Oceans and Seas

Introduction

Earth is an oceanic planet. An ocean is defined as a very large body of saline water—water that contains approximately 35,000 parts per million (ppm) of sodium chloride (NaCl) and trace amounts of many other molecules. In contrast to salty ocean water, the freshwater that people are able to drink contains below 1,000 ppm of

NaCl. More than 70% of Earth's surface is covered by ocean, an area of about 224 million sq mi (580 million sq km). To put this into perspective, the continental United States occupies an area of 3.54 million sq mi (9.2 million sq km).

The continents segregate the global ocean into several large regions, which have been named individually; the Atlantic and Pacific Oceans are two examples. A sea is considered to be a large inlet of the ocean. An example of a sea is the Bay of Fundy, which lies northeast of the coast of Maine and is enclosed by the Canadian provinces of Nova Scotia and New Brunswick. Other examples include the Mediterranean Sea, Black Sea, and Caribbean Sea.

In 2007, it is still accurate to say that scientists know more about the solar system than the ocean. The formidable depth of the ocean (more than half of the ocean is about 10,000 ft/3,000 m deep) has, until the era of manned and unmanned undersea robotic exploration, prevented extensive exploration. Even in 2007, despite centuries of undersea exploration and use of twentieth-century innovations such as sonar, less than 10% of the ocean floor has been mapped in detail. However, more is known and is being learned about the chemistry of the ocean, ocean currents, ocean-influenced wind and weather patterns, and, particularly in the latter several decades of the twentieth century, how the ocean is part of Earth's changing climate.

Historical Background and Scientific Foundations

There are five large oceans—the Atlantic, Pacific, Indian, Arctic, and Southern (or Antarctic) Oceans. Despite their different names, these waters merge into one large global ocean, in which the continents sit as immense islands.

The ocean is a global highway that has been used as a transportation route for centuries. For example, the

dating of material from the ruins of settlements found at L'Anse aux Meadows, located in the far north of the Canadian province of Newfoundland, has shown that they were built by a Viking expedition under the command of Leif Eriksson around AD 1000.

Trans-Pacific travel may have taken place even earlier. In 1947, inspired by evidence indicating that travel may have occurred around AD 500 between South Pacific regions (including New Zealand) and the west coast of South America, Norwegian adventurer Thor Heyerdahl (1914–2002) sailed the Kon-Tiki—a raft constructed as it would have been at that time—from South America to the Tuamotu Islands, a distance of over 4,000 mi (6,400 km). The journey, completed in 101 days, was the first of many trans-Pacific voyages, demonstrating that even 15 centuries ago far-flung ocean journeys were possible.

The ocean currents and prevailing ocean winds that made these and other voyages possible are also powerful contributors to climate. The development of regions of high and low pressure in areas of the tropical ocean can generate winds that spawn extreme storms, such as cyclones.

Some winds that blow over ocean waters are present regularly. These so-called prevailing winds blow in different directions, depending on the hemisphere and distance from the equator. Trade winds immediately north of the equator blow from the northeast to the southwest, while those south of the equator blow from the southeast to the northwest. In the middle northern and southern latitudes, westerlies blow from the southwest to the northeast and from the northwest to the southeast, respectively.

The friction between the moving air and the water molecules tends to push the water along in the direction of the wind, and actually causes the water to pile up. Although gravity tends to pull the piled up water downward, another factor called the Coriolis force, which is generated by the rotation of Earth, causes the water to rotate to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) in the Southern Hemisphere. The result is large mounds of water with a flow of water around them; these are known as gyres. Gyres produce currents in the Northern and Southern Hemispheres of the Atlantic and Pacific Oceans. The Gulf Stream is part of the North Atlantic Gyre.

Ocean currents, which make up about 10% of all ocean waters and are essentially rivers of water flowing within the surrounding ocean, influence climate depending upon whether the current is warmer or colder than the surrounding waters. The Gulf Stream is an example of a warm ocean current, which originates as a current in the Pacific Ocean. The United Kingdom enjoys a much more temperate climate than other regions at a similar northerly latitude because of the warm Gulf Stream waters that arc across the Atlantic and move southward past the country (along the European coast the current is known as the Carnary Current). Ultimately, this current flows back to the Pacific Ocean, and the cyclical global route of the current is repeated.

The flow of water in the ocean currents mixes colder waters nearer to the polar regions with the warmer waters in the mid-latitudes and equatorial regions. The warmer water from equatorial regions rises to the surface, while the colder polar waters sink to the bottom. The cold water moves to more tropical latitudes, where it warms and rises upward. This continuous cycling of water drives the formation of currents.

The air above currents will reflect the water temperature, so currents also help distribute warmer and colder air. In coastal areas, this movement of air influences weather. For example, in summer, coastal areas can be 10°F (5.5°C) or more cooler than inland areas that are not influenced by the ocean or the sea.

WORDS TO KNOW

CORIOLIS FORCE: The apparent tendency of a freely moving particle to swing to one side when its motion is referred to a set of axes that is itself rotating in space, such as Earth. The acceleration is perpendicular to the direction of the speed of the article relative to Earth's surface and is directed to the right in the Northern Hemisphere. Winds are affected by rotation of Earth so that instead of a wind blowing in the direction it starts, it turns to the right of that direction in the Northern Hemisphere; left in the Southern Hemisphere.

CYCLONE: An area of low pressure where winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

EL NIÑO: A warming of the surface waters of the eastern equatorial Pacific that occurs at irregular intervals of 2 to 7 years, usually lasting 1 to 2 years. Along the west coast of South America, southerly winds promote the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry. Near the end of each calendar year, a warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water. Because this condition often occurs around Christmas, it was named El Ninño (Spanish for boy child, referring to the Christ child). In most years the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Ninño conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous. El Ninño has been linked to wetter, colder winters in the United States; drier, hotter summers in South America and Europe; and drought in Africa.

GULF STREAM: A warm, swift ocean current that flows along the coast of the Eastern United States and makes Ireland, Great Britain, and the Scandinavian countries warmer than they would be otherwise.

GYRE: A zone of spirally circulating oceanic water that tends to retain floating materials, as in the Sargasso Sea of the Atlantic Ocean.

LA NIÑA: A period of stronger-than-normal trade winds and unusually low sea-surface temperatures in the central and eastern tropical Pacific Ocean; the opposite of El Ninño.

PHOTIC ZONE: Region of the ocean through which light penetrates and the place where photosynthetic marine organisms live.

PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photosynthesis in some plants. Plants create a very important reservoir for carbon dioxide.

SALINITY: The degree of salt in water. The rise in sea level due to global warming would result in increased salinity of rivers, bays, and aquifers. This would affect drinking water, agriculture, and wildlife.

SONAR: Sound Navigation and Ranging (SONAR) is a remote sensing system with important military, scientific, and commercial applications. Active SONAR transmits acoustic (i.e., sound) waves. Passive SONAR is a listening mode to detect noise generated from targets. SONAR allows the determination of important properties and attributes of the target (i.e., shape, size, speed, distance, etc.).

TRADE WINDS: Surface air from the horse latitudes (subtropical regions) that moves back toward the equator and is deflected by the Coriolis Force, causing the winds to blow from the Northeast in the Northern Hemisphere and from the Southeast in the Southern Hemisphere. These steady winds are called trade winds because they provided trade ships with an ocean route to the New World.

Similar to the atmosphere above Earth's surface, the ocean depths have also been classified into several regions (zones). The distinction between one zone and another depends on the physical conditions of the water and on the nature of the life in the particular region. Sunlight can penetrate to a maximum depth of about 650 ft (198 m). This region is called the photic zone. Some of the organisms that live in the photic zone can convert the energy of sunlight into chemical energy that they can use to live and grow. This process is known as photosynthesis. Because of the huge numbers of photo-synthesizing organisms that inhabit the photic zone, it is the region of greatest concentration and diversity of life in the ocean.

Life in the deeper ocean relies on organic material that floats down from the surface (marine snow) or on other food sources, such as other creatures. With increasing depth, the temperature of the water decreases and the pressure increases. For example, at the resting place of the sunken luxury liner Titanic, 12,400 ft (3,780 m) below the ocean surface, the temperature hovers just above the freezing point and the pressure is about 2,500 lbs per sq in (a literally bone-crushing 2,500 times greater than the air pressure experienced at sea level).

Impacts and Issues

Computer models that simulate the ocean and climate have indicated that the addition of freshwater to the North Atlantic Ocean, precisely what could happen if the ice caps in the Arctic and Greenland melt, would disrupt the northward flow of warm Gulf Stream water. Indeed, a scientific paper published in a 2005 edition of the journal Nature reported on measurements of the Gulf Stream that found a 30% decrease in the northerly flow of the warm water as compared to surveys done in 1957, 1981, and 1992. Whether this finding was temporary or is the beginning of a more permanent change was unclear as of 2007.

Other studies indicate that the computer models may be playing out in real life. Measurements of the Arctic sea-ice have shown that the polar ice cap has decreased in thickness by about half since 2001. As the ice melts, the freshwater encased in the ice (which is transported to the Arctic by Siberian and northerly North American rivers) is being added to the Arctic Ocean. Disruption of the Arctic Ocean currents may be a consequence of this increased input of freshwater.

In addition, evidence suggests that global warming could alter the cyclical global current that flows between the Pacific and Atlantic Oceans. If so, the consequence could be global weather changes. Some scientists have suggested that El Niño—an oscillation of the ocean-atmosphere system characterized by unusually warm ocean temperatures in the tropical Pacific—may be related to global-warming-induced changes in global ocean currents. As of 2007, however, this view is not shared by the majority of scientists.

Most scientists do agree that the ocean is rising at a rate of several centimeters every decade. Moreover, many climatologists agree that La Niña—an oscillation of the ocean-atmosphere system characterized by colder than normal ocean temperatures in equatorial regions of the Pacific Ocean—may be one of the triggers that has spawned the increased frequency of Atlantic hurricanes, by increasing the temperature in the Atlantic equatorial region.

Primary Source Connection

For centuries, explorers searched for a passage connecting the Atlantic Ocean and the Pacific Ocean across present-day Canada. The Northwest Passage, a sea route in the Arctic Ocean that goes through the Canadian Arctic Archipelago, was finally navigated in the twentieth century. While permanent ice pack made commercial navigation of the Northwest Passage impracticable, warming temperatures have melted seasonal ice, making navigation of the Northwest Passage possible.

MELTING ICE CLEARS NORTHWEST PASSAGE

LONDON, Sept. 15 (UPI) —Extreme melting has made the fabled Northwest Passage from Europe to Asia fully navigable.

It is the first time since record-keeping began in 1978 that the passage linking the Atlantic and Pacific Oceans has been free of ice, the European Space Agency said.

The steadily shrinking ice, being monitored by satellite, has raised concerns about an acceleration in global warming, the BBC reported Saturday.

IN CONTEXT: DEEP IMPACTS

“Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise.”

Statement of the Intergovernmental Panel on Climate Change (IPCC) as formally approved at the 10th Session of Working Group I of the IPCC in Paris, France, during February 2007.

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Ice also is melting in the Northeast Passage through the Russian Arctic, and that route now remains only partially blocked, the space agency said.

“melting ice clears northwest passage.” upi. september 15, 2007. <http://www.upi.com/newstrack/science/2007/09/15/melting_ice_clears_northwest_passage/5058/> (accessed december 4, 2007).

See Also Arctic Melting: Greenland Ice Cap; Arctic Melting: Polar Ice Cap; Coastlines, Changing; Great Barrier Reef; Gulf Stream; Red Tide; Sea Level Rise.

BIBLIOGRAPHY

Books

Bigg, Grant R. The Oceans and Climate. New York: Cambridge University Press, 2004.

Desonie, Dana. Oceans: How We Use the Seas. New York: Chelsea House Publications, 2007.

Fujita, Rodney. Heal the Ocean: Solutions for Saving Our Seas. Gabriola Island, British Columbia, Canada: New Society Publishers, 1998.

Roberts, Callum. The Unnatural History of the Sea. Washington, DC: Island Press, 2007.

Periodicals

Bryden, H. L., H. R. Longworth, and S. A. Cunningham. “Slowing of the Atlantic Meridional Overturning Circulation at 25° N.” Nature 438 (December 1, 2005): 655–657.

Web Sites

“An Ocean Warmer Than a Hot Tub.” Woods Hole Oceanographic Institute, February 17, 2006. <http://www.whoi.edu/page.do?pid=12455&tid=282&cid=10366> (accessed November 1, 2007).

Brian D. Hoyle