Anthropogenic factors of recent climatic change
The growing influence of human activities on the environment is being increasingly recognized and concern over the potential for global warming caused by such anthropogenic effects is growing. Four categories of climatic variable are subject to change and will now be considered in turn. Changes in atmospheric composition associated with the explosive growth of world population, industry and technology have led to drastic increases in the concentration of greenhouse gases. The tendency of these increases is to increase radiative forcing and global temperatures; the percentage apportionment of radiative forcing of these greenhouse gas has increased since the preindustrial era, together with the associated ranges of uncertainty and levels of confidence assigned to each factor. The radiative forcing effect of the minor trace gases is projected to increase steadily. Up to 1960 the cumulative CO2 contribution since AD ??1750 was about 67 per cent of the calculated 1.2W m-2 forcing, whereas for 1980-90 the CO2 contribution decreased to 56 per cent, with CFCs contributing 24 per cent and methane 11 per cent. For the entire period from AD +1765 to 2050, the CO2 contribution is projected to range from 4.15 W m-2, Out of a 6.5 W m-2 total (65 per cent), for a 'business-as-usual' scenario to 2.6 Wm-2, Out of a 4.0 Wm-2 total (65 per cent), if emission control policies are implemented rapidly.
The recent increase in global temperature forcing by the release of CFCs is particularly worrying. Ozone, which at high altitudes absorbs incoming short-wave radiation, is being dramatically destroyed above 25 km in the stratosphere by emissions of H2O and NOX by jet aircraft and by surface emissions of N2O by combustion and, especially, of CFCs. It is estimated that CFCs are now accumulating in the atmosphere five times faster than they can be destroyed by ultraviolet radiation. Ozone circulates in the stratosphere from low to high latitudes and thus the occurrence of ozone in polar regions is particularly diagnostic of its global concentration. In October 1984, an area of ??marked ozone depletion (the so-called 'ozone hole') was observed in the lower stratosphere (i.e. 12-24 km) centred on, but extending far beyond, the Antarctic continent. Ozone depletion is always greatest in the Antarctic spring, but in this year the ozone concentration was more than 40 per cent lower than that of October 1977. By 1990, Antarctic ozone concentrations had fallen to about 200 Dobson units in September-October, compared with 400 units in the 1970s. In the extreme years (1993-5), record minima of 116 D.U. have been recorded at South Pole. It has been estimated that, because of the slowness of the global circulation of CFCs and of its reaction with ozone, even a cut in CFC emissions to the level of that in 1970 would not eliminate the Antarctic ozone hole for at least fifty years.
The role of tropospheric aerosols in climate forcing and the magnitude of such effects are poorly known. There are four key aerosol types and these have a variety of effects:
1 black carbon - absorbs solar radiation; changes the vertical temperature gradient,
2 water-soluble inorganic species (SO2, NO3, NH4 - Backscatter of direct beam solar radiation, indirect effect of CCN on cloud albedo and cloud droplet lifetime.
3 condensed organic species - as (2)
4 mineral dust - as (1), (2) and absorption / emission of infrared radiation.
The global mean forcing exerted by the principal aerosols is as follows:
sulphate aerosols - 0.6 W m-2,
biomass burning aerosols - 0.8 W m-2.
mineral dust - 1.0 W m-2.
However, it should be emphasized that about 88 per cent of the total aerosols input is of natural origin. The indirect effects of cloud condensation nuclei (CCN) from anthropogenic sources are undetermined. Nevertheless, a ± 15 per cent change of CCN within marine stratus clouds, which cover about 25 per cent of the earth, could change the global energy balance by ± 1 W m-2.
Indirect anthropogenic factors, such as increasing population pressures leading to overgrazing and forest clearance, may increase desertification which also contributes to the increase of wind-blown soil. The 'dust-bowl' years of the 1930s in the United States and the African Sahel drought in the 1970s were observed. Evidence from the Soviet Union shows a sharp rise in dust-fall on mountain snow-fields from 1930sto 1960s, and atmospheric turbidity increased by 57 per cent over Washington, DC, over the period 1905-64, and by 85 per cent over Davos, Switzerland (1920-58). The presence of particles in the atmosphere increases the backscatеer of short-wave radiation, thereby increasing the planetary albedo and causing cooling, but the effect on infrared radiation is one of surface warming. The net result is complicated by the surface albedo. Man-made aerosols cause net warming over snow and ice and most land surfaces, but cooling over the oceans, which have a low albedo. Natural aerosols probably cause general cooling. The overall effect on global surface temperature remains uncertain.
Changes in surface albedo occur naturally with season, hut climatic forcing is also caused by anthropogenic vegetation changes. Human effects on vegetation cover have a long history. Deliberate burning of vegetation by Aborigines in Australia has been practised for perhaps 40,000 years. However, significant deforestation began in Eurasia during Neolithic times (C. 5000 BP), as evidenced by the appearance of agricultural species and weeds. Deforestation expanded in these areas between about AD 700 and 1700 as populations slowly grew, but it did not take place in North America until the westward movement of settlement in the eighteenth and nineteenth centuries. During the last half-century extensive deforestation has occurred in the tropical rainforests of South-east Asia, Africa and South America. Estimates of current tropical deforestation suggest losses of 105 km2/ Year, out of a total tropical forest area of ??9 x 106 km2. This annual figure is more than half the total land surface at present under irrigation and twice the annual loss of marginal land to desertification. Forest destruction causes an increase in albedo of perhaps 10 per cent locally, with consequences for surface energy and moisture budgets. However, the large-scale effect of deforestation in temperate and tropical latitudes on global surface albedo is estimated to be <0.001. It should also be noted that deforestation is difficult to define and monitor; it can refer to loss of forest cover with complete clearance and conversion to a different land use, or species 'impoverishment without major changes in physical structure. The term desertification, applied in semi-arid regions, creates similar difficulties. The process of vegetation change and associated soil degradation is not solely attributable to human-induced changes but is triggered by natural rainfall fluctuations.
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1750, 1.2 W m-2, The 1970s, 9 * 106 km 2, 5000BP.
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Ocean circulation | Deep ocean circulation | The Southern Oscillation | Water-Facts and Figures | Supply, Renewal and Use of Water Resources | Groundwater | Water Resource Problems | The U.S. Situation | Desertification: A Serious and Growing Problem | Climate forcing and feedbacks |