There were five extinctions of life on our planet. One of them, the Permian-Triassic event 251 million years ago cause the extinction of 96% of sea life and 70% of terrestrial vertebrate species. This was likely to have been caused by a sudden methane release from the ocean floor flooding the atmosphere with a greenhouse gas dramatically increasing global temperatures.
This happens because of sudden influx of greenhouse gases into the atmosphere. When it is slower the ocean and atmosphere is able to handle it. That is what happened at the Permian-Triassic, a sudden huge amount of methane thrown into the atmosphere. Skeptiks can say that there have been times with more CO2 or methane in the atmosphere without global warming, but that is because it is gradual. When it is sudden, as we are doing right now to the atmosphere, the oceans aren't able to handle it and we have severe global warming.
But is what is happening now anything like what happened 251 million years ago when 70% terrestrial life was extinguished? Well there is now evidence that something like what happened then can happen to us. There is now evidence of methane bubbling to the surface in the East Siberian Arctic Ocean. The Arctic is warming faster than other parts of the planet, and we are seriously in danger of something like what happened 251 million years ago. If so, civilization is certainly doomed.
Methane is much more short lived than CO2. The sudden influx of methane would bring the end of civilization, but also the end of the product of CO2. So then we might have the prospect of 30 generations of human survivors finding a congenial planet in about a thousand years, not much more than the length of the Dark Ages. So again we may have another Enlightenment, drawing not just on the Greeks but on the Industrial age. So perhaps we shouldn't be so depressed.
And maybe someday humans will halt this cycle of ignorance (dark ages) and knowledge (Enlightenment), observing that religion is the basis of the ignorance and science the basis of the knowledge.