It isn't necessarily just a metaphor. The idea that the human population being reduced to a tiny fraction of what it once was is totally survivable isn't undisputed fact. Us surviving times where there has been a narrow amount of human life have all happened when we were hunter-gatherers or at the very beginning of things approaching civilization. The vast majority of humans today are dependent on each other and technology. The climate is also different - the sun is warmer than the last time the green house gasses on this planet rose to their highest concentrations, and we know that runaway global warming can turn a planet completely uninhabitable for human life - just look at Venus.

Yes, there are plenty of scenarios where a small amount of humanity lives on and repopulates the world over time. It certainly can happen. But there's also plenty where we don't. The author doesn't bring up any accurate science to back up his claims, or make any real arguments beyond 'We've survived population bottlenecks in the past', so I'd say that 54% of Briton's are right - climate change is a threat that could cause extinction. It isn't guaranteed to, even unchecked... but the question isn't whether or not it's a guarantee of extinction for humanity, or a threat possible of causing it. And it certainly is.

> The idea that the human population being reduced to a tiny fraction of what it once was is totally survivable isn't undisputed fact.

That would mean it is a metaphor since this wouldn't end the human race.

I'm saying that if some event brings the human population to .1% immediately and then that .1% dies out in the future because it cannot sustain itself, that event might as well have directly ended the human race, even if some remnant lingers on temporarily.

It's 0.000004286% of the human population that would be considered a minimum viable population size, and of course that's considered stable. We know human population was less than that at least twice in history. For the vast majority of human history human population was about 0.000171429% of what it is now.

Both are vastly less than the .1% you claim to be fatal. What is your basis for claiming this number would end the human species ?