The planet isn't yet as warm as it was 12,000 years ago. There were no humans causing the previous warm eras.
For fuck's sake, is this a serious opinion? What exactly are you arguing here? Noone is denying that the climate of the earth changes for all sorts of reasons, and that over the course of millennia it changes significantly. Also, species become extinct over time, because they fail to adapt quickly enough to the changes. This is normal an natural.
What is happening at the moment is a hugely accelerated version of this, and whilst there are all sorts of reasons why this may be happening, te evidence overwhelmingly supports human activity as the cause of this unprecedented acceleration. The effect of this change will be the same as any natural change in climate - many species will fail to cope and after a period of decline will become extinct. But because the rate of change is so extreme, the number of species that will be affected will be vast. It's going to be chaos.
And even if we consider that extinction and environmental change is normal (albeit not at the current rate), it's still pretty rubbish if you are the species suffering. The planet will become smaller due to rising sea levels, water supplies will become unpredictable, agriculture will be extremely difficult in an unpredictable climate. Millions/billions of people will starve; global wars will be started over resources; it's going to be horrible. Not to mention all the wonderful diversity of life that will also be wiped out. Why would you want this to happen? Why would you not take steps to try to prevent this? Why just say 'oh well, these things happen naturally, so let's not worry about it'?! Even if there is doubt that we are causing it (and there really isn't), surely we should ere on the side of caution, because if we do take steps it's only a smallish effort that is needed, and if we don't it could be catastrophic.
I don't care if the world was uninhabitable for humans millennia ago; I don't want it to be uninhabitable now.