Table 2.4. Sources of nitrous oxide N2O. (Adapted from Houghton et al., 1990.)
Source Nitrous oxide Mt/year %
Natural
Oceans 1.4–3.0 17.2
Tropical soils 2.5–5.7 32.8
Temperate soils 0.5–2.0 11.5
Anthropogenic
Biomass burning 0.2–1.0 5.8
Cultivation 0.03–3.5 20.1
Industry 1.3–1.8 10.3
Cattle and feed 0.2–0.4 2.3
and is also involved in the degradation of ozone. The gas is produced naturally by the denitrification of nitrate by microbial activity in soil and sea. Nitrous oxide is produced in a sequence of reactions leading from nitrate to nitrogen gas and is shown below:
NO3> NO2> NO > N2O > N2 (2.1) The sources of nitrous oxide are given in Table 2.4. The addition of nitrogen-based fertilizer to soils increases the rate of denitrification. Nitrous oxide is mainly lost in the stratosphere by photodegradation:
2N2O+hv(light)= 2N2 + O2 (2.2)
Methane
Methane (CH4) is released from natural sources such as wetlands, termites, rumin- ants, oceans and hydrates (Table 2.5). Although the concentration of methane is
Table 2.5. Sources and sinks for methane. (Adapted from Houghton et al., 1990.)
Source Methane Mt/year
Natural
Wetlands 115
Rice paddies 110
Ruminants 80
Biomass burning 40
Termites 40 Oceans 10 Freshwaters 5 Anthropogenic
Gas drilling, venting 45
Coal mining 40
Hydrate distillation 5
Total 490 Removal
Soil 30 Reaction with hydroxyl in atmosphere ~500
over 200 times lower than carbon dioxide, it is 21 times more effective at adsorbing infrared radiation than carbon dioxide. Methane levels are over twice what they were in pre-industrial times and have been increased by human activities such as rice cultivation, coal mining, waste disposal, biomass burning, landfills and cattle farms (Fig. 2.7). Ruminants can produce up to 40 l of methane per day. Methane is mainly removed from the atmosphere through reaction with hydroxyl radicals where it is a significant source of stratospheric water vapour. The remainder is removed through reactions with the soil and loss into the stratosphere.
Methane hydrates have been proposed as a potential source of energy but the exploitation of these deposits has its problems. It has been estimated that methane hydrates represent 21 × 1015 m3 of methane (11,000 Gt carbon) (Kvenvolden, 1999).
The carbon content of these hydrates is greater than that contained in all the fossil fuels (Fig. 2.8) (Lee and Holder, 2001).
Methane 1750
1500
Methane(ppb)Nitrousoxide(ppb)Sulfate(mg/tice)
1250 1000 750
250 200
100
1000 1200 1400 1600
Year
1800 2000
0 270 290
310 Nitrous oxide
Sulfur (in Greenland ice)
Fig. 2.7. Global increases in methane, nitrous oxide and sulfur. (Redrawn from IPCC, 1996.)
As a result, any controlled release of methane from the hydrate deposits may have a significant effect on global warming. There is increasing evidence that major releases of methane from hydrates have occurred in the past and have been associated with warming events, although insufficient methane may have been released to be responsible for the full rise in temperature (Glasby, 2003). One consequence of global warming may be the dissociation of some of the shallow hydrate deposit, further increasing global warming. Slow release of methane in the sea would result in its oxidation before reaching the surface but large-scale sediment slumping, such as the Storegga slide off Norway displacing 3900 km3 of sediment, may release huge quanti- ties of methane.
Carbon dioxide
The carbon dioxide concentration in the atmosphere is low (368 ppmv; 0.03%) com- pared with oxygen and nitrogen but it is a greenhouse gas and is responsible for 55%
contribution to global warming. There is a continual flow between the atmosphere and organic and inorganic carbon in the soils and oceans (Fig. 2.3). Plants on land and in sea fix carbon dioxide in photosynthesis and this is balanced by carbon dioxide pro- duced by respiration of animal and plants and microbial decomposition of biological materials. Carbon dioxide is also locked up in plant and animal debris in soils and the oceans act as a very large sink where carbonate rocks and reefs also store carbon.
Over many millennia some of the plant and animal debris have been converted by high pressure and temperature into fossil fuels, oil, gas and coal. It is the burning of fossil fuels that is altering the balance of the atmospheric carbon dioxide.
Annual carbon dioxide emissions from the use of coal, gas and oil were above 23Gt in 2000 having risen from 15.7 Gt in 1973 and 0 in pre-industrial times (IEA, 2002). Carbon dioxide emissions depend on energy and carbon content of the fuel, which ranges from 13.6 to 14.0 Mt C/EJ for natural gas, 19.0 to 20.3 for oil and 23.0 to 24.5 for coal (Wuebbles et al., 1999).
The human activities that are responsible for greenhouse gas emissions are given in Fig. 2.9, from which it is clear that the energy sector dominates production.
10,000 5,000
1,400 980
830 566
Hydrate Fossil fuels Soil Water Land biota Others
Fig. 2.8. The carbon content (Gt) of methane hydrates compared with other sources.
(Redrawn from Lee and Holder, 2001.)
Agriculture produces in the main the greenhouse gases methane (CH4) and nitrous oxide (N2O) from cultivation and livestock. When considering carbon dioxide, we see that the energy sector produces 95% carbon dioxide when Annex 1 countries are surveyed, with 4% methane and 1% nitrous oxide. The carbon dioxide emissions of sectors of the energy sector are given in Fig. 2.10. This use of fossil fuels appears to be responsible for the rapid increase in atmospheric carbon dioxide since the 1800s, and the IPCC predict that the carbon dioxide levels will continue to increase to values of 700 ppm by the year 2100 if nothing is done (Fig. 2.11). A number of scenarios have been developed by the IPCC based on various assumptions on the degree of reduction in greenhouse emissions. The carbon dioxide emitted from the electricity sector is the largest followed by that from transport. Although coal only represents 25% of the fossil fuels used for electricity generation, it generates more carbon dioxide as it con- tains a higher carbon content (Fig. 2.12). Figure 2.12 shows the fuels used in the global energy supply and the proportion of carbon dioxide that these produce. It is clear that coal use produces the greater proportion of carbon dioxide.
The carbon dioxide emissions in the 1990s were estimated to be 6.3 ± 0.4 PgC/year (6.3 Gt), which resulted in an increase in atmospheric carbon dioxide of 3.2 ± 0.1 Pg/
year while the remainder was adsorbed by the oceans and land (Glasby, 2006). This equates to 25.2 Gt of carbon dioxide per year. There have been a number of estimates and calculations on the levels of carbon dioxide that would be obtained if various reduction scenarios were implemented (Fig. 2.11) (IPCC, 2007). The present-day global reserves of oil, gas and coal (Tables 1.9 and 1.10) are about 1091–1268 Gt carbon. If these reserves were all used, the final atmospheric carbon dioxide level would be 2000–2200 ppm (IPCC, 2006; Glasby, 2006). One of the targets is to hold atmospheric
2.50%
8%
5.50%
84%
Waste Agric Industry Energy Fig. 2.9. Anthropogenic sources of greenhouse gases. (From Quadrelli and Peterson, 2007.)
40
24 19
7
10
Elec and heat Transport Industry Residential Others
Fig. 2.10. A total of 26.5 Gt carbon dioxide emissions in the world by sectors.
(From Quadrelli and Peterson, 2007.)
carbon dioxide at 450 ppm, which represents an increase of 70 ppm from present values equivalent to 13.2 PgC or 3.8% of fossil fuel reserves. If the target is 750 ppm, an increase of 350 ppm, this would be equal to 66 PgC (19%) (Glasby, 2006).