Cosmic Connection

by Carl Sagan

  20. The Ice Age and the Cauldron

  On our tiny planet, spinning in an almost circular orbit at a nearly constant distance from our star, the climate varies, sometimes radically, from place to place. The Sahara is different from the Antarctic. The Sun’s rays fall directly on the Sahara and obliquely on the Antarctic, producing a sizable temperature difference. Hot air rises near the equator, cold air sinks near the poles–producing atmospheric circulation. The motion of the resulting air current is deflected by Earth’s rotation.

  There is water in the atmosphere, but when it condenses, forming rain or snow, heat is released into the atmosphere, which in turn changes the motion of the air.

  Ground covered by freshly fallen snow reflects more sunlight back to space than when it is snow-free. The ground becomes colder yet.

  When more water vapor or carbon dioxide is put into the atmosphere, infrared emission from the surface of the Earth is increasingly blocked. Heat radiation cannot escape from this atmospheric greenhouse, and the Earth’s temperature rises.

  There is topography on Earth. When wind currents flow over mountains or down into valleys, the circulation changes.

  At one point in time on one tiny planet, the weather, as we all know, is complex. The climate, at least to some degree, is unpredictable. In the past there were more violent climatic fluctuations. Whole species, genera, classes, and families of plants and animals were extinguished, probably because of climatic fluctuations. One of the most likely explanations of the extinction of the dinosaurs is that they were large animals with poor thermoregulatory systems; they were unable to burrow, and, therefore, unable to accommodate to a global decline in temperature.

  The early evolution of man is closely connected with the emergence of the Earth from the vast Pleistocene glaciation. There is an as yet unexplained connection between reversals of the Earth’s magnetic field and the extinction of large numbers of small aquatic animals.

  The reason for these climatic changes is still under serious debate. It may be that the amount of light and heat put out by the Sun is variable on time scales of tens of thousands or more years. It may be that climatic change is caused by the slowly changing direction between the tilt of the Earth’s rotational axis and its orbit. There may be instabilities connected with the amount of pack ice in the Arctic and Antarctic. It may be that volcanoes, pumping large amounts of dust into the atmosphere, darken the sky and cool the Earth. It may be that chemical reactions reduced the amount of carbon dioxide and other greenhouse molecules in the atmosphere, and the Earth cooled.

  There are, in fact, some fifty or sixty different and, for the most part, mutually exclusive theories of the ice ages and other major climatic changes on Earth. It is a problem of substantial intellectual interest. But it is more than that. An understanding of climatic change may have profound practical consequences–because Man is influencing the environment of the Earth, often in ways poorly thought-out, ill-understood, and for short-term economic profit and individual convenience, rather than for the long-term benefit of the inhabitants of the planet.

  Industrial pollution is churning enormous quantities of foreign particulate matter into the atmosphere, where they are carried around the globe. The smallest particles, injected into the stratosphere, take years to fall out. These particles increase the albedo or reflectivity of Earth and diminish the amount of sunlight that falls on the surface. On the other hand, the burning of fossil fuels, such as coal and oil and gasoline, increases the amount of carbon dioxide in the Earth’s atmosphere which, because of its significant infrared absorption, can increase the temperature of the Earth.

  There is a range of effects pushing and pulling the climate in opposite directions. No one fully understands these interactions. While it seems unlikely that the amount of pollution currently deemed acceptable can produce a major climatic change on Earth, we cannot be absolutely sure. It is a topic worth serious and concerted international investigation.

  Space exploration plays an interesting role in testing out theories of climatic change. On Mars, for example, there are periodic massive injections of fine dust particles into the atmosphere; they take weeks and sometimes months to fall out. We know from the Mariner 9 experience that the temperature structure and climate of Mars are severely changed during such dust storms. By studying Mars, we may better understand the effects of industrial pollution on Earth.

  Likewise for Venus. Here is a planet that appears to have undergone a runaway greenhouse effect. A massive quantity of carbon dioxide and water vapor has been put into its atmosphere, so cloaking the surface as to permit little infrared thermal emission to escape into space. The greenhouse effect has heated the surface to 900 degrees F or more. How did this greenhouse-overkill happen on Venus? How do we avoid its happening here?

  Study of our neighboring planets not only helps us to generalize the study of our own, but it has the most practical hints and cautionary tales for us to read–if only we are wise enough to understand them.

  21. Beginnings and Ends of the Earth

  Stars, like people, do not live forever. But the lifetime of a person is measured in decades, the lifetime of a star in billions of years.

  A star is born out of interstellar clouds of gas and dust. For a while, it stably converts hydrogen to helium in the thermonuclear furnaces of its deep interior. Then, in stellar old age, it encounters a set of minor or major catastrophes–a slow trickle or an explosive injection of star-stuff into space. During the more or less stable portion of the lifetime of the star, the hot interior region, converting hydrogen into helium, gradually eats its way outward from the very center. In the course of time, the star becomes slowly, almost imperceptibly, brighter.

  After the flares and other impetuosities of its early adolescence, our Sun settled down to a more or less constant radiation output. But four billion years ago it was about 30 percent dimmer than it is today. If we assume that four billion years ago the Earth had the same distribution of land and water, clouds and polar ice, so that it absorbed the same relative amount of sunlight as it does today, and if we also assume that it had the same atmosphere as it does today, we can calculate what its temperature would have been. The calculation reveals a temperature for the entire Earth significantly below the freezing point of seawater. In fact, even two billion years ago, under these assumptions, the Sun would not have been bright enough to keep the Earth above the freezing point.

  But we have a wide variety of evidence that this was not the case. There are in old mud deposits ripple marks caused by liquid water. There are pillow lavas produced by undersea volcanoes. There are enormous sedimentary deposits that can only be produced on ocean margins. There are biological products, called algal stromatolites, which can only be produced in water.

  So what is wrong? Either our theory of the evolution of the Sun is wrong or our assumption that the early Earth is like the present Earth is wrong. The theory of solar evolution seems to be in good shape. What uncertainties exist do not appear to affect the question of the Sun’s early luminosity.

  The most likely resolution of this apparent paradox is that something was different on the early Earth. After studying a wide range of possibilities, I conclude that what was different, two billion years ago and earlier, was the presence of small quantities of ammonia in the Earth’s atmosphere. Ammonia is present on Jupiter today; it is the form of nitrogen expected under primitive conditions. It absorbs very strongly at the infrared wavelengths that the Earth likes to emit to space. Ammonia on the primitive Earth would have held heat in, increasing the surface temperature through the greenhouse effect and keeping the global temperature of Earth at congenial levels–for the origin and early history of life and for liquid water to have been abundant early in the history of the planet. And ammonia is one of the atmospheric constituents needed for making the building blocks of life. The study of the Sun’s evolution leads us to information about the early history, chemical composition, and temperature of the Earth, and, therefore,
to the circumstances of its habitability. Stellar and biological evolution are connected.

  What about the future evolution of the Sun? The Sun is steadily growing brighter. About four billion years from now the Sun will be sufficiently brighter that there will be a greatly enhanced runaway greenhouse effect on Earth, just as there is today on Venus. Our oceans will boil, and carbon dioxide, now present as carbonates in the sedimentary rocks, will pour out into the atmosphere. The Earth will be an uninhabitable cauldron.

  It is conceivable that the technology of those remote times will be equal to the task of preventing such a runaway, but it will be an extremely difficult engineering job. However, remarkably, the same increase in the brightness of the Sun some billions of years from now will convert Mars from a place where the average temperature is 100 degrees F below zero to a place that has temperatures almost exactly the same as those on Earth today.

  When the Earth becomes uninhabitable, Mars will gain a balmy and clement climate. Our remote descendants, if any, may wish to take advantage of this coincidence.

  22. Terraforming the Planets

  Both subtly and profoundly, the activities of life have affected the environment of our planet. Our atmosphere is composed of 20 percent oxygen and 80 percent nitrogen. The oxygen is produced almost entirely by green-plant photosynthesis. Similarly, the most recent evidence suggests that nitrogen is almost entirely a product of the biological activity of soil micro-organisms, which convert nitrates and ammonia into the gas N2, molecular nitrogen. Not only are the principal constituents of our atmosphere closely controlled by biological activities, but the minor constituents are as well. To a significant extent, carbon dioxide is also buffered by the photosynthesis/respiration feedback loop. Even so minor a constituent of the Earth’s atmosphere as methane, CH4, is of biological origin.

  In fact, life on Earth, invisible to photography, could be detectable with a small telescope and an infrared spectrometer from the vantage point of Mars. The Martians, if any, could easily observe, at a wavelength of 3.33 microns in the infrared, a strong absorption feature that straightforward analysis would reveal to be due to one part per million of methane in the terrestrial atmosphere. It should not be difficult to deduce that the methane is probably of biological origin. Methane is chemically unstable in an excess of oxygen. It is oxidized rapidly to carbon dioxide:

  CH4+2O2=CO2+2H2O.

  The amount of methane that would be in equilibrium with the great excess of oxygen in our atmosphere is less than one billion billion billionth the amount actually observed. How can this be? Methane must be produced at a rate so rapid that there is not time enough for oxygen to reduce its abundance to the equilibrium amount. It might be that there are massive outpourings of methane from ancient petroleum fields on Earth. But because of the huge output required, this is a very unlikely hypothesis. It is far more likely that methane is produced by a biological process.

  And this is indeed the case. There seems to be a debate in the ecological literature on two possible sources of this methane. One source is methane bacteria, which live in swamps and marshes–hence the term “marsh gas” to refer to methane. The principal other habitat of methane bacteria is in the rumens of ungulates. There is at least one school of ecological thought that believes that more methane is produced from the latter source than from the former. This means that bovine flatulence–the intimate intestinal activities of cows, reindeer, elephants, and elk–is detectable over interplanetary distances, while the bulk of the activities of mankind are invisible. We would not ordinarily consider the flatulence of cattle as a dominant manifestation of life on Earth, but there it is.

  Inadvertently, with no conscious effort by mankind, life on Earth has reworked the environment in a major way. Through the effect of atmospheric pressure and composition on the climate, there is a feedback loop in which the climate itself may to some degree be controlled by the gas exchange reactions in which the life forms on Earth engage. In a way, life on Earth has terraformed Terra. It has to some extent made the Earth the way it is.

  Is it possible that at some time in the future we might be able similarly to terraform other planets, to convert a Mars or Venus, today inhospitable to Man, into a clement and habitable environment? Such a change, if possible at all, should be done only after the most careful and responsible examination of the consequences. We would first want to understand thoroughly the present environment of the planet before altering it. We must scrupulously guarantee that any indigenous organisms on the planet would not be disrupted by terraforming. If Mars, for example, has a population of indigenous organisms that would be extinguished by terraforming, we should never perform such terraforming. But if the planet is lifeless, or if the organisms survive better under conditions closer to our own, it might be reasonable at some time in the future to consider such an alteration of a planetary environment.

  Our motivations for planetary re-engineering must be clear. This is not a solution to the overpopulation problem. Several hundred thousand people are born every day on Earth. There is certainly no prospect in the immediate future of transshipping hundreds of thousands of people to other planets each day. In its entire history mankind has managed to launch one dozen people to another celestial body. Nor are we likely to see in the immediate future a thriving mining industry in which ores are extracted from another planet and transshipped to Earth: The freightage would be prohibitive.

  And yet the human spirit is expansive; the urge to colonize new environments lies deep within many of us. Such activities can be performed without cosmic imperialism, without the kind of arrogance that characterized the European colonization of the New World, or the encroachment on the Indians in the settling by whites of the American West. Interplanetary colonization can be consistent with the highest aspirations and goals of mankind.

  How would we do it? In the case of Venus, as we saw in Chapter 12, there is a crushing atmosphere, composed largely of carbon dioxide, and a searing surface temperature in excess of 900 degrees F. It would seem to be a formidable task indeed to convert this environment into one in which men could live and work without enormous technological assistance. But there is a bare possibility of reengineering Venus into a quite Earth-like place, a possibility I suggested with some caution in 1961. The method assumes that the high surface temperature is produced by a greenhouse effect involving carbon dioxide and water, a conjecture that is much more plausible now than it was then. The idea is simply to seed the clouds of Venus with a hardy variety of algae–a genus called Nostocacae was suggested–which would perform photosynthesis in the vicinity of the clouds. Carbon dioxide and water would be converted into organic compounds, largely carbohydrates, and oxygen. The algae would, however, be carried by the atmospheric circulation down to deeper and hotter levels in the Venus atmosphere, where they would be fried. Frying an alga releases simple carbon compounds, carbon, and water into the atmosphere. The water content of the atmosphere thus remains fixed, and the net result is the conversion of carbon dioxide into carbon and oxygen.

  The present greenhouse effect on Venus is due largely to carbon dioxide and water. The present total pressure on Venus is about ninety times that on the surface of Earth. The Venus atmosphere is largely composed of carbon dioxide. As the carbon dioxide is converted into carbon and oxygen, and the oxygen is chemically combined with the crust of Venus, the total pressure would decline, decreasing atmospheric infrared absorption, reducing the greenhouse effect, and lowering the temperature.

  It is possible, therefore, that the injection of appropriately grown algae into the clouds of Venus, algae able to reproduce there faster than they are fried, would in time convert the present extremely hostile environment of Venus into one much more pleasant for human beings.

  The amount of water vapor in the Venus atmosphere, if condensed on the surface of the planet, would give a layer of water about one foot high–not an ocean, but enough to do irrigation and to provide for other human needs. It is also possible that water
is available bound to the rocks on the surface of the planet.

  No one can estimate whether this is a very likely scenario, or how long it would take to re-engineer the second planet from the Sun. It is perfectly possible that there is some flaw in the idea. For example, the high surface temperature may not be due to a greenhouse effect, but I think this is unlikely.

  In any case, I think terraforming Venus is not impossible. The Nostoc scheme is an example of how human technology and science may, in periods quite short compared to geological time, rework the environment of another planet.

  For Mars, as we saw in Chapter 18, there is now evidence that in comparatively recent times conditions on that planet were much more Earth-like than they are today. We mentioned the likelihood that enormous quantities of carbon dioxide and water are locked in the Martian polar caps, trapped as permafrost and chemically bound to the surface material elsewhere on the planet. Much of this CO2 and H2O may be released from the polar caps into the atmosphere twice each precessional cycle of fifty thousand years. Drs. Joseph Burns and Martin Harwit of Cornell University have considered a variety of technological schemes to induce more clement conditions on Mars hundreds of years from now, rather than thousands. These schemes involve alteration of the orbits of the Martian satellites or of a nearby asteroid to change the precessional motion of the planet, or the installation of an enormous orbiting mirror over the polar cap to melt the material frozen there. Even easier, however, might be to sprinkle carbon black over the caps, heat up the poles, increase the atmospheric pressure, and warm the planet.