But even Gaia herself, poor lady, might not emerge unscathed from the fight. She may be robust, but she is not eternal. Look at this graph, from a paper by Franck, Bounama and Von Bloh,
As you see, the earth's biosphere, Gaia, peaked with the start of the Phanerozoic age, about 500 million years ago. Afterwards, it declined. Of course, there is plenty of uncertainty in this kind of studies, but they are based on known facts about planetary homeostasis. We know that the sun's irradiation keeps increasing with time at a rate of around 1% every 100 million years. That should have resulted in the planet warming up, gradually, but the homeostatic mechanisms of the ecosphere have maintained approximately constant temperatures by gradually lowering the concentration of CO2 in the atmosphere. However, there is a limit: the CO2 concentration cannot go below the minimum level that makes photosynthesis possible; otherwise Gaia "dies".
So, at some moment in the future, planetary homeostasis will cease to be able to stabilize temperatures. When we reach that point, temperatures will start rising and, eventually, the earth will be sterilized. According to Franck et al., in about 600 million years from now the earth will have become too hot for multicellular creatures to exist.
Of course, the extinction of the biosphere is not for tomorrow or, at least, the calculations say so. But it is like estimating one's lifespan from statistical data. Theoretically, the homeostatic mechanisms that operate your body could keep you alive until reach a respectable age; sure, but homoeostasis is never perfect. For instance, there are mechanisms in your body designed to reverse the effects of traumas. You may expect these mechanisms to work well if you are young but, if you are hit by a truck at full speed, well, you end up on the wrong side of the life expectancy statistics.
Similar considerations apply to Gaia. Theoretically, the planetary homeostatic mechanisms should keep Gaia alive for hundreds of millions of years, but what about major perturbations, some planetary equivalent of being hit by a truck? Would Gaia be able to recover from a human caused runaway greenhouse catastrophe?
We cannot say for sure. What we can say is that we are living in a period called the "sixth extinction," similar to other major past extinctions. In most cases, these extinctions appear to have been caused by an increase in the concentration of greenhouse gases in the atmosphere. The sixth extinction, too, is taking place in correspondence to a rise of the concentration of carbon dioxide in the atmosphere that may never have happened so fast in the history of the planet. This rapid rise is also taking place under a solar irradiation that has never been so high as it is today. We can't rule out that the sixth extinction will be the last one.
So, in the fight of man vs. Gaia neither one might be left standing. That's just a possibility, of course, but one thing is certain: in this fight, the enemy is us.
Thanks to Antonio Turiel for the link to humon's cartoon and to Weissbach for the link to George Carlin's movie
