The atmosphere

The Earth is surrounded by a relatively thin atmosphere consisting of a mixture of gases, primarily molecular nitrogen (77 percent) and molecular oxygen (21 percent). This gaseous envelope, commonly called the air, also contains much smaller amounts of gases such as argon, carbon dioxide, methane, and water vapour, along with minute solid and liquid particles in suspension.

It is not surprising that the Earth, as a small planet (with a rather weak gravitational field) at fairly warm temperatures (due to its proximity to the Sun), should lack the most common gases in the universe, hydrogen and helium. Whereas both the Sun and Jupiter are dominantly composed of these two elements, they could not be retained long on the Earth and would rapidly evaporate into interplanetary space. It is surprising, however, that more than 20 percent of the Earth's atmosphere is composed of oxygen, a highly reactive gas that, under most planetary conditions, would have combined with other chemicals. The two parts per million of methane in the atmosphere, which is far out of chemical equilibrium, is actually of biogenic origin (produced in the digestive tracts of cows, for example). The atmosphere extends from the surface of the Earth to heights of thousands of kilometres, where it gradually merges with the solar wind—a stream of charged atomic 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 kilometres.

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 15 kilometres. 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 vertical temperature gradient (i.e., temperatures decline with altitude) of roughly 6° C per kilometre (10.8° F per kilometre) through the troposphere. At the top of the troposphere, which is called the tropopause, temperatures fall to about −60° C (−76° F). The troposphere is the region where virtually all water vapour exists and where all weather occurs.

The dry, tenuous stratosphere lies above the troposphere and extends to an altitude of about 50 kilometres. 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 oxygen molecules; recombination of oxygen atoms with O₂ molecules into ozone (O₃) creates the ozone layer, which shields the lower ecosphere from harmful short-wavelength radiation.

Above the relatively warm stratopause is the even more tenuous mesosphere, in which temperatures again decline with altitude, reaching roughly −85° C at the mesopause. Temperatures then rise with increasing height through the overlying layer known as the thermosphere. Above about 100 kilometres, in the ionosphere, there is an increasing fraction of charged, or ionized, particles. Spectacular visible auroras are generated in this region, particularly along circular zones around the poles, by episodic precipitation of energetic particles.

The general circulation of the Earth's atmosphere is driven by solar energy, which falls preferentially in equatorial latitudes. Atmospheric redistribution of heat poleward is strongly affected by the Earth's rapid rotation and the associated Coriolis force at nonequatorial latitudes (which adds an east-west component to the direction of the winds), resulting in about three latitudinal cells of circulation in each hemisphere. Instabilities produce the characteristic high-pressure areas and low-pressure storms of the mid-latitudes 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, and their slowly changing currents and temperatures also influence weather and climate, as in the so-called El Niño episodes (see ocean: Impact of ocean-atmosphere interactions on weather and climate: The El Niño phenomenon).

The Earth's atmosphere is not a static feature of the environment. Rather its composition has evolved over time in concert with life and continues to change as human activities alter the ecosphere. Roughly halfway through the history of the Earth, the atmosphere's unusual complement of free oxygen began to develop owing to photosynthesis by blue-green algae and subsequently evolving plant life. Accumulation of oxygen eventually made it possible for respirating animals to move out onto the land.

The 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 particulates into the stratosphere, which remain suspended for years, decreasing atmospheric transparency and resulting in measurable cooling worldwide. Rare, giant impacts of asteroids and comets can have even more profound effects. The dominant climate variations observed in the recent geologic record are the ice ages, which are linked to small variations in the Earth's geometry with respect to the Sun.

The Sun is believed to have been less luminous during the early history of the Earth, so if other planetary conditions were identical with those of today, the oceans would have been frozen. But it is expected that there was much more carbon dioxide in the Earth's atmosphere during earlier periods, which would have enhanced greenhouse warming. In this phenomenon, heat radiated by the surface is trapped by gases such as carbon dioxide in the atmosphere and reradiated back to the surface, thereby warming it. There is presently 10⁵ times more carbon dioxide buried in carbonate rocks in the Earth's crust than in the atmosphere, in sharp contrast with Venus, whose atmospheric evolution followed a different course.

The amount of carbon dioxide in the atmosphere is rising steadily, however, and has increased by more than 10 percent in the last 30 years owing to 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. A further doubling by the middle of the 21st century could lead to a global warming of a few degrees, which would have profound effects on the sea level and on agriculture.

Of more immediate concern is the impact of human activities on the stratospheric ozone layer. Complex chemical reactions involving traces of man-made chlorofluorocarbons have recently created temporary holes in the ozone layer, particularly over Antarctica, during polar spring. More disturbing, however, is the discovery of a growing depletion of ozone over temperate latitudes, where a large percentage of the world's population resides, since the ozone layer serves as a shield against ultraviolet radiation, which has been found to cause skin cancer.

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The atmosphere and hydrosphere	\|	The hydrosphere

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