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. 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. 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. 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. 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!
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. 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.
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. We cannot escape the need to reach a better understanding of our global environment.
. 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.
. Adapted from L. Margulis and D. Sagan, Microcosmos, Summit Books, New York, 1986, p. 237.
 Ward and Brownlee, op. cit., pp. 71-86.
 R. Chris Wilson, Stephen Drury, and Jenny L. Chapman, The Great Ice Age, 2000. Cited by Ward and Brownlee, p. 82.
 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).
 J.E. Lovelock and M. Whitfield. 1982. “Life Span of the Biosphere.” Nature 296. Pp. 561-563.
 This account is borrowed from Ward and Brownlee, op. cit., pp. 101-116.
 Ward and Brownlee, op. cit., p. 75.