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). 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.
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. 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. 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.
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. 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.
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]
. 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.
. 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.
. Lovelock and Margulis. Op.cit.
 P.G. Falkowski, “The Ocean’s invisible forest,” Scientific American, Vol. 287, No. 2, August 2002, pp. 56-57.
 Peter D. Ward and Donald Brownlee, The Life and Death of Planet Earth, Times Books, 2002, p. 61.
 Ibid., p. 57.
. 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.