Appendix A Global Warming

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The surface temperature of earth is determined by the rate at which solar radiation reaches the planet, the surface albedo, and the rate of emission of infrared radiation by the earth into the atmosphere. The earth's atmosphere contains low levels of the so called "greenhouse" gases such as carbon dioxide and water vapor, that allow the incoming solar radiation to reach the earth unhindered but trap some of the heat radiated back from earth into the atmosphere49. In this regard, the greenhouse gases act similar to glass in a greenhouse, trapping the long wavelength infrared radiation from escaping the earth. Figure A.1 indicates the fate of solar radiation entering the earth's atmosphere. For the most part this "greenhouse effect" is a natural process indispensable to life on earth; if not for the biogenic greenhouse gases in our atmosphere the temperature of earth will be inhospitable (colder by about 33°C). However, increasing concentrations of carbon dioxide and other greenhouse gases in the atmosphere can cause a corresponding warming of the surface temperatures on earth. The concentration of greenhouse gases, particularly CO2 in the atmosphere has been steadily rising in the recent years mainly because of human activity. The consequent slow warming of earth measured over the recent decades (see Figure A.2) has been reliably attributed primarily to the accumulation of man-made greenhouse gases in the earth's atmosphere.

Outer space

8% Losses from

Outer space

8% Losses from

Shortwave radiation Longwave radiation

Figure A.1. The fate of solar radiation entering the earth's atmosphere.

Shortwave radiation Longwave radiation

Figure A.1. The fate of solar radiation entering the earth's atmosphere.

49 This is essentially the result of the large difference in the temperature of the sun (around 6000 K) and that of earth (about 286 K). Because of this large difference in temperature, the frequency of the infrared radiation emitted by the earth is much lower than that of incoming solar radiation, and is effectively absorbed by these greenhouse gases.

Year

Figure A.2. Increasing trend in the mean global temperature (compared to that in 1890) and atmospheric CO2 concentration (heavy line).

Year

Figure A.2. Increasing trend in the mean global temperature (compared to that in 1890) and atmospheric CO2 concentration (heavy line).

It is the burning of fossil fuels that is mostly responsible for the increased carbon dioxide levels in the present-day atmosphere to nearly 30% above that in pre-industrial times (~1850). Today, the atmospheric CO2 level of about 360 ppm is higher than that encountered by the earth at any time during at least the past 160,000 years [1]. Transportation and industrial uses account for most of this fossil fuel use, but CO2 emissions also originate from cement or lime facilities, incineration plants and the flaring of refinery gases. Deforestation is a second significant source of CO2 emissions [2]. The tropical forests were depleted at a rate as high as 12.6 million hectares per year worldwide during 1990-1995 [3]. The second key greenhouse gas is methane, released from cattle breeding operations, rice paddy cultivation, sewage treatment plants and from coal mining. Natural sources include wetland emissions, marine contributions, and termite activity. Levels of methane have more than doubled since the industrial era. A third greenhouse gas, nitrous oxide, N2O, is released from fertilizer used in agriculture as well as from internal combustion engines. Of the total greenhouse gas emissions in the US in 1998, CO2, CH4 and N2O account for 81.4, 9.9 and 6.5 percent respectively. The pivotal role of CO2 as a greenhouse gas is apparent from the long-term correlation between the CO2 level in the atmosphere and the average temperature on earth. Chlorofluorocarbons (CFCs) are particularly potent greenhouse gases and even at low concentrations have a very large impact in terms of global warming. Compared to a value of unity for CO2, the global warming potential (over a period of 100 years), GWP, of CH4 = 23 and that of N2O = 296. The comparable GWP value for CFC-11 and CFC-12 would be 4000 and 8500! The Table A.1 below shows data on the global warming potential of common greenhouse gases and chlorofluorocarbons relative to that of carbon

Table A.1 Direct Global Warning Potentials (GWPs) of Selected Gases Relative to Carbon Dioxide

Global Warming Potential3

(Time Horizon in Years) Gas Lifetime (Years) 20 yrs 100 yrs 500 yrs

Carbon dioxide

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