The trigger is actually around 2 degrees higher average global temperature. After that we lose control of the process: ending our own carbon-dioxide emissions would no longer be enough to stop the warming. We may end up trapped on an escalator heading up to +6 degrees with no way of getting off. And +6 degrees gives you the mass extinction.
There have been five mass extinctions in the past 500 million years, when 50 per cent or more of the species then existing vanished, but until recently the only people taking any interest in this were palaeontologists, not climate scientists. They did wonder what had caused the extinctions, but the best answer they could come up with was ''climate change''. It wasn't a very good answer.
...What they discovered was that there was indeed major warming at the time of all the other extinctions - and that the warming had radically changed the oceans. The currents that carry oxygen-rich cold water down to the depths shifted so that they were bringing down oxygen-poor warm water instead, and gradually the depths of the oceans became anoxic: the deep waters no longer had any oxygen.When that happens, the sulphur bacteria that normally live in the silt (because oxygen is poison to them) come out of hiding and begin to multiply. Eventually they rise all the way to the surface over the whole ocean, killing all the oxygen-breathing life. The ocean also starts emitting enormous amounts of lethal hydrogen sulphide gas that destroy the ozone layer and directly poison land-dwelling species. This has happened many times before.
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