For COP26 this is actually true in a very specific sense. The meeting in Glasgow really is the last chance for the world to embark on the drastic cuts in emissions required to achieve the Paris goal of keeping global temperatures from rising by more than 1.5 degrees Celsius.
The reason is that a 1.5C pathway means cutting emissions by about half by 2030. This isn't going to happen unless we start now. If we delay any longer, the required carbon cuts would become so drastic that they would require shutting down large sectors of the global economy in an even more dramatic way than during the Covid lockdown –permanently.
If we start in 2022, however, a smoother transition to clean technologies that allows modern life as we know it to continue remains possible, just about. But if we miss the COP26 deadline, we can pretty much forget about Paris.
What does failing to achieve 1.5C mean? Well, there is a good reason why the foremost advocates for 1.5C are the Climate Vulnerable Forum countries, the group of 48 developing nations, who are the most vulnerable to and least responsible for climate change, and also least able to adapt due to their lack of resources.
Accepting the fallback Paris goal of 2C would mean, according to the Intergovernmental Panel on Climate Change, a much faster rate of sea-level rise, which would permanently displace an additional ten million people and put 136 coastal megacities at increased risk of flooding.
The long-term viability of entire nations like the Maldives, Tuvalu and the Marshall Islands would be jeopardised, and Bangladesh would lose a substantial fraction of its territory with millions more displaced. An additional 1.7 billion people would be exposed to severe heatwaves, 420 million to extreme heatwaves and 65 million more to deadly heat.
World food supplies would be threatened, with net reductions projected for staple crops like maize, rice and wheat due to extreme heat and drought. Accepting 2C rather than 1.5C means losing 99 per cent of the world's coral reefs: the virtual extinction of an entire global biome.
Of course, current global policies don't just put us on course to miss 1.5C but also 2C, with the combined Paris pledges submitted by nations currently taking us to a 2.4 to 2.9C world.
Scientists are clear this climate future would be catastrophic: it would make the planet hotter than at any time since the Pliocene, more than three million years ago. That was a time when there was no ice at the North Pole and the Greenland ice sheet was drastically reduced, resulting in multi-metre sea-level rise (it would take centuries to melt Greenland and Antarctica, but rates of sea-level rise would accelerate dramatically this century).
At 3C vast areas of South Asia would become unliveably hot, with crop yields drastically reduced in all the world's major crop-producing regions. Much of the world's wildlife would be wiped out, with the Amazon rainforest tipping into full-scale collapse, and 12 million square kilometres of Arctic permafrost entering the melt zone and adding further methane and CO2 to the planet's atmosphere.
Many government climate pledges are never met, and if emissions continue to rise due to coal-burning in China and India – maybe with a 2024 return to power of Donald Trump leading to an American fossil-fuel renaissance – we could be on a pathway to 4C in the second half of the century.
Four degrees would make the southern US as hot as Death Valley in the summer, with large areas of the planet's tropical belt, home to two to three billion people, becoming biologically uninhabitable to humans. Southern Europe, Central America, much of Brazil, coastal Australia and southern China would become desertified and hyper-arid, with wildfires incinerating the world's last great forests, including the boreal belt in Siberia and Canada.
At 4C, virtually all ice has gone from the planet's great mountain ranges, while tropical cyclones hit areas like northern Europe. Lethal heatwaves sear the world's crops, with growing season temperatures above the thermal tolerance thresholds of all major food crops. With billions of people on the move, famine ensues. An additional trillion tonnes of carbon is now in the Arctic thaw zone, adding an additional upwards kick to Earth’s temperatures.
Let's say we're both unlucky with climate feedbacks and stupid with emissions – we carry on increasing coal consumption throughout the century, even as the heatwaves strike and the Arctic turns to mush. That way lies an unrecognisable world, 5C or even 6C hotter by century's end.
This is a world where most currently inhabited areas are uninhabitable because of heat, drought or a combination of the two. Colossal superstorms wreck coastal areas worldwide, while food production is only possible in cooler refuges in the high latitudes.
This is not a world where civilisation as we know it continues, or where eight or even ten billion people can be supported. We will have transformed our planet into a rocky, lifeless wasteland surrounded by hot, stagnant oceans. Warming at this level even imperils the survival of humans as a species. Over the longer term, heating this extreme raises the prospect of a runaway greenhouse effect that evaporates the oceans and sterilises the biosphere, turning the Earth into Venus a billion years too soon.
So don't let anyone tell you that COP26 doesn't matter. It won't decide our entire future, but it does represent a moment where we choose: will we begin to turn the carbon ship around, and move gradually into cooler waters, or will we carry on powering towards the rocks?
Glasgow is an opportunity for us to make the right decision. Make sure your voice is heard.
Mark Lynas is the author of Our Final Warning: Six Degrees of Climate Emergency