and 1,000 feet deep, flowing at the rate of four miles an hour, and having a mean temperature of not less than 65°—a temperature which gradually falls as the current goes north until it is cooled down to at least 40°. This estimate gives us 5,578,080,000,000 cubic feet of water per hour, and consequently the total quantity of heat transferred from the tropics by this current amounts to 154,959,300,000,000,000,000 foot-pounds per diem. Even if we reduce this estimate one half, the stoppage of the Gulf Stream would still deprive the Atlantic of a quantity of heat equal to one fourth of all that is received directly from the sun by that area.
It is beyond doubt, therefore, that the passage of so great a body of heated water into the North Atlantic, over the surface of which it spreads for so many thousands of square miles, must have a most powerful influence on the climate of those regions to which its heat is brought by prevalent winds. This is the case more especially with France, our own islands, Norway, the Faröes, and Iceland. With us the prevalent winds are westerly and southwesterly, and these, as they sweep across the Atlantic, filch warmth and moisture from the Gulf Stream, which they part with as they pass over us in their course toward the northeast. How very materially our climate is thus affected will be readily seen if we compare the mean winter and summer temperatures of places in the same latitudes of North America and Europe. Thus at Bordeaux, in north latitude 44° 50', we have a winter temperature of +41° Fahr., and a summer mean of +69·1° Fahr., while at Halifax (Nova Scotia), in the same latitude (north latitude 44·39°), the winter mean is +22·6°, and that of summer +63·5°. Again, at Killybegs (Ireland), in north latitude 54° 38', the winter temperature is +41·4, and that of summer +58·7° the winter and summer means of a corresponding latitude on the Labrador coast being +5° and +49° respectively. Once more, at Wick (north latitude 58° 25') the winter temperature is +38·5°, and that of summer +56·5°, while at Hebron (Labrador), in north latitude 58° 16', we get a winter mean of -5·1° and a summer mean of +46·1°. Now t, were the Gulf Stream to be stopped, the temperature of the maritime regions of Northwestern Europe would probably fall as low as that of corresponding latitudes in America. In the event of that stream being diverted out of the North Atlantic, the polar currents flowing south would of course occupy the greater portion of the oceanic surface to the west of us, and hence our west and southwest winds would be cold and ungenial. Were such a catastrophe to happen, we may be sure that the Scandinavian Peninsula and a large part of our own islands would not be habitable by civilised man, while Iceland would become as inaccessible as Franz-Josef Land is now. Our own shores would be frost-bound for a large part of the year, and perennial snow and glaciers would appear in our mountain districts. On the other hand, if we could suppose the body of warm water now flowing into the North Atlantic to be largely increased in volume, there can be just as little doubt that the climate of Northwestern Europe would be much improved, and even Spitzbergen and Nova Zembla might be rendered fit abodes for civilized communities.
Strange as it may appear, the records of geology assure us that such changes in the climate of Northwestern Europe have actually happened within a comparatively recent period—certainly within the human epoch. Thus it has been well ascertained that at a time when rude tribes of men, unacquainted with the use of metals, were hunting reindeer in Southern France, a vast area in Northern Europe was buried under a great sheet of ice—a veritable mer de glace—like those ice-sheets which cover up such extensive areas in the Arctic and Antarctic regions. Had any one at that time approached the shores of Northwestern Europe, he would have encountered, some fifty miles outside of the present western shores of Ireland and the Hebrides, a vast vertical wall of ice, like that which confronted Sir J. Clark Ross, Commodore Wilkes, and the Challenger expedition, and barred all approach to the Antarctic pole. Could our voyager have surmounted the Scoto-Hibernian ice-wall, he would have discovered that he stood on the verge of an interminable plain of snow and ice, extending eastward as far as the eye could reach. Here and there upon the far horizon he would have observed a few inconsiderable rocky hills, representing the upper portions of our highest mountains—for the ice which then buried our low grounds was not less than three thousand feet thick. Had he advanced inland until he reached those rocky heights he would have found the icy plain sweeping away in all directions, and bounded only by the horizon; and had he continued his journey toward the east, across what is now the German Ocean, he would have encountered nothing to break the monotonous level until he approached a low line of snow covered hills and mounds, marking the site of the ice-drowned Scandinavian Peninsula. After attaining these hills, had our imaginary wanderer now shaped his course toward the south, he must next have traversed a distance of more than five hundred miles before he reached the termination of the ice-sheet in Saxony. During this extraordinary condition of things the Arctic-Alpine plants and animals (which are now characteristic of lofty elevations in Central and Northwestern Europe, and of the low grounds of the higher latitudes) occupied the low-lying plains and valleys of middle Europe, a flora like that of Lapland growing in South Germany, Switzerland, etc., and pines and spruce-firs flourishing in Northeastern France, while reindeer, glutton, Arctic fox, lemming, marmot, and others were the prevailing animals of those regions. When we think of the conditions which now obtain in Europe, can we conceive of any stronger contrast than that which is suggested by the few facts which are here summarized? Instead of the splendid navies of war-ships, ocean-liners, and other craft which now crowd its waters, the English Channel had formerly its fleets of ice-floes, and the walrus haunted the coasts of Northern France. When such extreme conditions obtained, it can hardly be doubted that the Gulf Stream, as we know it now, had no existence in the North Atlantic. Now let us look at another picture of the past, but still within the human epoch. There was a time, then, when the European climate was so genial that many delicate southern species of plants nourished luxuriantly in regions where they can not now exist. Thus, in the neighborhood of Paris, the fig-tree, the Judas-tree, the laurel of the Canary Islands, and other southern species, found a congenial habitat. The Canary laurel does not grow farther north now than Toulon, on the borders of the Mediterranean. It flowers in winter, and repeated frosts would, therefore, prevent it reproducing its kind. That this plant formerly flourished near Paris is thus a striking proof of changed climatic conditions; we can not doubt that at one time the winters in Northern France must have been extremely genial. Moreover, we know, from the character of the plants with which the Canary laurel was associated in that region, that the climate must have been exempt from extremes—the summers were neither so dry nor so hot, and the winters were very much milder. The land and fresh-water shells which were contemporaneous with that remarkable flora in Northwestern Europe tell precisely the same tale, and this is still further illustrated and confirmed by the character of the mammalian fauna. Among the commoner animals at that time occupants of England, France, etc., were hippopotami, elephants, rhinoceroses, lions, tigers, hyenas, etc., and vast numbers of cervine and bovine animals which still occupy the temperate latitudes of Europe. That such genial climatic conditions were due in large measure to a great increase in the volume of warm water flowing into the North Atlantic seems just as certain as that the Arctic climate of the Glacial period was largely induced by a very considerable decrease, or even an entire stoppage, of that heat-bearing current. The presence of many Mediterranean shells in the ancient raised beaches of Scandinavia, the occurrence of mussel-banks in the coast-lands of Spitzbergen, the appearance here and there off the coast of Scotland, the Faröes, and Iceland, of southern species of shell-fish, and the presence of isolated colonies of southern mollusks in the Gulf of St. Lawrence, are all indicative of a former much greater influx of warm water into northern regions than is now the case. Those remarkable colonies of southern species are living evidence of the last epoch of extremely genial conditions experienced in Northwestern Europe—an epoch during which great forest growths overspread wide regions in the north—covering the British Islands, the Inner and Outer Hebrides, the Orkney and Shetland Islands, all Norway up to the extreme north, and most extensive areas which to-day lie submerged in the sea.
Thus, it will be seen how greatly the climate of Northwestern Europe has been, and may yet again be, modified by changes in the flow of the Gulf Stream. Now, if we glance at a map of America, it will be observed that only a narrow neck of land separates the Gulf of Mexico from the waters of the Pacific, and it is conceivable that, were this isthmus to be submerged, the great body of heated water, that at present finds its way north through the Straits of Florida, might flow directly into the Pacific Ocean. There is no evidence, however, to show that the barrier of Darien has been thus submerged within any recent geological period, and consequently the great changes of climate referred to above could not have been brought about in this way. They were most probably induced, as Dr. Croll has shown, by a remarkable series of physical mutations, which accompanied and were caused by the last great increase in the eccentricity of the earth's orbit. As the next period of high eccentricity will not occur for many thousands of years, we need have no apprehension that we shall be deprived of the genial influence of our friendly Gulf Stream in the immediate future. And when we reflect upon the magnitude of the submergence of Central America which would be required to divert that stream into the Pacific, we may be equally reassured that we are not likely to suffer from its loss in our day, for, before it could be sensibly diminished in volume by this means, the Isthmus of Darien would need to be sunk to a depth of not less than 800 feet, and perhaps even 1,000 feet. We may therefore contemplate with perfect composure the formation of any number of canals across that isthmus. The great work of the famous French engineer will have as much effect upon the Gulf Stream and the climate of North western Europe as the emptying of a teapotful of boiling water into the Arctic Ocean would have in raising the annual temperature in Greenland.—British Trade Journal.