How long have we been saying this?
Let's see, I first noticed global climate change in 1974, in the Snowy Range of Wyoming. I talked to biologists, geologists, foresters and archaeologists. They all agreed that, yes, the climate has been getting warmer and drier since the end of the Pleistocene, some 12,000 years ago.
So why is it such a big surprise, with a dramatic global gasp of astonishment, that, yes indeed, the global climate is changing and this change has dire consequences for human "civilization" (if that's what it is, which I doubt)?
Where has everyone been the past 12,000 years, out for a soda and a packet of crisps? Why did my Earth Science professor tell me in 1972 that human agriculture was developed during an unusually clement period of history, and that, soon, it would change back to its normal state, one in which human agriculture would not be so much at home? How did he know (and we, after he told us) and no one else knew?
Whether or not human industrial activity has "caused" climate change is sort of a moot point. The fact is clear that the climate is changing and, at this point, there's not a whole hell of a lot humans can do about it, especially considering current political and economic realities. The best we can hope for is to accommodate the inevitable changes ahead and hope that our societies can withstand massive economic collapse and precipitous population declines.
It's not even particularly important that we recycle more, drive more fuel efficient cars and turn down our air conditioners a notch. The change is set in motion and no small changes on our part will make one whit of difference in the outcome.
What will make a difference is when our industrial society collapses under its own weight, when unbridled human growth and consumption slows and reverses, and when humans once again take their place beside all other life on Earth, rather than on top of them. Then global climate change will proceed at its own pace, the remaining humans will accommodate to it, and in a thousand years or so, everything will stabilize into a dynamic equilibrium once again.
Some human societies will continue, those that live within natural limits of resource exploitation and waste production. Some human societies will change and adapt... most will not.
It doesn't look like anyone is going to invent a time machine any time soon. At least no one has shown up to see what we were up to in the 21st Century. So it's up to our descendants, if we have any, to work their way into the future.
And curse us in the past.