The late astronomer and author Carl Sagan (1934– 1996) famously described Earth when viewed from deep space as “a pale blue dot.” His description, which was intended to highlight the fragility of the planet, points out the visual effect of Earth's atmosphere. Viewed from space, the optical properties of the atmosphere surrounding Earth haloes the planet in a thin film of blue.
Compared to Earth's diameter, which averages about 7,800 mi (12,550 km), the atmosphere, which peters out to the near-vacuum of space at an altitude of approximately 620 mi (1,000 km), is paper-thin. Furthermore, most of the planet's weather is accounted for by the regions of the atmosphere within 35 mi (56 km) of the surface.
The atmosphere of the primordial Earth was different in composition from the atmosphere that cocoons the planet now. The appearance and evolution of life on Earth influenced atmospheric structure. The susceptibility of the atmosphere to change is the root of global warming, which all but a small minority of climate researchers now concede is a consequence of human activities.
Historical Background and Scientific Foundations
Earth is about 4.5 billion years old. The newly formed planet had no atmosphere, but as the planet cooled, the release of various gases created an atmosphere that was likely very different from that of the present day. Although the composition of this primordial atmosphere is still debatable, the majority of scientists who study the early climate agree that the atmosphere was probably rich in carbon (C) and nitrogen (N), and lacked oxygen (O). As life began, the atmosphere changed, with carbon dioxide (CO2) decreasing and oxygen appearing and accumulating.
The present-day atmosphere consists predominantly of nitrogen (an average of 78% of the total material) and oxygen (average of 21%). The remaining 1% of the atmosphere consists of the so-called trace gases—argon (Ar), helium (He), hydrogen (H), krypton (Kr), neon (Ne), methane (CH4), ozone (O3), and xenon (Xe)—as well as carbon dioxide and water vapor.
The atmosphere is not a single layer. Rather, it consists of regions that are separated from one another by narrow zones of transition. The atmosphere gives way to space at an altitude of approximately 620 mi (1,000 km).
The atmosphere is also not uniform in the density of the constituent gases. Instead, over 99% of the mass of the atmosphere is concentrated within 25 mi (40 km) of Earth's surface. Finally, the atmosphere is not uniform in temperature. As anyone who has climbed a mountain can attest, air temperature decreases with altitude, as the heat-absorbing gases become more dilute. Atmospheric temperature drops by about 11°F(6°C) for every 0.6 mi (1 km) of altitude in the atmospheric layer immediately above Earth's surface.
This layer is called the troposphere. The upper range of the troposphere varies depending on latitude. At higher latitudes, it is about 5 mi (8 km) high, while at the equator it is upwards of 11 mi (18 km) high. The troposphere contains almost all (99%) of the atmospheric water vapor. Again, there is geographic variation, with water vapor concentration being up to 3% of total atmospheric content above the equator, but less toward the poles.
Weather occurs exclusively in the troposphere. Indeed, the meaning of the word troposphere (“region of mixing”) reflects the importance of air currents in this layer. Pollutants that enter the troposphere will be evenly dispersed within days; some of the chemicals will return to the surface in precipitation, as occurs in acid rain.
The troposphere is separated from the next layer of the atmosphere, the stratosphere, by a thin transition region called the tropopause. The stratosphere is approximately 25 mi (40 km) thick. It begins about 6.3 mi (10 km) above Earth's surface, 1.5 mi (2.4 km) above the peak of Mt. Everest. Commercial aircraft cruise at altitudes that are in the lower to middle portions of the stratosphere.
The temperature within the stratosphere also varies with height, but in a different pattern to that of the tropo-sphere. The temperature does not vary up to an altitude of about 15 mi (24 km), after which it gradually increases until reaching the next atmospheric transition zone, which is called the stratopause. This temperature pattern, with warmer air overlying colder air, is known as an inversion. Glimpsing a towering thunderhead on a summer's day provides a visual example of the influence of the inversion; the thunderhead flattens off when the warm rising air in the cumulus cloud contacts the cooler air in the lower stratosphere, which halts the rising of the air.
The increasing temperature with altitude in the stratosphere acts to make this layer more stable than the underlying troposphere. Another contributor to this stability, and the reason for the stratospheric temperature inversion, is ozone. Ozone is a three-oxygen compound that absorbs incoming ultraviolet radiation from sunlight. This retention of heat is what maintains temperature with increasing altitude.
Beyond the stratopause lies the mesosphere. This atmospheric layer extends to approximately 50 mi (80 km) above the surface. There is little water vapor or ozone in this layer, hence, temperatures are low and keep decreasing with altitude. As well, the levels of oxygen and nitrogen are far less than in the troposphere and stratosphere; mesopheric air pressure (the number of atoms per given area) is 1,000 times less than air pressure at sea level.
A transition layer called the mesopause separates the mesosphere from the thermosphere. The thermosphere extends to about 75 mi (121 km) above Earth's surface. The thermosphere is home to the International Space Station and orbits of the space shuttle.
The final layer of the atmosphere is the exosphere. Beyond lies the near-vacuum of space.
Impacts and Issues
Because regions of the atmosphere determine weather patterns and the global climate, atmospheric changes can be profound. The documented increase in the atmospheric levels of carbon dioxide, chlorofluorocarbons (CFCs), methane, and nitrous oxide (N2O), which are collectively known as greenhouse gases, is driving an increase in the temperature of the troposphere that has been termed global warming.
WORDS TO KNOW
ACID RAIN: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.
CHLOROFLUOROCARBONS: Members of the larger group of compounds termed halocarbons. All halocarbons contain carbon and halons (chlorine, fluorine, or bromine). When released into the atmosphere, CFCs and other halocarbons deplete the ozone layer and have high global warming potential.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
INVERSION: A type of chromosomal defect in which a broken segment of a chromosome attaches to the same chromosome, but in reverse position.
OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.
TRACE GASES: Gases present in Earth's atmosphere in trace (relatively very small) amounts. All greenhouse gases happen to be trace gases, though some are more abundant than others; the most abundant greenhouse gases are CO2(0.037% of the atmosphere) and water vapor (0.25% of the atmosphere, on average).
WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.
The final greenhouse compound is ozone. Degradation of ozone in the stratosphere has been accelerated from the naturally occurring rate due to the presence of human-made compounds including CFCs and hydro-chlorofluorocarbons (which are used in air conditioners, refrigerators, and aerosol cans), halons (an ingredient of fire extinguishers), methyl chloroform (C2H3Cl3), and methyl bromide (CH3Br) is allowing more ultraviolet light to reach Earth's surface.
The energy of ultraviolet light is sufficient to permit the light to penetrate into the upper layers of the skin and even to slice apart the genetic material inside cells. Consequences include skin damage such as sunburn and, more ominously, the increased tendency of the genetically damaged cells to become cancerous.
Although in the past it was argued that global warming was a natural phenomenon, only a tiny minority of scientists continue to hold this view. The vast majority of scientists now accept that human activities are at the heart of global warming today.
For example, carbon dioxide released into the atmosphere by the burning of fossil fuels and the burning of felled lumber from deforested regions, as two examples, account for almost half of the atmospheric warming caused by human activity. In another example, the build-up of CFCs not only stimulates ozone breakdown, but increases the retention of heat, since CFCs are a powerful greenhouse gas. Indeed, one molecule of CFC has about 20,000 times the heat-trapping power as a molecule of carbon dioxide.
The pollution of the atmosphere near Earth's surface with noxious compounds can be unhealthy. An example from 2007 is Beijing, China. Air pollution in Beijing, which is mainly caused by the millions of vehicles operating daily in the mega-city, has become a great concern to officials of the International Olympic Committee responsible for ensuring that Beijing is ready to host the Summer Olympics in 2008. Events such as the marathon may need to be shifted to early morning, when air pollution is less. Alternatively, the government has proposed a ban on all vehicles in Beijing during the games.
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Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Introduction to Meteorology. New York: Prentice Hall, 2006.
Trefil, Calvo. Earth's Atmosphere. Geneva, IL: McDougal Littell, 2005.
Ward, Peter. Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere. Washington, DC: Joseph Henry Press, 2006.