Green houses

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CHAPTER 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 anthropogenicincreases 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 thechemical 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

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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 surfacewarming; the uncertainty lies in the magnitude of the response. It is well established that the global mean surface temperature of the Earth has increased over the past century by about 0.6 K. The evidence comes from direct temperature observations (Figure 7-2, top panel) and also from observations of sea-level rise and glacier recession. According to current climate models, this observedtemperature rise can be explained by increases in greenhouse gases. The same models predict a further 1-5 K temperature rise over the next century as greenhouse gases continue to increase.
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Figure 7-2 Trend in the surface temperature of the Earth at northern midlatitudes over the past 150,000 years. Each panel from the top down shows the trend over an increasingly longer time span, with the shaded area corresponding to the time span for the panel directly above. The record for the past 300 years is from directtemperature measurements and the longer-term record is from various proxies. From Graedel, T.E., and P.J. Crutzen, Atmospheric Change: an Earth System Perspective, New York: Freeman, 1993.

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Examination of the long-term temperature record in Figure 7-2 may instill some skepticism, however. Direct measurements of temperature in Europe date back about 300 years, and a combination of various proxiescan provide a reliable thermometer extending back 150,000 years. From Figure 7-2 (second panel from top), we see that the warming observed over the past century is actually the continuation of a longer-term trend which began in about 1700 AD, before anthropogenic inputs of greenhouse gases became appreciable. This longer-term trend is thought to be caused by natural fluctuations in solar...
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