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 forces 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.[28] 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.