From THE LIFE AND DEATH OF PLANET EARTH, mit comments by M.E.
Certainly we have shown that the conditions for the Earth’s rich diversity did not pop into being, but instead were the result of billions of years of geologic processes, the modification of the atmosphere, and evolution. A complex system has arisen in which plants and animals provide services to one another and in which photosynthesis and animal respiration have kept oxygen levels roughly constant at 21 percent of our atmosphere for the past 400 million years. Life, in a sense, has terraformed the planet, and it is our hope and belief that we inhabit a young and vigorous world where life’s greatest moments are still ahead. But what if the earth system that sustains this planetary perfection begins to break down? What if the Earth is already well into old age? This is something that the planetary “doctors” are beginning to believe. What is the prognosis from the Earth’s last “checkup”?
A trip to the doctor’s office means - and involves - different things depending on our stage of life. As babies we are in for observation and inoculations. At this stage no doctor is overly worried about the squalling baby being too fat. But we are concerned about those babies who are not fat enough. Predictions are usually made during such visits, predictions that are based on a series of observations through time: the child will reach such and such height or weight at the cessation of growth - predictions usually about the future success of the organism. Later in childhood, trips to the doctor take place only in reaction to events or changes: mainly childhood injuries or the occasional severe flu. Even as young adults we rarely see the insides of a doctor’s office unless the uncommon accidental cancer or life-threatening injury intrudes. But at some indefinable age - that strange ontogenetic stage known as “middle age” - visits to our doctor change in nature and importance. For the first time the emphasis switches to a new and more ominous goal - predicting when and how your life will end, and how to delay that inevitability. Now the doctor’s ministrations deal with measuring specific attributes of organ system function as a means of predicting when they might fail. Those of us in this time know these travails all too well: blood pressure, cholesterol, kidney function, and liver function. The search for cancers. The workings of the heart. The degradation of the reproductive system. The changing chemistry of the body and its hormones. When examined year after year, these readings provide the doctor with a rough idea of how the particular body being examined is declining toward death - for that is the central fact of life in middle age, that a slow decline has set in, with an invariable end. The only question is how long it will take, which systems will fail first, and what variables will either extend or subtract from the time yet allotted.
In a rough sense this analogy extends to planet Earth as well. What we might call the “Age of Animals” is well into middle age. It began, as we have seen, only some 500 million to 600 million years ago - on a planet 4.6 billion years of age [sic - at that time, Earth was actually 4.1-4.0 billion years old; subtract the beginning of the “Age of Animals” from 4.6 billion, the age of the Earth now, to get that latter figure. - Monty Eisenstein]. There are specific tests and measures that astrobiologists - the doctors of planets - can take (and have been taking) that, when averaged over time, yield important clues about how much time is left not only for animal life but for all life on Earth, and even for the Earth itself. And in some ways it is easier to predict the ends of the Earth than the end of specific organ systems of a human, for the Earth has fewer major systems that control the fate of life on the planet. Ultimately there are but two essential measures that will control the fate of life on the planet: surface temperature and the amount of carbon dioxide in the atmosphere. And like slightly mad doctors prodding a particularly interesting patient, the planetary doctors have been measuring vital signs in the present day - and comparing these data to their equivalents stored in the Earth’s rock record. The result is almost medical: there is a good record of the temperature of patient Earth through time, and of its vital chemistries. Predictions of how long this patient has, and what will end its various components, can indeed be made - and have been made. The results are surprising - but such is always the case when a diagnosis of approaching death is made.
The Earth, as a habitat for animal life, is in old age and has a fatal illness. Several, in fact. It would be happening whether humans had ever evolved or not. But our presence is like the effect of an old-age patient who smokes many packs of cigarettes per day - and we humans are the cigarettes.
- Peter D. Ward and Donald Brownlee, The Life and Death of Planet Earth: How the New Science of Astrobiology charts the Ultimate Fate of Our World (Times Books, 2002; ISBN 978-0805075120; http://www.amazon.com/Life-Death-Planet-Earth-Astrobiology/dp/0805075127,, pp. 45-47