The term “greenhouse effect” was coined by the 19th century scientist Jean Fourier. The greenhouse effect occurs when GHGs allow sunlight to reach the Earth's surface but absorb or scatter most of the heat emanating from the Earth's surface, thus retaining heat in the atmosphere. Without the natural greenhouse effect, the average temperature would be about -18^oC, or -0.4^oF [23] [24]. Although the terms “greenhouse effect” and “global warming” are used due to their familiarity, the concept at issue here is actually “infrared absorptivity” of greenhouse gases [25].

Scientists have been concerned with the greenhouse effect regarding not only the quality of life on Earth, but also its effect on other planets. Twenty-six million miles from Earth, in an orbit much closer to the Sun, Venus spins through space with a furnace-like surface temperature of more than 800^o Fahrenheit (F) (426.5^o Celsius [C]), which is much hotter than its proximity to the Sun would explain. Scientists used to believe that Venus fell victim to the greenhouse effect because 96 percent of its atmosphere is carbon dioxide, with nitrogen accounting for almost all the remainder [26]. It is now generally agreed within the planetary atmospheres community that carbon dioxide alone would lead to an average temperature of less than 25^oC. The primary reason that Venus is warmer than this is the presence of sulfuric acid cloud cover over the entire planet, extending from about 50 kilometers to 70 kilometers from the surface.

Unlike Venus, Earth is blessed with an atmosphere rich in nitrogen and oxygen. Dry air near Earth's surface currently contains 78.08 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, 0.0353 percent carbon dioxide, and trace amounts of methane, nitrous oxide, carbon monoxide, ozone, hydrogen, helium, neon, ammonia, hydrogen sulfide, and sulfur dioxide (Table 2) [27] [28]. Water can vary roughly from 0 percent to 4 percent of the atmosphere (by volume) depending on location, altitude, and atmospheric conditions, with the global average being around 3 percent.

Table 2. Composition of the Atmosphere
Constituent	Fraction by Volume in Dry Air	Total Amount
		moles	grams
Nitrogen	78.08%	1.38 x 1020	3.87 x 1021
Oxygen	20.95%	3.7 x 1019	1.19 x 1021
Argon	0.93%	1.65 x 1019	6.59 x 1019
Water Vapor	variable	9.4 x 1017	1.7 x 1018
Carbon Dioxide	353 ppmv	6.27 x 1016	2.76 x 1018
Neon	a18.18 ppmv	3.2 x 1015	6.48 x 1016
Helium	a5.24 ppmv	9.28 x 1014	3.71 x 1015
Methane	a1.72 ppbv	3.1 x 1014	4.9 x 1015
Krypton	a1.14 ppmv	2.0 x 1014	1.69 x 1016
Hydrogen	a500 ppbv	9.0 x 1013	1.8 x 1014
Ozone	avariable	6.88 x 1013	3.3 x 1015
Nitrous Oxide	a310 ppbv	5.2 x 1013	2.3 x 1015
Carbon Monoxide	a120 ppbv	2.1 x 1013	5.9 x 1014
Xenon	a87 ppbv	1.54 x 1013	2.02 x 1015
Ammonia	a100 ppbv	1.76 x 1012	3.0 x 1013
Nitrogen Dioxide	a1 pptv	1.76 x 1011	8.1 x 1012
CFC-12	a480 pptv	8.3 x 1010	1.0 x 1013
CFC-11	a280 pptv	4.95 x 1010	6.8 x 1012
Sulfur Dioxide	a200 pptv	3.59 x 1010	2.3 x 1012
Hydrogen Sulfide	a200 pptv	3.5 x 1010	1.2 x 1012
aValues of trace constituents valid in 1990. CFC-12 = CCl2F2. CFC-11 = CCl3F. ppmv = Parts per million by volume. ppbv = Parts per billion by volume. pptv = Parts per trillion by volume. Note: One mole of a gas is equal to the amount of a substance that contains as many elementary units (6.023 x 1023 molecules or atoms) as there are atoms in 12 grams of carbon-12. Source: D.C. Hartmann, Global Physical Climatology (San Diego: Academic Press, 1994), p. 8.

The Earth's atmosphere has been transformed slowly, as human activity has pumped into it billions of tons of greenhouse gases (GHGs) such as carbon dioxide, water vapor, and large amounts of other gases that absorb the heat energy emitted from Earth's surface, not to mention the addition of anthropogenic heat (i.e., direct heat generated by human activities) from burning of fossil fuels, including transportation fuels, and operation of almost all equipment.

The principal force driving Earth's weather and climate change comes from the Sun. Although the Earth receives only about one two-billionth of the energy emitted by the Sun, it is this energy that heats the Earth, drives ocean currents, and creates weather patterns. The heat output of the Sun has varied by about one-third since life on Earth began, and continues to vary during the solar cycles. Despite this change, global temperatures have remained in a narrow range suitable for life. It appears that some form of climate regulation is in operation.

During the past decade or so, people have become concerned with how human activity may be affecting the world's climate. This concern has focused largely on anthropogenic GHGs--that is, GHGs generated by human activity such as the combustion of fuel for transportation. Anthropogenic [29] GHGs intensify the natural greenhouse effect because they absorb infrared radiation emitted from the Earth's surface, increasing the heat trapped inside the atmosphere (Figure 2). GHGs occur naturally in the atmosphere, and they are essential to life on Earth in its present form. The concern is that human activity may be increasing the concentration of atmospheric GHGs enough to alter the climate worldwide.

D

Most of the radiation reaching the Earth's surface and atmosphere is visible and infrared light. About 70 percent of the radiation reaching the Earth's atmosphere and surface is absorbed. The Earth's atmosphere and surface reflect the remainder. Molecules always emit lower energy than they absorb. Therefore, when the visible light of solar radiation is absorbed, it is emitted as long wavelength infrared heat waves. Part of the total heat emitted as a result of visible and infrared light absorption is absorbed by GHG molecules and clouds and re-emitted in all directions. Some of the re-emitted radiation is absorbed by the surface. The remainder of the total heat escapes through the atmosphere and into space (Figure 2).

The major GHGs are water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), fluorocarbons [30], and ozone (O₃). Of these, carbon dioxide is the most commonly discussed. However, water vapor is the most important GHG due to its abundance (it represents about 3 percent of the gases in the Earth's atmosphere). Carbon dioxide and water vapor are the two major products of all hydrocarbon fuel combustion.

Water vapor is the predominant absorber of incoming solar radiation and a major contributor to the natural greenhouse effect. Scientists at the National Oceanic and Atmospheric Administration have reported that the atmospheric water vapor content in the stratosphere at mid-latitudes in the northern hemisphere has been increasing over the last 14 years [31]. Water vapor in the stratosphere can harm the ozone layer by stimulating the formation of polar clouds, which help pollutants such as oxides of nitrogen and halocarbons destroy ozone. At the tropopause, a rather distinct boundary between the troposphere and the stratosphere located at an altitude fluctuating around 15 kilometers, there is a sharp change in the concentration of water vapor (that is, the variation in concentration within the stratosphere is minimal) (Table 3). Lindzen and others [32] argue that water vapor between 2 kilometers (800 millibars) and 16 kilometers from Earth's surface (the tropopause) is the primary determinant of the greenhouse effect. However, Shine and others [33] argue that water vapor concentration in the lower troposphere is an equally important contributor to the greenhouse effect. Currently, it is believed that the impact of anthropogenic water vapor from the surface sources such as fuel combustion is minimal on the atmospheric water vapor concentrations.

Table 3. Atmospheric Water Vapor Concentrations, 1981-1994
Altitude (kilometers)	Concentration (parts per million)	Average Annual Increase (percent)
10-12	59.2	1.03
12-14	11.88	0.49
14-16	4.66	0.54
16-18	3.87	0.73
18-20	3.85	0.84
20-22	4.07	0.54
22-24	4.21	0.38
24-26	4.29	0.34
Source: S.J. Oltmans and D.J. Hoffman, “Increase in Lower-Stratospheric Water Vapor at Mid-Latitude Northern Hemisphere Site from 1981-1994,” Nature, 374 (1995): 146-149.

Methane (CH₄) is a product of organic decay. The largest natural source of methane is the world's wetlands, although it is also the major constituent of natural gas and a potent GHG. Although methane occurs in the atmosphere in one two-hundredth of the quantity of carbon dioxide, it has 5-10 times the heat-trapping potential per molecule [34] [35]. Methane is increasing in the atmosphere at an annual rate of 1 percent, double the rate of increase for carbon dioxide. Activities that release methane are rice-paddy agriculture, waste treatment, biomass burning, livestock production, and venting during natural gas and coal exploration and production activities. Methane is also released during the transport of natural gas.

Approximately 90 percent of atmospheric methane is chemically destroyed in the troposphere. Chemical destruction of methane through oxidation occurs by hydroxyl radicals. Although the concentration of hydroxyl radicals is very small (0.04 parts per trillion by volume) [36], they are the main oxidants of atmospheric methane, carbon monoxide [37], oxides of nitrogen, and non-methane hydrocarbons. Atmospheric hydroxyl radicals are produced by dissociation of water vapor and reaction between water vapor and other trace gases in the atmosphere. These reactions are controlled by pressure, temperature, atmospheric pH, altitude, and reactants concentration.

Nitrous oxide (N₂O) is also a powerful GHG. It stays in the atmosphere for 150-180 years, eventually floating up into the stratosphere where it helps destroy the ozone layer. Its concentration is increasing by 0.2 percent to 0.3 percent per year. Its main source is the tropics, but roughly 20 percent of nitrous oxide emissions result from manufacturing and using chemical fertilizers and from burning fossil fuels. The increased use of emission control devices like catalytic converters in internal combustion engines contributes further to these emissions. The use of fertilizers in growing corn for ethanol is the major component of the ethanol fuel cycle's high nitrous oxide emission.

Although chlorofluorocarbons (CFCs) are not products of engine combustion, these chemicals are associated with vehicle activity because they result from the production and use of air cooling devices. Automotive CFCs are being phased out of new models. Concentrations of CFCs are rising approximately 0.5 percent per year, and these chemicals typically persist in the atmosphere from 75 to 180 years. CFCs are 20,000 times more potent than carbon dioxide in trapping Earth's thermal radiation. However, some scientists believe that CFCs might have countervailing effects that cause cooling, so on balance, the effect of CFCs is not clear.

Ozone is not produced directly from the combustion of transportation fuels; however, combustion products like NO_x, hydrocarbons, and water vapor play a major role in its formation. Ozone is a beneficial GHG in the stratosphere and a harmful pollutant in the troposphere. Ozone survives anywhere from a few hours to a few days in the upper troposphere and for only an hour in the stratosphere [38] (see Appendix B for a discussion of ozone's stability). Thinning the stratospheric ozone layer increases the amount of harmful UV-C radiation reaching the Earth's surface. This will not only increase UV-induced diseases, but also aid the production of ozone in the troposphere. It is beneficial when ozone stays in the stratosphere because ozone shields the Earth's surface from harmful ultraviolet rays of the Sun. Because of its oxidizing power, ozone is hazardous to health. Therefore, ozone is classified as a criteria pollutant in the troposphere. Throughout the atmosphere, however, ozone acts as a greenhouse gas.

Greenhouse gases have the ability to absorb infrared radiation (radiation with a wavelength of 1 micrometer or more) and also have, in general, longer residence times in the atmosphere than criteria pollutants. Although some compounds (e.g., carbon monoxide, oxides of nitrogen [except nitrous oxide], and nonmethane volatile organic compounds) have properties of greenhouse gases in terms of absorption of infrared radiation, they are also called criteria pollutants because they are hazardous to health.

Proceed to Chapter 3