The bottom line conclusion is that there is a small - of order 5% - risk of global warming creating a situation in which a large fraction of the planet was uninhabitable (in the sense that if you were outside for an extended period during the hottest days of the year, even in the shade with wet clothing, you would die). To give you a feeling for the likely uninhabitable regions... it includes most of the eastern US, much of inland Brazil and Latin America, tropical Africa, pretty much all of India, portions of northern China, and most of Australia. Plenty to qualify as a "Risk to Global Civilization", I think.We are, it's clear, currently in a "do nothing serious about climate change" mode. If this continues, we're looking at CO2 concentrations going up, close to quadrupling pre-industrial levels, and putting us very much in the apocalypse. 1000 ppm would certainly ruin the oceans from acidification alone. This paper just demonstrates that some of the most densely populated parts of the planet would also die.
This is all conceivable by 2100, 2150 tops. That's the status quo.