The Earth's climate depends among other parameters (see later) on the chemical composition of the atmosphere. Thus, any variation in the composition raises the possibility of climatic change. First of all, the chemical composition regulates the radiation balance of the Earth-atmosphere system. However, since differences in radiation balance in various geographical regions control the atmospheric circulation, there is also a relationship between composition and dynamic processes. In this chapter we shall deal mainly with the effects of compositional variations on the radiation balance. Moreover, the significance of so-called feedback mechanisms will also be stressed.

In the last decades the study of climatic change has become one of the most important problems of atmospheric science because of the possible variation of the climate due to the effects of anthropogenic pollution on our environment.1 Since it is evident that we cannot evaluate the importance of the present variations without the understanding of past climates, independent of human influences, this interest has also given an impetus to the investigation of the Earth's climate during geological times. The problem of climatic change was discussed in detail by an international group of experts in 1971 in Sweden. The main aim of this study was to evaluate the role of man in the present climatic variations. The conclusions of their discussion were published in an excellent report (SMIC, 1971 )2 which was summarized in a shorter form by Schneider and Kellogg (1973). This chapter is also based in many respects on that report.

The history of the Earth dates back nearly 5 billion years. We have reliable information, however, concerning only the last 500 million years or so. This information has been obtained mostly from studies of minerals and oceanic sediments, as well as from studies of the forms of mountains and the ocean floor. The understanding of the climate of more recent eras was facilitated by the investigation

1 Human activities can also influence the climate in other ways: modification of the surface, energy production, etc. We shall limit our discussion, however, to the effects of air pollution in view of the scope of this book.

2 SMIC: Study of Man's Impact on Climate.

11 Mészâros of ice layers in Greenland and Antarctica, but important results were provided also by biological studies.

This research, together with other studies, demonstrated that for 90 % of the time from the beginning of the Cambrian period (500-600 million years ago) the poles of the Earth were free of ice. The first important glaciation began at the end of the Paleozoic era (250—300 million years ago), which was followed by a warmer era in the Triassic era. Approximately 150 million years ago the temperature started to drop. Thus, according to studies made by Knauth and Epstein (1976) on hydrogen and oxygen isotope ratios in nodular and bedded cherts in the western and central parts of North America, the average temperature decreased from about 34 to 20 C through the Paleozoic period, increased to 35-40 C in the Triassic period and then decreased again to its Tertiary period value (~ 17 C). The glaciation of Antarctica began about 5 million years ago; the corresponding date for Greenland is 2 million years.

Fig. 52

Variation of ,80/160 ratio (¿) as a function of time B. P. according to Dansgaard etal.(see SMIC, 1971). (By courtesy of Massachusetts Institute of Technology)

Fig. 52

Variation of ,80/160 ratio (¿) as a function of time B. P. according to Dansgaard etal.(see SMIC, 1971). (By courtesy of Massachusetts Institute of Technology)

In the Pleistocene era, warmer and cooler intervals alternated. There is some indication that each glaciation built up over a period of about 90 000 years and terminated in a shorter time of about 10000 years. The last "ice age" reached its maximum about 20000 years ago and had largely disappeared 10000 years later. We have much reliable information on the average temperature for the last 14 000 years as a result of the 180/160 isotopic studies made in ice layers deposited in Greenland in different epochs. An increase in this ratio indicates a warmer temperature in the time of ice and snow deposition. Figure 52 represents the results of such an investigation, carried out by Dansgaard and his co-workers (see SM1C. 1971). Blackened parts on the right-hand side of the smoothed curve refer to warmer periods, while white parts to the left indicate cooler intervals. The amplitude of the variations is around 1-2 °C. It can be seen that, in spite of these fluctuations, the temperature in the last 8000 years has been practically constant. Since the beginning of our era, a short cold period was noted after 1200 and between 1550-1700 ("little ice age").

Different authors attribute the above climatic variations to the following causes:

(1) variations in solar activity;

(2) fluctuations in the Earth's orbit;

(3) fluctuations of the quantity of C02 or volcanic dust in the atmosphere;

(4) changes of the position and size of the continents;

(5) variations in oceanic circulation;

(6) intransitivity of the Earth-atmosphere system (see later).

Among these causes only those mentioned in point 3 are within the scope of this book. We shall discuss them in some detail in Section 6.5 in connection with present climatic variations. Of the other factors we shall clarify here only point 6, which is probably less known than the others. If in our climatic model (see later) the random modification of initial conditions results in a nonzero probability that the climate remains unchanged, then the climatc is called stable. If the system has only one single stable climate (and possibly many unstable ones) it is said to be transitive (see more detail in SMIC, 1971). Otherwise, i? is said to be intransitive. According to Lorenz (1968) the climate of the Earth is intransitive. This means that climate can go from one stable state to another without the modification of external or initial conditions, that is, variations are due to the "internal" fluctuations of the system.

The above list of possible causes of climatic changes starts with the most deterministic explanation, according to which the variations are totally due to an external factor, namely fluctuations of solar activity. The list ends with the least deterministic theory predicting that climatic variations are caused by internal random fluctuation of the Earth-atmosphere system.

After this discussion the reader is surely curious to know wich factor is the most important in the control of climatic variations. Unfortunately the experts are not able to make such a choice at present. Furthermore, it cannot be excluded that the interaction of several factors produced past climatic variations.

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