Global warming will not necessarily mean a slow, steady rise in temperature, to which we can gradually adjust. Climate history contains sudden, lurching reconfigurations. Says the IPCC, as quoted by the U.S. EPA, "complex systems, such as the climate system, can respond in non-linear ways and produce surprises."
We might be able to adapt to an incremental warming trend, but such a surprise — e.g., the shutdown of the thermohaline circulation — would be likely be catastrophic.
What are the chances of a sudden shift? No one knows for sure. Low, we think, for now, probably. Such things are, almost by definition, difficult to predict with any certainty. What we do know is that we increase the probability of such a catastrophe with every ton of CO2 we put in the atmosphere.
We are rolling dice with humanity’s future.