While the IPCC talks about "tipping points," it does not mean "cascades" that "threaten the existence of human civilization." It's important to understand that what the IPCC means by "tipping points" is certain irreversible changes in things like sea ice coverage, or collapse of maize crops in "certain areas." Those things might be bad, but they don't imply "runaway climate change" or collapse of global agricultural systems, or anything like that. See Table 3.7.
The aggregate effect of these local effects is estimated in box 3.6. For the United States, it says:
> The results for the baseline no-policy case indicate that economic damages along median temperature change and median damages (median-median) reach 4.5% of GDP by 2100, with an uncertainty range of 2.5% and 8.5% resulting from different combinations of temperature change and damages. Avoided damages from achieving a 1.5°C temperature limit along the median-median case are nearly 4% (range 2–7%) by 2100.
That's the bottom-line figure quoted in the article I linked. Avoided damages from keeping climate change to 1.5C are 2-7% of GDP by 2100. That means significant disruptions to the structure of our economy in response to climate change will almost certainly cost us more than what we avoid in damage from climate change.
The last section of the Nature article says that in the authors' view, the aggregate effect of these tipping points could be worse than the IPCC projects. (Note that the above GDP figures do attempt to account for tipping point effects.) That may or may not be true, but that conclusion is not part of the IPCC consensus.