Other greenhouse gases Methane

Methane is the main component of natural gas. Its common name used to be marsh gas because it can be seen bubbling up from marshy areas where organic material is decomposing. Data from ice cores show that for at least 2000 years before 1800 its concentration in the atmosphere was about 700 ppb. Since then its concentration has more than doubled (Figure 3.6a) to a value that the ice core record shows is unprecedented over at least the last 650 000 years. During the 1980s it was increasing at about 10 ppb per year but during the 1990s the average rate of increase fell to around 5 ppb per year11 and close to zero from 1999 to 2005. Although the concentration of methane in the atmosphere is much less than that of carbon dioxide (only 1.775 ppm in 2005 compared with about 380 ppm for carbon dioxide), its greenhouse effect is far from negligible. That is because the enhanced greenhouse effect caused by a molecule of methane is about eight times that of a molecule of carbon dioxide.12

The main natural source of methane is from wetlands. A variety of other sources result directly or indirectly from human activities, for instance from leakage from natural gas pipelines and from oil wells, from generation in rice paddy fields, from enteric fermentation (belching) from cattle and other livestock, from the decay of rubbish in landfill sites and from wood and peat burning. Details of estimates of the sizes of these sources during the 1990s are shown in Table 3.2. Attached to many of the numbers is a wide range of uncertainty. It is, for instance, difficult to estimate the amount produced in paddy fields averaged on a worldwide basis. The amount varies enormously during the rice growing season and also very widely from region to region. Similar problems arise when trying to estimate the amount produced by animals. Measurements of the proportions of the different isotopes of carbon (see box on page 44) in atmospheric methane assist considerably in helping to tie down the proportion that comes from fossil fuel sources, such as leakage from mines and from natural gas pipelines.

The main process for the removal of methane from the atmosphere is through chemical destruction. It reacts with hydroxyl (OH) radicals, which are present in the atmosphere because of processes involving sunlight, oxygen, ozone and water vapour. The average lifetime of methane in the atmosphere is determined by the rate of this loss process. At about 12 years13 it is much shorter than the lifetime of carbon dioxide.

Although most methane sources cannot be identified very precisely, the largest sources apart from natural wetlands are closely associated with human activities.

Rice paddy fields have an adverse environmental impact because of the large quantities of methane gas they generate. World methane production due to paddy fields has been estimated to be in the range of 30 to 90 million tonnes per year.

It is interesting to note that the increase of atmospheric methane (Figure 3.6a) follows very closely the growth of human population since the Industrial Revolution. However, even without the introduction of deliberate measures to control human-related sources of methane because of the impact on climate change, it is not likely that this simple relationship with human population will continue. The IPCC Special Report on Emission Scenarios (SRES) presented in Chapter 6 include a wide range of estimates of the growth of human-related methane emissions during the twenty-first century - from approximately doubling over the century to reductions of about 25%. In Chapter 10 (page 305) ways are suggested in which

500-

500-

Figure 3.6 Change in (a) methane and (b) nitrous oxide concentration (mole fraction in ppb) over the last 10 000 years (insets from 1750) determined from ice cores (symbols with different colours from different studies) and atmospheric samples (red lines). Radiative forcing since the pre-industrial era due to the increases is plotted on the right-hand axes.

5000

Time (before 2005)

10 000

5000

Time (before 2005)

Figure 3.6 Change in (a) methane and (b) nitrous oxide concentration (mole fraction in ppb) over the last 10 000 years (insets from 1750) determined from ice cores (symbols with different colours from different studies) and atmospheric samples (red lines). Radiative forcing since the pre-industrial era due to the increases is plotted on the right-hand axes.

10 000

Table 3.2 Estimated sources and sinks of methane in millions of tonnes per year. The first column of data shows the best estimate from each source; the second column illustrates the uncertainty in the estimates by giving a range of values

Best estimate

Uncertainty

Sources

Natural

Wetlands

150

(90-240)

Termites

20

(10-50)

Ocean

15

(5-50)

Other (including hydrates)

15

(10-40)

Human-generated

Coal mining, natural gas, petroleum industry

100

(75-110)

Rice paddies

60

(30-90)

Enteric fermentation

90

(70-115)

Waste treatment

25

(15-70)

Landfills

40

(30-70)

Biomass burning

40

(20-60)

Sinks

Atmospheric removal

545

(450-550)

Removal by soils

30

(15-45)

Atmospheric increase

22

(35-40)

For some more recent estimates see Table 7.7 in Denman, K. L., Brasseur, G. et al., Chapter 7, in Solomon et al. (eds.) Climate Change 2007: The Physical Science Basis. The figure for atmospheric increase is an average for the 1990s; note that from 1999 to 2005 the increase was close to zero.

methane emissions could be reduced and methane concentrations in the atmosphere stabilised. Also see box on page 48-9 for possible destabilisation of methane emissions from methane hydrates especially at high latitudes.

Nitrous oxide

Nitrous oxide, used as a common anaesthetic and known as laughing gas, is another minor greenhouse gas. Its concentration in the atmosphere of about 0.3 ppm is rising at about 0.25% per year and is about 16% greater than in pre-industrial times (Figure 3.6b). The largest emissions to the atmosphere are associated with natural and agricultural ecosystems; those linked with human activities are probably due to increasing fertiliser use. Biomass burning and the chemical industry (for example, nylon production) also play some part. The sink of nitrous oxide is photodissociation in the stratosphere and reaction with electronically excited oxygen atoms, leading to an atmospheric lifetime of about 120 years.

Chlorofluorocarbons (CFCs) and ozone

The CFCs are man-made chemicals which, because they vaporise just below room temperature and because they are non-toxic and non-flammable, appear to be ideal for use in refrigerators, the manufacture of insulation and aerosol spray cans. Since they are so chemically unreactive, once they are released into the atmosphere they remain for a long time - 100 or 200 years - before being destroyed. As their use increased rapidly through the 1980s their concentration in the atmosphere has been building up so that they are now present (adding together all the different CFCs) in about l ppb (part per thousand million - or billion - by volume). This may not sound very much, but it is quite enough to cause two serious environmental problems.

The first problem is that they destroy ozone.14 Ozone (O3), a molecule consisting of three atoms of oxygen, is an extremely reactive gas present in small quantities in the stratosphere (a region of the atmosphere between about 10 km and 50 km in altitude). Ozone molecules are formed through the action of ultraviolet radiation from the Sun on molecules of oxygen. They are in turn destroyed by a natural process as they absorb solar ultraviolet radiation at slightly longer wavelengths - radiation that would otherwise be harmful to us and to other forms of life at the Earth's surface. The amount of ozone in the stratosphere is determined by the balance between these two processes, one forming ozone and one destroying it. What happens when CFC molecules move into the stratosphere is that some of the chlorine atoms they contain are stripped off, also by the action of ultraviolet sunlight. These chlorine atoms readily react with ozone, reducing it back to oxygen and adding to the rate of destruction of ozone. This occurs in a catalytic cycle - one chlorine atom can destroy many molecules of ozone.

The problem of ozone destruction was brought to world attention in 1985 when Joe Farman, Brian Gardiner and Jonathan Shanklin at the British Antarctic Survey discovered a region of the atmosphere over Antarctica where, during the southern spring, about half the ozone overhead disappeared. The existence of the 'ozone hole' was a great surprise to the scientists; it set off an intensive investigation into its causes. The chemistry and dynamics of its formation

Ozone depletion can be seen by comparing ozone levels in September 1980 and September 2008. The dark blue and purple areas denote where the ozone layer is thinnest.

turned out to be complex. They have now been unravelled, at least as far as their main features are concerned, leaving no doubt that chlorine atoms introduced into the atmosphere by human activities are largely responsible. Not only is there depletion of ozone in the spring over Antarctica (and to a lesser extent over the Arctic) but also substantial reduction, of the order of 5%, of the total column of ozone - the amount above one square metre at a given point on the Earth's surface - at mid latitudes in both hemispheres.

Because of these serious consequences of the use of CFCs, international action has been taken. Many governments have signed the Montreal Protocol set up in 1987 which, together with the Amendments agreed in London in 1991 and in Copenhagen in 1992, required that manufacture of CFCs be phased out completely by the year 1996 in industrialised countries and by 2006 in developing countries. Because of this action the concentration of CFCs in the atmosphere is no longer increasing. However, since they possess a long life in the atmosphere, little decrease will be seen for some time and substantial quantities will be present well over 100 years from now.

So much for the problem of ozone destruction. The other problem with CFCs and ozone, the one which concerns us here, is that they are both greenhouse gases.15 They possess absorption bands in the region known as the longwave atmospheric window (see Figure 2.5) where few other gases absorb. Because, as we have seen, the CFCs destroy some ozone, the greenhouse effect of the CFCs is partially compensated by the reduced greenhouse effect of atmospheric ozone.

First considering the CFCs on their own, a CFC molecule added to the atmosphere has a greenhouse effect 5000 to 10 000 times greater than an added molecule of carbon dioxide. Thus, despite their very small concentration compared, for instance, with carbon dioxide, they have a significant greenhouse effect. It is estimated that radiative forcing due to CFCs is about 0.3 W m-2 - or about 12% of the radiative forcing due to all greenhouse gases. This forcing will only decrease slowly in the twenty-first century.

Turning now to ozone, the effect from ozone depletion is complex because the amount by which ozone greenhouse warming is reduced depends critically on the height in the atmosphere at which it is being destroyed. Further, ozone depletion is concentrated at high latitudes while the greenhouse effect of the CFCs is uniformly spread over the globe. In tropical regions there is virtually no ozone depletion so no change in the ozone greenhouse effect. At mid latitudes, very approximately, the greenhouse effects of ozone reduction and of the CFCs compensate for each other. In polar regions the reduction in the greenhouse effect of ozone more than compensates for the greenhouse warming effect of the CFCs.16

As CFCs are phased out, they are being replaced to some degree by other halocarbons - hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). In Copenhagen in 1992, the international community decided that HCFCs would also be phased out by the year 2030. While being less destructive to ozone than the CFCs, they are still greenhouse gases. The HFCs contain no chlorine or bromine, so they do not destroy ozone and are not covered by the Montreal Protocol. Because of their shorter lifetime, typically tens rather than hundreds of years, the concentration in the atmosphere of both the HCFCs and the HFCs, and therefore their contribution to global warming for a given rate of emission, will be less than for the CFCs. However, since their rate of manufacture could increase substantially their potential contribution to greenhouse warming is being included alongside other greenhouse gases (see Chapter 10, page 296).

Concern has also extended to some other related compounds which are greenhouse gases, the perfluorocarbons (e.g. CF4, C2F6) and sulphur hexafluoride (SF6), which are produced in some industrial processes. Because they possess very long atmospheric lifetimes, probably more than 1000 years, all emissions of these gases accumulate in the atmosphere and will continue to influence climate for thousands of years. They are also therefore being included as potentially important greenhouse gases.

Ozone is also present in the lower atmosphere or troposphere, where some of it is transferred downwards from the stratosphere and where some is generated by chemical action, particularly as a result of the action of sunlight on the oxides of nitrogen. It is especially noticeable in polluted atmospheres near the surface; if present in high enough concentration, it can become a health hazard. In the northern hemisphere the limited observations available together with model simulations of the chemical reactions leading to ozone formation suggest that ozone concentrations in the troposphere have doubled since pre-industrial times - an increase which is estimated to have led to a global average radiative forcing of about 0.35 W m-2 (Figure 3.11). Ozone is also generated at levels in the upper troposphere as a result of the nitrogen oxides emitted from aircraft exhausts; nitrogen oxides emitted from aircraft are more effective at producing ozone in the upper troposphere than are equivalent emissions at the surface. The radiative forcing in northern mid latitudes from aircraft due to this additional ozone17 is of similar magnitude to that from the carbon dioxide emitted from the combustion of aviation fuel which currently is about 3% of current global fossil fuel consumption.

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