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How the Ocean Works An Introduction to Oceanography

Throughout history, men and women have been drawn to the sea. In ancient times, people felt the same desire you and I feel today, the urge to stand and gaze upon the ocean. Then, as now, the ocean meant different things to different people. For the hungry, the sea was a source of abundant food: fish and lobsters, seals and seaweeds, whales and shrimp. For the audacious, the ocean was a route to faraway lands, an avenue of commerce and conquest. For the bereaved, the ocean epitomized grief: the oblivion into which their loved ones sailed, never to return, or the source of waves that consumed their villages. For the lucky, the sea was a source of pleasure and recreation.

These historical perspectives persist, but if you go down to the shore today and stare out to sea, your perspective is likely tempered by a modern point of view. With our recently acquired ability to travel into space, we now see the earth and its oceans as a whole (figure 1.1). Using satellite cameras we can locate storms and guide ships out of harm's way. We can count the phytoplankton that fuel the fisheries and direct fishermen to fertile grounds. Our sensors detect climatic anomalies as they begin, allowing us to predict their consequences. These are exciting times for humankind's interactions withthe sea.

But this new perspective has two dangers. First, our capacity to see the entire earth can foster a sense of arrogance. It is easy to forget that the ability to observe is only the first step toward the ability to understand and control. As we learn how the ocean works, we must be careful to note the limits of our knowledge. Second, the grand view from space has the tendency to diminish our sense of personal contact. Awe-inspiring as it is, figure 1.1 cannot convey the infinite expanse of the ocean at night when viewed from the deck of a small ship, the tang of salt on an ocean breeze, the crash of a breaking wave, or the shiver of water against your skin. To arrive eventually at a full appreciation of earth's seas, we first need to anchor our perspective in human experience. To that end, we retrace the steps of our ancestors and explore the arduous path by which human society discovered the oceans. In this chapter, we have time only for an outline of the full journey-a mad dash through history. So, brace yourself as we begin with legends.

Ancient Myths

In many cultures, myths of the sea tell mostly of destruction. From the islands of the Pacific to the coasts of Central America, India, and the Middle East, when the ancient gods were displeased with men or women, they often chose the ocean rather than fire or wind as the instrument of their wrath. In the Bible, for instance, forty days and forty nights of rain caused the sea to rise and wipe the earth clean, sparing only Noah and those on his ark. Similar legends of earth-cleansing floods are common among civilizations in the Middle East. The Babylonians, for example, had a flood myth similar to that of the Bible, with a man named Utnapishtim playing the role of Noah. Likewise, the Sumerians had King Ziusudra.

Given this ubiquitous mythical reference to floods, some historians have suggested that there are historical bases for these tales. For example, legends of a great flood among coastal Indians of the Pacific Northwest may refer to tsunamis (tidal waves) caused by earthquakes, and the flood myths of Pacific islanders may describe waves resulting from volcanic eruptions.

A potential, although controversial, source of the Middle Eastern flood legends concerns the Black Sea. Today, the Black Sea is connected to the Mediterranean Sea by the Bosporus Strait, a narrow passage adjacent to Istanbul (figure 1.2). However, about ten thousand years ago at the end of the last Ice Age, a sill (essentially an earthen dam) closed the Bosporus, and the Black Sea was a freshwater lake. As the glaciers receded and earth's climate warmed, the level of the Black Sea dropped due to evaporation, while the level of the Mediterranean rose as the world's oceans absorbed the water from melting ice. Eventually, the Mediterranean broke through the sill separating it from the Black Sea, and the consequent flooding would have been catastrophic to the villages along the Black Sea's coast-an event worthy of a legend.

Regardless of their precise origin, the flood myths convey the mystery and fear that tinged ancient encounters with the sea.

Commerce and Expansion

For all its destructive potential, the ocean has always tempted humans to risk its dangers in search of food and rapid transport. Boats built of wood and reeds traveled the waters of the Nile River in Egypt as long ago as 4000 BC, and many ancient civilizations of the Middle East used boats for fishing and coastal commerce. In particular, the Phoenicians were adventuresome merchants and adept sailors. In the first and second millennium BC, they developed a complex web of trade routes around the Mediterranean, and sailed as far north as the British Isles, trading for tin to use in making bronze. Similarly, in the Far East, coastal commerce using sampans and junks flourished in China. In the Arctic, the Inuit developed sea-going kayaks and umiaks, capable vessels from which they could hunt walrus and whales. The indigenous people of Chile, Peru, and Ecuador used small reed boats and large rafts for fishing and commerce.

In fact, some anthropologists suggest that the expansion of humans into North and South America occurred most rapidly not by land, but rather by sea. Experts agree that humans spread from Asia into North America after the last Ice Age, and have long assumed that humans spread throughout North America before subsequently expanding to Central and South America. It has recently been proposed, however, that the leading edge of this expansion was not through the middle of the continent, but rather along its western shore as groups used small boats to travel south. For example, a village at Monte Verde on the coast of Chile probably dates back to at least 15,000 BC, a time when humans were only beginning to populate central North America. Clearly, from very early times, our fear of the ocean has competed with-and often lost to- our urge to go exploring in boats.

Perhaps the best example of this wanderlust is the spread of humanity to islands in the Pacific. Starting in the Philippines about five thousand years ago, modern humans rapidly expanded their range in the open expanse of the Pacific Ocean. By 1600 BC they had reached New Guinea and the nearby Bismarck Archipelago. Forced onward by population growth, Polynesians next sailed to Samoa around 1200 BC, and by 500 AD, they had traveled all the way to Hawai'i and Easter Island (figure 1.3).

It is clear that this expansion was not the accidental result of a few fisher folk being blown offshore and ending up on other islands. Hawai'i, for example, was settled by sailors traveling from the Marquesas across 3700 kilometers (2300 miles) of open ocean. That monumental leap required ocean-going outrigger canoes large enough to carry not only people but also the plants and animals of their culture. And finding Hawai'i required exceptional working knowledge of navigation and the sea. Each step in the Polynesian expansion involved skill, planning, and a group effort.

Even for the Polynesians, however, there were limitations. They sailed from one island to another within a circumscribed area of the Pacific, but this expansion was constrained by the availability of uninhabited islands. As well planned and skillfully executed as they were, Polynesian expeditions were incapable of invading previously occupied mainland territory. For instance, if prehistoric Polynesians reached South America-the next step east from Easter Island-their numbers were far too small to gain a foothold among the indigenous people who had arrived millennia before. Thus, despite their sailing prowess, the Polynesians could not move beyond the Pacific, and as a result, they had no knowledge of the entirety of the world ocean.

The course of Polynesian exploration in the Pacific was mirrored by that of the Vikings in the northern Atlantic. In the first few centuries AD, the climate in Scandinavia was unusually warm, allowing crops to flourish and populations to grow. As the countryside filled up, Norwegians and Danes looked for opportunities elsewhere. Utilizing their superior shipbuilding technology, the Vikings sent raiding parties eastward along the rivers of Russia and southward along the coasts of Europe, at times ranging as far as Constantinople (now Istanbul). These raids significantly impacted European history. For example, William the Conqueror defeated the English at the Battle of Hastings in 1066-the sole successful invasion of the British Isles-in large part because the English troops were exhausted from doing battle a month before with Viking invaders.

In addition to raiding established societies, the Vikings undertook an island-hopping expansion westward into uninhabited territory (figure 1.4), first to the Orkney, Shetland, and Faeroe Islands (around 800 AD), then to Iceland (by 874 AD) and southern Greenland (in about 980 AD). Ships plying the trade routes between Iceland and Greenland were occasionally blown off course, and one of these (captained by Bjarni Herjolfsson in 986 AD) sighted a forested coast to the west of Greenland. Spurred on by the lure of abundant timber, which had become scarce in Greenland, Leif Ericsson bought Herjolfsson's boat and formed an expedition to explore this new land. Ericsson and his crews, accompanied by women and livestock, traveled along the coast of Baffin Island and Labrador and built a small outpost at L'Anse aux Meadows in Newfoundland. However, they skirmished with the local Indians, and in light of the prospect of continued conflict, the Vikings abandoned their attempt to colonize the New World.

Like the Polynesians, the Vikings explored the ocean not out of a thirst for knowledge, but rather in response to the demands of population growth and commerce. And, like the Polynesians, they were stymied in their expansion by the overwhelming size of indigenous populations. Viking travels were thereby limited, and they never developed an understanding of the scope of the world ocean.

In summary, as a result of its expanding population, by 1000 AD, humankind had dipped its toes into each of earth's seas. Many cultures used coastal waters for fishing and commerce, and the Polynesians and Vikings traveled a few open-ocean routes. But our knowledge of the sea was piecemeal: we hadn't truly discovered the ocean. That achievement resulted from a different journey, one that began with the Greeks.

Science and the Greeks

Growth of Greek civilization marked the beginning of abstract scientific thought. Whereas the Chinese, Egyptians, and Babylonians were masters of technology and astronomy, the Greeks were masters of concept. This distinction is evident in the way the different civilizations viewed the world. Early Chinese maps, for instance, give detailed information regarding the disposition of armies and the location of rivers and cities in the Middle Kingdom, but they show nothing beyond China's boundaries. In contrast, by 2500 years ago, the Greeks were actively speculating on the shape of the entire earth.

In fact, their speculations were highly logical and specific. For example, Aristotle (384-322 BC), the famous Greek scientist and philosopher, deduced that earth was a sphere, an assumption he based on four observations. First, there was the appearance of ships as they sailed into the distance. If the earth were flat, a ship sailing toward the horizon would appear smaller and smaller until it vanished from view. Instead, ships descended into the horizon, their hulls disappearing first, then their masts, suggesting that the surface of the water (and thus, of the earth) was curved.

The same logic applies from the perspective of the ship: as it travels away from shore, the coastline descends into the horizon. This perspective led Aristotle to note that stars appear and disappear on the horizon as one travels north or south, further evidence of earth's spherical shape.

Lunar eclipses provided a third clue. It was apparent from simple observations that the moon is a sphere. For instance, as the moon goes through its phases, the line separating dark from light changes shape as it travels across the moon's face. It is curved when the moon is a crescent, and straight when the moon is half full: what one would expect for a sphere lit from different angles. Shadows similarly reveal the shape of the earth. During a lunar eclipse, the shadow of the earth falling on the moon is always curved, as it should be if earth is a sphere.

And finally, there was the matter of aesthetics. To Greek mathematicians, the sphere was the perfect shape, the only object uniform about its center. The sun was clearly spherical, as was the moon. Why should the earth be any less perfect?

Acceptance of the earth as a sphere immediately led Greek scientists to two important questions: how big is the sphere, and where on its surface is Greece? In an extraordinary example of cultural genius, they devised answers to both.

In the fourth century BC, Alexander the Great (a pupil of Aristotle's) conquered much of the known world, sending many of the treasures he obtained to his museum in Alexandria, near the mouth of the Nile. After Alexander's death, the museum was supported by the royal rulers of Egypt and continued to acquire and catalog the fruits of civilization. To that end, it established a "think tank" (Euclid and Archimedes worked there, among others) and a vast library. The second Librarian of Alexandria, a man named Eratosthenes (ca. 276-196 BC), took it upon himself not only to accumulate the written knowledge of the world (as any good librarian would), but also to synthesize that knowledge. One of the things he contemplated was the size of the earth.

It came to Eratosthenes' attention that each year a notable event occurred in the city of Syene, due south of Alexandria. At noon on the summer solstice- the day of the year when the sun is highest in the sky-sunlight shone directly down into the wells at Syene, indicating that the sun was precisely overhead. Eratosthenes found this curious. In Alexandria there were vertical stone pillars called gnomons. If the sun were also directly overhead in Alexandria, the gnomons would not cast a shadow, but at noon on the summer solstice, shadows persisted. Clearly the angle at which sunlight approached the ground differed between Alexandria and Syene.

Enter the notion of a spherical earth (figure 1.5). Eratosthenes deduced that the difference in the angle of sunlight resulted from the different locations of Alexandria and Syene. At noon on the summer solstice, Syene (near the present-day city of Aswan) was at a point on earth directly under the sun, whereas Alexandria, north of Syene, was at a point on earth's curve where a vertical gnomon pointed off at an angle. Eratosthenes' genius was to use this realization to measure the size of the earth.

To do so, he needed two measurements. First, he had to measure the angle between Alexandria and Syene (angle A in figure 1.5). This was easily done. Noting that angle A equals angle B, Eratosthenes realized he could use the shadow cast by a gnomon to measure the angle between Alexandria and Syene. At noon on the solstice, he wandered over to the nearest gnomon, measured its height and the length of its shadow, and used geometry to calculate first angle B, and then angle A. It turned out to be 7.12[degrees], almost exactly 1/50 of an entire circle. This angle in turn meant that the distance from Syene to Alexandria was 1/50 the entire distance around the earth. Thus, if Eratosthenes could measure the distance from Syene to Alexandria, he could measure the earth's circumference.

Surprisingly, Eratosthenes didn't even need to leave home to measure the distance from Syene to Alexandria. As Librarian, he was privy to all sorts of information, including the fact that caravans of camels typically required 50 days to travel from Syene to Alexandria. It was common knowledge in those times that camels can travel 100 stadia per day, so he calculated that the distance from Syene to Alexandria is approximately 50 x 100 = 5000 stadia.

(Continues...)

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Excerpted from How the Ocean Works by Mark Denny
Copyright © 2008 by Princeton University Press. Excerpted by permission.
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