The term "global warming" refers to an increase in Earth's mean global temperature because a part of Earth's outgoing infrared radiation is retained by several trace gases in the atmosphere whose concentrations have been increasing because of human industrial, commercial, and agricultural activities. These gases have the ability to absorb radiation, leading to the tendency of the atmosphere to create warmer climates than would otherwise be the case. The most important naturally occurring trace gases that have the ability to absorb infrared radiation are water vapor (H 2 O), carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), and ozone (O 3 ). In addition, there are some industrial gases that are extremely effective absorbers of the radiation. Important among these are chlorofluorocarbons (CFCs) , perfluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF 6 ). These gases are analogous to the glass in a greenhouse, which also leads to the net trapping of infrared radiation, hence the terms "greenhouse gases" (GHGs) and " greenhouse effect ." These GHGs act as a partial blanket for the thermal radiation from the surface and make the atmosphere warmer than it would otherwise be. Before human intervention, Earth's radiation balance was in equilibrium , resulting in a mean average temperature of Earth at 15°C (59°F). Without the presence of naturally occurring GHGs, Earth's average surface temperature would have been −18°C (−0.4°F). This difference of 33°C (91.4°F) is due to the natural greenhouse effect, which has made Earth a habitable planet. Although the level of naturally occurring GHGs may change naturally over time, their concentration has steadily increased since the Industrial Revolution that began around 1750, because of fossil fuel combustion , deforestation, biomass burning, drainage of wetlands, conversion of natural into agricultural ecosystems, and plowing/cultivation of soil.
The concentration of CO 2 has increased from about 280 parts per million by volume (ppmv) in the preindustrial era to about 370 ppm in 2000, and is increasing at the rate of about 1.5 ppm/yr (0.4 percent/yr). The atmospheric concentration of CH 4 has increased from 700 parts per billion by volume (ppbv) to 1,720 ppbv, and is increasing at the rate of 10 ppbv/yr (0.6 percent/yr). Similarly, the concentration of N 2 O has increased from 275 ppbv to 312 ppbv and is increasing at the rate of about 0.8 ppbv/yr (0.25 percent/yr) (Bruce, Lee, and Haites 1996). Because of the successful implementation of the Montreal Protocol in 1987, the concentration of industrial gases has been decreasing.
There are three anthropogenic human-derived sources of atmospheric enrichment of CO 2 : (1) fossil fuel combustion; (2) cement manufacturing; and (3) land use change involving deforestation, biomass burning, and cultivation. Fossil fuel combustion annually emits 6.22 Pg C (1 Pg = 1 billion metric tons) as CO 2 , 46 to 155 Tg of C (1 Tg = 1 million metric tons) as CH 4 , and 0.7 to 1.8 Tg N as N 2 O and NO x . The fossil fuel emission has steadily increased over the last 150 years. The CO 2 -C emission from fossil fuel combustion was negligible in 1850, 1,850 Tg/yr in 1900, 1.7 Pg/yr in 1950, and 6.2 Pg/yr in 1995 (Hansen, et al. 1998, pp. 12753–12758). Cement manufacturing emits 0.2 Pg C/yr as CO 2 . Tropical deforestation and soil cultivation annually emit 0.6 to 2.6 Pg C as CO 2 , 160 to 460 Tg of C as CH 4 , and 2.2 to 6.8 Tg N as NO x (Harvey 2000, pp. 16–20). From 1850 to 1998, approximately 270 (±30) Pg C has been emitted as CO 2 by fossil fuel combustion and cement production. During the same time, about 136 (±55) Pg has been emitted as a result of deforestation and land use change, of which 78 (±17 Pg) is due to depletion of the soil organic carbon pool (Watson, et al. 2000, p. 4; Lal 1999, p. 317).
The alteration in Earth's radiation budget because of an increase in atmospheric concentration of GHGs is referred to as "radiative forcing," and is measured in w/m 2 . The radiative forcing of three gases (CO 2 , CH 4 , and N 2 O) since the preindustrial era is 2.45 w/m 2 , due to the accelerated greenhouse effect or global warming. The GHGs differ with regard to their radiative forcing and their life span, or residence time in the atmosphere. This relative ability of GHGs is called the global warming potential or GWP. The GWP is computed relative to CO 2 , and is 1 for CO 2 , 21 for CH 4 , 210 for N 2 O, 1,800 for O 3 , and 4,000 to 12,000 for CFCs. It is estimated that the mean global temperature has increased by about 0.5°C (32.9°F) since the preindustrial era. With business as usual, the radiation budget of Earth may change within a short span of several decades to a century, with an attendant increase in Earth's mean temperature of 1 to 4°C (33.8 to 39.2°F). The projected increase will be less in the tropics than in the boreal, temperate, and cold regions. The greenhouse effect is tolerable (i.e., the biomes or ecological communities comprising plants and animals can adapt) if the rate of increase in Earth's mean temperature is about 0.1°C (32.18°F) per decade.
World soils constitute the third largest global C pool (after oceanic and geologic), and comprise 1,550 Pg of soil organic carbon (SOC) and 750 Pg of soil inorganic carbon (SIC). Thus, the soil C pool is 3.2 times the atmospheric pool (720 Pg), and 4.1 times the biotic pool (560 Pg). The C depleted from the SOC pool can be resequestered through adoption of appropriate land use and soil/crop/vegetation management practices (Lal and Bruce 1999, p. 182; Lal 2001a, pp. 171–172). Restoration of degraded soils and ecosystems and desertification control have a potential to sequester C in soil and the biota and to decrease the rate of enrichment of GHGs in the atmosphere (Lal 2001b, p. 52; 2001c, p. 23).
There are short-term and long-term strategies of mitigating the accelerated greenhouse effect. In the short term, it is important to improve energy use efficiency and to identify strategies of CO 2 sequestration. In the long term, it is important to develop noncarbon fuel sources. Carbon sequestration in soil and vegetation through restoration of degraded soils and the ecosystem and adoption of appropriate land uses is a winning strategy.
SEE ALSO Air Pollution .
Bruce, James P.; Lee, Hoesung; and Haites, Erik F., eds. (1996). Climate Change 1995:Economic and Social Dimensions of Climate Change. Cambridge, U.K.: Cambridge University Press.
Hansen, J. E.; Sato, M.; and Lacis, A.; et al. (1998). "Climate Forcing in the Industrial Era." Proceedings of the National Academy of Sciences 95:12753–12758.
Harvey, Leslie Daryl Danny (2000). Global Warming: The Hard Science. Harlow, U.K.: Longman.
Lal, Rattan (1999). "Soil Management and Restoration for C Sequestration to Mitigate the Accelerated Greenhouse Effect." Progress in Environmental Science 1:307–326.
Lal, Rattan (2001a). "World Cropland Soils as a Source or Sink for Atmospheric Carbon." Advances in Agronomy 71:145–191.
Lal, Rattan (2001b). "Potential of Desertification Control to Sequester Soil Carbon and Mitigate the Greenhouse Effect." Climate Change 15:35–72.
Lal, Rattan (2001c). "The Potential of Soils of the Tropics to Sequester Carbon and Mitigate the Greenhouse Effect." Advances in Agronomy 74:23, 155–192.
Lal, Rattan, and Bruce, J. (1999). "The Potential of World Cropland to Sequester C and Mitigate the Greenhouse Effect." Environment Science & Policy 2:177–185.
Watson, Tina, et al. (2000). Land Use, Land Use Change, and Forestry. Cambridge, U.K.: Cambridge University Press.