(a) Evolution of global surface temperature (solid green line). The green dashed line denotes a second possible evolutionary path triggered by a temperature perturbation in the Neoproterozoic era. (b) Evolution of the cumulative biosphere pools for procaryotes (red), eucaryotes(green), and complex multicellular life (brown). Franck, et al., 2005

By Ugo Bardi
17 June 2011

[…] Gaia herself, poor lady, might not emerge unscathed from the fight. She may be robust, but she is not eternal. Look at this graph [above], 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 earth' temperature. 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 you 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? …

Man vs. Gaia

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