Sea level change

For at least tens of thousands of years, changes in sea level seem to be a natural part of the earth's environment . Up until a few hundred years ago, these changes, both up and down, occurred due to land movement, ice melting from glaciers, and an increase or decrease in the amount of water trapped in the polar ice caps. In most cases, the changes were very gradual in human terms. But in the past several decades, many scientists have become alarmed at the rapid increase in ocean levels.

The reason is because the earth is heating up, which causes sea water to expand in volume and ice caps to melt. Over the past 100 years, scientists have measured a mean sea level rise of about 4 in (10 cm). They blame it on an average increase of 1.8°F (1°C) in world-wide surface temperatures of the planet. With the advent of satellites and other technology in the last half of the twentieth century, scientists have been able to more precisely measure the ocean levels. They have found that sea levels are rising at a rate of about 0.4–1.2 in (1–2 cm) per year. A 1-ft (30-cm) rise in sea level would place at least 100 ft (30 m) of beach width underwater.

Why the earth is heating up is the subject of much discussion and disagreement among scientists. Some believe it is due to the heavy burning of fossil fuels , such as coal and oil, which causes increases the amounts of certain gases, particularly carbon dioxide , in the atmosphere . Other scientists believe the current global warming is mostly a natural phenomenon and a part of the normal cycle of the planet's environment.

Scientists are studying the ice cap on Antarctica to determine if, in fact, the earth's climate is warming due to the burning of fossil fuels. The global warming hypothesis is based on the atmospheric process known as the greenhouse effect , in which pollution prevents the heat energy of the earth from escaping into the outer atmosphere. Global warming could cause some of the ice caps to melt, raising the sea level and flooding many of the world's largest cities, including New York, Los Angeles, Tokyo, and London, and other lowland areas. Nearly half of the world's population live in coastal areas. Because the polar regions are the engines that drive the world's weather system, this research is essential to identify the effect of human activity on these regions.

Many scientists are concerned about the increasing levels of carbon dioxide in Earth's atmosphere. With more carbon dioxide in the atmosphere, they say, more heat will be trapped. Earth's annual average temperature will begin to rise. Some estimates suggest that a doubling of atmospheric carbon dioxide will result in an increase of 4.5°F (2.5°C) in the planet's annual average temperature.

While that number may seem small, it could have disastrous effects on the world's economies. One result might be the melting of Earth's ice caps at the North and South poles, with a resulting increase in the volume of the ocean's water. Were that to happen, many of the world's largest cities, those located along the edge of the oceans, might be flooded. Some experts predict dramatic changes in climate that could turn currently productive croplands into deserts, and deserts into productive agricultural regions.

As with many environmental issues, experts tend to disagree about one or more aspects of anticipated climate change. Some authorities are not convinced that the addition of carbon dioxide to the atmosphere will have any significant long-term effects on Earth's average annual temperature. Others concede that Earth's temperature may increase, but that the changes predicted are unlikely to occur. They point out that other factors, such as the formation of clouds, might counteract the presence of additional carbon dioxide in the atmosphere. They warn that nations should not act too quickly to reduce the combustion of fossil fuels since that will cause serious economic problems in many parts of the world. They suggest it would be prudent to wait for a while to see if greenhouse factors really are beginning to change.

[Ken R. Wells ]

RESOURCES

BOOKS

Douglas, B. C. ed, et al. Sea Level Rise: History and Consequences. San Diego: Academic Press, 2000.

PERIODICALS

Laber, Emily. "Meltdown." The Sciences (July 1999): 6.

Middleton, Nick. "The Heat is On." Geographical (January 2000): 44.

Moore, Curtis A. "Awash in a Rising Sea."International Wildlife (January–February 2002).

Spalding, Mark. "Danger on the High Seas." Geographical (February 2002): 15–16.

Foley, Grover. "The Threat of Rising Seas." The Ecologist (March–April 1999): 76–79.

ORGANIZATIONS

National Oceanic and Atmospheric Administration, 14th St. and Constitution Ave. NW, Room 6013, Washington, DC USA 20230 (202)482-6090, Fax: (202)482-3154, Email: [email protected], <http://www.noaa.gov>