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The carbon budget in the atmosphere is of critical importance to climate and to life. Carbon appears in Earth’s atmosphere primarily as carbon dioxide (CO2) produced naturally by the respiration of living organisms, the decay of these organisms, the weathering of carbon-containing rock strata, and volcanic emissions. Plants utilize CO2, water, and solar insolation to convert CO2 to diatomic oxygen (O2). This process, known as photosynthesis, can result in local reductions of CO2 of tens of parts per million within vegetation canopies. In contrast, nighttime respiration occurring when photosynthesis is not active can increase CO2 concentrations. These concentrations may even double within dense tropical forest canopies for short periods before sunrise. On the global scale, seasonal variations of about 1 percent occur as a result of CO2 uptake from photosynthesis, plant respiration, and soil respiration. Atmospheric CO2 is primarily absorbed in the Northern Hemisphere during the growing season (spring to autumn). CO2 is also absorbed by ocean waters; the rate of exchange to the ocean is greater for colder than for warmer waters. Currently CO2 makes up about 0.03 percent of the gaseous composition of the atmosphere.
In the geologic past, CO2 levels have been significantly higher than they are today and have had a significant effect on both climate and ecology. During the Carboniferous Period (360 to 300 million years ago), for example, moderately warm and humid climates combined with high concentrations of CO2 were associated with extensive lush vegetation. After these plants died and decomposed, they were converted to sedimentary rocks that eventually became the coal deposits currently used for industrial combustion.
In the atmosphere, certain wavelengths of long-wave radiation are absorbed and then reemitted by CO2. Since the lower levels of the atmosphere are warmer than layers higher up, the absorption of upward-propagating electromagnetic radiation, and a reemission of a portion of it back downward, permits the lower atmosphere to remain warmer than it would be otherwise. The association of higher concentrations of CO2 in the air with a warmer lower troposphere is commonly referred to as the greenhouse effect. (The name is inaccurate—an actual greenhouse is warmed primarily because solar radiation enters through the glass, which retains the heated air and prevents the mixing of cooler air into the greenhouse from above.) In recent years, there has been increasing concern that the release of CO2 through the burning of coal and other fossil fuels will warm the lower atmosphere, a phenomenon commonly referred to as global warming. Average carbon dioxide concentrations rose from about 316 parts per million by volume (ppmv) of dry air in 1959 to approximately 417 ppmv in 2022. Water vapour is a more efficient greenhouse gas than carbon dioxide. However, since H2O is ubiquitous, occurring in its three phases (solid, liquid, and gas), and since CO2 is also a biogeochemically active gas, global temperature changes are both explained and predicted by changes in the atmospheric concentration of CO2.
Vertical structure of the atmosphere
Earth’s atmosphere is segmented into two major zones. The homosphere is the lower of the two and the location in which turbulent mixing dominates the molecular diffusion of gases. In this region, which occurs below 100 km (about 60 miles) or so, the composition of the atmosphere tends to be independent of height. Above 100 km, in the zone called the heterosphere, various atmospheric gases are separated by molecular mass, with the lighter gases being concentrated in the highest layers. Above 1,000 km (about 600 miles), helium and hydrogen are the dominant species. Diatomic nitrogen (N2), a relatively heavy gas, drops off rapidly with height and exists in only trace amounts at 500 km (300 miles) and above. This decrease in the concentration of heavier gases with height is largest during periods of low Sun activity, when temperatures within the heterosphere are relatively low. The transition zone, located at a height of about 100 km between the homosphere and heterosphere, is called the turbopause.
The atmosphere can be further divided into several distinct layers defined by changes in air temperature with increasing height. These layers are described below in order of increasing height above the surface.
