The thing with this type of scenarios is that they assume a fundamental instability - the existence of a hidden self-reinforcing feedback loop, or "positive" fe

The thing with this type of scenarios is that they assume a fundamental instability - the existence of a hidden self-reinforcing feedback loop, or "positive" feedback (as opposed to "negative", or self-reducing) in the environment. Once you release an agent capable of exploiting that pathway, there's no stopping it. But in the billions of years past, if such a feedback loop existed, it would have been exploited already. Perhaps the Great Oxygenation Event was one such loop being exploited. http://en.wikipedia.org/wiki/Great_Oxygenation_Event http://en.wikipedia.org/wiki/Great_Oxygenation_Event Would the release of the first hydrogen bomb trigger a mass fusion through the entire atmosphere and hydrosphere? You could calculate the physics, as you should, but you could also notice that this requires a positive feedback loop, so it's extremely unlikely. The examples could continue.