Green houses
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113CHAPTER 7. THE GREENHOUSE EFFECT
We examine in this chapter the role played by atmospheric gases in controlling the temperature of the Earth. The main source of heat to the Earth is solar energy, which is transmitted from the Sun to the Earth by radiation and is converted to heat at the Earth’s surface. To balance this input of solar radiation, the Earth itself emits radiation to space. Some of this terrestrial radiation is trapped by greenhouse gases and radiated back to the Earth, resulting in the warming of the surface known as the greenhouse effect. As we will see, trapping of terrestrial radiation by naturally occurring greenhouse gases is essential for maintaining the Earth’s surface temperature above the freezing point. There is presently much concern that anthropogenic increases in greenhouse gases could be inducing rapid surface warming of the Earth. The naturally occurring greenhouse gases CO2, CH4, and N2O show large increases over the past century due to human activity (Figure 7-1). The increase of CO2 was discussed in chapter 6, and the increases of CH4 and N2O will be discussed in chapters 11 and 10 respectively. Additional greenhouse gases produced by the chemical industry, such as CFC-11, have also accumulated in the atmosphere over the past decades and added to the greenhouse effect (Figure 7-1).
CO2 CONCENTRATION (ppmv) CH4 CONCENTRATION (ppbv) 360 CARBON DIOXIDE 340 320 300 280 260 1750 1800 METHANE 1600 1400 1200 1000 800 600 1750
1800
1850 1900 YEAR
1950
2000
1800
1850 1900 YEAR
1950
2000
CFC11 CONCENTRATION (ppbv)
N2O CONCENTRATION (ppbv)
310 NITROUS OXIDE
0.3 CFC11
300
0.2
290
0.1
280 1750
1800
1850 1900 YEAR
1950
2000
0.0 1750
1800
1850 1900 YEAR
1950
2000
Figure 7-1 Rise in the concentrations of greenhouse gases since the 18th century
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As we will see in section 7.3, simple theory shows that a rise in greenhouse gases should result in surface