Greenhouse Gases
From Thermal-FluidsPedia
The link between carbon dioxide and atmospheric temperatures was presumed as early as the 19th century, but it remained merely speculation until the 1980s when enough data had been collected to show strong correlation between the two. In order to understand the global warming phenomenon, we must understand the greenhouse effect. Greenhouses heat to temperatures exceeding their surroundings because window glass (and some plastics) is transparent to incoming solar radiation but blocks most of the reflected infrared radiation from the plants and surroundings. Certain gases such as water vapor, carbon dioxide, methane, nitrous oxides, and CFCs behave in a similar fashion; they allow solar radiation to pass through to reach and heat up the ground and air above it but absorb and reradiates the reflected terrestrial (IR) radiation, blanketing the earth. Because no air is present outside the atmosphere, heat cannot escape by conduction and cooling can only occur by radiation. Carbon dioxide is the main product of the combustion of fossil fuel. Methane is leaked into the air from coal mines and pipelines, but is mostly produced by cattle and termites, or in wetlands, municipal waste dumps, and rice patties. Nitric oxides come primarily from fertilizers and animal waste. CFCs are non-toxic, inexpensive, and energy-efficient substitutes for highly toxic, potentially explosive ammonia and, until recently, were marketed under the trade name Freon, being widely used in fire extinguishers, refrigeration systems, and aerosol spray cans.
The degree to which greenhouse gases contribute to global warming depends not only on the nature of the gases and their concentrations, but also on the length of time that they remain in the atmosphere. Some gases remain for short periods of time while others, such as CFCs, may remain there for several hundred years (a). Contrary to what some may suspect carbon dioxide is not the most damaging greenhouse gas. Methane traps 20 times more heat per molecule than carbon dioxide, nitric oxide 200 times more, and CFCs could be 10,000 times more damaging than carbon dioxide. Carbon dioxide has been cited as the most important greenhouse gas merely because it is the most abundant; it is responsible for roughly one-half of measurable global warming.
Question: Water vapor is an effective greenhouse gas. Why isn’t it cited as the main cause of global warming?
Answer: Indeed, water vapor is the largest contributor to greenhouse gases in the atmosphere. When we burn fossils, we are producing about twice as much water vapor as carbon dioxide; however, its emission is directly linked to the emission of carbon dioxide. Unfortunately, some politicians and lobbyists cite the role of water vapor as proof that global warming is a natural phenomenon having nothing to do with human activity.
An analysis of air bubbles trapped in Antarctic ice caps provides evidence that, for thousands of years, carbon dioxide concentrations in the atmosphere has been essentially constant at around 280 parts per million (ppm). Only in the last two hundred years, following industrialization and the rapid increase in the rate of consumption of fossil fuels, has man-made carbon dioxide caused these levels to increase (2). The concentration is now roughly 370 ppm and is expected to rise to over 500 ppm by the middle of this century. As Figure 2 shows, the increase in carbon dioxide emission in the last few decades closely correlates with the consumption of fossil fuels. The seasonal variation of the background carbon dioxide concentration can be explained by noting that in spring and summer (in the northern hemisphere), when photosynthesis is more predominant, plants use some atmospheric carbon dioxide as food. It is returned to the atmosphere in fall and winter as plants die.
Until now, most greenhouse gases have been released by industrial countries, mainly the United States, and distributed roughly equally among the transportation, residential and commercial heating and cooling, and industry sectors (Figure 3). As developing countries become more industrialized and their consumptions of fossil fuels increase, it is expected that the percentage of greenhouse gases contributed by these countries will increase, causing global warming to accelerate in the foreseeable future (b).
Question: Estimate the volume of carbon dioxide released into the atmosphere as a result of fossil combustion in the United States. Answer: The latest data indicates that the US consumes about 112 quads4 of energy annually. A simple calculation can be made to convert this to the total mass of fossil fuel consumed. Referring to the energy conversion tables given in Appendix A, we have: 112 quads = 1.12x1017 BTU = 2.81x1010 barrels of oil = 1.18x1012 gallons of oil = 4.47x1012 liters of oil
The United States satisfies about 86% of its energy needs with fossil fuels. Assuming a fuel specific gravity of 0.8, the total mass of fossil fuels consumed will be 4.16x1012x0.86x0.8 = 2.9x1012 kg fossil. Considering that, on average, hydrocarbon fuels produce about three times their mass in carbon dioxide, the total amount of carbon dioxide emissions is 9x1012 kilograms (9 billion tons). This figure accounts for about one quarter of all the carbon dioxide produced in the world.
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References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
(2) Neftel, A., E.,et al., “Evidence From Polar Ice Cores for the Increase in Atmospheric CO2 in the past Two Centuries”. (Nature, 315, 45. 1985)
Additional Comments
(a) For example, CFC-12, common in many refrigeration systems, has a half-life of 100-150 years. CFC-115 can stay in the atmosphere for up to 800 years.
(b) Recent data suggests that for the first time China surpassed the United States in total emission of greenhouse gases. Per capita production remains well below US, however. (International Herald Tribune, June 20, 2007)
Further Reading
Gore, A., An Inconvenient Truth, Penguin Books, 2007.
Roleff, T., Pollution: Opposing viewpoints, Greenhaven Press, 2000.
Walsh, P. J., Dudney, C. S., Copenhave, E. D., Indoor Air Quality, CRC Press, 1984.
Environmental Science and Technology, published by the American Chemical Society.
External Links
Environmental Protection Agency (http://www.epa.gov).
Occupational Safety and Health Administration (OSHA) (http://www.osha.gov).
Intergovernmental Panel on Climate Control (IPCC), (http://www.ipcc..ch).
United Nations Environment Programme (http://www.unep.org).
World Health Organization (WHO) (http://www.who.ch).