If Earth is our dwelling place, air is our enfolding mantle. We spend our lives bathed in air, enveloped in its encompassing embrace. It provides our every breath, it supports our every activity. Every morsel of food we consume is a gift of air, a product of the ancient alchemy of photosynthesis, by which plants withdraw one component of the air (carbon dioxide) and so create another (free oxygen). Air encloses the water of the planet, evaporating here, precipitating there, responding in its movements to the radiant energy of the Sun as it drives the water cycle and powers the winds. Air is everywhere—above the ground, on the ground, within the ground—supporting all Earth’s living things, participating in all its ceaseless surface changes, acting and reacting endlessly as the chief participant in all the great mediating systems of the planet.

We have already discussed some aspects of the air in chapter 5. We reviewed its layering, its distribution and extent, its nature, and its broad composition. We described its probable origin as Earth’s “third atmosphere.” And we reviewed its changing temperature through time, including especially its recent warming, and the factors—natural and human—that influence those changes in temperature. We’ve also considered atmospheric changes that present us with a challenge: the hole in the stratospheric ozone layer caused by CFCs and the spread of acid rain and snow, for example. The attention we’ve already devoted to the atmosphere in each of these different contexts indicates how significant it has been and still is in the economy of the Earth and its life.

(p.229) Earth and its atmosphere have developed together over their long history, with the atmosphere evolving in its composition, changing in its temperature, and responding to the planet’s slow structural development and changing geographies and also to the influence of its successive inhabitants. It was, for example, simple bacteria that contributed to the development of an oxygenating atmosphere some 3½ billion years ago. Major volcanic explosions continue to leave their atmospheric imprint, injecting particulates so high into the atmosphere that they persist there for a year or more, changing the weather, even influencing the quality of harvests. Sandstorms darken the skies over vast areas on the fringes of the world’s deserts to such an extent that they can be seen from space. To these changes—both the long-term natural changes in the Earth and its inhabitants as well as its short-term local perturbations—the atmosphere has responded: evolving, adapting, and accommodating to each.

And in spite of the carelessness of our present patterns of human pollution, Earth will, no doubt, continue to absorb these latest changes and, on its own terms and in its own time, accommodate and adjust to them. The question is not whether the atmosphere is sustainable: it is. The question is whether the atmosphere, changed as it must then become, can continue to sustain us. It is we and our fellow creatures who are at risk from the very changes imposed by our own pattern of living. The hazard we face is whether the atmospheric changes we are inducing may now have become so severe and so rapid that they will overwhelm the atmosphere’s capacity to provide a habitable environment for us and for all its other creatures. We have assumed that we could funnel and pipe away all our global gaseous garbage. Building even higher smokestacks for our factories and power plants, fitting quadruple exhaust pipes on our automobiles, we have assumed that, once piped away, our toxic fumes and combustion smoke are gone.

But nothing is “gone.” In one form or another, our refuse comes back to us, as acid rain or snow, corroding buildings and bridges and damaging crops; or as contaminating particulates, producing a spreading pattern of pulmonary illness and other chronic ailments. Though some pollutant gases are short-lived, others remain in the atmosphere for a century or more.

And in contrast to other atmospheric hazards—tornadoes, thunderstorms, hurricanes, and so on—we cannot escape indoors for protection from pollution, for, even as we slam the door on the threat, the same atmosphere awaits us within our buildings. In fact, the air within many of our buildings—schools, homes, factories, offices, and even hospitals—can be even more threatening to our health and safety than it is outside. Consider, for example, the discovery in the 1980s of the link between asbestos—a naturally occurring, fibrous mineral, once widely used in building construction and insulation—and mesothelioma, a serious form of lung cancer. Or consider the “sick building syndrome” of more recent years, or the continuing concerns about “radon houses,” constructed over areas of natural (p.230) background seepage of radon, an odorless, tasteless, colorless gas, said to be second only to smoking in causing lung cancer. Like it or not, the atmosphere is everywhere and we are everywhere dependent on its quality for our life support. A century or less ago, atmospheric pollution tended to be local in extent, confined to the neighborhood or village, and thus limited in its effect, because of Earth’s capacity to absorb and dilute contaminants. Now, in contrast, the explosion in human numbers and the impact of growing industrialization and urbanization of society make it a far more global threat.

Cities, for example, tend to have more atmospheric pollution than rural areas, but because there is continued growth in the proportion of the world’s population who are city dwellers (now approaching 50 percent), urban pollution is a growing health hazard. In spite of differences between one region and another, atmospheric degradation is now a global issue. Sustainability requires a habitable atmosphere for the whole planet.

Sources and Causes of Atmospheric Degradation

The harsh reality is that much of our atmospheric pollution is the product of our basic means of subsistence. The major sources are all too familiar: heavy industry,

18.1 Common air pollutants in the United States by source.

Courtesy of EPA.

(p.231) power generation, transportation, space heating, agriculture, garbage disposal, and so on. But we can no more eliminate those activities as a basis for modern society than we can cut out eating as a basis for human life.

Nor can we take any action that will guarantee immediate improvement. Consider the atmospheric lifetimes of the various compounds that have been identified as the chief sources of atmospheric degradation. Thus carbon dioxide has an atmospheric lifetime of 5 to 200 years, nitrous oxide 114 years, and hydrofluorocarbons up to 260 years. In contrast, water vapor, though it is the most abundant of the greenhouse gases, has an average residence time of only about nine days. Its changing concentration appears to arise from climatic feedback associated with increasing temperature rather than from industrialization. Its effect seems to be to amplify the influence of other greenhouse gases, though the details are far from clear.

Carbon dioxide (CO₂), the chief greenhouse gas after water vapor, is released from the combustion of fossil fuels, from cement production, and from deforestation. In effect, we are producing this by taking carbon from the Earth, where it is stored in fossil fuels and carbonate rocks, and burning it to produce not only heat but also CO₂, water vapor, SO₂, and traces of other gases. Present levels of atmospheric CO₂ are around 350 parts per million by volume (ppmv), and at current levels of increase this is likely to reach 500 ppmv by 2100. The preindustrial level is thought to have been in the region of 270 ppmv.¹

Nitrogen oxides (NO_x), including both nitric oxide (NO) and nitrogen dioxide (NO₂), are formed from fossil fuel combustion and the burning of biomass. Though having a residence time of only days in the atmosphere, they contribute to the formation of acid rain and photochemical smog.

Nitrous oxide (N₂O) is formed not only by combustion, biomass burning, cattle lots, and deforestation but also by nitrogen-based fertilizers. An important greenhouse gas, it has a residence time in the atmosphere of 114 years.

Sulfur dioxide (SO₂) is produced chiefly by power plants and factories by the combustion of high-sulfur coal and fuel oils. It is highly toxic and corrosive, and forms, with nitric oxide, secondary pollutants (sulfuric acid and nitrate and sulfur salts) in the form of acid rain.

Chlorofluorocarbons (CFCs) were once widely used as major components of aerosol propellants, as well as in refrigeration and cooling equipment. Now banned, they survive for several thousand years in the high atmosphere, where they break down, releasing chlorine, which attacks ozone in the stratosphere and so damages the ozone layer. CFCs are also derived from some industrial solvents, Teflon polymers, and foam. The industrial use of these products has been greatly reduced by legislation that was passed in the 1980s. (p.232)

18.2 Level of CO₂ in atmosphere 1750–present. Squares represent values determined from ice cores; triangles represent direct (Hawaiian) atmospheric measurements; the straight line is an approximation to a smooth curve value; and shading represents likely measurement error.

From Jonathan Cowie, Climate Change: Biological and Human Aspects (Cambridge: Cambridge University Press) and A. Lacey, ed., Bioresources: Some UK Perspectives (London: Institute of Biology, 1992), based on NOAA data; used with permission.

Methane (CH₄) is released from combustion of fossil fuels and from rice paddies, animal waste, landfills, and domestic sewage. Its concentration has increased markedly since the Industrial Revolution. Methane also reacts with chlorine in the atmosphere and contributes to ozone depletion. It has a residence time of twelve years.

Not all these gases are harmful, especially in low concentrations. CO₂, as we have seen, is as vital to plants as O₂ is to animals. In fact, as we have seen, many of them occur naturally in the atmosphere. Without their presence, Earth’s comfortable 59°F average temperature would be closer to the frigid average—13°F—of the moon.

Studies of direct temperature readings over the last century and a half and proxy records from ice cores from earlier periods reveal the steady increase in concentration of these greenhouse gases and their close, but not perfect, correlation with increasing global temperatures. The balance of evidence strongly suggests a causal link between the two, though there is still some debate on this point. The correlation over the last century between proxies for solar activity and temperature is regarded by some as better, especially in reflecting the temperature decrease from 1940 to 1970. (p.233)

18.3 Temperature over last 160,000 years, with CO₂ plots. Top: atmospheric carbon dioxide; bottom: temperature change.

From Jonathan Cowie, Climate and Human Change: Disaster or Opportunity? (New York: Parthenon, 1998), based on NOAA data; used with permission.

Now, a further temperature increase of a few degrees would not be unprecedented. We have seen, for example, that the Earth was about 4°C (7.2°F) warmer at the start of the Pleistocene period, 2 million years ago. The previous interglacial period (the Eemian-Sangamon-Ipswichian), which took place about 120,000 years ago and lasted some 20,000 years, was alSO₂.0–2.5°C warmer than at present, and the still earlier and shorter (Hoxnian-Holstein-Yarmouthian) interglacial, some 220,000 years ago, was also warmer. Within all such glacial and interglacial periods there were many smaller episodes of advances and retreats of the ice in response to smaller fluctuations in temperature.

The last 150 years have clearly been a warming period and GGEs have been increasing. Most informed observers accept that continuation of this climate change poses a serious threat. That is not to suggest complete unanimity in the scientific community about the longer-term consequences of these temperature increases, and especially about the precise future timing, magnitude, impact, rate, and extent of their influence. Nor will all these changes have negative consequences for all areas. They may well prove beneficial for some regions, especially those in mid to high latitudes, by extending growing seasons, although they may also produce more extensive drought. There is also some debate about the extent to which future reductions in GGEs, changes in land use, and other remedial actions can reduce temperature increases that are already “built into” the climate system. (p.234)

18.4 Global temperature anomalies, 1880–2010.

Courtesy of NASA GISS.

Confronting Atmospheric Changes

A major source of debate in all discussions of the impact of climate change is the reliability of the elaborate general circulation models (GCMs) that have been developed as a means of predicting future climate patterns. Alternative methods, such as the use of past warm periods of Earth history to serve as analogues for future climate, have not, as yet, proved satisfactory. This is partly because of the unknown influence of differences between past rates of terrestrial elevation and patterns of oceanic circulation in comparison with those of the present. But even though our predictive skills are limited, the potential threats are so serious that a policy of sensible reduction of system risk would seem to provide the most prudent response. This policy would seek to balance the potential benefits of reducing the various threatening trends with the economic and social costs of their mitigation.

Within such a context, if we are to reduce our vulnerability to the threats presented by atmospheric changes, as to other natural threats, there are two broad categories of response available to us. The first involves mitigation: reducing or eliminating the potentially harmful threat. Most of the atmospheric changes we have described cannot now be prevented, because some continuing change is already incorporated in Earth’s oceans and systems, but they can be reduced. And reduction can provide great benefits. Mitigation will be costly if we embrace it on a global scale, but we need a careful calibration of those costs against the potential (p.235) benefits of mitigation. Mitigation in this case would require us to reduce GGEs, develop alternative energy sources, redesign our buildings and communities, improve efficiency of our use of fuels, and reduce our dependence on polluting materials by replacing them with substitute fuels or other materials. All these are worthwhile goals in their own right, quite apart from their influence on climate change.

The second response involves adaptation. Although we cannot precisely predict the climate future, we have seen that some temperature increase may already be “built into” the global system and thus be inevitable, and we need to adapt as best we can to its consequences. By implementing programs to reduce deforestation, improve agricultural practices, replace forests, strengthen food defenses, and develop drought-, heat-, and frost-resistant crops, we can make a significant difference. And, again, all these programs are beneficial in their own rights.

The magnitude and extent of programs such as these will require the most extensive scientific and social analysis and financial review. Any plans adopted will have substantial economic and societal consequences. And they will certainly have to be refined and corrected in the light of continuing experience. We shall later review the political context in which such major policy questions will need to be considered.

Even as we wrestle with these problems of greenhouse gases and particulate emissions, the number of coal-fired power plants and gasoline-powered automobiles continues to grow, and it will be decades before they are replaced by some other form of clean power generation and clean transportation. In the meantime, how do we limit further damage to the atmosphere? Air circulation is so pervasive that there is no possibility of remediation on any significant scale. We can’t clean up the world’s air by remediation. Certainly within a house or a store or a factory we can “treat” the air, dehumidifying, filtering, purifying, heating, or cooling it as we wish. But for any but the smallest point source of outdoor space, that is not possible, as the Beijing Olympics of 2008 demonstrated. The only means of reducing such emissions is the elimination or reduction of the sources of emissions. Unlike soil or water, where contaminants can persist for many years, air composition and quality responds rapidly to reduction at the source of the “polluting” agent. That “solution” has proved to be wholly effective in its outcome, surprisingly rapid in its benefits, but remarkably difficult in its implementation. This may be, in part, because the costs are largely local while the benefits are not only local, but also to a degree regional and global. It is also, chiefly, this same imbalance of costs and benefits that lies behind the inability of the federal government to develop a preventative strategy, in contrast to the agreement of states and regions to enact a common approach. The technical means of reducing GGEs, for example, are demonstrably effective and readily available. The financial incentives and statutory requirements to implement them, however, are not. That (p.236) means that any effective “clean air” program has to be built on a framework of local, regional, and international partnership. The seeds of just such a program already exist in the Kyoto and other protocols, but we still have a long way to go in their implementation. Most of the signatories to that protocol have not met the goals they established.

Until we can develop such international agreements, the most critical need is to reduce emissions, especially from power plants and automobiles, which account respectively for 40 percent and 33 percent of carbon emissions. We must also improve the efficiency of every machine, appliance, and vehicle that is manufactured, from toasters to tankers, in order both to reduce energy consumption and to limit emissions.

Two major initiatives are now commonly promoted as being particularly promising in this effort. First, emissions of carbon dioxide can be reduced by programs of carbon capture and sequestration (CCS). This involves the injection of CO₂ from large emitters into secure underground reservoirs, rather than releasing it into the atmosphere. It is estimated that the present sum total of human activity is responsible for the emission of some 7 billion tons of carbon dioxide a year: that’s about one ton per person. Virtually all of this escapes into the atmosphere, but pilot projects to reduce this pollution are now under way in several countries. Secure underground aquifers in which carbon dioxide may be stored are relatively plentiful. One recent study suggests it should be possible to store all the CO₂ generated by fossil fuel power plants in Europe over the next eight hundred years in a single saline aquifer beneath the North Sea.

Though the cost is likely to be high, the present levels of European carbon taxes could make such sequestration attractive. A possible alternative might be the injection of CO₂ into oil field reservoirs, where the gas can contribute to the secondary recovery of petroleum from depleted oil fields. Recent studies have also shown that injection of CO₂ into underground coal seams can enhance recovery of methane, an important component of natural gas. Coal has an affinity for CO₂, which it absorbs. Other possible methods of sequestration also exist, including injecting CO₂ into the deep oceans to produce icelike clathrate hydrates (crystalline solids in which small gas molecules such as hydrogen, carbon dioxide, methane, and others are trapped inside cages of water molecules) and combining CO₂ with olivine and serpentine-rich rocks to produce carbonates. These are, however, theoretical schemes, which have yet to be demonstrated or tested.

Two factors seem to me to make large-scale carbon sequestration a questionable strategy: present costs and future dangers. The costs of sequestration are likely to be high; they include not only the immediate costs of carbon disposal but also the potential costs of dealing with sequestered CO₂ and possibly leaking reservoirs long after we have finished using fossil fuels. The security of underground reservoirs is not something for which anyone can provide a long-term guarantee.

(p.237) Nor is there, as yet, any general agreement that carbon sequestration is a safe, necessary, or cost-effective method of reducing greenhouse gases. Some geologists believe the associated costs and hazards are likely to exceed the benefits.

Carbon sequestration is not the only method of reducing carbon emissions. Improved carbon capture in power plants and similar sources of CO₂ emissions can make a major contribution to this reduction. Various effective methods of pre-and postcombustion capture already exist. Most involve additional energy use and thus additional costs to the consumer, but these costs may not be greater than those of electricity derived from other sources, including nuclear plants and other “low carbon” power plants.

The second major initiative is to mitigate the effects of GGEs by encouraging increased forestation and decreased deforestation. Unlike carbon sequestration, this is an established and effective program. It is increasingly constrained, however, by the lack of land available for reforestation, especially as global population continues to increase, together with growing numbers of those who depend on slash-and-burn agriculture for their subsistence.

The remedies proposed to reduce potentially harmful atmospheric emissions may sound simple. They are not. They involve difficult trade-offs and painful priority choices. The recent experience in California, after the governor and legislature agreed on plans to curb carbon emissions by 5 percent by 2020, has highlighted the concerns. Relying on a mixture of cap-and-trade schemes, regulations, and incentives, the program has created sharp divisions in both the business and environmental communities. In the absence of a federal plan, other states are creating their own piecemeal plans, but there is great debate about the effects of regulation on competitiveness, costs, and the overall economy. Some point to the application of the Kyoto Protocol by European countries as a failure from which we have yet to extract the lessons.

Preserving the quality of the air we breathe will be as big a challenge as preserving the land, lakes, and rivers. We are more likely to succeed by a continuing, rigorous balance between clear benefits and real costs (social as well as economic). Redesign and improved efficiency of agricultural, industrial, and domestic practices and products, judicious remediation and restoration, and incentive-based regulation seem to offer our best, if tenuous, hope.