There are two main sources of human-caused greenhouse-gas emissions, namely energy production and land-use change. Today, most energy is produced by burning fossil fuels, especially coal, oil, and natural gas. Burning these fuels—whether to run vehicles, heat buildings or industrial processes, or generate electricity—releases carbon dioxide (CO2), the most important greenhouse gas. The two other large-scale sources of energy are nuclear power and hydroelectric dams. Other energy sources, such as biofuels, thermal and photovoltaic solar power, geothermal energy, and windmills, produce a small but rapidly growing fraction of world energy supply.
All forms of energy production have environmental impacts, but these impacts vary widely. In some cases, such as nuclear power, their nature is disputed by supporters and opponents of the technology. With regard only to the release of greenhouse gases, not other possible environmental harms, fossil fuels are by far the worst offenders. However, large hydroelectric dams emit significant methane (CH4), biofuels may or may not reduce greenhouse emissions significantly depending on how they are produced, and windmills, solar cells, and nuclear power plants do not emit greenhouse gases while in operation because they do not emit waste gases, but do emit greenhouse gases at least during manufacture.
Today, debate swirls around the question of whether a massive new round of investment in nuclear power is essential to mitigating climate change, or would set back mitigation by absorbing funds that could be more effectively spent fighting greenhouse emissions by increasing energy efficiency. All energy generation schemes, not only nuclear power, can be seen as diverting funds from efficiency measures that can reduce energy demand more cheaply than that demand can be met by any primary source.
Historical Background and Scientific Foundations
Human beings have been burning fuel, usually wood, for almost 800,000 years. Fossil fuels—most of which consist of the transformed remnants of ancient forests (coal) or tiny oceanic organisms (petroleum and natural gas)—were first widely used in the Middle Ages in Europe, after the forests had been depleted for fuel and timber. In Britain, where the forests were smaller originally, the crisis arrived sooner and the practice of mining for coal underground began first, in the 1200s. Coal was heaped up in furnaces or fireplaces to heat spaces or boil liquids; it was not used to make steam or run machinery. Europe's increased reliance on coal was the beginning of large-scale human emissions of greenhouse gases.
Although burning or decaying wood also releases CO2, this does not increase the total amount of carbon in the atmosphere. Growing trees extract carbon from the air, and burning wood returns that carbon to the air. The carbon in fossil fuels was also originally obtained from the air by plants, but this happened so many millions of years ago that its reappearance in the atmosphere today has the effect of adding brand new carbon to Earth's environment.
For centuries after coal-burning became important, the amount of coal burned was small by modern standards. Mining techniques were crude and could not reach the largest, deepest deposits, and demand was relatively low. This began to change in the 1700s, when the switch to more complex, energy-intensive technologies began, the phase of history called the Industrial Revolution. In 1712, British inventor Thomas Newcomen (1663–1729) was the first to demonstrate a coal-powered steam engine for pumping water, allowing coal mines to be kept open more cheaply. Though Newcomen's steam engine was inefficient it was still far cheaper than the horse-powered pumps that had been standard up to that time.
By the end of the 1700s, more sophisticated steam engines were being used to run factory machinery, a practice that became universal during the 19th century in the industrializing countries of Europe and North America. The beginning of the modern or post-industrial increase in greenhouse gas concentrations in the atmosphere, especially of CO2, is usually dated to about 1750. Greenhouse-gas emissions have increased ever since, with the most recent growth being the most rapid. In the 1800s, coal began to be used not only for heat and factory work but to fuel locomotives and steamships. In the early 1900s, trucks and cars became essential to industrialized economies and were dependent on gasoline refined from petroleum. Atabout the same time, coal and other fuels began to be used to generate large amounts of electricity. Finally, from the 1950s onward, large quantities of natural gas also became essential to modern civilization.
WORDS TO KNOW
BIOFUEL: A fuel derived directly by human effort from living things, such as plants or bacteria. A biofuel can be burned or oxidized in a fuel cell to release useful energy.
CARBON SEQUESTRATION: The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide, release the oxygen, and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon until burned.
DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosyn-thesis are no longer present and contributing to carbon storage.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
GEOTHERMAL ENERGY: Energy obtained from Earth's internal heat, which is maintained by the breakdown of radioactive elements. Geothermal means, literally, Earth-heat. Geothermal energy may be used either directly as heat (e.g., to heat buildings or industrial processes) or to generate electricity.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
INDUSTRIAL REVOLUTION: The period, beginning about the middle of the eighteenth century, during which humans began to use steam engines as a major source of power.
INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.
METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH4. It is the simplest hydro-carbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.
PHOTOVOLTAIC SOLAR POWER: Electricity produced by photo-voltaic cells, which are semiconductor devices that produce electricity when exposed to light. Depending on the cell design, between 6% and (in laboratory experiments) 42% of the energy falling on a photovoltaic cell is turned into electricity.
RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).
RENEWABLE ENERGY: Energy obtained from sources that are renewed at once, or fairly rapidly, by natural or managed processes that can be expected to continue indefinitely. Wind, sun, wood, crops, and waves can all be sources of renewable energy.
None of the old fuels, since the replacement of wood with coal, have ever become truly obsolete. In the early 2000s, the world is using more coal than ever, more oil than ever, and more natural gas than ever. Although nuclear power and renewable energy sources such as wind and solar have contributed some energy in recent decades and renewable energy production is growing rapidly, only a relatively small slice of world primary energy comes from these sources.
The Energy Usage Picture: Primary Supply and Usage by Sector
Primary energy is the energy content of a fuel or other source. As of 2004, world primary energy supply was obtained from the following sources: oil (petroleum), 34.3%; coal, 25.2%; natural gas, 20.9%; renewables, 13.1%; nuclear power, 6.5%. The 13.1% of supply from renewables could be further broken down into combustibles, 10.6%; hydroelectric dams, 2.2%; and tidal, wind, solar, and geothermal energy, 0.5%.
Most primary energy is wasted, not used. How much of a primary energy fuel is wasted depends on how the fuel is used: for example, burning one gallon (3.8 l) of gas in an efficient car produces more useful work than burning it in an SUV, but the gallon of gas contains the same amount of primary energy in either case, namely about 124,000 British thermal units. (In metric units, gasoline's primary energy density is 34.6 million joules per liter.) Since 70% of primary fuel energy is wasted in standard centralized electric-generating plants, whether coal-fired or nuclear—about 30% is turned into electricity, and the rest is thrown away as waste heat—the energy sources that produce electricity directly, without producing and discarding heat (wind, solar, hydroelectric dams), actually supply a greater share of useful energy than their primary energy share.
Most coal—in the United States, about 92%—is burned to generate electricity, although a significant fraction is burned in kilns to make cement and a growing amount is used to make liquid fuel. Petroleum is refined to produce a variety of specialty fuels, including gasoline, kerosene, diesel fuel, aviation fuel, heating oil, and liquefied petroleum gas (mostly propane). Almost the entire modern transportation system depends on petroleum fuels, but very little oil is burned to generate electricity—only about 2.4% of oil usage in the United States as of 2005. Natural gas, which consists mostly of methane, is burned as-is, after the addition of a scenting agent to alert users to leaks. It is used to generate about 20% of the nation's electricity. Most of the remainder is used for residential space heat and industrial processes and space heat.
Greenhouse Gas Emissions
As of 2007, atmospheric CO2 was the most important greenhouse gas, accounting for 63% of the radiative forcing involved in anthropogenic (human-caused) climate change. Radiative forcing is the amount of energy retained by Earth, rather than radiated to space, as a result of a greenhouse gas's presence in the atmosphere. About 84% of CO2 emissions were from fossil-fuel combustion, with the remainder coming from land-use changes, mostly deforestation. CO2 from fossil-fuel energy was the largest single contributor to both historic and ongoing accumulation of greenhouse gases in the atmosphere. In the United States, CO2 emissions were 86% of greenhouse pollution in 2006.
Globally, coal and oil contribute about equally to annual global emissions of anthropogenic CO2: oil's share was 40% in 2004 and coal's share was 39%. The remainder of CO2 emissions, about 20%, was from deforestation and natural gas. However, coal's contribution was expected to climb. The U.S. Energy Information Administration (EIA) predicted in 2007 that by 2030, oil's share would be down to 36% and coal's up to 43%, with natural gas' share remaining about the same.
Historically, the first populations to industrialize— Europe and the United States—have been the greatest producers of greenhouse gases from energy fuels. As of 2004, the developing and least-developed countries, containing about 80% of the world's population, had produced only 23% of all emissions released since the beginning of the Industrial Revolution. The emissions share of the developing nations was growing rapidly; they accounted for 73% of emissions growth in 2004. However, this still only amounted to 41% of global emissions. Most of these emissions were from fuel-burning.
The EIA predicted that total amounts of CO2 would continue to rise, despite statements by NASA scientist James Hansen and some other climatologists that global emissions of CO2 and other greenhouse gases must be reduced by 80% below present-day levels by 2050 in order to prevent potentially catastrophic climate change. So far, the EIA's prediction of rising emissions has the virtue of describing actual trends; as of 2004, CO2 emissions were accelerating, not slowing or decreasing. (The EIA's results were released in 2007, but referred to 2004 because energy analysis figures often lag reality by several years due to the time it takes to collect and analyze data.)
CO2 emissions grew at 1.1% per year from 1990 to 1999, but at more than 3% per year from 2000 to 2004. This post-2000 growth rate was greater than in the most pessimistic, fossil-fuel-intensive scenario posited by the Intergovernmental Panel on Climate Change (IPCC) in the late 1990s and was driven mostly by increases in fossil-fuel combustion, including industrial process heat, transportation, space heat, and flaring of gas from wells.
CO2 emissions in the United States fell in 2006 by about 1.3%. The drop was unusual: in only three out of the 16 years from 1990 to 2006 had CO2 emissions dropped rather than risen, with typical changes being about +1% per year. Long-term growth (or shrinkage) of CO2 emissions is influenced by overall economic growth, the energy intensity of the economy (how much energy is used per unit of business done), and the carbon intensity of the energy supply (how much CO2 is emitted from the mix of primary energy sources). The drop in CO2 emissions in the United States for 2006 illustrates the interaction of all these factors with weather and climate across energy-consumption sectors: residential CO2 emission fell 3.7%, mostly because of warm weather, commercial and industrial emissions fell by about 1%, and transportation emissions stayed about constant. Warm weather and increased use of natural gas, the least carbon-intensive fossil fuel, accounted for the 2006 CO2 dip.
Greenhouse gas emissions can be described by fuel, as discussed earlier. They can also be described by economic sector. Global CO2 emissions in 2000 by energy sector were as follows:
Electricity generation: 29.5%
Industrial processes: 20.6%
Residential and commercial use: 12.9%
Processing of fossil fuels: 8.4%
Biomass burning and deforestation: 9.1%
Impacts and Issues
Nonzero Emissions from Zero Emissions Sources?
Greenhouse gas emissions are produced directly by fossil-fuel burning, but not by renewables, hydroelectric power, or nuclear power. However, these other forms of energy do not have zero environmental or even climate impact, partly because energy must be invested to build the systems that produce the renewable energy. For example, large amounts of concrete, steel, and electricity must be consumed to build and fuel a nuclear power plant, and windmills consist mostly of steel, which requires energy to manufacture. Since most of the existing energy supply comes from fossil fuels, building non-emitting energy sources therefore releases greenhouse gases. This makes it incorrect to characterize windmills, nuclear power plants, and similar systems as zero-emissions energy sources, at least as long as most primary energy still comes from fossil fuels. Over the lifetime of a unit, however, the reduction in emissions versus generating the same energy from fossil fuels may be great. Net lifetime emission gains vary widely between technologies. In cases where a complex life-cycle of mining, refinement, construction, safeguards, waste disposal, and decommissioning is involved, as in the case of nuclear power, the subject is highly contentious, with expert estimates varying sharply.
Emissions from Dams
Hydroelectric dams do not use fuel: their energy is indirectly solar. Sun-driven evaporation lifts water above sea level as vapor and drops it as snow or rain inland, where it runs downhill in streams and rivers and gives up some of its gravitational potential energy to the turbines that produce hydroelectricity. Dams appear, therefore, to be perfect non-emitters of greenhouse gases.
Recent research, however, has shown that many hydroelectric dams are actually large greenhouse-gas emitters. When a dam is built, it causes water to back up and fill an artificial lake. This lake generally drowns a large area of forest. In areas where trees are only killed by shallow water, not submerged, their decay releases CO2. Submerged trees, which must decay under oxygen-free (anaerobic) conditions, release methane. Also, vegetation flourishes in the vast mud flats that are exposed during low-water or “drawdown” periods; this vegetation is submerged when the water rises again and decays, also releasing methane. Methane release from many dams, especially in tropical and subtropical areas, is thus an ongoing process, not a one-time pulse from drowned trees after construction.
A study published in 2004 found that in 1990 the Curuá-Una Dam in Brazil, 13 years after its lake filled with water, was producing 3.6 times as much global greenhouse warming, because of its methane emissions, as if the same amount of electricity were being produced by burning oil. Some researchers have suggested that at least some of this methane could be collected and burned to produce electricity, both extracting useful energy and converting the methane to CO2. Burning methane reduces its greenhouse potential by about 95%, since methane is about 21 times as powerful a greenhouse gas as CO2. However, dam-methane recovery technology had not yet been demonstrated as of 2007.
If emissions of greenhouse gases are to be stabilized or reduced, energy must be either produced with fewer emissions, used more efficiently and sparingly, or both. Lower emissions are claimed for natural gas, nuclear power, renewable energy sources, and advanced coal technologies that sequester CO2 underground rather than spewing it into the air.
All of these technologies have their supporters and critics. For example, although it is a fact that natural gas releases less carbon at the point of combustion for each unit of energy produced, a Carnegie Mellon University team claimed in 2007 that importing liquefied natural gas (LNG) to the United States from other countries could produce 35% more greenhouse gas emissions than domestic coal burned in projected (but not yet built) facilities that use carbon sequestration. The reason is that imported LNG is a complex and roundabout technology, which requires that gas be first extracted, chilled to -264°F (-163°C) to make it a liquid, shipped in large tankers over thousands of miles, heated again to a gaseous state, and finally distributed through pipelines.
The coal technologies to which LNG was compared by the Carnegie Mellon analysts, however, were still speculative: no large-scale coal plant with carbon sequestration had yet been built in 2007. In October 2007, the U.S. Department of Energy announced permits for three regional-scale sequestration demonstration projects. It remains to be seen whether the technology will prove both effective and affordable.
In 2007, the United Nations released an analysis of the global potential for energy efficiency as a way of abating global climate change. The report said that to reduce rising global greenhouse-gas emissions to 2007 levels by 2030 would require only 0.3–0.5% of the projected 2030 global gross domestic product. Greater savings could be realized at higher cost. In general, it is cheaper to save a unit of energy than to generate it by any means. The UN Climate Change Secretariat, Yvo de Boer, stated: “Energy efficiency is the most promising means to reduce greenhouse gases in the short term.”
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