Greenhouse effect is a warming of the lower atmosphere and surface of a planet by a complex process involving sunlight, gases, and particles in the atmosphere. On the earth, the greenhouse effect began long before human beings existed. However, recent human activity may have added to the effect. The amounts of heat-trapping atmospheric gases, called greenhouse gases, have greatly increased since the mid-1800’s, when modern industry became widespread. Since the late 1800’s, the temperature of the earth’s surface has also risen. The greenhouse effect is so named because the atmosphere acts much like the glass roof and walls of a greenhouse, trapping heat from the sun.

The natural greenhouse effect. The atmosphere reflects toward space about 30 percent of the energy in incoming sunlight. The atmosphere absorbs about another 30 percent, and the remaining 40 percent or so reaches the earth’s surface.

The earth’s surface reflects about 15 percent of the solar energy that reaches it back toward space. The remaining energy heats the lands and seas. The warmed lands and seas then send most of the heat back into the atmosphere, chiefly as infrared rays and in evaporated water. Infrared rays are much like light waves but are invisible to the human eye.

When the rays from the lands and seas strike certain substances in the atmosphere, such as greenhouse gases and particles, those substances absorb the rays. As a result, the gases and particles are heated. They then are cooled by sending out infrared rays of their own. Some of the rays go into space. The remainder radiate back toward the earth’s surface, adding to the warming of the surface layer of air. Without the natural greenhouse effect, the average temperature of the earth’s surface would be about 59 degrees Fahrenheit (33 degrees Celsius) colder than it is now.

The chief greenhouse gases are made up of atoms of carbon (C), hydrogen (H), and oxygen (O). These gases are water vapor, carbon dioxide, methane, and ozone. The greenhouse particles include cloud droplets, soot, and dust.

Increases in greenhouse gases. Since the early to mid-1800’s, the amount of carbon dioxide in the atmosphere has increased by about 25 percent and the methane concentration has risen by about 150 percent. Most of the increase has been due to human activities-chiefly the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land. Fossil fuels contain carbon, and burning them creates carbon dioxide. Trees and other plants absorb the gas through the process of photosynthesis. As land is cleared and forests are cut down, carbon dioxide levels rise.

The average temperature of the earth’s surface has increased by about 0.9 to 1.6 degrees Fahrenheit (0.5 to 0.9 degree Celsius) since the late 1800’s. Scientists have not yet proved that an increase in atmospheric carbon dioxide has raised the surface temperature. But in the likely event that this relationship does exist, the eventual results could be severe. Many scientists estimate that by about 2050, the amount of carbon dioxide in the atmosphere will have doubled from the preindustrial level. If this increase were to add to the natural greenhouse effect, the earth’s surface temperature might rise 2.5 to 10.4 degrees Fahrenheit (1.4 to 5.8 degrees Celsius) by 2100.

The increase in surface temperature, which is called global warming, could alter the ecology of many parts of the earth. For example, global warming could change rainfall patterns, melt enough polar ice to raise the sea level, increase the severity of tropical storms, and lead to shifts in plant and animal populations. Ocean currents and wind patterns could change, making some areas cooler than they are now. One remote possibility is that a warming of northern regions will result in more winter snowfall, causing some ice sheets to advance.

Studying the greenhouse effect. Researchers use high-speed computers to study how carbon dioxide concentration may affect surface temperature. The computers manipulate mathematical models, sets of equations that describe relationships between changeable factors. Scientists do not have enough data to prove that variations in carbon dioxide and other human-caused changes to atmospheric composition cause shifts in surface temperature. They may need until the 2010’s to gather enough data. But certain models suggest that the 2010’s may be too late to avoid some damage from global warming.

Scientists have also examined evidence from the distant past to determine whether changes in carbon dioxide concentration cause temperature changes. Cores of ice drilled from great depths in Greenland and Antarctica provide a record for the past 160,000 years. During those years, the climate warmed and cooled several times. Researchers analyzed the gases and other substances that were trapped in the ice when it formed. During the cooler periods, the atmosphere contained about 30 percent less carbon dioxide and 50 percent less methane than during the warmer periods.