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The Environmental Imperative for Renewable Energy: An Update Climate Change |
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To the now-familiar litany of problems associated with conventional air pollution comes the additional threat of global warming and resultant climate change. Because the climate naturally exhibits both statistical variability and long-term patterns of change, it remains difficult to distinguish "natural" from human-caused climate effects. (See Box 2 on physical evidence of climate change.) Yet many scientists now conclude that human activity has altered natural climactic processes at a geologically rapid pace by boosting atmospheric concentrations of several greenhouse gases. In the words of the Intergovernmental Panel on Climate Change (IPCC), "the balance of evidence suggests a discernible human influence on global climate." 41 Underlying the threat is a well-known phenomenon, the greenhouse effect. That phrase describes the tendency of several gases to trap heat in the atmosphere, much like the transparent walls of a greenhouse. Visible sunlight passes through Earth's layer of "greenhouse gases" unhindered, but much of the resultant heat (characterized by infrared wavelengths longer than those of visible light) radiating upward from the warming planet cannot. Instead, the gas layer absorbs and re-emits some of the heat back down to Earth's surface. The planet's natural greenhouse process forms part of a complex pattern that creates the conditions that allow life to flourish. In particular, carbon, which as carbon dioxide (CO2 ) represents a major greenhouse gas, 42 follows a cycle of storage and circulation, passing among living plants and animals, the atmosphere and oceans, and earthbound mineral deposits - which prominently include coal, oil, natural gas, and other fossilized substances used as fuels. The industrial age has seen the following: 43
Greenhouse gases remain in the atmosphere for varying lengths of time; for example, perhaps 12 years for methane, 50-200 years for CO2 , and 50,000 years for carbon tetrafluoride. 44 For this reason, even a decision to cut drastically all greenhouse gas emissions (say, in response to a series of dramatic climactic disasters) might not return the climate to its previous state for a century or longer.
Table 4 lists U. S. emissions of greenhouse gases released by human activity (" anthropogenic" greenhouse gases). For ease of comparison, the table lists each gas in tons of CO2 equivalent, 45 calculated from each gas's global warming potential. In the United States, energyrelated activity accounts for about 86% of anthropogenic greenhouse gas emissions. As Table 4 indicates, electricity use as a whole accounts for about 36% of U. S. greenhouse gas emissions. 46
A. CARBON DIOXIDE Rising carbon concentrations largely reflect increasing use of fossil fuels. 47 Globally, fossil fuel use results each year in the release of about 6 billion metric tons of carbon (GtC), growing recently at about 2% annually. The United States accounts for about 23% of global carbon emissions. 48 Among the fossil fuels, petroleum and natural gas contain respectively about 75% and 55% as much carbon per unit of energy as coal. The higher efficiency of gasburning technology enhances its inherent carbon advantage; a gasfired combinedcycle turbine with 48% efficiency releases only half the CO2 of a conventional coal plant of the same capacity and 38% efficiency. 49 As a result, natural gas represents a climate "winner" when it replaces coal-burning capacity, but part of the climate problem when it meets new energy demand or replaces zeroemission facilities such as decommissioned nuclear plants. Given the huge projected need for energy, especially in the developing world, even an energy system reliant on natural gas would impose substantial stress on the climate - albeit less than one reliant on coal and oil. 50 Geologists have identified approximately 1,050 GtC from coal, oil, and natural gas that can be economically recovered with current technology; improved technology may ultimately bring a total of 4,100 GtC from fossil fuels into that economically recoverable resource base. 51 For purposes of comparison, the IPCC estimates that the emission of 1,030 GtC between 1991 and 2100 will raise atmospheric concentrations of CO2 to more than double preindustrial levels, from 280 to 650 ppmv. Emissions of 1,410 GtC during that period would almost quadruple concentrations compared to preindustrial levels, to 1,000 ppmv. 52
B. METHANE Like all fossil fuels, natural gas releases CO2 when burned. In addition, the primary constituent of natural gas, methane (CH4 ), is itself a much more potent greenhouse gas than carbon dioxide. On the other hand, methane remains in the atmosphere for a relatively short time, perhaps a dozen years, compared with 50-100 years for CO2 . Given these countervailing factors, the IPCC calculates the 100-year global warming potential of methane as 21 times that of CO2 . 53 In other words, one unit by mass of methane will heat the Earth as much in 100 years as 21 units of CO2 . Energyrelated sources account for almost one third of U. S. methane emissions. Sources, and their contribution to 1997 U. S. methane emissions, include: 54
In addition to its physical presence in deposits of solid and fluid fossil fuels, methane is also formed by the anaerobic (i. e., without oxygen) breakdown of organic matter - for example, garbage rotting in landfills, or plant matter decaying underwater. Some 37.1% of U. S. methane emissions seep out of landfills, suggesting a useful step toward staunching greenhouse emissions: landfill owners currently capture and burn almost 15% of total landfill methane to generate electricity. 55 While this practice releases CO2 , it remains both environmentally and often economically preferable to venting the gas. 56 More than a third of U. S. methane emissions seep out of landfills. Capturing and burning this gas to generate electricity provides a useful product and lowers the landfill's net environmental impact. A potentially important aspect of the methane problem, about which little certain information exists, concerns large hydropower facilities. The World Commission on Dams (WCD), set up by the World Bank and the World Conservation Union (IUCN) to assess the role of dams in energy and water development, notes that oxygen-poor hydropower reservoirs can vent substantial quantities of methane, especially early in their lifetime, due to rotting vegetation beneath the waterline. 57 (Well-oxygenated reservoirs emit CO2 rather than methane.) For this reason, large, shallow hydropower reservoirs that inundate large quantities of biomass may not be justifiable on the basis of contributions to the mitigation of climate change. For example, the Petit-Saut hydroelectric dam in French Guiana, which supplies power to the launch site of Europe's Ariane rocket program, submerged 365 square kilometers of tropical forest. One study reckons that during its first 20 years Petit-Saut will emit the equivalent of 66 million metric tons of CO2 - about 85% from methane and the remainder from CO2 itself - making French Guiana one of the world's largest per-capita emitters of greenhouse gases. 58 While nontropical and more mature hydro-electric systems (including most U. S. facilities) have much lower emission rates, it is clear that tropical hydropower does not necessarily provide climatesafe energy, and may represent an important and poorly understood part of the climate problem. 59
C. OTHER ENERGY-RELATED SOURCES OF GREENHOUSE GAS Like those of methane, nitrous oxide emissions by weight are low compared with CO2 emissions. However, the substance packs a greenhouse wallop about 310 times more powerful than CO2 on a per-weight basis. 60 Some 3.8% of America's total nitrous oxide emissions result from chemical reactions initiated by fossil fuel combustion in stationary sources, chiefly power plants. 61 One final greenhouse culprit merits mention here. Sulfur hexafluoride has the highest global warming potential of any substance yet evaluated - 23,900 times more potent than CO2 on a per-weight basis. 62 Once leaked or released, it persists in the atmosphere for an extremely long time. Eighty percent of SF6 in use worldwide insulates electrical transmission and distribution lines. 63
D. THE GREENHOUSE IMPACT OF RENEWABLES As plants grow, they absorb carbon dioxide. When burned, they relinquish an equal quantity of CO2 back to the atmosphere. For this reason, many analyses consider biopower roughly greenhouse neutral. For instance, U. S. emission figures (see Table 4) do not include the 57 million metric tons of CO2 released by biomass combustion during 1997. 64 In general, biopower can be near-neutral over reasonably short time periods, assuming that it is not fueled by old-growth forests and provided that forest soils are managed so as to permit continuing regrowth. In fact, biopower may provide net greenhouse advantages. Most biomass fueling the biopower industry would otherwise decompose in landfills, releasing CO2 and methane. Natural forest fires and purposeful burning would account for the rest, also releasing CO2 . (Lack of pollution controls means that these fires emit copious conventional pollutants as well.) The National Renewable Energy Laboratory (NREL) suggests that although the California biopower sector releases 1,330 grams of CO2 equivalent per kWh directly, avoiding the alternative outcomes, especially the emission of methane, actually makes biopower a net greenhouse winner, at -1,747 grams of CO2 equivalent/ kWh. According to the NREL report, subtracting the emissions that would otherwise be released by burning natural gas to generate an equivalent amount of electricity further lowers the figure, to -2,802 grams CO2 equivalent/ kWh. 65 Depending on the technology used, geothermal power may also have a greenhouse impact, although a more modest one than fossil fuel plants. Geothermal power exploits reservoirs of hot, underground fluid. Composed primarily of steam (90-99%), these fluids can also contain varying fractions of carbon dioxide, hydrogen sulfide, methane, and ammonia. One type of geothermal technology, binary systems, never exposes geothermal fluids to the air, and therefore has no direct emissions; binary plants account for about 260 MW of total U. S. geothermal capacity of 2,900 MW. 66 A second geothermal technology, flashed steam systems, does vent the working fluids, but the actual greenhouse impact of flashed steam plants varies. California (which accounts for about 85% of U. S. capacity) requires plants to control hydrogen sulfide. Several facilities, including the largest in the country, the 1300MW Geysers complex, do so by removing the H2 S, incinerating it, and reinjecting the combustion products into the reservoir. Serendipitously, this process also removes methane (as well as other gases and contaminants such as mercury, arsenic, and selenium). However, all flashed steam plants release the native CO2 . 67 In some future circumstances, geothermal power production may offer net climate advantages. New technology allows the extraction of minerals including zinc, manganese, and silica from the geothermal brine, offsetting greenhouse emissions that would otherwise be produced by mining and processing. For example, the mining-related energy use avoided by a geothermal zinc extraction plant now under construction in California may cancel out half the flashed steam plant's reservoir-derived greenhouse gas emissions. 68 Mineral coproduction is being examined at several other geothermal sites, as well.
E. GLOBAL WARMING Since climate change first appeared in the news, scientists have refined and to some extent moderated initial estimates of future warming. (See Table 5.) It would be a mistake to interpret falling estimates as evidence of a dissipating threat. For one thing, estimates have started to creep back up. Perhaps more significant, scientists became more convinced in the 1990s that climate change is indeed occurring. Computer-generated climate models have gradually improved their ability to generate today's known conditions when given data about the past - a crucial test of a model's accuracy regarding the future. Models prepared in 1990 were based on emissions of greenhouse gases dating to the rise that began in the 19th-century industrial era. Their "predictions" of the present were too warm, and they generated distorted temperature maps. Subsequent models from the mid-1990s incorporated the cooling effect of sulfate aerosols strewn into the air by volcanoes and burning coal, which tend to reflect sunlight back into space and thereby inhibit the greenhouse effect. 69 While more accurate, these models nevertheless produced snapshots of "today" that were too cool, as well as being spatially disproportionate. The current generation of models, however, adds a third factor: periodic variation in the strength of the sun itself. Not only do the resultant temperature maps resemble current conditions on Earth, but the temperature predicted by the models matches what we know about the past fairly well. In short, there is better reason to trust today's predictions than there was 10 or even 5 years ago. 70 Nevertheless, climate modeling remains a challenge. In addition to its complexity, the climate may not behave as a linear system. Just as a ball rolling toward a table edge may alter its velocity and direction precipitously, steady stress on the climate may provoke sudden, large effects as the system crosses a threshold. Such changes may be out of proportion to the incremental pressure. Many scientists accept that we may be seeing the first signs of climate change. In 2000, the National Research Council found evidence that surface temperatures have risen about 0.4-0.8° C (0.7-1.7° F) over the last century. 71 The report's authors acknowledged that the upper atmosphere does not seem to be warming, and may in fact be cooling, and they characterized this difference as evidence that scientists do not yet fully know how Earth's climate works. But the report dismissed the possibility that the scientific community might be mistaken about surface warming: the phenomenon, the report concludes, is "undoubtedly real." 72 In recent years, scientists have catalogued a variety of circumstantial evidence consistent with a changing climate. None of these factors prove or disprove a link between human behavior and long-term climate change. Considered as a group, however, they are suggestive. To take just a few examples:
A changing climate could cause myriad other environmental shifts. For example, sea levels would rise, most obviously because of melting alpine glaciers and polar ice caps, but also because warming water expands. Yet the mechanics of the climate are complex: greenhouse heating might carry warmer, wetter air to Antarctica, adding to the ice pack. ZZZ Tom Wigley's report for the Pew Center on Climate Change suggests that seas might rise 46-58 centimeters by 2100. 78 The prospect of such changes alarms coastal communities and, especially, inhabitants of island nations. Most monitors in the South Pacific now record rises of up to 25 millimeters per year. 79 Closer to home, rising salt seas could pollute fresh aquifers, for instance in Florida and Long Island. Global warming would lead to climate change primarily by affecting evaporation and precipitation. Computer modelers have only recently become able to hypothesize specific regional changes - although with substantial caveats and uncertainty. For instance, with respect to the continental United States, one recent report suggests overall warmer weather, especially during winters. 80 The frequency of hot spells will increase, and that of cold spells will decrease. A warmer climate may increase the frequency of intense rain and snowstorms, and also of dry days and longer dry spells. Although a warming climate might produce more frequent, wetter, windier North Atlantic hurricanes and tropical storms, most scientists remain cautious regarding the ability of current climate science to predict this with any accuracy.
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The Environmental Imperative for |
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