The Atmosphere

Earth is surrounded by a relatively thin atmosphere (commonly called air) consisting of a mixture of gases, primarily molecular nitrogen (78 percent) and molecular oxygen (21 percent). Also present are much smaller amounts of gases such as argon (nearly 1 percent), water vapour (averaging 1 percent but highly variable in time and location), carbon dioxide (0.037 percent [370 parts per million] and presently rising), methane (0.00015 percent [1.5 parts per million]), and others, along with minute solid and liquid particles in suspension.

Because Earth has a weak gravitational field (by virtue of its size) and warm atmospheric temperatures (due to its proximity to the Sun) compared with the giant planets, it lacks the most common gases in the universe that they possess: hydrogen and helium. Whereas both the Sun and Jupiter are composed predominantly of these two elements, they could not be retained long on early Earth and rapidly evaporated into interplanetary space. The high oxygen content of Earth's atmosphere is out of the ordinary. Oxygen is a highly reactive gas that, under most planetary conditions, would be combined with other chemicals in the atmosphere, surface, and crust. It is in fact supplied continuously by biological processes; without life, there would be virtually no free oxygen. The 1.5 parts per million of methane in the atmosphere is also far out of chemical equilibrium with the atmosphere and crust: it, too, is of biological origin, with the contribution by human activities far outweighing others.

The gases of the atmosphere extend from the surface of Earth to heights of thousands of kilometres, eventually merging with the solar wind—a stream of charged particles that flows outward from the outermost regions of the Sun. The composition of the atmosphere is more or less constant with height to an altitude of about 100 km (60 miles), with particular exceptions being water vapour and ozone.

The atmosphere is commonly described in terms of distinct layers, or regions. Most of the atmosphere is concentrated in the troposphere, which extends from the surface to an altitude of about 10-15 km (6-9 miles), depending on latitude and season. The behaviour of the gases in this layer is controlled by convection. This process involves the turbulent, overturning motions resulting from buoyancy of near-surface air that is warmed by the Sun. Convection maintains a decreasing vertical temperature gradient—i.e., a temperature decline with altitude—of roughly 6 °C (10.8 °F) per km through the troposphere. At the top of the troposphere, which is called the tropo-pause, temperatures have fallen to about -80 °C (-112 °F). The troposphere is the region where nearly all water vapour exists and essentially all weather occurs.

The dry, tenuous stratosphere lies above the troposphere and extends to an altitude of about 50 km (30 miles). Convective motions are weak or absent in the stratosphere; motions instead tend to be horizontally oriented. The temperature in this layer increases with altitude. In the upper stratospheric regions, absorption of ultraviolet light from the Sun breaks down molecular oxygen (O2); recombination of single oxygen atoms with O2 molecules into ozone (O3) creates the shielding ozone layer.

Above the relatively warm stratopause is the even more tenuous mesosphere, in which temperatures again decline with altitude to 80-90 km (50-56 miles) above the surface, where the meso-pause is defined. The minimum temperature attained there is extremely variable with season. Temperatures then rise with increasing height through the overlying layer known as the thermosphere. Also above about 80-90 km there is an increasing fraction of charged, or ionized, particles, which from this altitude upward defines the ionosphere. Spectacular visible auroras are generated in this region, particularly along approximately circular zones around the poles, by the interaction of nitrogen and oxygen atoms in the atmosphere with episodic bursts of energetic particles originating from the Sun.

Earth's general atmospheric circulation is driven by the energy of sunlight, which is more abundant in equatorial latitudes. Movement of this heat toward the poles is strongly affected by Earth's rapid rotation and the associated Coriolis force at latitudes away from the equator (which adds an east-west component to the direction of the winds), resulting in multiple cells of circulating air in each hemisphere. Instabilities (perturbations in the atmospheric flow that grow with time) produce the characteristic high-pressure areas and low-pressure storms of the midlatitudes as well as the fast, eastward-moving jet streams of the upper troposphere that guide the paths of storms. The oceans are massive reservoirs of heat that act largely to smooth out variations in Earth's global temperatures, but their slowly changing currents and temperatures also influence weather and climate, as in the El Niño/Southern Oscillation weather phenomenon.

Earth's atmosphere is not a static feature of the environment. Rather its composition has evolved over geologic time in concert with life and is changing more rapidly today in response to human activities. Roughly halfway through the history of Earth, the atmosphere's unusually high abundance of free oxygen began to develop, through photosynthesis by blue-green algae (also known as cyanobac-teria) and saturation of natural surface sinks of oxygen (e.g., relatively oxygen-poor minerals and hydrogen-rich gases exuded from volcanoes). Accumulation of oxygen made it possible for complex cells, which consume oxygen during metabolism and of which all plants and animals are composed, to develop.

Earth's climate at any location varies with the seasons, but there are also longer-term variations in global climate. Volcanic explosions, such as the 1991 eruption of Mount Pinatubo in the Philippines, can inject great quantities of dust particles into the stratosphere, which remain suspended for years, decreasing atmospheric transparency and resulting in measurable cooling worldwide. Much rarer, giant impacts of asteroids and comets can produce even more profound effects, including severe reductions in sunlight for months or years, such as many scientists believe led to the mass extinction of living species at the end of the Cretaceous period, 65 million years ago. The dominant climate variations observed in the recent geologic record are the ice ages, which are linked to variations in Earth's tilt and its orbital geometry with respect to the Sun.

The physics of hydrogen fusion leads astronomers to conclude that the Sun was 30 percent less luminous during the earliest history of Earth than it is today. Hence, all else being equal, the oceans should have been frozen. Observations of Earth's planetary neighbours, Mars and Venus, and estimates of the carbon locked in Earth's crust at present suggest that there was much more carbon dioxide in Earth's atmosphere during earlier periods. This would have enhanced warming of the surface via the greenhouse effect and so allowed the oceans to remain liquid.

Today there is 100,000 times more carbon dioxide buried in carbonate rocks in Earth's crust than in the atmosphere, in sharp contrast to Venus, whose atmospheric evolution followed a different course. On Earth, the formation of carbonate shells by marine life is the principal mechanism for transforming carbon dioxide to carbonates; abiotic processes involving liquid water also produce carbonates, albeit more slowly. On Venus, however, life never had the chance to arise and to generate carbonates. Because of the planet's location in the solar system, early Venus received 10-20 percent more sunlight than falls on Earth even today, despite the fainter young Sun at the time. Most planetary scientists believe that the elevated surface temperature that resulted kept water from condensing to a liquid. Instead, it remained in the atmosphere as water vapour, which, like carbon dioxide, is an efficient greenhouse gas. Together the two gases caused surface temperatures to rise even higher so that massive amounts of water escaped to the stratosphere, where it was dissociated by solar ultraviolet radiation. With conditions now too hot and dry to permit abiotic carbonate formation, most or all of the planet's inventory of carbon remained in the atmosphere as carbon dioxide. Models predict that Earth may suffer the same fate in a billion years, when the Sun exceeds its present brightness by 10 to 20 percent.

Between the late 1950s and the end of the 20th century, the amount of carbon dioxide in Earth's atmosphere increased by more than 15 percent because of the burning of fossil fuels (e.g., coal, oil, and natural gas) and the destruction of tropical rain forests, such as that of the Amazon River basin. Computer models predict that a net doubling of carbon dioxide by the middle of the 21st century could lead to a global warming of 1.5-4.5 °C (2.7-8.1 °F) averaged over the planet, which would have profound effects on sea level and agriculture. Although this conclusion has been criticized by some on the basis that the warming observed so far has not kept pace with the projection, analyses of ocean temperature data have suggested that much of the warming during the 20th century actually occurred in the oceans themselves—and will eventually appear in the atmosphere.

Another present concern regarding the atmosphere is the impact of human activities on the stratospheric ozone layer. Complex chemical reactions involving traces of man-made chlorofluorocarbons (CFCs) were found in the mid-1980s to be creating temporary holes in the ozone layer, particularly over Antarctica, during polar spring. Yet more disturbing was the discovery of a growing depletion of ozone over the highly populated temperate latitudes, since the short-wavelength ultraviolet radiation that the ozone layer effectively absorbs has been found to cause skin cancer. International agreements in place to halt the production of the most egregious ozone-destroying CFCs will eventually halt and reverse the depletion, but only by the middle of the 21st century, because of the long residence time of these chemicals in the stratosphere.

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