Industrial gases and the greenhouse effect

A major element of the Industrial Revolution was the switch from the use of sustainable fuels to the use of coal (and later, oil) as a source of power. Between the middle of the 19th and the middle of the 20th century the burning of fossil fuels, together with extensive deforestation, added about 9 X 1010 tonnes of carbon dioxide (CO2) to the atmosphere and even more has been added since. The concentration of CO2 in the atmosphere before the Industrial Revolution (measured in gas trapped in ice cores) was about 280 ppm, a fairly typical interglacial 'peak' (Figure 2.23), but this had risen to around 370 ppm by around the turn of the millennium and is still rising (see Figure 18.22).

Solar radiation incident on the earth's atmosphere is in part reflected, in part absorbed, and part is transmitted through to the earth's surface, which absorbs and is warmed by it. Some of this absorbed energy is radiated back to the atmosphere where atmospheric gases, mainly water vapor and CO2 absorb about 70% of it. It is this trapped reradiated energy that heats the atmosphere in what is called the 'greenhouse effect'. The greenhouse effect was of course part of the normal environment before the Industrial Revolution and carried responsibility for some of the environmental warmth before industrial activity started to enhance it. At that time, atmospheric water vapor was responsible for the greater portion of the greenhouse effect.

Figure 2.21 An example of long-distance environmental pollution. The distribution in Great Britain of fallout of radioactive caesium (Bq m-2) from the Chernobyl nuclear accident in the Soviet Union in 1986. The map shows the persistence of the pollutant on acid upland soils where it is recycled through soils, plants and animals. Sheep in the upland areas contained more caesium-137 (137Cs) in 1987 and 1988 (after recycling) than in 1986. 137Cs has a half-life of 30 years! On typical lowland soils it is more quickly immobilized and does not persist in the food chains. (After NERC, 1990.)

Figure 2.22 The history of the diatom flora of an Irish lake (Lough Maam, County Donegal) can be traced by taking cores from the sediment at the bottom of the lake. The percentage of various diatom species at different depths reflects the flora present at various times in the past (four species are illustrated). The age of the layers of sediment can be determined by the radioactive decay of lead-210 (and other elements). We know the pH tolerance of the diatom species from their present distribution and this can be used to reconstruct what the pH of the lake has been in the past. Note how the waters have been acidified since about 1900. The diatoms Fragilaria virescens and Brachysira vitrea have declined markedly during this period, while the acid-tolerant Cymbella perpusilla and Frustulia rhomboides have increased. (After Flower et al., 1994.)

Figure 2.22 The history of the diatom flora of an Irish lake (Lough Maam, County Donegal) can be traced by taking cores from the sediment at the bottom of the lake. The percentage of various diatom species at different depths reflects the flora present at various times in the past (four species are illustrated). The age of the layers of sediment can be determined by the radioactive decay of lead-210 (and other elements). We know the pH tolerance of the diatom species from their present distribution and this can be used to reconstruct what the pH of the lake has been in the past. Note how the waters have been acidified since about 1900. The diatoms Fragilaria virescens and Brachysira vitrea have declined markedly during this period, while the acid-tolerant Cymbella perpusilla and Frustulia rhomboides have increased. (After Flower et al., 1994.)

Figure 2.23 Concentrations of CO2 and methane (CH4) in gas trapped in ice cores from Vostok, Antarctica deposited over the past 420,000 years. Estimated temperatures are very strongly correlated with these. Thus, transitions between glacial and warm epochs occurred around 335,000, 245,000, 135,000 and 18,000 years ago. BP, before present; ppb, parts per billion; ppm, parts per million. (After Petit et al., 1999; Stauffer, 2000.)

Age BP (years)

Figure 2.23 Concentrations of CO2 and methane (CH4) in gas trapped in ice cores from Vostok, Antarctica deposited over the past 420,000 years. Estimated temperatures are very strongly correlated with these. Thus, transitions between glacial and warm epochs occurred around 335,000, 245,000, 135,000 and 18,000 years ago. BP, before present; ppb, parts per billion; ppm, parts per million. (After Petit et al., 1999; Stauffer, 2000.)

In addition to the enhancement of greenhouse effects by increased CO2, other trace gases have increased markedly in the atmosphere, particularly methane (CH4) (Figure 2.24a; and compare this with the historical record in Figure 2.23), nitrous oxide (N2O) and the chlorofluorocarbons (CFCs, e.g. trichlorofluoromethane (CCl3F) and dichlorodifluoromethane (CCl2F2)). Together, these and other gases contribute almost as much to enhancing the greenhouse effect as does the rise in CO2 (Figure 2.24b). The increase in CH4 is not all explained but probably has a microbial origin in intensive agriculture on anaerobic soils (especially increased rice production) and in the digestive process of ruminants (a cow produces approximately 40 litres of CH4 each day); around 70% of its production is anthropogenic (Khalil, 1999). The effect of the CFCs from refrigerants, aerosol propellants and so on is potentially great, but international agreements at least appear to have halted further rises in their concentrations (Khalil, 1999).

It should be possible to draw up a balance sheet that shows how the CO2 produced by human activities translates into the changes in concentration in the atmosphere. Human activities only CO2

Figure 2.24 (a) Concentration of methane (CH4) in the atmosphere through the 20th century. (b) Estimates of global warming over the period 1850-1990 caused by CO2 and other major greenhouse gases. (After Khalil, 1999.)

Figure 2.24 (a) Concentration of methane (CH4) in the atmosphere through the 20th century. (b) Estimates of global warming over the period 1850-1990 caused by CO2 and other major greenhouse gases. (After Khalil, 1999.)

release 5.1-7.5 X 109 metric tons of carbon to the atmosphere each year. But the increase in atmospheric CO2 (2.9 X 109 metric tons) accounts for only 60% of this, a percentage that has remained remarkably constant for 40 years (Hansen et al., 1999). The oceans absorb CO2 from the atmosphere, and it is estimated that they may absorb 1.8-2.5 X 109 metric tons of the carbon released by human activities. Recent analyses also indicate that terrestrial vegetation has been 'fertilized' by the increased atmospheric CO2, so that a considerable amount of extra carbon has been locked up in vegetation biomass (Kicklighter et al., 1999). This softening of the blow by the oceans and terrestrial vegetation notwithstanding, however, atmospheric CO2 and the greenhouse effect are increasing. We return to the question of global carbon budgets in Section 18.4.6.

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