Critical Depletion of Planetary Oxygen Levels

  Oct 2, 2004

 Links that reference variation in planetary atmosphere Oxygen levels

Link  "Since we have begun to measure in 1989, there has been a steady decline of free oxygen in our atmosphere. And while this is nothing more than expected, since every molecule of additional carbon dioxide locks up two oxygen atoms, the free oxygen decline is greater than the carbon dioxide lock-up. The greater than expected overall free oxygen decline is proof that the Earth's photosynthetic capacity has declined. And since there has been no measurable decline in plankton, and consequently, in marine photo synthesis, as long expected and measured due to the increase of hard UV-B radiation at surface level, the decline points straight at the only other source of free oxygen - the forests and green cover of the continents." 

Link: "The decline of atmospheric free oxygen will become progressively more noticeable, human habitation at high altitudes will become untenable, while Life at low altitudes will first take on the aerobic characteristics of high altitude living, and then too becomes progressively more untenable."

 Link  "Scientists were stunned recently when it was revealed that air bubbles trapped in fossilized amber had been analyzed and found to contain oxygen levels of 38%. Yet today it is well known that the average content of the air is only 19% to 21%. It appears that since the early history of our earth there has been a stunning decrease of 50% in the average oxygen content of the air we breathe. Worse yet, analysis of the air in various parts of the world today reveals the frightening fact that the oxygen content continues to decline. In fact in some of the larger and therefore more polluted cities the oxygen levels have been measured at a disturbing level of 12 to 15%. Scientists claim that anything under 7% oxygen content in the air is too low to support human life, even for short periods."

Link  "Air as described in the American Heritage dictionary in 1982 states: A colorless, odorless, tasteless, gaseous mixture, mainly nitrogen (approximately 78 percent) and oxygen (approximately 21 percent) with lesser amounts of argon, carbon dioxide, hydrogen, neon, helium, and other gases. Since 1990 it has averaged between 17 and18 percent and recently dropped as low as 15 percent. At 15 percent you begin to suffocate."

Link  "(Atmospheric Oxygen, Giant Paleozoic Insects and the Evolution of Aerial Locomotor Performance, R. Dudley, JExB), and Dudley shows a high of about 35% just before the beginning of the Permian, with a rapid decline to a low of about 13-14% near the beginning of the Triassic, then a small spike at about 17% in mid Triassic, another drop to about 14-15% early in the Jurassic, a sudden climb to about 21% by mid-Jurassic, then a gentle climb to about 26% early in the Tertiary, and a rather constant, steady decline to the present "20.9%." 

Link  "Normal air has about 16% oxygen."  Note: "science" usually quotes the O2 content as 20-21%, as if it will 'never change' - that's a misconception - the level varies with time. If 16% is 'normal' and we start to suffer hypoxia beginning at 15%, the writing is on the wall.

Link  "Between 1989 and 1994 the O2 content of the atmosphere decreased at an average annual rate of 2 parts per million ... Just how O2 came into being remains a mystery"

Link (a product site) "Scientists have determined that the oxygen content in our atmosphere is being reduced at an alarming rate - about 0.85% approximately every 15 years. Since the United States became a nation in 1776, it is estimated that our planet has seen a decline of almost 11% of its available oxygen. At this rate, considering the destruction of the rain forests and increased pollution, the future decline in oxygen may be even more alarming."

Link " By analyzing air bubbles trapped in fossils, scientists have recently proven that the earth's atmosphere used to have 40% oxygen. By comparison, today's air only contains 16% to 19% oxygen. This is one half of what your body was designed for! Even more startling, analysis of the air in different parts of the world shows that this number is continuing to decline and many large cities now have oxygen levels as low as 12%. Medical researchers say that if falls below 7% the human race will perish. In 1905, virtually no one had cancer. the oxygen level at that time has been estimated to have been at 30%. In the 1940's the oxygen in the atmosphere measured 25% and one out of twenty Americans had cancer. Today, one third of Americans now have cancer and within the next five years it has been estimated that half of the American population will have some form of cancer. There appears to be a clear relevance with the decrease in atmospheric oxygen and the increase in cancer."

Rise and Fall of the Dinosaurs - The Oxygen Theory

Beldeu Singh

The appearance and disappearance of species is a natural phenomenon that has occurred throughout the history of Earth. In all instances, it can be traced to adaptation or a failure to adapt to changes in the environment at the pace at which elements in the environment or material environmental changes take place. 
Species are at all times subject to natural selection and many species have faced natural extinction or more recently have become extinct due to the actions of man on their habitat or extermination. But, the extinction of dinosaurs has always been of special interest and may have implications to life on earth, provided we reexamine it, not merely from the viewpoint of their extinction but from the larger perspective of rise and fall of dinosaurs. That might be more meaningful and more thought provoking. 

There are several theories that attempt to explain the extinction of dinosaurs and some people have their favourite theory. One common theory is the meteor impact attendant with large scale fires or even a meteorite shower but they all have serious shortcomings that do not satisfactorily explain mass extinction of species or certain species on a global scale. To my mind, a "good" theory ought to explain the rise of dinosaurs, their increase in size upto 90-100 tons and rapid extinction which can then be followed up with a proper research design to investigate it scientifically. 

The "Oxygen Theory" (OT) does offer an interesting insight into the rise and fall of dinosaurs as it also offers a possible link to the physiology that may support the increases in body mass of dinosaurs over a period of time stretching millions of years. 

According to the Oxygen Theory, Earth became green at around 350-330 million years due to climatic changes. During this period, the sea was also teeming with algae and aerobic bacteria flourished, causing the atmospheric oxygen level to rise slowly. The atmospheric oxygen level, then began to rise at a more rapid rate, until at around 200-250 million years ago, it peaked at around 38-40% just before the Cretaceous period. After this period, climatic changes began the slow process of decline of vegetation and later on started the process of desert formation in some areas. In turn, it led to lowering of the populations of aerobic bacteria as well as sea algae. 

This resulted in falling levels of atmospheric oxygen, a process that may have begun at around 200 million years ago, declining from 35-40% to 11% at 65 million years ago. This had a drastic impact on life on earth and provides interesting clues of the role of levels of atmospheric oxygen on life and the long term existence of human beings on this planet. 

The evidence of different or changing levels of atmospheric oxygen can be found in trapped bubbles of air in crystals or amber. This evidence is critical and it ought to be examined more carefully from the various strata and arranged in chronological order to test the validity of the Oxygen Theory or its possible role in the rise and fall of dinosaurs. Air trapped in amber found in geological formations dated at around 230 million years ago show a level of about 35% of oxygen. 

According to the Oxygen Theory, the dinosaurs probably began to appear about 310 million years, when the atmospheric oxygen level rose above 11%. As the atmospheric oxygen level rose higher, it was able to support larger bodied dinosaurs. The level of atmospheric oxygen became the critical factor in natural selection and those that could not adapt to the increasing levels of atmospheric oxygen died out from oxygen toxicity as higher and higher levels of atmospheric oxygen meant that more of it is absorbed by the blood through the lung tissue due to the high oxygen gradient. 

As atmospheric oxygen levels rose, the larger bodied dinosaurs began to emerge and their size increased over 100-130 million years to coincide with the peak period of atmospheric oxygen levels during the Cretaceous Period. This may help to explain one very critical enigma in animal physiology - the size of dinosaur hearts and their blood pressure. Linear extrapolations on the size of dinosaur hearts and their consequent increase in blood pressure to supply blood to all the organs and tissue in 90-100 ton organisms are unrealistic simply because such tremendous blood pressure would rupture the capillaries in the brain and lung tissue. 

However, if the blood was so highly oxygenated as it would be if the atmospheric oxygen level was 38-40%, then the large heart need to contract only slowly, thus enabling the flow of large volumes of blood at significantly low blood pressure to oxygenate the tissues at 'normal' levels. This physiological adaptation served as a mechanism to prevent the rupture of the tiny capillaries while at the same time preventing oxygen toxicity in the tissues. The cretaceous dinosaur heart may have had a heart beat of around 30-35 beats per minute. 

So, the level of atmospheric oxygen as an element of natural selection had a dual effect which is another element that can be tested for in order to determine the adequacy of the Oxygen Theory in the rise and fall of dinosaurs. And naturally, later on when the atmospheric oxygen levels began to fall after peaking at around 40%, it again became a critical factor in natural selection and quite possibly, the dinosaurs that could adapt better would be the ones with smaller body mass. During this phase, the physiology that served the large bodied dinosaurs so well during the peak period of atmospheric oxygen, could not cope the the decreasing levels of atmospheric oxygen - it affected their performance, survival and it affected the hatching success rate of their eggs and sealed their fate as atmospheric oxygen levels dropped below the critical level of 20%. 

Later when the atmospheric oxygen levels declined to 11%, at around 65 million years ago, it ended the era of large dinosaurs and nature brought forth the small bodied dinosaurs in the form of birds and the small warm blooded animals began to appear. As atmospheric oxygen levels began to rise again, modern plants and the large bodied mammals emerged. As atmospheric oxygen levels was rising to about 28% shortly after the Cretaceous period there was diversification of mammals and large bodied mammals appeared. But these mammals, such as the sabre toothed tiger and the woolly mammoth disappeared again, as atmospheric oxygen level started to decline towards 21%, giving way to the smaller mammals such as the lions and tigers and the African elephant which may be 'older' than the relatively smaller Asian elephant. 

If life on Earth is so inextricably linked to changes in the atmospheric levels of oxygen, the recent discovery of the pygmy elephant in Borneo, which is only about half the size of the other elephant species does raise an alarm in the mind and the need to examine the Oxygen Theory closely. 

More importantly, if such a theory has an inherent element of truth or reality, we need to relook at our energy sources and in particular the internal combustion engine that revolutionized life on earth even as it competes with humans and life on Earth for oxygen and reduce or stamp out other oxygen depleting activities. If atmospheric oxygen dropped to the Cretaceous level of 11%, humans may have to go the way the dinosaurs went 200-65 millions ago! Thats unpalatable food for thought. 


Global Biosphere Cataclysms

Professor Mikhail I. Budyko

Abstract

Climate biosphere cataclysms, which occurred many times in the geological past, provoked climate changes that led to the extinction of populations of many fauna and flora species. The study of these cataclysms allows a conclusion that climate systems are very sensitive to relatively small changes in climate-forcing factors (atmospheric transparency, large glaciations, etcetera). It is important to take this conclusion into account while estimating the possible consequences of the presently occurring anthropogenic warming caused by the increase in greenhouse gas concentrations in the atmosphere. 

Introduction 
Climate catastrophes occur with large-scale environmental changes, which cause mass deaths of living organisms. In comparison with global climatic catastrophes, local ones induced by atmospheric factors occur much more frequently. These local catastrophes, because of their limited duration, are associated in many cases with weather changes and not with climate variations. However, in the absence of a universally adopted time scale distinguishing the synoptic processes determining weather and the climate-forming processes, it is not always easy to discriminate between weather and climate catastrophes. We will consider several examples of frequently occurring local ecological catastrophes caused by atmospheric factors. 

The best-known example of this kind is large-scale droughts covering areas of thousands of square kilometers. During such droughts, a considerable part of the natural vegetation cover and crops perish. Most animals inhabiting the drought area also die, or if possible, move to other regions, which also results in the death of a part of forced emigrants. 

The largest droughts in developing countries caused mass mortality in agricultural populations. Often epidemics spread far beyond the boundaries of the drought regions. The catastrophic droughts in Ethiopia's Sahelian region and in other countries are widely known recent examples. 

Aftereffects of strong thermal variations disastrous for animate nature are also known and are frequently distributed over far greater areas than those affected by drought. These weather changes result less in mortality (although this always increases with excessive temperature variations), but inadvertently in mass deaths of animals and plants belonging to species sensitive to the thermal conditions of the environment. 

Such an event is often observed in the mid-latitudes over vast areas. Long-term, strong reductions in temperature cause a considerable decrease in the number of many animal species — even full extinction. Particularly strong cooling reduces the area occupied by less frost-resistant plants. 

One more example of comparatively frequent local climatic catastrophes is associated with the anomalous development of atmospheric processes when warm tropical waters move southward along the western coast of South America — the so-called El Nino phenomenon. In this case, the nutrient- and oxygen-rich cold waters of the Peruvian current are overlapped over long distances by warm waters, which destroys the nutrient links supporting the existence of large fish populations. The disappearance of fish results in the mass deaths of birds feeding on fish, and creates the threat of starvation for the populations of coastal regions whose existence depends on fishery. Without dwelling on further aspects of the El Nino effect on atmospheric and ecological processes, we mention that this same effect can be observed at great distances from the region where this current is located. 

The general feature of all local climatic catastrophes is that, although in a number of cases these catastrophes result in disastrous consequences, they rarely cause complete extinction of various species of animals and plants. First, this can be explained by the fact that most living organisms can renew their population even after great reductions and, second, that these organisms can survive in areas with more favorable environmental conditions. The exception to this rule may be less numerous organisms occupying small areas, ones that were on the verge of extinction before the climatic catastrophe. 

Passing to global climatic catastrophes, these are associated with the extinction of many species of organisms as a result of drastic changes in climate either over the entire globe or over a part that is so great that it covers areas where a number of representatives of animal and plant kingdoms live. It should be particularly emphasized that the time factor is of great importance in the occurrence of global climatic catastrophes. 

Even comparatively great climatic changes developing for a period of many thousands or even millions of years have not led, as paleontological data show, to mass extinction of organisms. The most striking examples of these climatic changes pertain to cases of vast glacier advances, in particular to well-studied Pleistocene glaciations. 

In glacial epochs of this time, the climatic conditions drastically changed in those regions of middle and high latitudes where glaciations advanced. At the same time, the climate changed at all latitudes, including the tropics, where, with some cooling, the moisture conditions varied noticeably. 

These considerable climatic changes, greatly affecting living nature, did not result in the mass extinction of organisms. The cause of this is very simple: with slow climatic changes ecological systems had not been ruined. These systems were preserved in certain geographical zones, which shifted as the climate changed. In middle latitudes of the Northern Hemisphere these zones usually shifted southward. 

At the same time, gradual changes developed both in the structure of geographical zones and ecological systems, which favored the evolution of the organisms involved in these systems. However, in these cases there was no mass extinction of organisms. 

It is easy to imagine what would have happened to animals and plants if such climatic changes had developed not over thousands of years but in the course of one or two years, which sometimes can be a consequence of explosive volcanic eruption. Undoubtedly, in this case a massive ecological catastrophe associated with the extinction of a host of animal and plant species would have occurred. Discussion 
Benjamin Franklin was the first to pay attention to the possible climatic effects of volcanic gases and dust. He proposed that a large eruption of the Lacki volcano in Iceland in 1783 resulted in "dry fog," a haze that caused a cold summer and poor harvests in Europe. Later, Savinov (1913), Kimball (1918), Kalitin (1920) and other authors established that after an explosive volcanic eruption, solar radiation to the Earth's surface decreases drastically. In these cases, the value of direct radiation averaged over a large area can decrease by 10 to 12% over several months or years. 

Far greater explosive eruption occurred in 1815 in Indonesia (Tambora). From the limited observations available at that time, it is difficult to estimate accurately the average air temperature reduction after the Tambora eruption. However, it is clear that this reduction was uneven and, in a number of regions, attained several degrees. In particular, in the summer of 1816 in Europe and North America, the temperature was so low that the year was called "a year without summer" (the cause of this was unknown at that time). The eruption of Tambora deserves attention, because it is the eruption nearest in time that induced a climatic change which, in spite of its comparatively short duration, caused noticeable damage to living nature. 

In particular, due to drastic decreases in crop yield in a number of regions far from the volcano, many thousands of people died of starvation (Stommel and Stommel, 1979, 1983). 

It is possible that the climatic change after eruption of the volcano on the island of Santorin in the eastern part of the Mediterranean Sea, which took place approximately in 1500 B.C., was much more significant. Although the ecological aftereffects of this eruption are difficult to estimate, it is assumed that they were very considerable and led, in particular, to an abrupt decline of the highly developed Cretan civilization that had flourished up to that time. It is possible that this eruption is recorded in folk tales about the "outer darkness" recounted in the Bible (Rust, 1982). 

There is no doubt that, for far longer time intervals comparable with geological epochs and periods, the effects of volcanic eruptions on climate and the biosphere were much greater than the effects of the eruptions that have occurred over the last several thousand years. 

It was known long ago that possible deviations from the norm of intensity indices of many natural processes increase as the time interval under consideration increases. Thus, for example, a catastrophic earthquake, whose probability is very low for a short time interval, is quite probable over a longer time interval. 

It is natural to assume that massive anomalies in natural processes — that could not be observed over the comparatively short period of mankind's existence and for the short time of environmental study — had occurred throughout the long geological history of the Earth. 

On the basis of this concept, the theory was formulated of the occurrence of aerosol climatic catastrophes in the past. This theory is based on the following ideas. 

If the influence of individual explosive eruptions on the Earth's temperature is comparatively small, due to the limited amount of aerosol that the stratosphere gains after every eruption, it is obvious that the Earth's temperature changes much more drastically when many explosive eruptions occur one after another within a short time interval. The possibility of such coincidences for long periods of time, as a consequence of general statistical rules, increases noticeably with variations in the mean level of volcanic activity. Similar to this, the greatest amount of aerosol entering the stratosphere during one volcanic eruption will rise as the studied time interval grows, due to the same causes that induce the greater frequency of eruptions. 

By analyzing empirical data on the influx of aerosol to the stratosphere over the last 100 years (Lamb, 1969), a formula has been derived that relates the maximum aerosol amount entering the atmosphere within a certain time (t) depending on the total duration of the period (T), for which the corresponding analysis is carried out. It follows that the maximum influx of aerosol with T>>t is proportional to the logarithm of ratio T/t. 

Using this formula, it was deduced that for sufficiently long periods of time (thousands or millions of years), the amount of aerosol coming into the atmosphere during 10 years could exceed by 10 to 20 times the amount of aerosol contained in the stratosphere after the Krakatoa eruption (1883). 

The analysis confirmed the conclusion as to the possibility of the formation of aerosol layers over sufficiently long time intervals, with the mass inducing the reduction of global mean surface-air temperature by 5 to 10 degrees centigrade or more. Since, in this case, air temperature on the continents decreased by far greater values than for the planet as a whole, such coolings could have led to the extinction of numerous species of animals and plants. 

The hypothesis of the possibility of aerosol climatic catastrophes as a result of the collision of celestial bodies with the Earth was proposed at the end of the 1970s. Here we shall briefly explain this hypothesis. One of the results of the falling of large meteorites should be a considerable increase in aerosol layer optical density in the atmosphere. Preserved traces of meteor craters on the Earth's surface allow us to assume that during the long history of the Earth, meteorites, of a size up to several hundred meters or even more, collided with the Earth. If, as a result of the explosion caused by a meteorite falling on the Earth's surface, the stratosphere gained aerosol particles of even only a small portion of its mass, it would be sufficient to cause an abrupt decrease in the solar radiation reaching the Earth's surface. 

By calculation it was concluded that after the falling of a sufficiently large meteorite, the temperature would be lowered by approximately 5 to 10 degrees centigrade for many months, and this would catastrophically affect various living organisms. The question of effects in the biosphere caused by the collision of celestial bodies with the Earth attracted great attention after the appearance of publications by Alvarez et al (1980). 

A detailed calculation of climatic effects of the Earth's collision with a large asteroid was carried out by Toon and coauthors (Toon et al., 1982; Pollack et al., 1983). In these studies, it was concluded that, due to coagulation and sedimentation of the aerosol particles produced by the explosion of the asteroid, the concentration of these particles remains high only for several months after the asteroid falls. At this time, the solar radiation influx decreases to a level insufficient to support photosynthesis. This calculation shows that attenuating the solar radiation influx would reduce the mean surface-air temperature over the ocean surface by 2 or 3 degrees centigrade for two or more years, and over the continents by several tens of degrees for half a year. It was also determined in these calculations that for 10 months after the falling of an asteroid, the global mean temperature is reduced by 9 degrees centigrade and in 20 months, by 6 degrees centigrade, on average. These results are close to values obtained in the calculation by Budyko (1980). 

The presented materials indicate that, in the geological past, large short-term climatic changes occurred, which could have affected living nature. 

The possibility of these catastrophes occurring follows from the fact that a reduction of mean air temperature after the large explosive eruptions of the past centuries (Krakatoa and Tambora) was less in order of magnitude than would have resulted in a massive aerosol climatic catastrophe. Taking into account that volcanic activity changes considerably with time, and considering in terms of statistics the above possibility of the coincidence of a series of eruptions over short intervals, it is clear that throughout the Earth's history an increase in the density of the volcanic aerosol layer, by at least one order of magnitude, could have taken place many times. 

It is also beyond doubt that a considerable increase in the density of the aerosol layer occurred after the fall of large celestial bodies on the Earth's surface, although it is much more difficult to estimate a decrease of atmospheric transparency after these events. 

The recent investigations made it possible to assess the consequences of the falling of an asteroid that caused extinction of animals in the late Cretaceous. In particular, it was supposed that this collision resulted in aerosol emission into the stratosphere three orders of magnitude greater than that due to the largest volcanic eruption over the last 100 to 150 years (Krakatoa, 1883). 

It was hypothesized that increased density of the stratospheric aerosol layer caused a drastic decrease in the influx of solar radiation to the Earth's surface. This resulted in a decrease in photosynthetic activity, which, in turn, caused the extinction of vegetation and then, of phytivorous animals. This hypothesis, as well as the assessments of some other consequences of the formation of an aerosol screen in the stratosphere, seems to be incorrect. The most probable cause of climatic catastrophes in the late Cretaceous was surface-air temperature decreases by several degrees due to the decrease in solar radiation influx resulting from the increased density of the aerosol layer after the collision of the asteroid with the Earth or increased volcanic activity due to the destruction of a part of the Earth's crust after that collision. 

Discussing the possible causes of aerosol climatic catastrophes, the narrowness of the "climatic zone of life" (the range of physical and chemical conditions in the atmosphere and hydrosphere under which living organisms can exist) on our planet should be taken into account. 

During the millions of years before the catastrophe of the late Cretaceous, climatic conditions at all latitudes of the planet differed comparatively insignificantly, being either warm or hot. Consequently, living organisms were stenothermal and a short-term, small temperature decrease (by several degrees) could cause mass extinction of many plants and animals. 

Such changes in the biosphere did occur over the last 100 million years, such as in the late Cretaceous and Cenozoic periods. Information about the temperature regime over this period is presented in Table 1 (Budyko, Ronov and Yanshin, 1985), where Mc (%) is the bulk concentration of CO2 in the atmosphere, DS/S is the ratio of the difference between solar radiation in the past and present to the modern solar constant, and DA is the difference between the Earth's albedos, expressed in the parts of unity. 

Taking into account that present mean surface-air temperature equals 15 degrees centigrade, and assuming that a radiation increase of 1% with constant albedo causes a mean temperature increase of 1.4 degrees centigrade, while an albedo increase of 0.01 causes a temperature decrease of 2 degrees centigrade, the difference between the mean air temperature in the geological past and present can be determined from the data in Column DT in degrees of centigrade. 

Over the relatively short Quaternary period, CO2 concentration in the atmosphere fluctuated considerably and the tendency for a CO2 concentration decrease, which appeared long before the beginning of the Cenozoic, continued to intensify. In the coldest Pleistocene epochs, when glaciation developed, the mean surface-air temperature was about 5 degrees centigrade lower than the modern one. 

Calculations using climate theoretical models show that in the warm epochs of the Phanerozoic the climate changed comparatively little in the tropical latitudes, while in the middle and especially in the high latitudes, air temperature was much higher than at present (Manabe and Bryan, 1985). As a result, the climate in the polar regions was comparatively warm. Paleogeographic investigations show that in the Mesozoic and early Cenozoic, forests consisting of evergreen species existed in the high latitudes. 

The comparison of empirical data on mean air-temperature fluctuations with the changes in CO2 concentration and albedo confirm the conclusion that those were climate-forming factors for the intervals mentioned in Table 1. 

The idea of the influence of atmospheric chemical composition on nature was put forward in the early 19th century by Geoffroy St.-Hilaire, who wrote: "Let us suppose that with time gradual changes occurred in the ratio between different elements of the atmosphere and it was an unavoidable consequence that these changes affected the fauna" (E. Geoffroy St.-Hilaire, 1833). For a long time the idea of Geoffroy St.-Hilaire did not attract any attention due to the absence of reliable information about the changes in atmospheric chemical composition in the geological past. In recent years, after new data on atmospheric chemical composition changes in the geological past had been received, these data were compared with the history of natural evolution. 

It is probable that, of the different components of the external environment, the amount of oxygen in the air is one of the most essential factors of life activities for terrestrial animals (or for aquatic animals, the amount of oxygen in the water, which depends on the atmospheric oxygen concentration). The level of metabolism of aerobic animals is directly dependent on the amount of oxygen in the environment, increasing, other things being equal, with an increase in oxygen content. 

Additional energy received by an animal in the environment with increased oxygen content may be used for different purposes, including the development of a more complex structure of the organism, or its individual organs and tissues, in the course of evolution. Especially important, in this case, might be the improvement of organs maintaining metabolism, such as circulatory and respiratory systems in vertebrates. 

Of considerable interest is the question about general principles of animal evolution throughout a long period from the end of the Carboniferous to the end of the Triassic, when the atmospheric oxygen level decreased for the longest time in the Phanerozoic. As a result, at the end of the Permian and throughout a part of the Triassic, oxygen was only 20% to 25% of its present level, which is close to the average Late Proterozoic amount of oxygen. The lower (exothermal) vertebrates that used relatively little oxygen could more easily adapt to such conditions. It is very doubtful whether the higher (endothermal) vertebrates could exist at such low oxygen concentrations. 

It is evident that with a decrease in atmospheric oxygen content, the life activities of animals and their mutual relations with other components of ecological systems must have been changing. A gradual deterioration of the conditions for the existence of animals must have been reflected in the specific features of the evolutionary process in the period in question. 

These features were long noted by paleontologists, although their causes were unknown. In particular, Simpson (1961) wrote: "It is interesting that no phylum has expanded steadily from the time of its appearance to the present day. The most nearly general feature is that most of the phyla contracted in the Permian, Triassic or both." 

The principle indicated by Simpson was also confirmed by many other investigators. For instance, Raup and Stanley (1971) present a scheme of temporal variations in the number of taxons of fossil animals in the Phanerozoic, which shows that against a gradual increase in the number of taxons, their number abruptly declined in only one single period, in the Triassic. The authors indicate that, in fact, this contraction began in the middle of the Permian. Robinson (1971) states that from the Permian to the Late Triassic the number of genera of therapsids became ten times less and the number of genera of sauropsids increased considerably. This shows that conditions were unfavorable for the existence of mammal-like reptiles for the greatest part of the Triassic. Worth noting is also a tendency that appeared in the Triassic toward a decrease in size of the therapsids, which belonged to the most progressive groups. This tendency may be due to a growing shortage of oxygen, since the maintenance of the metabolism of large animals requires more oxygen than for small animals, other things being equal. 

It might be supposed that a decrease in oxygen content during the Late Permian and the Triassic greatly retarded the process of the formation of mammals, which extended over an enormous interval of time, about one-hundred-million years. This class of vertebrate spread only with a new rise in atmospheric oxygen, which began in the second half of the Triassic. 

Another considerable decrease in oxygen concentration occurred during the second half of the Cretaceous. At this time, the oxygen content decreased by more than 1.5 times and approached a level somewhat lower than the modern one. Although this level was rather high compared to the average conditions of the Phanerozoic, it is probable, however, that the oxygen decrease in the Cretaceous was of importance for the fauna of that time, and, in particular, caused the extinction of some groups of animals at the end of the Mesozoic. 

Following these considerations, it might be concluded that the formation of modern classes of vertebrates was determined to a great extent by global changes in the environment, one of the causes of which was variations in the degassing of the Earth's mantle. 

As mentioned earlier, with increases in the amount of oxygen in the atmosphere in the range of its variations throughout the Phanerozoic, possibilities emerged for the new groups of animals, which use more energy in the course of their vital activity, to evolve. Therefore, it is natural to suppose that in the epochs of increased oxygen concentrations, the diversity of fauna grew larger, while in epochs of decreased oxygen content their diversity diminished. 

Turning to the problem of the influence of carbon dioxide changes in the atmosphere on animate nature, let us note that in some cases this influence was combined with that of atmospheric oxygen changes. Thus, in particular, in the example of the dissemination of aerobic organisms with the formation of oxygen atmosphere, the oxygen concentration in the atmosphere started to grow after a decrease in atmospheric carbon dioxide content below a certain limit, which resulted from a relevant decrease in the influx rate of carbon monoxide and other not fully oxidized gases in the atmosphere. 

During the Phanerozoic, carbon dioxide fluctuations considerably influenced photosynthetic productivity and climatic conditions. These fluctuations undoubtedly influenced the life of plants and, to a lesser extent, that of animals. 

The comparison of variations in carbon dioxide with principal events in the development of vegetation is impeded by the lack of a generally accepted chronology of the considerable changes in the nature of vegetation. Krasilov (1977) identified four major epochs in the formation of the higher taxonomic groups of plants: the second half of the Devonian, the Permian to the beginning of the Triassic, the Cretaceous and the Miocene. The first of these epochs is associated with the formation of progymnospermous forests, the second with the expansion of coniferous forests, the third with the emergence of archaic flowering plants and the fourth with the appearance of steppe plant communities. 

It may be noted that these epochs correspond to four out of the five maxima in carbon dioxide concentration that were enumerated. This testifies that an increased productivity of photosynthesis considerably affected the progressive development of plants. 

Since the existence of all living organisms depends on the atmospheric chemical composition, the question naturally arises: why did the atmospheric chemical composition vary for about four-billion years within a range permitting not only continuous preservation of life on the Earth (the continuous existence of the biosphere), but also the progressive development of organisms, among which many achieved a high level of complexity in the course of a long evolution? 

Although this question has rarely been discussed in scientific studies, it could be answered in two different ways. One of them is the so-called "Hypothesis of Gaia," which suggests that living organisms have the ability to control the environment, and to maintain a state favorable for their life activity (Margulis and Lovelock, 1974; Lovelock, 1979). The authors of this hypothesis have not adduced any definite proofs in favor of their idea, and it is mainly based on the apparent impossibility of otherwise accounting for the long existence of the biosphere. 

The presented conclusions about the mechanism of the evolution of the atmosphere have indicated that variations in the physical state and chemical composition of the atmosphere depend mostly on two external factors, namely, the evolution of the Sun, which leads to a gradual increase in solar radiation, and the evolution of the Earth, in the course of which the process of degassing of the upper mantle gradually attenuates. The first of these processes is entirely independent of the activity of terrestrial organisms and the second almost so. 

The causes of the antiquity of the Earth's biosphere can therefore be explained otherwise. In might be thought that this antiquity is a result of random coincidence (independent of the existence of organisms) of the direction and rate of the processes of the Sun and the Earth's evolution, which are not connected to each other. Since the probability of such a coincidence is extremely low, it means that life (and particularly its higher forms) in the Universe is an exceptionally rare phenomenon. 

This point of view has been developed in a number of works (Budyko, 1977, 1980, 1984; Hart, 1978, 1979). As has been noted in these works, the atmosphere in which life on any planet can exist must have a specific physical state and chemical composition. 

Let us conclude that life could originate on Earth and be preserved for billions of years because of the coincidences of several factors. It might well be considered that the probability of each of these coincidences was very low. 

Conclusion 
In the early 1970s, an estimate of the increase in the carbon dioxide concentration and mean surface-air temperature expected to occur in the next century was presented (Budyko, 1972). In recent years, many forecasts of changes in mean air temperature in the next century have been published. It should be noted that these forecasts, as a rule, are in good agreement. The investigations show that in the nearest decades, due to the effect of man's economic activity on the atmospheric chemical composition, considerable climate changes are to occur. In the past, it took thousands of years for climate changes of such scale to develop. As these climate changes would influence different economical objects, which are now being projected or built, it is clear that information about the forthcoming climate changes is of great practical importance. 


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