ANTHROPOGENIC CHANGE IN THE ATMOSPHERE

14.1. ANTHROPOGENIC CHANGE IN THE ATMOSPHERE

There is a very strong connection between life forms on earth and the nature of earth’s climate, which determines its suitability for life. As proposed by James Lovelock, a British chemist, this forms the basis of the Gaia hypothesis, which con-

tends that the atmospheric O 2 /CO 2 balance established and sustained by organisms determines and maintains earth’s climate and other environmental conditions. 1 Ever since life first appeared on earth, the atmosphere has been influenced by the metabolic processes of living organisms. When the first primitive life molecules were formed approximately 3.5 billion years ago, the atmosphere was very different from its present state. At that time it was chemically reducing and thought to contain nitrogen, methane, ammonia, water vapor, and hydrogen, but no elemental oxygen. These gases and water in the sea were bombarded by intense, bond-breaking ultraviolet radiation which, along with lightning and radiation from radionuclides, provided the energy to bring about chemical reactions that resulted in the production of relatively complicated molecules, including even amino acids and sugars. From this rich chemical mixture, life molecules evolved. Initially, these very primitive life forms derived their energy from fermentation of organic matter formed by chemical and photochemical processes, but eventually they gained the capability to produce

organic matter, “{CH 2 O},” by photosynthesis,

(14.1.1) and the stage was set for the massive biochemical transformation that resulted in the

CO 2 + H 2 O + h ν → {CH 2 O} + O 2 (g)

production of almost all the atmosphere’s oxygen. The oxygen initially produced by photosynthesis was probably quite toxic to primitive life forms. However, much of this oxygen was converted to iron oxides by reaction with soluble iron(II):

(14.1.2) The enormous deposits of iron oxides thus formed provide convincing evidence for

4Fe 2+ + O

2 + 4H 2 O → 2Fe 2 O 3 + 8H

the liberation of free oxygen in the primitive atmosphere. Eventually enzyme systems developed that enabled organisms to mediate the reaction of waste-product oxygen with oxidizable organic matter in the sea. Later this mode of waste-product disposal was utilized by organisms to produce energy for respiration, which is now the mechanism by which nonphotosynthetic organisms obtain energy.

In time, oxygen accumulated in the atmosphere, providing an abundant source of O 2 for respiration. It had an additional benefit in that it enabled the formation of an

ozone shield against solar ultraviolet radiation (see Section 11.6). With this shield in place, the earth became a much more hospitable environment for life, and life forms were enabled to move from the protective surroundings of the sea to the more exposed environment of the land.

Other instances of climatic change and regulation induced by organisms can be cited. An example is the maintenance of atmospheric carbon dioxide at low levels through the action of photosynthetic organisms (note from Reaction 14.1.1 that pho-

tosynthesis removes CO 2 from the atmosphere). But, at an ever accelerating pace during the last 200 years, another organism, humankind, has engaged in a number of activities that are altering the atmosphere profoundly. As noted in Chapter 1, human influences are so strong that it is useful to invoke a fifth sphere of the environment, the anthrosphere. The effects of human activities and the anthrosphere on the atmosphere are summarized below:

• Industrial activities, which emit a variety of atmospheric pollutants

including SO 2 , particulate matter, photochemically reactive hydrocarbons, chlorofluorocarbons, and inorganic substances (such as toxic heavy metals)

• Burning of large quantities of fossil fuel, which can introduce CO 2 , CO, SO 2 , NOx, hydrocarbons (including CH 4 ), and particulate soot, polycyclic aromatic hydrocarbons, and fly ash into the atmosphere

• Transportation practices, which emit CO 2 , CO, NOx, photochemically reactive (smog forming) hydrocarbons, and polycyclic aromatic hydro- carbons

• Alteration of land surfaces, including deforestation • Burning of biomass and vegetation, including tropical and subtropical

forests and savanna grasses, which produces atmospheric CO 2 , CO, NOx, and particulate soot and polycyclic aromatic hydrocarbons

• Agricultural practices, which produce methane (from the digestive tracts of domestic animals and from the cultivation of rice in waterlogged

anaerobic soils) and N 2 O from bacterial denitrification of nitrate-fertilized soils.

These kinds of human activities have significantly altered the atmosphere, particularly in regard to its composition of minor constituents and trace gases. Major effects have been the following:

• Increased acidity in the atmosphere • Production of pollutant oxidants in localized areas of the lower

troposphere (see Photochemical Smog, Chapter 13) • Elevated levels of infrared-absorbing gases (greenhouse gases) • Threats to the ultraviolet-filtering ozone layer in the stratosphere • Increased corrosion of materials induced by atmospheric pollutants

In 1957 photochemical smog was only beginning to be recognized as a serious problem, acid rain and the greenhouse effect were scientific curiosities, and the ozone-destroying potential of chlorofluorocarbons had not even been imagined. In

that year, Revelle and Suess 2 prophetically referred to human perturbations of the earth and its climate as a massive “geophysical experiment.” The effects that this experiment may have on the global atmosphere are discussed in this chapter.