Atmospheric chemistry and climate change

The Earth’s global temperature, as well as the amount of solar radiation reaching its surface, can be significantly influenced by changes in the concentrations of chemicals naturally present in the atmosphere, such as natural GHGs, and by anthropogenic chemicals, such as CFCs.

Background

The significance of the Earth’s atmosphere is vastly disproportionate to its size. Although its thickness relative to Earth’s sphere is comparable to an apple’s skin, it is essential for life. It was not until the eighteenth century that scientists began to understand the role of atmospheric gases such as oxygen and in plant and animal life, and it was not until the end of the nineteenth century that scientists grasped the details of how soil microorganisms utilized atmospheric nitrogen to create compounds necessary for the health of plants and animals. Throughout the twentieth century, climatologists, atmospheric chemists, and others gathered information about how such anthropogenic gases as CO₂, methane, and nitrous oxide were increasing Earth’s and elevating the planet’s average global temperature. This enhanced greenhouse effect fosters climate changes that are potentially so devastating that some scholars have called climate change the most important issue of the twenty-first century.

Chemical Composition of the Earth’s Atmosphere

Approximately three-quarters of the Earth’s air mass is located in the troposphere, and dry air in this region is 78.1 percent nitrogen, 20.9 percent oxygen, and 0.93 percent argon by volume. The also contains trace amounts of many other gases, such as methane, various nitrogen oxides, ammonia, sulfur dioxide, and ozone, and these come from both natural and anthropogenic sources. Human activities have not changed the concentrations of the major gases in the atmosphere—nitrogen and oxygen—but scientific evidence accumulated over the past century indicates that human beings, particularly in advanced industrialized societies, are dramatically affecting the concentrations of certain trace gases. Examples of these include CO₂, methane, nitrous oxide, carbon monoxide, chlorofluorocarbons (CFCs), and sulfur dioxide. Some of these atmospheric trace gases, such as CFCs, result from certain industries and their products, such as refrigerants and aerosols. Others, such as CO₂ and sulfur dioxide, are produced by burning fossil fuels. Agricultural practices are also significant sources of such gases as methane and nitrous oxide.

Although the Earth’s contains much less matter than the troposphere, it contains similar proportions of such gases as nitrogen and oxygen. It differs markedly from the troposphere, however, in its concentrations of water vapor and ozone. Stratospheric water-vapor concentrations are only about one-thousandth of tropospheric conentrations, but ozone concentrations are much higher in the stratosphere. Ozone is localized in a layer ranging from about 15 to 35 kilometers above Earth’s surface. This ozone layer, whose molecules are created when oxygen interacts with high-energy solar radiation, prevents about 95 percent of the Sun’s ultraviolet radiation from reaching Earth’s surface, where it could damage living organisms. The also prevents tropospheric oxygen from being converted to ozone, which, in the lower atmosphere, is a dangerous air pollutant.

Chemical Reactions in the Troposphere

Besides being home to such major gases as nitrogen and oxygen, the troposphere contains hundreds of other distinctive molecules, leading to myriad chemical reactions, some of which have an influence on climate change. Because oxygen is such a reactive species, many of these reactions are oxidations, and some scientists see these reactions as constituting a low-temperature combustion system. Fueling this combustion are chemicals released from both natural and artificial sources. For example, methane enters the troposphere in large amounts from swamp and bog emissions, termites, and ruminant animals. Human activities contribute a large number of organic compounds, and CO₂ and water are the end results of their oxidation. CO₂ and water vapor are powerful greenhouse gases(GHGs).

Atmospheric chemists have also been attempting to work out in detail the influence of chemical radicals on tropospheric gases. Such charged groups of atoms as the hydroxyl radical (composed of hydrogen and oxygen) play an important role in the daytime chemistry of the troposphere, and the nitrate radical (composed of nitrogen and oxygen) is the dominant nighttime oxidant. Fossil-fuel combustion is a significant contributor to tropospheric pollution. Particulates such as soot were a factor in some “killer smogs,” and scientists have recently discovered that particulates contribute to global dimming, a lessening of sunlight’s ability to penetrate particle-filled hazes and reach the Earth’s surface. Sulfur dioxide, which is produced by the combustion of certain kinds of coal and oil, can be a primary air pollutant, since it is toxic to living organisms as well as damaging to buildings. It can also be a secondary air pollutant, because it reacts with water vapor to create sulfuric acid, which is an acid-rain component, causing harm to various lifeforms, including trees and fish.

Chemical Reactions in the Stratosphere

Just as in the lower atmosphere, chemical reactions in the upper atmosphere exhibit great variety, and some of these reactions have an important influence on climate change. Over the past decades, the chemical species that has received the most attention has been ozone. Scientists paid heightened attention to the chemical reactions in the ozone layer when, in the late 1980s, a hole was discovered in this layer above the Antarctic. During the 1970s scientists had found a threat to the ozone layer when they worked out the reactions between chlorine-containing radicals and ozone. These reactions changed ozone molecules into diatomic oxygen molecules, thus weakening the ability of the ozone layer to protect Earth’s surface from high-energy solar radiation.

A primary source of these catalytic, chlorine-containing species turned out to be CFCs. General Motors had introduced CFCs in 1930, and they proved to be successful in such products as refrigerator and air-conditioning coolants, as well as aerosol propellants. Because of the widespread and accelerating use of CFCs, the tropospheric concentrations of these chemicals increased from the 1930s to the 1970s, when Mexican chemist Mario Molina and American chemist F. Sherwood Rowland showed that CFCs, although seemingly inert in the troposphere, became very reactive in the stratosphere. There, ultraviolet radiation split the CFCs into highly reactive radicals that, in a series of reactions, promoted the debilitation of the protective ozone shield.

The exhaust from aircraft and spacecraft also helped deplete stratospheric ozone. Despite attempts, such as the (1987), to reduce concentrations of CFCs and other ozone-depleting chemicals in the atmosphere, the Antarctic continued to grow in the 1990s and early twenty-first century. This meant that countries near Antarctica began experiencing higher levels of ultraviolet solar radiation.

Atmospheric Chemistry and Global Climate Change

Humans tend to be most aware of weather—that is, a local area’s short-term temperature and precipitation variations. Scientists such as atmospheric chemists tend to concentrate on climate, or a large region’s long-term variations in temperature, precipitation, and cloud cover. Because of discoveries revealing the extreme complexity of chemical reactions in the atmosphere, has become a profoundly interdisciplinary field, depending on new facts and ideas found by physicists, meteorologists, climatologists, oceanographers, geologists, ecologists, and other scientists.

Paleoclimatologists have studied changes in Earth’s atmosphere over hundreds of millions of years, while other environmental and atmospheric chemists have focused on such pivotal modern problems as global warming. These studies have led to research aimed at understanding the causes of global warming and the development of theories to explain existing data. Particularly useful has been computerized modeling of Earth’s atmosphere, through which experiments can be performed to help scientists understand likely future effects of climate change. These theoretical predictions have placed pressure on various governments to make important changes in policy, such as taxing fossil-fuel use to motivate reductions in GHG emissions.

Atmospheric chemists have come to realize that the goal of their research on is to understand the relevant chemical species in the atmosphere, their reactions, and the role of anthropogenic chemicals, especially GHGs, in bringing about global warming. Many atmospheric chemists believe that the greenhouse effect is a certainty, and they are also highly confident that human activities generating GHGs are a significant element in the recent rise in average global temperatures. Less certain are predictions about the future.

Computer models developed to synthesize and test theories about the complex chemical interactions in the troposphere and stratosphere necessarily involve assumptions and simplifications. For example, the numbers of chemical compounds and their reactions have to be reduced to formulate even a crude working model of the Earth’s atmosphere. However, according to NOAA's Climate Program Office, scientists made an important discovery in 2024 that significantly improved their modeling procedures for atmospheric chemistry's influence on global climate change. Previous models overestimated the amount of hydroxyl radicals, the molecules used for breaking down methane, in Earth's atmosphere. They concluded that atmospheric methane breaks down slower than previous models predicted. This was critical in understanding how methane contributes to climate change.

Context

Atmospheric chemists’ discoveries have had a major influence on how environmentalists and other scientists understand the gravity, interrelatedness, and complexity of atmospheric problems. Many atmospheric chemists educate their students and the public about issues relating to global climate change, while others have been carefully monitoring the changes in the Earth’s atmosphere. They have also participated in international discussions and agreements about controlling GHG emissions, developing substitutes for CFCs, and passing local and international laws that would lessen the likelihood of some catastrophic scenarios predicted by various computer models. Just as the many components and reactions in the atmosphere make a full understanding of these complexities very difficult, so, too, environmental chemists find themselves in an even more complex milieu in which they have to integrate their understanding with those of other scientists, industrialists, and government officials in both developed and developing countries. Therefore, though global climate change is, at root, a physical and chemical issue, to solve the problem of global climate change will require an integrated, multidisciplinary, and international approach that, though daunting, appears to be increasingly necessary.

Key Concepts

• anthropogenic: caused by humans
• chlorofluorocarbons (CFCs): compounds of chlorine, fluorine, and carbon, popularly known by the trade name Freon
• greenhouse effect: result of atmospheric trace gases that allow high-energy sunlight to reach the terrestrial surface but absorb low-energy heat that is radiated back
• greenhouse gases (GHGs): tropospheric gases such as carbon dioxide, methane, and water vapor that cause the greenhouse effect
• ozone layer: a stratospheric region containing relatively high concentrations of triatomic oxygen (ozone) that prevents much ultraviolet solar radiation from reaching Earth’s surface
• primary air pollutants: harmful substances that are emitted directly into the atmosphere
• secondary air pollutants: harmful substances that result from the reaction of with principal atmospheric components
• stratosphere: an atmospheric region extending from about 17 to 48 kilometers above the Earth’s surface
• troposphere: an atmospheric region extending from the Earth’s surface to about 17 kilometers high over equatorial regions and to about 8 kilometers high over polar regions

Bibliography

Birks, John W., Jack G. Calvert, and Robert E. Sievers, eds. The Chemistry of the Atmosphere: Its Impact on Global Change—Perspectives and Recommendations. Washington, D.C.: American Chemical Society, 1993.

Jacob, Daniel J. Introduction to Atmospheric Chemistry. Princeton, N.J.: Princeton University Press, 2007.

Makhijani, Arjun, and Kevin R. Gurney. Mending the Ozone Hole: Science, Technology, and Policy. Cambridge, Mass.: MIT Press, 1996.

"New Discovery in Atmospheric Chemistry Helps Predict Methane's Role in Climate Change." Climate.gov, Climate Program Office, 23 July 2024, www.climate.gov/news-features/feed/new-discovery-atmospheric-chemistry-helps-predict-methanes-role-climate-change. Accessed 11 Dec. 2024.

Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. New York: John Wiley & Sons, 1998.

Wayne, Richard P. Chemistry of Atmospheres. 3d ed. New York: Oxford University Press, 2000.