Greenhouse Effect
The greenhouse effect, the cause of global warming, is an unprecedented, and possibly irreversible, environmental condition in which damaging human-produced gases build up and trap heat within the Earth's protective atmospheric shield, called the ozone layer.
The threat of global warming was first recognized in 1896 by Swedish chemist Svante August Arrhenius, who suggested that the burning of fossil fuels might have a serious impact on the Earth's temperature. But scientists at that time could not have predicted that it would become one of the world's most pressing environmental issue less than one hundred years later.
Arrhenius's early warning of the dangers of carbon dioxide was not taken seriously until 1938 when G. S. Callendar, an English physicist, pointed to meteorological records showing the gradual warming since 1880 of North America and Northern Europe. Callender was the first scientist to gather data from several sources on the danger of increasing carbon dioxide levels. Even with this data, Callender's study did not garner significant support because many scientists believed that the excess carbon dioxide would be absorbed in the oceans, not in the atmosphere. Also, the warming trend was inexplicably replaced by a temporary temperature decline around 1940. Callender remained adamant, however, and noted in 1958 that the warming had resumed in 1942. Finally, in the 1960s, concern about atmospheric pollution was taken seriously as the link between air pollution and the Earth's temperature became abundantly clearer.
The Earth's fragile atmosphere is comprised primarily of nitrogen, oxygen, and carbon dioxide. These three ingredients, along with methane, provide Earth with a protective blanket that regulates how much of the Sun's enormous heat reaches the Earth, as well as how much heat leaves our atmosphere. For this reason, these gases are called greenhouse gases, and they appeared to be balanced until the advent of the industrial revolution.
Through this delicate system, the Earth's atmosphere allows the visible and infrared wavelengths of radiation from the Sun to reach the ground. Once it has hit the surface, this visible light is absorbed and reflected by the Earth as infrared radiation that cannot be seen but which can be felt as heat. If not for the presence of atmospheric gases such as carbon dioxide, water vapor, and other greenhouse gases, the heat would escape out beyond the Earth's atmosphere. Instead, it is absorbed by the greenhouse gases, and much of it is reemitted down towards the surface, resulting in extra heat.
A great deal of the carbon dioxide that is released by industrialized societies is absorbed by forests, oceans, and the process of limestone deposition. However, these resources are either limited or depleting, and extra output of carbon dioxide collects in the atmosphere. Global warming from the build-up of greenhouse gases is also exacerbated by ozone depletion, which is allowing harmful radiation to reach the Earth's surface.
Recently, computers have allowed scientists to develop models that estimate the consequences of global warming while suggesting ways to slow the process. Measurements now show that between 1957 and 1975, the amount of carbon dioxide in the air has increased from 312 to 326 parts per million, a jump of approximately five percent. These measurements were collected all around the world from the top of the highest mountain, Mauna Loa in Hawaii, to the South Pole, where air was gathered through airplane air-intake systems. Overall, carbon dioxide concentrations are up about 30% since preindustrial times. Methane is up 145%, and nitrous oxide is up 15%. A car emits about 20 pounds of carbon dioxide for every gallon of gas it burns.
Most climatologists—scientists who study the earth's climate—now believe it is clear that this human-produced greenhouse effect is changing the earth's climate. One recent comprehensive study used tree rings (which show temperature variations as well as precipitation variations), ice core samples (where trapped air can give clues about past environments), and coral records to trace climate patterns over the last 600 years. Researchers combined this evidence with temperature readings, which have been available for only about the last 150 years, and with historical records, and concluded that the 20th century has been the warmest century in the last 600 years. Moreover, they concluded that the warmest years in all of that period were 1990, 1995, and 1997.
Although it is difficult to precisely predict exactly how greenhouse effect processes will affect the Earth's temperature in the future, one computer model estimates that by the year 2050 the temperature level could rise by as much as two degrees celsius (about four degrees Fahrenheit). This average increase would be unevenly distributed, ranging from as little as less than a degree at the equator to up to six degrees at higher latitudes. Although this may not sound like a large increase, it may be enough to affect the rate of glacial melting, raising sea levels, and to otherwise have an impact on the Earth's climate. Scientists believe the global sea level has already risen by about 10 to 25 centimeters (4 to 10 inches) over the past 100 years. By the year 2100, it is expected to rise an additional 50 centimeters (20 inches), doubling the number of people in the way of storm surges. Warmer climates would affect growing seasons, shift crop zones, and could increase the risk of certain tropical diseases like malaria.
Valuable research continues as scientists try to compare today's atmospheric and air quality with much older historical data.
As research continues, industries and governments are trying to establish programs that would help protect the environment. At the 1997 climate-change conference in Kyoto, Japan, delegates from 159 nations agreed to a pact that takes the first steps toward legal regulations that will reduce industrial gases--Europe by eight percent, Japan by six percent, and the United States by seven percent. As these same gases are often behind the engines and industries of every country's economies, conflicts will need to be resolved between industrial users and environmental advocates.
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