Greenhouse Effect Changes

preindustrial times, the atmospheric concentration of methane was about 0.8 ppm by volume, increasing to 1.75 ppm in 1999 and still increasing, although the rate of increase appears to have decreased during the past two decades. The recent changes in CO2 concentration in the atmosphere were shown in Figures 2-2 and 2-3. At any rate, there is definitely a positive correlation between the concentration of these gases and climate.

Because of the known correlation between the concentrations of carbon dioxide and methane and climate, even though the actual connection is quite complicated, the concept of relative global warming potentials (GWPs) has been developed for all the so-called greenhouse gases, those that absorb infrared radiation in the "atmospheric windows." This concept takes into account (1) the effect of an increase in concentration of the gas in the present

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Age (thousand years before 1990)

FIGURE 3-13 Analysis of air trapped in Antarctic ice cores shows that methane and carbon dioxide concentrations were closely correlated with the local temperature over the last 160,000 years. The concentration of carbon dioxide is indicated on the left ordinate. Redrawn from J. Jager and H. L. Ferguson, eds., Climate Change: Science, Impacts and Policy. Cambridge University Press, New York. Copyright © 1991. Used by permission of the Intergovernmental Panel on Climate Change.

0 40 80 120 160

Age (thousand years before 1990)

FIGURE 3-13 Analysis of air trapped in Antarctic ice cores shows that methane and carbon dioxide concentrations were closely correlated with the local temperature over the last 160,000 years. The concentration of carbon dioxide is indicated on the left ordinate. Redrawn from J. Jager and H. L. Ferguson, eds., Climate Change: Science, Impacts and Policy. Cambridge University Press, New York. Copyright © 1991. Used by permission of the Intergovernmental Panel on Climate Change.

atmosphere both in absolute terms and relative to carbon dioxide, (2) the effect of the different times that the various gases remain in the atmosphere, and (3) some of the reactions that these gases undergo in the atmosphere (see Chapter 5) over long periods. The results of factor 3 are called "indirect effects'' and are not always included in the calculations of the GWP. At any rate, the GWP is thus the time-integrated warming effect expected from the instantaneous release of 1 kg of a given gas in today's atmosphere, relative to that expected from an equal mass of carbon dioxide. Table 3-2 shows the total GWPs, based on both direct and indirect effects, calculated over 20,100, and 500 years for some gases with respect to the earth's atmospheric composition in 1990 along with the atmospheric lifetimes of these gases as used in the calculations. Carbon dioxide has the lowest GWP in the table, but its contribution to the greenhouse effect, and thus potential global warming, is very large because this contribution depends on the product of the GWP and the number of kilograms of the gas actually emitted. The CFC's in Table 3-2 are discussed at greater length in Section 5.2.3.2 because of their ability to destroy the ozone in the stratosphere. It is important, by the way, not to consider the greenhouse effect and the GWP in isolation from the other possible effects on climate, because both positive and negative feedback influences are possible, as mentioned in Section 3.3.5.

TABLE 3-2

Atmospheric Lifetimes and Total Global Warming Potentials of Some Greenhouse Gases (Based on the Atmospheric Composition in 1990)

Time horizon (years)

Atmospheric lifetime Gas (years) 20 100 500

Time horizon (years)

Atmospheric lifetime Gas (years) 20 100 500

C02

1

1

1

ch4

10.5

63

21

9

n2o

132

270

290

190

CFC-11a

55

4500

3500

1500

CFC-12a

116

7100

7300

4500

HCFC-22a

16

4100

1500

510

aCFC's are chlorofluorocarbons while HCFC's are hydrochlorofluorocarbons. The composition of each one can be determined by using the Rule of 90 as follows: add 90 to the number in the name of the gas. A number larger than 100 will be obtained, for example, 101 for CFC-11. The first digit is the number of C atoms, the second is the number of H atoms, the third is the number of F atoms, and the number of Cl atoms is obtained by assuming that the molecule contains no double or triple bonds or rings. Thus, CFC-11 is CFO3, CFC-12 is CF2Q2, and HCFC-22 is CHF2CL

Source: J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds., Climate Change: The IPCC Scientific Assessment. Cambridge University Press, Cambridge, U.K., 1990.

Let us note that not all the carbon dioxide emitted into the atmosphere remains there. For example, it has been calculated that in the period 19581968, enough fossil fuels were burned to increase the carbon dioxide content of the atmosphere by 1.24 ppm per year. The actual increase in C02 content was 0.64ppm/year. This is an indication of the enormous buffering effect of the earth's oceans on the C02 concentration in the atmosphere. This buffering effect was discussed in Chapter 2 and is considered further in Sections 9.2.2 and 11.2.

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