That “one thing” I referenced in the title is NOT in the picture.
The one thing is in the alt-text at XKCD that you normally only get to read if you hover your mouse over the picture there. It says:
“The good news is that according to the latest IPCC report, if we enact aggressive emissions limits now, we could hold the warming to 2°C. That’s only HALF an ice age unit, which is probably no big deal.”
In other words, even if global warming is human-caused, it can’t be human-stopped except with human-extinction.