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15-30 400-600

Source: Our Future World, Natural Environment Research Council (NERC) (1989).

Figure 2.7 Methane concentration (parts per million by volume) in air bubbles trapped in ice dating back to 1000 years bp obtained from ice cores in Greenland and Antarctica and the global average for ad 2000 (X).

Source: Data from Rasmussen and Khalil, Craig and Chou, and Robbins; adapted from Bolin et al. (eds) The Greenhouse Effect, Climatic Change, and Ecosystems (SCOPE 29). Copyright ©1986. Reprinted by permission of John Wiley & Sons, Inc.

Figure 2.7 Methane concentration (parts per million by volume) in air bubbles trapped in ice dating back to 1000 years bp obtained from ice cores in Greenland and Antarctica and the global average for ad 2000 (X).

Source: Data from Rasmussen and Khalil, Craig and Chou, and Robbins; adapted from Bolin et al. (eds) The Greenhouse Effect, Climatic Change, and Ecosystems (SCOPE 29). Copyright ©1986. Reprinted by permission of John Wiley & Sons, Inc.

inates primarily from microbial activity (nitrification) in soils and in the oceans (4 to 8 X 109 kg N/year), with about 1.0 X 109 kg N/year from industrial processes. Other major anthropogenic sources are nitrogen fertilizers and biomass burning. The concentration of N2O has increased from a pre-industrial level of about 285 ppbv to 316 ppbv (in clean air). Its increase began around 1940 and is now about 0.8 ppbv per year (Figure

2.8A). The major sink of N2O is in the stratosphere, where it is oxidized into NO .

Chlorofluorocarbons (CF2Cl2 and CFCl3), better known as 'freons' CFC-11 and CFC-12, respectively, were first produced in the 1930s and now have a total atmospheric burden of 1010 kg. They increased at 4 to 5 per cent per year up to 1990, but CFC-11 is declining slowly and CFC-12 is nearly static as a result of the

Montreal Protocol agreements to curtail production and use substitutes (see Figure 2.8B). Although their concentration is <1 ppbv, CFCs account for nearly 10 per cent of the greenhouse effect. They have a residence time of 55 to 130 years in the atmosphere. However, while the replacement of CFCs by hydrohalocarbons (HCFCs) can reduce significantly the depletion of stratospheric ozone, HCFCs still have a large greenhouse potential.

Ozone (O3) is distributed very unevenly with height and latitude (see Figure 2.3) as a result of the complex photochemistry involved in its production (A.2, this chapter). Since the late 1970s, dramatic declines in springtime total ozone have been detected over high southern latitudes. The normal increase in stratospheric ozone associated with increasing solar radiation in spring apparently failed to develop. Observations in Antarctica show a decrease in total ozone in September to October from 320 Dobson units (DU) (10-3 cm at standard atmospheric temperature and pressure) in the 1960s to around 100 in the 1990s. Satellite measurements of stratospheric ozone (Figure 2.9) illustrate the presence of an 'ozone hole' over the south polar region (see Box 2.2). Similar reductions are also evident in the Arctic and at lower latitudes. Between 1979 and 1986, there was a 30 per cent decrease in ozone at 30 to 40-km altitude between latitudes 20 and 50°N and S (Figure 2.10); along with this there has been an increase in ozone in the lowest 10 km as a result of anthropogenic activities. Tropospheric ozone represents about 34 DU compared with 25 pre-industrially. These changes in the vertical distribution of ozone concentration are likely to lead to changes in atmospheric heating (Chapter 2C), with implications for future climate trends (see Chapter 13). The global mean column total decreased from 306 DU for 1964 to 1980 to 297 for 1984 to 1993 (see Figure 2.3). The decline over the past twenty-five years has exceeded 7 per cent in middle and high latitudes.

The effects of reduced stratospheric ozone are particularly important for their potential biological damage to living cells and human skin. It is estimated that a 1 per cent reduction in total ozone will increase ultraviolet-B radiation by 2 per cent, for example, and ultraviolet radiation at 0.30 |m is a thousand times more damaging to the skin than at 0.33 |m (see Chapter 3A). The ozone decrease would also be greater in higher latitudes. However, the mean latitudinal and altitudinal gradients of radiation imply that the effects of a 2 per cent UV-B

Figure 2.8 Concentration of: (A) nitrous oxide, N2O (left scale), which has increased since the mid-eighteenth century and especially since 1950; and of (B) CFC-11 since 1950 (right scale). Both in parts per billion by volume (ppbv).

Source: After Houghton et al. (1990 and 2001).

400 350

U 150

100 50

Source: After Houghton et al. (1990 and 2001).

400 350

U 150

100 50

Jul Aug Sep Oct Nov Dec

Figure 2.9 Total ozone measurements from ozonesondes over South Pole for 1967 to 1971, 1989, and 2001, showing deepening of the Antarctic ozone hole.

Source: Based on Climate Monitoring and Diagnostics Laboratory, NOAA.

Jul Aug Sep Oct Nov Dec

Figure 2.9 Total ozone measurements from ozonesondes over South Pole for 1967 to 1971, 1989, and 2001, showing deepening of the Antarctic ozone hole.

Source: Based on Climate Monitoring and Diagnostics Laboratory, NOAA.

increase in mid-latitudes could be offset by moving poleward 60 km or 100 m lower in altitude! Recent polar observations suggest dramatic changes. Stratospheric ozone totals in the 1990s over Palmer Station, Antarctica (65°S), now maintain low levels from September until early December, instead of recovering in November. Hence, the altitude of the sun has been higher and the incoming radiation much greater than in previous years, especially at wavelengths <0.30 |m. However, the possible effects of increased UV radiation on biota remain to be determined.

Aerosol loading may change due to natural and human-induced processes. Atmospheric particle con-

MAM SON

Figure 2.10 Changes in stratospheric ozone content (per cent per decade) during March to May and September to November 1978 to 1997 over Europe (composite of Belsk, Poland, Arosa, Switzerland and Observatoire de Haute Provence, France) based on umkehr measurements.

Source: Adapted from Bojkov et al. (2002), Meteorology and Atmospheric Physics, 79, p. 148, Fig. 14a.

% change per decade

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