Figure 7.5. Schematic of temperature profiles before and after the addition of greenhouse gases. The total outgoing longwave radiation must remain the same because this radiation balances the incoming solar radiation, and so the emissions temperature, Te, stays the same. However, the emissions height must increase (from Zl to Z2) because of the increased absorptivity of the atmosphere. Hence, if the temperature gradient in the vertical remains similar, the surface temperature must increase.

levels can have an important effect even if levels of water vapor are already quite high. In this case, it might be thought that since the lower atmosphere is already quite opaque to longwave radiation, the addition of C02 would have little effect. However, even in a warm, wet climate the upper atmosphere is quite dry, so that the addition of CO2 still adds to the emissivity of the upper atmosphere and so raises the emissions height. Because the temperature decreases with height, the overall temperature increases, as in figure 7.5. The question, of course, is how much? That is to say, is the measured increase in greenhouse gases actually responsible for the observed increase in temperature of just under 1°C over the past century?

A partial answer to this question is that the increase that has been observed is entirely and quantitatively consistent with the increase in greenhouse gases. Radiative calculations indicate that the increased downward infrared radiation, DR, varies logarithmically with CO2 concentration, C, according to DR c 5.3 ln(C/C0), where C0 is the preindustrial CO2 level, excluding water vapor and other feedbacks (Myhre et al. 1998). Thus, the radiative forcing increases about 3.7 W/m2 for each doubling of CO2, and this rate is sufficient to cause the temperature increase observed. More complete calculations using comprehensive general circulation models of the climate show good quantitative agreement with the observed temperature rise when proper account is taken of greenhouse gases and other anthropogenic influences on climate (such as the fact that we are also putting particu-late matter, or aerosols, into the atmosphere), but poor agreement otherwise. A fair question to ask is, if greenhouse gases have been increasing monotonically over the past century, why did the temperature not rise over the middle of the century, from about 1940 to 1975? Most likely, the flat temperature record in midcentury arose because of an increased aerosol concentration in the atmosphere from volcanoes and pollutants, increasing Earth's albedo, possibly in conjunction with natural variability. Regarding the most recent warming, the decade 2000-2009 was 0.2°C warmer than the previous one and about 0.4°C above the 1961-90 average. It was the warmest decade since 1880, and most likely the warmest of the past millennium.7 The year 1998 was warmed to the tune of about 0.5°C by El Niño, and there have been no comparable El Niño years since, but 2005 and 2010 were nevertheless virtually as warm.

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