Let's quote a bit more from this study:
In light of such findings, we should be asking when abrupt change will happen, what the impacts will be, and how we can prepare ñ not whether it will really happen. In fact, the climate record suggests that abrupt change is inevitable at some point, regardless of human activity. Among other things, we should:
Notice the "inevitable...regardless of human activity."
In sum, the risk of abrupt climate change remains uncertain, and it is quite possibly small. But given its dire consequences, it should be elevated beyond a scientific debate. Action now matters, because we may be able to reduce its likelihood of happening, and we can certainly be better prepared if it does. It is time to recognize it as a national security concern.
Notice that this risk of cataclysmic change is "uncertain" and "quite possibly small".