If we don't start now, when? It's the kind of thing that you can always put off for another million years.
Why? Tons of other species have been wiped out very easily, and they didn't even contribute to their own demise as much as we did.
No other species on the planet has attained a comparable ability to bend the environment to its needs like we have. Everything's had to evolve to live in differing conditions. We start fires, invent and build air conditioners, etc.
Most of them due to an external force they could not begin to comprehend actively or accidentally trying to kill them (ie humans). Also we are on the whole smarter, more creative and better with tools than they where.
they didn't even contribute to their own demise as much as we did.
The fact the we actively contributed to our (potential) demise gives us a massive head start in that we understand and can see the problem coming in a way no other species could. Realistically anything that could wipe out humans would have to be something so completely alien and beyond our current understanding or control that we cannot even currently imagine it. Basically anything we can comprehend we can control for and counter.
And so in the same way 'it is easier to imagine the end of civilization than to imagine the end of capitalism' the idea that we could survive for even a year without exploiting the planet beyond steady-state seems so unrealistic as to not be mentionable.
Sure, 99% of the population might die, the equator could become uninhabitable, and we might lose advanced tech in the ensuing wars, but it'll take much more than climate change to completely wipe out homo sapiens.
We do need to be colonizing other stars within a quarter billion years or so though. The sun won't last forever!