The Greenhouse Effect
Earth's climate is a delicate balance of energy input, chemical and biological processes, and physical phenomena. The Earth's atmosphere plays a critical role in planetary surface temperature. Some gases, such as carbon dioxide (CO2) and methane (CH4), absorb and maintain heat in the same way that glass traps heat in a greenhouse. These greenhouse gases in Earth's atmosphere allow temperatures to build up, keeping the planet warm and habitable to the life forms that have evolved here. This phenomenon is called the greenhouse effect. Figure 3.1 shows how the greenhouse effect causes elevation of surface temperatures on Earth.
The "natural greenhouse effect" creates a climate in which life can exist. It maintains the mean temperature of Earth's surface at approximately 33 degrees warmer than if natural greenhouse gases were not present. Without this process, Earth would be frigid and uninhabitable. However, an "enhanced greenhouse effect," sometimes called the "anthropogenic effect," refers to the increase in Earth's surface temperature due to human activity.
A Revolutionary Idea
Earth's atmosphere was compared to a glass vessel in 1827 by the French mathematician Jean-Baptiste Fourier. In the 1850s British physicist John Tyndall measured the heat-trapping properties of various components of the atmosphere. By the 1890s scientists had concluded that the great increase in combustion in the Industrial Revolution had the potential to change the atmosphere's load of carbon dioxide. In 1896 the Swedish chemist Svante Arrhenius made the revolutionary suggestion that the rapid increase in coal use during the Industrial Revolution could increase carbon dioxide concentrations and cause a gradual rise in temperatures. For almost six decades, his theory stirred little interest.
FIGURE 3.1
The greenhouse effect
Then in 1957 studies at the Scripps Institute of Oceanography in California showed that, in fact, half the carbon dioxide released by industry remained permanently trapped in the atmosphere. Atmospheric concentrations of carbon dioxide were shown to be at their highest level in 160,000 years.
More recent studies have provided evidence that levels of other greenhouse gases are also rising due to human activity:
- Methane (CH4). Its atmospheric concentration is now about 150 times higher than in the pre-industrial era. Some estimates indicate that methane's concentration in the atmosphere could double again during the next 100 years.
- Nitrous oxide (N2O). This gas comes from fertilizers used in agriculture, combustion of fossil fuels and solid waste, and industrial processes. Scientists estimate that there is 16 percent more nitrous oxide in the atmosphere presently than there was in 1750.
Man-made greenhouse gases, particularly fluorinated compounds, also contribute to global warming. These include:
- Chlorofluorocarbons (CFCs). The Montreal Protocol on Substances That Deplete the Ozone Layer phased out, with a few exceptions, the use of these popular aerosols, refrigerants, and solvents.
- Hydrochlorofluorocarbons (HCFCs). Designed as substitutes for the ozone-depleting CFCs, they have about one-fifth the stratospheric ozone depletion potential of CFCs and most of the same uses.
- Hydrofluorocarbons (HFCs). These low-cost and often energy-efficient compounds are used in insulation, air conditioning, refrigeration, fire suppression, and medical metered dose inhalers.
Most man-made gases are present in the atmosphere now at concentrations about 25 percent greater than 150 years ago. The Intergovernmental Panel on Climate Change (IPCC) has documented increases in the levels of a number of greenhouse gases. The panel has conducted studies of ice cores obtained from Antarctica and Greenland—as ice gradually accumulates over millenia, it traps tiny air bubbles that yield information on past atmospheric conditions. Some of the ice cores examined required drilling to depths of thousands of feet.
Earth's Increasing Temperature
As of 2004 experts are almost certain that human-induced climate change is occurring due to increased levels of greenhouse gases. The IPCC, sponsored jointly by the United Nations Environmental Programme (UNEP) and the World Meteorological Organization, was formed to study climate change and to advise policymakers worldwide.
Evidence that global temperatures have been increasing comes from sources as diverse as fossils, corals, ancient ice, and growth rings in trees. The IPCC published a report in 2001 that shows global surface temperatures as measured by thermometers in the last 140 years. A marked rise of approximately one degree has occurred during the 140-year time course. There are extremely striking increases documented in the twentieth century. Data used to document these increases are derived from tree rings, corals, ice cores, and historical temperature records. Climate models suggest that the Earth will warm another two to six degrees between 2000 and 2100. If this happens, it will be the warmest Earth has been for millions of years.
Greenhouse Gases and Emission Trends
Total greenhouse gas emissions from 1991–2001 are shown in Figure 3.2. Emissions increased sharply through the 1990s, but dropped between 2000 and 2001. This drop is attributed to slow economic growth in the U.S. that year, as well as to reduced heating demands due to an unusually warm winter in 2001.
Although carbon dioxide is the primary driver of global warming, several other gases also impact temperatures significantly. Figure 3.3 shows U.S. greenhouse gas emission by type of gas in 2002.
CARBON DIOXIDE.
Carbon dioxide, a naturally occurring component of Earth's atmosphere, is generally considered the major cause of global warming. Carbon is an essential component of all living organisms. The carbon
FIGURE 3.2
Change in U.S. greenhouse gas emissions, 1991–2001
cycle, which shows the movement of carbon from organic to inorganic forms, is illustrated in Figure 3.4. Plants perform the essential function of taking carbon dioxide from the atmosphere and converting it to organic matter, a form that can be used by other living species.
The Energy Information Administration of the U.S. Department of Energy reported in 2002 that carbon dioxide accounted for 82.8 percent of greenhouse gas emissions in the United States (shown in Figure 3.3). The burning of fossil fuels by industry and motor vehicles is by far the leading source of carbon dioxide, accounting for more than 96 percent of carbon dioxide emissions. (See Table 3.1 and Figure 3.5.) As populations and economies expand, they use ever-greater amounts of fossil fuels. Consequently, most carbon dioxide emission comes from the developed world.
Contributions from the developing world are expected to increase as these countries industrialize. (See Figure 3.6 and Figure 3.7.) The United States, despite having only 5 percent of the world's population, accounts for 25 percent of the world's energy use, making it the most carbon-intensive country on Earth. Figure 3.8 and Figure 3.9 document the sources of energy in the United States—the bulk is derived from fossil fuels (petroleum, natural gas, and coal).
METHANE.
Methane is second only to carbon dioxide in its contribution to global warming, contributing 8.9 percent of greenhouse gases in 2002 (shown in Figure 3.3). While there is less methane than carbon dioxide in the atmosphere, scientists estimate that it may be 21 times more effective at trapping heat. Since the 1800s, the amount of methane in the atmosphere has more than doubled. Scientists attribute this rise to human sources,
FIGURE 3.3
U.S greenhouse gas emissions by gas, 2002
including landfills, natural gas systems, agricultural activities, coal mining, and wastewater treatment. Sources of methane in 2001 are shown in Figure 3.10.
NITROUS OXIDE.
Nitrous oxide is a greenhouse gas with natural biological sources as well as human sources. Although nitrous oxide makes up a much smaller portion of greenhouse gases than carbon dioxide (4.9 percent in 2002), it is as much as 310 times more powerful than carbon dioxide at trapping heat.
CHLOROFLUOROCARBONS.
Chlorofluorocarbons (CFCs), an important class of modern industrial chemicals, caused some of the anthropogenic greenhouse effect and global warming experienced during the 1980s. CFCs are also responsible for depletion of the ozone layer in the stratosphere, which has resulted in increased levels of damaging ultraviolet radiation on Earth. The United States is the leading producer of CFCs. Beginning in the 1970s the United States and some other nations banned the use of CFCs in aerosol sprays. In 1987 leaders of many world nations met in Montreal, Canada, and agreed to cut CFC output by 50 percent by the year 2000. In 1989, 82 nations signed the Helsinki Declaration, pledging to completely phase out five CFCs.
Forests and Oceans as Carbon Sinks
Because plants naturally take in carbon dioxide from the atmosphere for photosynthesis, large forests act as sinks, or repositories, for carbon. There has been some debate about whether forests are capable of soaking up the excess carbon dioxide emitted through human activity. Some scientists have also argued that the increasing levels of carbon dioxide in the atmosphere might be better tolerated
FIGURE 3.4
The carbon cycle
if not for the additional complicating factor of global deforestation. (See Figure 3.11.)
Oceans may also have a profound effect on climate change, both because of their tremendous heat storage capability and because they affect levels of atmospheric gases. The ocean is by far the largest reservoir of carbon in the carbon cycle. It holds approximately 50 times more carbon than the atmosphere and 20 times more than the terrestrial reservoir. Ocean currents also transport stored heat, causing heating and cooling in different parts of the world. It is still unclear, however, what effects oceans may have on global warming.
Other Factors Affecting the Global Climate
VOLCANOES.
Volcanic activity, such as the 1991 eruption of Mount Pinatubo in the Philippines, can temporarily offset global warming. Volcanoes spew vast quantities of particles and gas into the atmosphere. Sulfur dioxide, a frequent product of eruptions, combines with water to form tiny super-cooled sulfuric acid droplets. These create a long-lasting global haze that reflects sunlight, reducing the amount of heat absorbed and cooling the planet. (See Figure 3.12.) The effects of the Mount Pinatubo cloud—the largest volcanic cloud of the twentieth century—were felt for years. It not only blocked a significant portion of the impinging sunlight but affected wind and weather patterns. Weather anomalies such as cooler summers and warmer winters, as well as an overall cooling effect, were observed for several years. Similarly, the explosion of the El Chichon volcano in Mexico in 1982 depressed global temperatures for about four years.
CLOUDS.
Clouds also contribute to global climate patterns. Clouds can either reflect sunlight, cooling the Earth, or cause the planet to retain heat. These differing effects depend largely on the brightness and thickness of the clouds in question. Marine stratocumulus clouds, which occur at low altitudes over the ocean, are known to reflect solar energy, resulting in a cooling of the Earth. (See Figure 3.13.) Other clouds, however, such as the cirrus clouds that occur at high altitudes, enhance global warming. A
TABLE 3.1
Trends in U.S. greenhouse gas emissions and sinks, 1990 and 1995–2001
| Gas/source | 1990 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | 2001 |
| CO2 | 5,003.7 | 5,334.4 | 5,514.8 | 5,595.4 | 5,614.2 | 5,680.7 | 5,883.1 | 5,794.8 |
| Fossil fuel combustion | 4,814.8 | 5,141.5 | 5,325.8 | 5,400.0 | 5,420.5 | 5,488.8 | 5,692.2 | 5,614.9 |
| Iron and steel production | 85.4 | 74.4 | 68.3 | 71.9 | 67.4 | 64.4 | 65.8 | 59.1 |
| Cement manufacture | 33.3 | 36.8 | 37.1 | 38.3 | 39.2 | 40.0 | 41.2 | 41.4 |
| Waste combustion | 14.1 | 18.5 | 19.4 | 21.2 | 22.5 | 23.9 | 25.4 | 26.9 |
| Ammonia manufacture & urea application | 19.3 | 20.5 | 20.3 | 20.7 | 21.9 | 20.6 | 19.6 | 16.6 |
| Lime manufacture | 11.2 | 12.8 | 13.5 | 13.7 | 13.9 | 13.5 | 13.3 | 12.9 |
| Natural gas flaring | 5.5 | 8.7 | 8.2 | 7.6 | 6.3 | 6.7 | 5.5 | 5.2 |
| Limestone and dolomite use | 5.5 | 7.0 | 7.6 | 7.1 | 7.3 | 7.7 | 5.8 | 5.3 |
| Aluminum production | 6.3 | 5.3 | 5.6 | 5.6 | 5.8 | 5.9 | 5.4 | 4.1 |
| Soda ash manufacture and consumption | 4.1 | 4.3 | 4.2 | 4.4 | 4.3 | 4.2 | 4.2 | 4.1 |
| Titanium dioxide production | 1.3 | 1.7 | 1.7 | 1.8 | 1.8 | 1.9 | 1.9 | 1.9 |
| Carbon dioxide consumption | 0.9 | 1.1 | 1.1 | 1.2 | 1.2 | 1.2 | 1.2 | 1.3 |
| Ferroalloys | 2.0 | 1.9 | 2.0 | 2.0 | 2.0 | 2.0 | 1.7 | 1.3 |
| Land-use change and forestry (sink)1 | (1,072.8) | (1,064.2) | (1,061.0) | (840.6) | (830.5) | (841.1) | (834.6) | (838.1) |
| International bunker fuels2 | 113.9 | 101.0 | 102.3 | 109.9 | 112.9 | 105.3 | 99.3 | 97.3 |
| CH4 | 644.0 | 650.0 | 636.8 | 629.5 | 622.7 | 615.5 | 613.4 | 605.9 |
| Landfills | 212.1 | 216.1 | 212.1 | 207.5 | 202.4 | 203.7 | 205.8 | 202.9 |
| Natural gas systems | 122.0 | 127.2 | 127.4 | 126.0 | 124.0 | 120.3 | 121.2 | 117.3 |
| Enteric fermentation | 117.9 | 123.0 | 120.5 | 118.3 | 116.7 | 116.6 | 115.7 | 114.8 |
| Coal mining | 87.1 | 73.5 | 68.4 | 68.1 | 67.9 | 63.7 | 60.9 | 60.7 |
| Manure management | 31.3 | 36.2 | 34.9 | 36.6 | 39.0 | 38.9 | 38.2 | 38.9 |
| Wastewater treatment | 24.1 | 26.6 | 26.8 | 27.3 | 27.7 | 28.2 | 28.3 | 28.3 |
| Petroleum systems | 27.5 | 24.2 | 23.9 | 23.6 | 22.9 | 21.6 | 21.2 | 21.2 |
| Rice cultivation | 7.1 | 7.6 | 7.0 | 7.5 | 7.9 | 8.3 | 7.5 | 7.6 |
| Stationary sources | 8.1 | 8.5 | 8.7 | 7.5 | 7.2 | 7.4 | 7.6 | 7.4 |
| Mobile sources | 5.0 | 4.9 | 4.8 | 4.7 | 4.6 | 4.5 | 4.4 | 4.3 |
| Petrochemical production | 1.2 | 1.5 | 1.6 | 1.6 | 1.6 | 1.7 | 1.7 | 1.5 |
| Field burning of agricultural residues | 0.7 | 0.7 | 0.7 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
| Silicon carbide production | + | + | + | + | + | + | + | + |
| International bunker fuels2 | 0.2 | 0.1 | 0.1 | 0.1 | 0.1 | 0.1 | 0.1 | 0.1 |
| N2O | 397.6 | 430.9 | 441.7 | 440.9 | 436.8 | 430.0 | 429.9 | 424.6 |
| Agricultural soil management | 267.5 | 284.1 | 293.2 | 298.2 | 299.2 | 297.0 | 294.6 | 294.3 |
| Mobile sources | 50.6 | 60.9 | 60.7 | 60.3 | 59.7 | 58.8 | 57.5 | 54.8 |
| Manure management | 16.2 | 16.6 | 17.0 | 17.3 | 17.3 | 17.4 | 17.9 | 18.0 |
| Nitric acid | 17.8 | 19.9 | 20.7 | 21.2 | 20.9 | 20.1 | 19.1 | 17.6 |
| Human sewage | 12.7 | 13.9 | 14.1 | 14.4 | 14.6 | 15.1 | 15.1 | 15.3 |
| Stationary combustion | 12.5 | 13.2 | 13.8 | 13.7 | 13.7 | 13.7 | 14.3 | 14.2 |
| Adipic acid | 15.2 | 17.2 | 17.0 | 10.3 | 6.0 | 5.5 | 6.0 | 4.9 |
| N2O product usage | 4.3 | 4.5 | 4.5 | 4.8 | 4.8 | 4.8 | 4.8 | 4.8 |
| Field burning of agricultural residues | 0.4 | 0.4 | 0.4 | 0.4 | 0.5 | 0.4 | 0.5 | 0.5 |
| Waste combustion | 0.3 | 0.3 | 0.3 | 0.3 | 0.2 | 0.2 | 0.2 | 0.2 |
| International bunker fuels2 | 1.0 | 0.9 | 0.9 | 1.0 | 1.0 | 0.9 | 0.9 | 0.9 |
| HFCs, PFCs, and SF6 | 94.4 | 99.5 | 113.6 | 116.8 | 127.6 | 120.3 | 121.0 | 111.0 |
| Substitution of ozone depleting substances | 0.9 | 21.7 | 30.4 | 37.7 | 44.5 | 50.9 | 57.3 | 63.7 |
| HCFC-22 production | 35.0 | 27.0 | 31.1 | 30.0 | 40.2 | 30.4 | 29.8 | 19.8 |
| Electrical transmission and distribution | 32.1 | 27.5 | 27.7 | 25.2 | 20.9 | 16.4 | 15.4 | 15.3 |
| Semiconductor manufacture | 2.9 | 5.9 | 5.4 | 6.5 | 7.3 | 7.7 | 7.4 | 5.5 |
| Aluminum production | 18.1 | 11.8 | 12.5 | 11.0 | 9.0 | 8.9 | 7.9 | 4.1 |
| Magnesium production and processing | 5.4 | 5.6 | 6.5 | 6.3 | 5.8 | 6.0 | 3.2 | 2.5 |
| Total | 6,139.6 | 6,514.9 | 6,707.0 | 6,782.6 | 6,801.3 | 6,849.5 | 7,047.4 | 6,936.2 |
| Net emissions (sources and sinks) | 5,066.8 | 5,450.7 | 5,646.0 | 5,942.0 | 5,970.9 | 6,008.5 | 6,212.7 | 6,098.1 |
| +Does not exceed 0.05 Tg CO2 Eq. | ||||||||
| 1For the most recent years, a portion of the sink estimate is based on historical and projected data. Parentheses indicate negative values (or sequestration). | ||||||||
| 2Emissions from International Bunker Fuels are not included in totals. | ||||||||
| Note: Totals may not sum due to independent rounding. | ||||||||
| SOURCE: "Table ES-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.)," in U.S. Emissions Inventory 2003, U.S. Environmental Protection Agency, Washington, DC, April 2003 [Online] http://yosemite.epa.gov/oar/globalwarming.nsf/content/ResourceCenterPublicationsGHGEmissionsUSEmissionsInventory2003.html [accessed February 11, 2004] | ||||||||
2000 study at NASA's Goddard Institute for Space Studies reported that global warming results in the formation of thinner clouds that are less capable of reflecting sunlight.
SOLAR CYCLES.
Finally, the sun itself is not an entirely steady source of energy. The sun has seasons, storms, and characteristic patterns of activity. Sunspots and flares appear in cycles of roughly eleven years, and may well contribute to climate change on Earth. Future studies of each stage of this cycle are expected to produce a wealth of new data that will increase our understanding of these phenomena.
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