Musings 2

Chapter 4D

Musings 2

Nevertheless it follows from Lovelock’s considerations that life influences to a large extent the climate of the planet. Life largely determines chemical composition, and chemical composition largely determines the range of many crucial physical factors such as freezing and boiling points, which in turn determine whether several substances will be in gaseous, liquid, or solid form. In the deep cold of Titan, the large moon of Saturn, there are large lakes, because the dense atmosphere, mostly nitrogen, contains 1% methane, and methane can exist as a liquid at those pressures and temperatures.[1] Those physical factors will also affect the patterns of heating (e.g., the greenhouse effect) and cooling (e.g., glaciation), and thus the transfer of energy throughout the system. It is not surprising, then, that by removing gases from the atmosphere, or by replenishing them, life on Earth exerts much influence upon the climate.

Stephen H. Schneider criticizes the notion of Gaia as a regulating mechanism, a sort of giant planetary thermostat, by drawing attention to the fact that on occasion life gives a positive feedback to the climatic changes taking place. He assumes too narrow an interpretation of Lovelock’s idea, however, for Gaia is a metaphor not merely for life on Earth, but for the whole interaction of life and the rest of the environment. Some parts of the total environment may thus temporarily accelerate climatic changes, as we have seen, but at some point other parts of the system bring about stability and often reversal. Had it not been that way so far, life would have disappeared from Earth long ago.

Still, the most Lovelock can conclude is that life on this planet is very resilient. But after this point is acknowledged, we still have much reason for concern. For example, he points out that the present industrial pollution of the planet cannot compare with the massive poisoning of the atmosphere by oxygen a billion years ago. When the free hydrogen in the atmosphere had been used up or escaped into space, bacteria began to withdraw it from H₂O in photosynthesis. The waste product, oxygen, was extremely toxic to most of the bacteria that dominated the Earth in those days.[2] Sure enough, life overcame this threat by developing organisms that used oxygen. But we should find no comfort in learning that those evolutionary solutions led to the replacement of one kind of life by another. If we foul up our own planet, life may survive – but we might not. Industrial waste fertilizes the water in our lakes and rivers and leads to the rapid growth of algae. This kills the fish, the frogs, and the water lilies. It is not much consolation to know that upon the ruins of the present order life will adapt and produce a new kingdom of scum.

The pessimists urge us to think about the Earth in the long run, and particularly about the drastic changes that we may bring about in the next few centuries because of our use of fossil fuels. But we may also wish to put their own concerns in the context of the truly long-run, not just centuries, but thousands of years, and then tens and hundreds of millions of years.

For the past 400,000 years, according to the temperature record found in deep ice in Antarctica, our planet has been going through glacial cycles of about 90,000 years, punctuated by short warm interglacial periods (about 11,000 years each). Indeed, our planet has been very cold for the last 2.5 million years and is likely to remain so for the next 2-10 million. As Peter D. Ward and Donald Brownlee point out in their book The Life and Death of Planet Earth, the present interglacial period has been unusually long already.[3] This prolonged period of warm may be accounted for by Milutin Milankovich’s theory of the influence of Earth’s orbit on its climate (to be discussed later in the chapter). But according to some calculations, the present warm period will give way to another glaciation in a few thousand years at most.[4] The temperature record shows, however, that the switch from warm to ice can take place almost suddenly, so we may be due for another “ice age” at any time. Of course, the descent of sheets of ice two miles high upon the Northern hemisphere would devastate our present civilization far more than the current warming of the atmosphere is likely to. Ironically, the much-maligned man-made global warming may stave off the ice for another 50,000 years![5]

In the truly long run, but long before the sun becomes a red giant, the Earth’s thermostat is likely to malfunction. It was Lovelock himself who realized that Gaia would eventually fail as the planet’s self-regulatory mechanism. In a 1982 article with M. Whitfield, he argued that life was steadily removing CO₂ from the atmosphere – it actually has been doing so for the last 400 million years, when plants conquered the land – and in about 100 million years the level of CO₂ will go below 150 parts per million (ppm)of air.[6] This level is important because most plants require at least that much atmospheric CO₂ to survive. Newer forms of plants – grass, palm trees – use slightly different mechanisms for photosynthesis and can go well below the 150 ppm. The flora of the future, then, will have a very different view: gone will be the apple orchards and the rose gardens, replaced by new and exotic varieties of plants. But, eventually, the level of atmospheric CO₂ will go below 10 ppm and photosynthesis will come to an end altogether. More recent studies following on Lovelock and Whitfield’s footsteps have revised their estimate to between 500 million and a billion years.[7]

The loss of plants will be a catastrophe for animals, obviously, but also for marine life, since it depends so much on the run-off of the soil nutrients that result from the presence of plants. Those few animals that can manage to survive will be obliterated in a few million years by the rising temperatures, for eventually significant levels of atmospheric CO₂ will rise in the atmosphere by geological processes but will no longer be kept in check by photosynthetic organisms. Several scenarios have been proposed to explain what will happen after that point. To me it seems simple to imagine that a highly increased level of solar energy coupled with high levels of atmospheric CO₂ will quickly lead to the sort of runaway greenhouse effect that vaporized Venus’ oceans.

It is possible, nonetheless, that Lovelock and those who refined his prediction of doom have not given Gaia enough credit. After all, just as cyanobacteria were able to survive in small, protected pockets the flooding of the planet by oxygen, a few plants may just barely survive near vents that outgas CO₂, lie low until the CO₂ rises again, and then explode once more through ocean and land. Other photosynthetic life on land and in the ocean will thrive also, as their ancestors now do, and together with the plants will begin to regulate the climate and, literally, give the Earth a new lease on life.

Of course, this new, though not improved, version of the planetary thermostat gives even less reassurance than the previous one. Indeed, it turns out that the thermostat has allowed the entire planet to become a ball of ice once or twice in the eons before the Cambrian explosion.[8] We cannot escape the need to reach a better understanding of our global environment.

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[1]. Methane on Titan would thus be the counterpart of water on Earth. Other possible counterparts, in other planets, would be ammonia, which freezes at -78 degrees Celsius and boils at -33 degrees Celsius, and methyl alcohol, whose range is from -94 to +65 degrees Celsius.

[2]. Adapted from L. Margulis and D. Sagan, Microcosmos, Summit Books, New York, 1986, p. 237.

[3] Ward and Brownlee, op. cit., pp. 71-86.

[4] R. Chris Wilson, Stephen Drury, and Jenny L. Chapman, The Great Ice Age, 2000. Cited by Ward and Brownlee, p. 82.

[5] Ward and Brownlee, op. cit., p. 83. On the other hand, global warming, according to some speculations, might trigger the ice age by shutting off the water circulation patterns in the Atlantic (warm water going north closer to the surface and cold water going south closer to the bottom of the ocean).

[6] J.E. Lovelock and M. Whitfield. 1982. “Life Span of the Biosphere.” Nature 296. Pp. 561-563.

[7] This account is borrowed from Ward and Brownlee, op. cit., pp. 101-116.

[8] Ward and Brownlee, op. cit., p. 75.

Conceptual Musings on CO2

CHAPTER 4C

Conceptual Musings on CO₂

At first sight it seems that increases in CO₂ lead to increases in temperature by trapping infrared radiation. And the higher temperature will lead to more water vapor, which will lead to higher temperatures and so on. If this were the whole story, any increase in CO₂ would inevitably lead to a runaway greenhouse effect. If that were to happen the temperature of the Earth would be measured in the hundreds of degrees Celsius (as in Venus) and life would have long disappeared from the planet. But surely there have been fluctuations in the level of CO₂. Indeed, it seems that in the past the level of CO₂ has been much higher than it is now: 10 to 30 times higher in some eras, and also a bit lower than it is now (two thirds of today’s level during the last ice age).[1] Thus we should expect the Earth to be lifeless by now. But of course it is not.

We know, therefore, that the Earth itself must have mechanisms that regulate the level of CO₂ and its effect on temperature. It is clear, then, that to understand the significance of the increase of CO₂ we first need to understand those mechanisms.

Three mechanisms are particularly influential in the regulation of carbon and thus of temperature: plate tectonics, stone weathering, and terrestrial life.

Let us begin with life. As CO₂ increases, plants and bacteria that thrive on it will displace others that do not. But as these life forms become very successful and proliferate, there will be more of them to remove CO₂ from the atmosphere, and thus the temperature will begin to go down.[2]

It used to be thought that plants were by far the main biological sink of CO₂ in the planet, even though as Lynn Margulis and James Lovelock argued many years ago, bacteria have a much greater influence on the composition of the atmosphere than plants do.[3] This particular controversy has been pretty much settled by the launching in 1997 of a satellite capable of keeping a close watch on the world’s populations of phytoplankton (Sea Wide Field Sensor). Whereas plankton remove as much as 45 billion to 50 billion metric tons of inorganic carbon, plants handled about 52 billion metric tons.[4] We should see this pattern repeated time and again: Firm knowledge about problems of the global climate seems more likely when we can investigate them with space science and technology.

Plants, however, contribute to the removal of CO₂ not merely by photosynthesis, but also as part of the so-called “silicate-carbonate geochemical cycle,” which works by taking the calcium living beings produce and combining it with carbonic acid to make limestone. As astrobiologists Peter D. Ward and Donald Brownlee explain

Here we have a wonderful partnership. Animals such as coral are harnessing calcium. The roots of plants exude an acid that helps to break down rocks, accelerating weathering by the wind and rain generated by the atmosphere and oceans, creating the [carbonic] acid necessary to convert the calcium to limestone. All combined are working together to take excess carbon dioxide out of the atmosphere and bury it in “reservoirs” of rock within the Earth, and thus balance temperature.[5]

To understand the regulation of CO₂ we also need to determine the ways in which CO₂ is put back into the atmosphere. The oxygen produced by the organisms that remove CO₂ is itself taken up by other organisms, which end up producing more CO₂ as waste. Plankton stores carbon in the ocean, but much of that carbon is returned to the surface via upwelling and ocean currents in a few hundred years at the most.[6] Most of those carbonates that fall to the bottom of the ocean become part of the crust, and because of the spreading of the ocean floors, through plate tectonics, they are eventually pressed onto continental shells as plates collide (in subduction zones) and finally find their way into volcanic eruptions as CO₂ again. Of course there is a long lag in this geologic cycle, perhaps in the millions of years.

Let us continue, then, with the scenario in which the planet begins to cool. As more water freezes in the polar caps, the level of the oceans drops, and more land is exposed to the wind and the rain. Phosphates and calcium in great abundance come to the oceans. The phosphates feed the plankton, which will then make more carbonates from atmospheric CO₂. As a result, the temperature will decline even more (a case of positive feedback). Will the Earth finally become like Europa, the Jovian satellite, a beautiful ball of ice?

No (but see below). As the ocean surface is reduced, the ability of the planet to support plankton is also reduced. A saturation point is eventually reached, and the temperature becomes stable. After a long time the combination of plate tectonics and volcanism will begin to increase the level of CO₂ and the temperature will begin to rise again (although big volcanic eruptions put up large amounts of dust that initially may cool the planet more instead).

It would be a terrible mistake to conclude from these speculations that the planet's mechanisms are bound to take care of any and all environmental consequences of burning fossil fuels. As a first approximation we might say that all such mechanisms working together have served as if the Earth had a thermostat. But all the mechanisms guided by thermostats operate successfully only within limits. On a very hot summer day, the air conditioner may be stretched beyond its specifications and be unable to bring the temperature down anywhere near the thermostat setting. Likewise, if the energy that we receive from the sun were to increase continuously, as it presumably will – up to the apocalyptic end in a few billion years when the sun will become a red giant – the feedback mechanisms of the planet will moderate the temperature somewhat, but the heat from the sun will eventually overwhelm, and finally scorch into oblivion all: life, oceans, and air.

Short of that calamity, many changes in the environment can be disastrous enough. At first sight, however, we find reason for optimism in the work of some observers. In presenting his famous Gaia hypothesis, J. Lovelock has compared the totality of Earth's life, the biota, to a super-organism with a fierce instinct for self-preservation. As his “Gaia” metaphor is pressed to explain the natural history of our planet, it does become clear that life has always managed to adapt to profound changes in the environment. It also becomes clear that the biota is a very effective mechanism in the regulation of that environment.

Indeed, if a spaceship were exploring our solar system, it might be able to determine from a long distance that life was plentiful on Earth. The reason is that our atmosphere is not chemically stable. For example, oxygen forms 21% of the volume of the atmosphere. From a purely chemical point of view this high percentage is very surprising, for oxygen is a very reactive element (it combines easily to form compounds); thus it should be swept up in a rather short time. Life, however, replenishes the free molecular oxygen that is lost to chemical reactions. And given the large amount of oxygen, the percentage of other gases would be impossible except for the action of life. Methane, for instance, is 10²⁹ times more abundant than it ought to be. According to Lovelock and Margulis, nitrogen is one billion times and nitrous oxide ten trillion times more abundant than they would be, given chemistry alone.[7]

Since nitrogen makes up nearly 75% of the atmosphere, the impact of life on the composition of the atmosphere cannot be underestimated. Although nitrogen would not be detectable in the Earth’s spectrum, the exploring spaceship might still be able to determine the existence of life on Earth, long before arriving, from the extremely high concentrations of trace gases such as methane and nitrous oxide.[viii]

The converse is also true. By spectral analysis, Lovelock determined in the early sixties that life on Mars would be very unlikely. His determination was confirmed when the two Viking spacecraft landed in 1975 and found that the chemistry in the Martian soil could not support life. Lovelock had played the role of the visiting spaceship scientist and drew the appropriate conclusions from the fact that Mars' atmosphere is in chemical equilibrium.[ix] As we will see later, however, our enthusiasm for this approach should be tempered by the realization that the atmosphere of Venus is not quite in equilibrium, even though Venus is lifeless.[x]

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[1]. For an account see H.D. Holland, B. Lazar, and M. McCaffrey, "Evolution of the Atmosphere and Oceans," in Nature. Vol. 320, March 6, 1986. p. 33. For variations in CO2 levels throughout the history of the planet see E.T. Sundquist and W.S. Broecker, The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. American Geophysical Union, Washington, D.C. 1985.

[2]. The importance of life to the climate has been particularly emphasized by J. Lovelock and L. Margulis. See their "Atmospheres and Evolution," in J. Billingham, (ed.), Life in the Universe, MIT Press, 1981, pp 79-100. See also their "Atmospheric Homoestasis by and for the Biosphere: The Gaia Hypothesis," Tellus 26:2, 1973. Most of the scenarios described in the pages below are offered merely to illustrate the many factors that may play a role in the behavior of the climate. They should not be ascribed to any one particular investigator.

[3]. Lovelock and Margulis. Op.cit.

[4] P.G. Falkowski, “The Ocean’s invisible forest,” Scientific American, Vol. 287, No. 2, August 2002, pp. 56-57.

[5] Peter D. Ward and Donald Brownlee, The Life and Death of Planet Earth, Times Books, 2002, p. 61.

[6] Ibid., p. 57.

[7]. Lovelock and Margulis, "Atmospheres and Evolution." Op.cit. p.81.

[viii]. This is a recurring theme in Lovelock's work. See his article in Nature, London, No. 207, p.568.

[ix]. See Lovelock's analysis of J. and P. Connes (J. Opt. Soc. Am., 1966, No.9, p. 896) in Lovelock and A.J. Watson, "The Regulation of Carbon Dioxide and Climate: Gaia or Geochemistry," Planetary and Space Science, vol. 30, no. 8, p 795.

[x] Although the Viking controversy is not entirely settled, as we will see in Ch. 6.