There are six more or less permanent centers of high pressure over the oceans, three in each hemisphere. Not only do these areas play a controlling part in the climate of surrounding lands, but they affect the whole world because they are the birthplaces of most of the dominant winds of the globe. The trade winds originate in high-pressure belts of the Northern and Southern hemispheres. Over all the vast extent of ocean across which they blow, these great winds retain their identity; it is only over the continents that they become interrupted, confused, and modified.
In other ocean areas there are belts of low pressure, which develop, especially in winter, over waters that are then warmer than the surrounding lands. Traveling barometric depressions or cyclonic storms are attracted by these areas; they move rapidly across them or skirt around their edges. So winter storms take a path across the Icelandic ‘low’ and over the Shetlands and Orkneys into the North Sea and the Norwegian Sea; other storms are directed by still other low-pressure areas over the Skagerrak and the Baltic into the interior of Europe. Perhaps more than any other condition, the low-pressure area over the warm water south of Iceland dominates the winter climate of Europe.
And most of the rains that fall on sea and land alike were raised from the sea. They are carried as vapor in the winds, and then with change of temperature the rains fall. Most of the European rain comes from evaporation of Atlantic water. In the United States, vapor and warm air from the Gulf of Mexico and the tropical waters of the western Atlantic ride the winds up the wide valley of the Mississippi and provide rains for much of the eastern part of North America.
Whether any place will know the harsh extremes of a continental climate or the moderating effect of the sea depends less on its nearness to the ocean than on the pattern of currents and winds and the relief of the continents. The east coast of North America receives little benefit from the sea, because the prevailing winds are from the west. The Pacific coast, on the other hand, lies in the path of the westerly winds that have blown across thousands of miles of ocean. The moist breath of the Pacific brings climatic mildness and creates the dense rain forests of British Columbia, Washington, and Oregon; but its full influence is largely restricted to a narrow strip by the coast ranges that follow a course parallel to the sea. Europe, in contrast, is wide open to the sea, and ‘Atlantic weather’ carries hundreds of miles into the interior.
By a seeming paradox, there are parts of the world that owe their desert dryness to their nearness to the ocean. The aridity of the Atacama and Kalahari deserts is curiously related to the sea. Wherever such marine deserts occur, there is found this combination of circumstances: a western coast in the path of the prevailing winds, and a cold coastwise current. So on the west coast of South America the cold Humboldt streams northward off the shores of Chile and Peru—the great return flow of Pacific waters seeking the equator. The Humboldt, it will be remembered, is cold because it is continuously being reinforced by the upwelling of deeper water. The presence of this cold water offshore helps create the aridity of the region. The onshore breezes that push in toward the hot land in the afternoons are formed of cool air that has lain over a cool sea. As they reach the land they are forced to rise into the high coastal mountains—the ascent cooling them more than the land can warm them. So there is little condensation of water vapor, and although the cloud banks and the fogs forever seem to promise rain, the promise is not fulfilled so long as the Humboldt rolls on its accustomed course along these shores. On the stretch from Arica to Caldera there is normally less than an inch of rain in a year. It is a beautifully balanced system—as long as it remains in balance. What happens when the Humboldt is temporarily displaced is nothing short of catastrophic.
At irregular intervals the Humboldt is deflected away from the South American continent by a warm current of tropical water that comes down from the north. These are years of disaster. The whole economy of the area is adjusted to the normal aridity of climate. In the years of El Niño, as the warm current is called, torrential rains fall—the downpouring rains of the equatorial regions let loose upon the dust-dry hillsides of the Peruvian coast. The soil washes away, the mud huts literally dissolve and collapse, crops are destroyed. Even worse things happen at sea. The cold-water fauna of the Humboldt sickens and dies in the warm water, and the birds that fish the cold sea for a living must either migrate or starve.
Those parts of the coast of Africa that are bathed by the cool Benguela Current also lie between mountains and sea. The easterly winds are dry, descending winds, and the cool breezes from the sea have their moisture capacity increased by contact with the hot land. Mists form over the cold waters and roll in over the coast, but in a whole year the rainfall is the meagerest token. The mean rainfall at Swakopmund in Walvis Bay is 0.7 inches a year. But again this is true only as long as the Benguela holds sway along the coast, for there are times when the cold stream falters as does the Humboldt, and here also these are years of disaster.
The transforming influence of the sea is portrayed with beautiful clarity in the striking differences between the Arctic and Antarctic regions. As everyone knows, the Arctic is a nearly landlocked sea; the Antarctic, a continent surrounded by ocean. Whether this global balancing of a land pole against a water pole has a deep significance in the physics of the earth is uncertain; but the bearing of the fact on the climates of the two regions is plainly evident.
The ice-covered Antarctic continent, bathed by seas of uniform coldness, is in the grip of the polar anticyclone. High winds blow from the land and repel any warming influence that might seek to penetrate it. The mean temperature of this bitter world is never above the freezing point. On exposed rocks the lichens grow, covering the barrenness of cliffs with their gray or orange growths, and here and there over the snow is the red dust of the hardier algae. Mosses hide in the valleys and crevices less exposed to the winds, but of the higher plants only a few impoverished stands of grasses have managed to invade this land. There are no land mammals; the fauna of the Antarctic continent consists only of birds, wingless mosquitoes, a few flies, and microscopic mites.
In sharp contrast are the arctic summers, where the tundra is bright with many-colored flowers. Everywhere except on the Greenland icecap and some of the arctic islands, summer temperatures are high enough for the growth of plants, packing a year’s development into the short, warm, arctic summer. The polar limit of plant growth is set not by latitude, but by the sea. For the influence of the warm Atlantic penetrates strongly within the Arctic Sea, entering, as we have seen, through the one large break in the land girdle, the Greenland Sea. But the streams of warm Atlantic water that enter the icy northern seas bring the gentling touch that makes the Arctic, in climate as well as in geography, a world apart from the Antarctic.
So, day by day and season by season, the ocean dominates the world’s climate. Can it also be an agent in bringing about the long-period swings of climatic change that we know have occurred throughout the long history of the earth—the alternating periods of heat and cold, of drought and flood? There is a fascinating theory that it can. This theory links events in the deep, hidden places of the ocean with the cyclic changes of climate and their effects on human history. It was developed by the distinguished Swedish oceanographer, Otto Pettersson, whose almost century-long life closed in 1941. In many papers, Pettersson presented the different facets of his theory as he pieced it together, bit by bit. Many of his fellow scientists were impressed, others doubted. In those days few men could conceive of the dynamics of water movements in the deep sea. Now the theory is being re-examined in the light of modern oceanography and meteorology, and only recently C. E. P. Brooks said, ‘It seems that there is good support for Pettersson’s theory as well as for that of solar activity, and that the actual variations of climate since about 3000 B.C. may have been to a large extent the result of these two agents.’
To review the Pettersson theory is to review also a pageant of human history, of men and nations in the control of elemental for
ces whose nature they never understood and whose very existence they never recognized. Pettersson’s work was perhaps a natural outcome of the circumstances of his life. He was born—as he died 93 years later—on the shores of the Baltic, a sea of complex and wonderful hydrography. In his laboratory atop a sheer cliff overlooking the deep waters of the Gulmarfiord, instruments recorded strange phenomena in the depths of this gateway to the Baltic. As the ocean water presses in toward that inland sea it dips down and lets the fresh surface water roll out above it; and at that deep level where salt and fresh water come into contact there is a sharp layer of discontinuity, like the surface film between water and air. Each day Pettersson’s instruments revealed a strong, pulsing movement of that deep layer—the pressing inward of great submarine waves, of moving mountains of water. The movement was strongest every twelfth hour of the day, and between the 12-hour intervals it subsided. Pettersson soon established a link between these submarine waves and the daily tides. ‘Moon waves,’ he called them, and as he measured their height and timed their pulsing beat through the months and years, their relation to the ever-changing cycles of the tides became crystal clear.
Some of these deep waves of the Gulmarfiord were giants nearly 100 feet high. Pettersson believed they were formed by the impact of the oceanic tide wave on the submarine ridges of the North Atlantic, as though the waters moving to the pull of the sun and moon, far down in the lower levels of the sea, broke and spilled over in mountains of highly saline water to enter the fiords and sounds of the coast.
From the submarine tide waves, Pettersson’s mind moved logically to another problem—the changing fortunes of the Swedish herring fishery. His native Bohuslan had been the site of the great Hanseatic herring fisheries of the Middle Ages. All through the thirteenth, fourteenth, and fifteenth centuries this great sea fishery was pursued in the Sund and the Belts, the narrow passageways into the Baltic. The towns of Skanor and Falsterbo knew unheard-of prosperity, for there seemed no end of the silvery, wealth-bringing fish. Then suddenly the fishery ceased, for the herring withdrew into the North Sea and came no more into the gateways of the Baltic—this to the enrichment of Holland and the impoverishment of Sweden. Why did the herring cease to come? Pettersson thought he knew, and the reason was intimately related to that moving pen in his laboratory, the pen that traced on a revolving drum the movements of the submarine waves far down in the depths of Gulmarfiord.
He had found that the submarine waves varied in height and power as the tide-producing power of the moon and sun varied. From astronomical calculations he learned that the tides must have been at their greatest strength during the closing centuries of the Middle Ages—those centuries when the Baltic herring fishery was flourishing. The sun, moon, and earth came into such a position at the time of the winter solstice that they exerted the greatest possible attracting force upon the sea. Only about every eighteen centuries do the heavenly bodies assume this particular relation. But in that period of the Middle Ages, the great underwater waves pressed with unusual force into the narrow passages to the Baltic, and with the ‘water mountains’ went the herring shoals. Later, when the tides became weaker, the herring remained outside the Baltic, in the North Sea.
Then Pettersson realized another fact of extreme significance—that those centuries of great tides had been a period of ‘startling and unusual occurrences’ in the world of nature. Polar ice blocked much of the North Atlantic. The coasts of the North Sea and the Baltic were laid waste by violent storm floods. The winters were of ‘unexplained severity’ and in consequence of the climatic rigors political and economic catastrophes occurred all over the populated regions of the earth. Could there be a connection between these events and those moving mountains of unseen water? Could the deep tides affect the lives of men as well as of herring?
From this germ of an idea, Pettersson’s fertile mind evolved a theory of climatic variation, which he set forth in 1912 in an extraordinarily interesting document called Climatic Variations in Historic and Prehistoric Time.* Marshalling scientific, historic, and literary evidence, he showed that there are alternating periods of mild and severe climates which correspond to the long-period cycles of the oceanic tides. The world’s most recent period of maximum tides, and most rigorous climate, occurred about 1433, its effect being felt, however, for several centuries before and after that year. The minimum tidal effect prevailed about A.D. 550, and it will occur again about the year 2400.
During the latest period of benevolent climate, snow and ice were little known on the coast of Europe and in the seas about Iceland and Greenland. Then the Vikings sailed freely over the northern seas, monks went back and forth between Ireland and ‘Thyle’ or Iceland, and there was easy intercourse between Great Britain and the Scandinavian countries. When Eric the Red voyaged to Greenland, according to the Sagas, he ‘came from the sea to land at the middle glacier—from thence he went south along the coast to see if the land was habitable. The first year he wintered on Erik’s Island …’ This was probably in the year 984. There is no mention in the Sagas that Eric was hampered by drift ice in the several years of his exploration of the island; nor is there mention of drift ice anywhere about Greenland, or between Greenland and Wineland. Eric’s route as described in the Sagas— proceeding directly west from Iceland and then down the east coast of Greenland—is one that would have been impossible during recent centuries. In the thirteenth century the Sagas contain for the first time a warning that those who sail for Greenland should not make the coast too directly west of Iceland on account of the ice in the sea, but no new route is then recommended. At the end of the fourteenth century, however, the old sailing route was abandoned and new sailing directions were given for a more southwesterly course that would avoid the ice.
The early Sagas spoke, too, of the abundant fruit of excellent quality growing in Greenland, and of the number of cattle that could be pastured there. The Norwegian settlements were located in places that are now at the foot of glaciers. There are Eskimo legends of old houses and churches buried under the ice. The Danish Archaeological Expedition sent out by the National Museum of Copenhagen was never able to find all of the villages mentioned in the old records. But its excavations indicated clearly that the colonists lived in a climate definitely milder than the present one.
But these bland climatic conditions begin to deteriorate in the thirteenth century. The Eskimos began to make troublesome raids, perhaps because their northern sealing grounds were frozen over and they were hungry. They attacked the western settlement near the present Ameralik Fiord, and when an official mission went out from the eastern colony about 1342, not a single colonist could be found—only a few cattle remained. The eastern settlement was wiped out some time after 1418 and the houses and churches destroyed by fire. Perhaps the fate of the Greenland colonies was in part due to the fact that ships from Iceland and Europe were finding it increasingly difficult to reach Greenland, and the colonists had to be left to their own resources.
The climatic rigors experienced in Greenland in the thirteenth and fourteenth centuries were felt also in Europe in a series of unusual events and extraordinary catastrophes. The seacoast of Holland was devastated by storm floods. Old Icelandic records say that, in the winters by the early 1300’s, packs of wolves crossed on the ice from Norway to Denmark. The entire Baltic froze over, forming a bridge of solid ice between Sweden and the Danish islands. Pedestrians and carriages crossed the frozen sea and hostelries were put up on the ice to accommodate them. The freezing of the Baltic seems to have shifted the course of storms originating in the low-pressure belt south of Iceland. In southern Europe, as a result, there were unusual storms, crop failures, famine, and distress. Icelandic literature abounds in tales of volcanic eruptions and other violent natural catastrophes that occurred during the fourteenth century.
What of the previous era of cold and storms, which should have occurred about the third or fourth century B.C., according too the tidal theory? There are shadowy hints in
early literature and folklore. The dark and brooding poetry of the Edda deals with a great catastrophe, the Fimbul-winter or Götterdämmerung, when frost and snow ruled the world for generations. When Pytheas journeyed to the seas north of Iceland in 330 B.C., he spoke of the mare pigrum, a sluggish, congealed sea. Early history contains striking suggestions that the restless movements of the tribes of northern Europe—the southward migrations of the ‘barbarians’ who shook the power of Rome—coincided with periods of storms, floods, and other climatic catastrophes that forced their migrations. Large-scale inundations of the sea destroyed the homelands of the Teutons and Cimbrians in Jutland and sent them southward into Gaul. Tradition among the Druids said that their ancestors had been expelled from their lands on the far side of the Rhine by enemy tribes and by ‘a great invasion of the ocean.’ And about the year 700 B.C. the trade routes for amber, found on the coasts of the North Sea, were suddenly shifted to the east. The old route came down along the Elbe, the Weser, and the Danube, through the Brenner Pass to Italy. The new route followed the Vistula, suggesting that the source of supply was then the Baltic. Perhaps storm floods had destroyed the earlier amber districts, as they invaded these same regions eighteen centuries later.