E

6 Q3

1860 1880 1900 1920 1940 1960 Fig. 53

Changes in temperature of surface air compared to the value observed in 1880 according to Mitchell (see SM1C, 1971). (By courtesy of Massachusetts Institute of Technology)

1860 1880 1900 1920 1940 1960 Fig. 53

Changes in temperature of surface air compared to the value observed in 1880 according to Mitchell (see SM1C, 1971). (By courtesy of Massachusetts Institute of Technology)

decreased with a higher rate than the above value. Figure 53 represents these variations as a function of latitude belts according to Mitchell (see SMIC, 1971; Schneider and Kellogg, 1973). The horizontal line plotted in this figure gives the mean temperature of the surface air calculated from observations carried out between 1931 and 1960. The dotted line is Mitchell's prediction. The predicted trend was later confirmed by measurement (black square). The plotted curves show that the amplitude of variation increases with increasing latitude. This is an important finding, since temperature fluctuations at higher latitudes have a much larger

Fig. 54

Variation of temperature (/ 2) and solar radiation intensity (J) according to Budyko (1969). I: annual mean: 2: ten year mean. (By courtesy of Tellus)

Fig. 54

Variation of temperature (/ 2) and solar radiation intensity (J) according to Budyko (1969). I: annual mean: 2: ten year mean. (By courtesy of Tellus)

It is obvious that there is a direct relationship between radiation intensity and temperature in the atmosphere. In Fig. 54 the trend of average temperature and radiation flux measured in surface air is plotted according to Budyko (1969). Data referring to midlatitudes of the Northern Hemisphere are expressed in absolute (temperature) and relative (radiation) deviations from the average values. One can see from this figure that the two parameters have varied together since the beginning of this century. For this reason Budyko speculates that temperature variations are determined either by changes in turbidity (extinction caused by aerosol particles) of the air or by fluctuations in the intensity of solar radiation reaching the upper layers of the atmosphere.

Budyko (1969) argues that the study of present climatic variations is particularly important since the thermal state of the Earth-atmosphere system as well as the present ice cover are in unstable equilibrium (during 90 % of the Earth's history from the beginning of the Paleozoic era our planet was free of ice, see Section 6.1). A relatively small change in temperature would lead to irreversible change (positive feedback, see previous section).

During the last hundred years the parameters characterizing the circulation also underwent significant fluctuations. Lamb demonstrated (see SMIC, 1971) that the number of days with westerly winds over the British islands increased until the middle of twenties while it decreased until about 1960. Lamb speculated but did not prove that these variations in the circulation pattern are connected with changes in temperature and precipitation fields.

6.5 Possible explanation for present climatic variations.

Connection of climatic variations with the air pollution

6.5.1 Variations in the chemical composition of the stratosphere

The aim of this section is to discuss the modifications of the atmospheric composition which can be related to the variations of the radiation balance and temperature observed in this century. We shall first deal with the relation between short-range modifications of atmospheric (stratospheric and tropospheric) composition and the transfer of incoming radiation. After this discussion, atmospheric factors influencing the absorption of infrared radiation emitted by the Earth's surface will be presented briefly.

From the foregoing parts of this book it is clear that solar radiation in the stratosphere is primarily attenuated by ozone (see Subsection 3.4.3) and at a lesser extent by the stratospheric sulfate aerosol layer (see Subsection 4.4.3). This means that any change in the stratospheric 03 burden or aerosol concentration involves modification of radiative transfer in this atmospheric domain. We should remember that the residence time of trace constituents above the tropopause is rather long because of the thermal structure and the absence of wet removal. Furthermore at these altitudes the density of the air is low as compared to that of lower layers. For this reason even an insignificant quantity of pollutants can produce relatively long and significant effects.

The study of the possible modification of the stratospheric ozone "shield" seems to be particularly important since ozone absorbs radiation dangerous to living species. Furthermore, this substance plays an important role in the control of the heat balance of the stratosphere.

One possible agent of inadvertent stratospheric modification is NOA emitted by supersonic transport aircraft. As discussed in Subsection 3.4.3 NO^ takes part in photochemical processes in the stratosphere in such a way that it catalyzes the reaction of O and 03 molecules (see also Fig. 9). That this reaction occurs was confirmed in individual cases by measuring the vertical profile of 03 over an area (Berlin) where supersonic transport is heavy (Grasnick, 1974). It is much more difficult, however, to assess long-range global effects. One estimate (Rowland, 1976) states that if 100 supersonic aircraft of models available at present were operated, the increase in ultra-violet radiation erythermally effective would be 0.04 % at the

Earth's surface, which is equivalent to an ozone decrease of 0.02 % (there is a factor of two between the two parameters, see WMO, 1976). Another calculation (Rowland, 1976) suggests that a 500-plane fleet of Boeing supersonic aircraft would result in an ozone depletion of 16 % in the Northern and 18 % in the Southern Hemisphere. However, "currently planned supersonic transport aircraft, due to their lower flight altitudes of 17 km and their limited numbers (30-50 projected) are not predicted to have an effect that would be significant or that could be distinguished from natural variations" (WMO, 1976). More recent calculations suggest that the previous models may have erred with regard to the sign of the effect. Nevertheless studies in this area must continue, and international agreements on flight altitudes, numbers of flights and NOx emission standards may yet become necessary.

According to various estimates the effect of halogenated hydrocarbons on the stratospheric ozone layer may exceed that of nitrogen oxides. As we have seen (Subsection 3.3.3), these anthropogenic species are emitted into the atmosphere at the surface but they reach the stratosphere by mixing. There they photolyze, forming chlorine, which may reduce the ozone quantity (see Subsection 3.4.3). Figure 55, published by Wofsy et al. (1975), reproduces the results of model

Fig. 55

Calculated effects of fluorocarbons on global ozone according to Wofsy et a/. (1975). A: production held constant at present rate; B : production ceases in 1978; C: production increases by 10 "„ per year, ceases in 1995; D: production increases by 10 "'„ per year; E: production increases by 22 °0 per year, ceases in production increases by 22 "„per year. (Copyright 6.2.1979 by the American Association for the Advancement of Science)

Fig. 55

Calculated effects of fluorocarbons on global ozone according to Wofsy et a/. (1975). A: production held constant at present rate; B : production ceases in 1978; C: production increases by 10 "„ per year, ceases in 1995; D: production increases by 10 "'„ per year; E: production increases by 22 °0 per year, ceases in production increases by 22 "„per year. (Copyright 6.2.1979 by the American Association for the Advancement of Science)

calculations aimed at estimating the chemical effects of fluorocarbons in the stratosphere. Curve A gives the computed ozone depletion in percentage if production remains constant at the present rate (see Subsection 3.3.3). One can see that according to this prediction the ozone decrease would reach 10 % in less than hundred years. This would result in an increase of 20 % of the intensity of UV radiation at the surface and in a decrease of about 10 °C of the temperature of the upper stratosphere (WMO, 1976). Furthermore, Curve B represents ozone depletion values if production of fluorocarbons had ceased in 1978. Even in this case around 1990 the ozone reduction would be several percent. Further curves in this figure give different combinations of production rate and cessation time (see the caption of the figure). This figure preceeded the evidence (see Subsection 3.4.3) that the interaction of chlorine and nitrogen species can considerably limit the effect of halogenated hydrocarbons on stratospheric ozone. The chlorine nitrate chemistry (and some new rate constants) were recently considered by Miller et al. (1978). They also took into account the multiple scattering of solar radiation by different molecules. By using a one-dimensional model these workers obtained the results given in Fig. 56. The figure represents two cases in which fluorocarbon production is stopped at the end of 1976 and 1978, respectively. The maximum ozone depletion is 0.62 and 0.77 "0, respectively. Miller et al. (1978) also calculated that the ozone depletion in the indefinite future (in several hundred years) with a steady state release at the 1975 level is 5.1 %. However, it should be noted that other model calculations carried out recently give a higher ozone depletion. Thus, in discussing new US reports on this problem and averaging the results of different estimates Rowland (1979) showed that the steady state ozone depletion is around 20 due to the increase of the rate constant of the reaction [3.30], Should further study confirm such a high value, the cessation of the anthropogenic production of halocarbons may become necessary.

Fig. 56

Calculated ozone depletion as a function of time for two cases of stopping fluorocarbon production at the end of 1976 and at the end of 1978 (Miller et al. 1978). (By courtesy of Atmospheric Environment)

Fig. 56

Calculated ozone depletion as a function of time for two cases of stopping fluorocarbon production at the end of 1976 and at the end of 1978 (Miller et al. 1978). (By courtesy of Atmospheric Environment)

It follows from this discussion that the inadvertent ozone depletion in the past and at present is not expected to be measurable. This conclusion is supported by observational data on total ozone showing a net increase during recent years. This is confirmed by observations carried out at various places on the Earth's surface. Thus, according to Komhyr et al. (1971) between 1958 and 1970 the rate of this increase has been as large as several percent per decade (see Fig. 57). It is to be noted in this respect that Mastenbrock (1971) found that the stratospheric water vapour burden over the U.S.A. also increased significantly in recent years and this was attributed to the water vapour emission of supersonic aircraft. In Subsection 3.4.3 we mentioned that free radicals formed from water vapour can play a certain role in ozone photochemistry. However, the temporal rise of total ozone cannot be explained by this effect, which is considered to be negligible (see Fig. 9). Furthermore, Birrer (1974) demonstrated that the value of the ozone trend decreases with an increasing period of observation. On the basis of data obtained beginning in 1926 in Arosa, Switzerland, this author was able to show that in the past, when the possibility of man-made effects was excluded, total ozone underwent through much more significant variations than since 1960. He also found a periodicity of 42 months in the ozone data. It follows from this finding that we have to be cautious in estimating possible anthropogenic modifications in stratospheric photochemistry.

It was mentioned above that the modifications of the stratospheric aerosol layer can also induce climatic variations. For this reason the following part of this

Huancayo

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