From the preceding discussion, it is clear that rising CO₂ has both direct effects on the physiology of land and sea organisms and indirect effects related to climate change and ocean chemistry. In order to ensure our environmental future, scientists must understand the climate system and biological responses to global change much better than they do now. Because the principal source of rising CO₂ is the burning of fossil fuels, the solutions we develop as citizens will clearly focus on energy—how we obtain it and how we use it. As the global population continues to increase and people aspire to higher standards of living, human impact on the carbon cycle will continue to grow, at least if we continue business as usual. Business as usual, however, may not be a viable option for the long term because population growth may simply outstrip resource availability, especially for petroleum. Ultimately, our success in both slowing rates of climate change and sustaining energy supplies will depend on our ability to use energy more efficiently and generate new forms of power, most likely from the sun, wind, and nuclear sources.

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Half a century ago, geologist M. King Hubbard predicted from patterns of oil use and the rate at which new oil fields are discovered that petroleum production in the United States would peak in the 1970s and decline thereafter. Global production, he suggested, would peak later, in the early decades of the 21st century. As King predicted, conventional petroleum production in the United States peaked in 1972 and has fallen since then to about half its peak rate of production. On the other hand, unconventional petroleum resources, made available in no small part by hydraulic fracturing, or fracking, is expected to restore total petroleum production in the USA to near-1970s levels by 2020. (Fracking is a technique in which large volumes of fluid and sand are injected into shale rock, fracturing the rock and thereby releasing trapped oil or gas.) After 2020, petroleum production will resume its decline, both in the United States and globally. Coal, natural gas, and unconventional reserves such as the oil sands found widely in western Canada can quench the world’s thirst for fossil fuels for another century or more, and biofuels—fuels generated from plant biomass—may also help conserve petroleum, but the energy produced will be expensive, and climate change will continue.

In particular, natural gas (methane, CH₄) looms large in estimates of 21st century energy generation. Fracking has revolutionized natural gas extraction; in the United States alone, shale gas production has increased by 700% since 2007. Of all fossil fuels, natural gas generates the most energy for each mole of CO₂ produced by combusting it; thus, generating electricity by natural gas has advantages over burning coal. CO₂ emissions do continue, however, and gas leaks from expanding wells and pipelines add more methane, a strong greenhouse gas, to the atmosphere. Scientists and citizens debate whether the toxic fluids used in fracking will contaminate ground water in regions where shale gas is produced.

Like fossil fuels, alternative energy sources—wind, the sun, tidal energy, and nuclear power (Fig. 49.15)—all have important pros and cons, not the least of which, in the case of nuclear power, is the problem of long-term storage of radioactive waste. On the other hand, all these sources have the potential to benefit humanity at least three ways: by reducing the amount of CO₂ emitted per kilowatt of energy generated, by conserving finite natural resources, and by helping to contain the cost of energy production. The development of more fuel-efficient cars, airplanes, and buildings will also help greatly to maintain energy-intensive lifestyles while decreasing their environmental consequences. As individuals, we can choose energy-efficient transportation and appliances and develop habits that decrease energy waste. When one person turns off the light in an empty room, the energy saved is tiny, but when millions of people do it, the benefits add up.

Wise choices by citizens and consumers can slow the growth of atmospheric CO₂, but the CO₂ already in the atmosphere isn’t going to go away quickly. As noted earlier, we have dramatically increased the rate at which CO₂ is added to the atmosphere, but we have not changed the rate at which it is removed. The geologic processes that control atmospheric CO₂ work on longer timescales. Therefore, even if we could cut CO₂ emissions by 90% tomorrow, CO₂ would not return to nineteenth-century levels for hundreds or even thousands of years.

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Another possibility is to actively remove CO₂ from the air. Reforestation of previously cleared landscapes removes carbon from the atmosphere because plants build biomass from CO₂ during photosynthesis (Chapter 8). It is also possible to capture CO₂ as it rises upward through smokestacks, although it is currently expensive to do so. We do not yet know how to scrub CO₂ directly from the atmosphere, but if we could do so at reasonable cost, it would apply a significant brake to climate change.

Clearly, forestry and 21st-century technology have potentially important roles to fill in undoing the consequences of the past hundred years. For the most part, however, gaining control over our environmental future will entail wise personal and governmental choices about how we obtain and use energy.