Characteristics of the atmosphere

The atmosphere (and ocean) are thin film of fluid on the spherical Earth under the influence of gravity, Earth’s rotation, and differential heating by solar radiation.  

We discuss the equation of state of air (the connection between pressure, density, and temperature) and key properties of moist air. In particular, we will learn that warm air is generally more moist than cold air, a fact that has enormous implications for the climate of the planet.

CHEMICAL COMPOSITION OF THE ATMOSPHERE

Air is a mixture of ‘‘permanent’’ gases (N2, O2) in constant ratio together with minor constituents (see below Table). The molecular weight of the mixture that makes up air is 28.97, so that 22.4 liters of air at standard temperature and pressure (STP; T = 273 K and p = 1013 h Pa) weighs 28.97 g.


TABLE  The most important atmospheric constituents. The chlorofluorocarbons (CFCs) CCl2F2 and CCl3F are also known as CFC-12 and CFC-11, respectively. [N.B. (ppm, ppb, ppt) = parts per (million, billion, trillion)] The concentrations of some constituents are increasing systematically because of human activity. For example, the CO2 concentration of 380 ppm was measured in 2004; CFCs are now decreasing in concentration following restrictions on their production.

FIGURE - A north-south section of topography relative to sea level (in meters) along the Greenwich meridian (0◦ longitude). Antarctica is over 2 km high, whereas the Arctic Ocean and the south Atlantic basin are about 5 km deep. Note how smooth the relief of the land is compared to that of the ocean floor.

The composition of air is a direct consequence of the supply of elements from the Earth’s interior and the presence of life on the surface. Photosynthesis by plants makes O2; nitrogenous compounds from living organisms are returned to the atmosphere as N2 from metabolism. Lightning converts N2 into usable molecules for life. Two of the most important minor constituents are H2O and CO2; they play a central role in controlling the temperature of the Earth’s surface and sustaining life (living material is primarily composed of C, H and O).

Atmospheric water vapor is present in variable amounts (typically 0.5% by volume). It is primarily the result of evaporation from the ocean’s surface. Unlike N2 and O2, water vapor and to a lesser degree CO2 is of great importance in radiative transfer (the passage of radiation through the atmosphere), because it strongly absorbs and emits in the infrared, the region of the spectrum (wavelengths about 10 μm)  at which Earth radiates energy back out to space. The CO2 concentration in the atmosphere is controlled by such processes as photosynthesis and respiration, exchange between the ocean and the atmosphere, and, in the modern world, anthropogenic activities.

It is important to note that the proportion of some constituents (especially chemically or physically active species, such as H2O) is variable in space and time. Moreover, several crucially important constituents (e.g., H2O, CO2, O3) are present in very small concentrations, and so are  sensitive to anthropogenic activity. 

FIGURE -  Atmospheric CO2 concentrations observed at Mauna Loa, Hawaii (19.5◦ N, 155.6◦ W). Note the seasonal cycle superimposed on the long-term trend. The trend is due to anthropogenic emissions. The seasonal cycle is thought to be driven by the terrestrial biosphere: net consumption of CO2 by biomass in the summertime (due to abundance of light and heat) and net respiration in wintertime.

For example, above Fig. shows the CO2 concentration measured at the Hawaiian island of Mauna Loa. Atmospheric CO2 concentration has risen from 315 ppm to 380 ppm over the past 50 years. Preindustrial levels of CO2 were around 280 ppm; it is thought that over the course of Earth’s history, CO2 levels have greatly fluctuated. Atmospheric CO2 concentrations were probably markedly different in warm as opposed to cold periods of Earth’s climate. 

For example, at the last glacial maximum 20,000 years ago, CO2 concentrations are thought to have been around 180 ppm. Reconstructions of atmospheric CO2 levels over geologic time suggest that CO2 concentrations were perhaps five times the present level 220 million years ago, and perhaps 20 times today’s concentration between 450 and 550 million years ago. If the curve shown in above Fig. continues its exponential rise, then by the end of the century, CO2 concentrations will have reached levels—perhaps 600 ppm—not seen since 30 million years ago, a period of great warmth in Earth history.