The point you're missing is that in the past, the CO2 level has ALSO been higher (in some cases MUCH higher) than it is today. The rate of change means zip. What matters is the concentration in the atmosphere.
Not quite accurate, WW. The climate system has a lag time of response, mainly due to the heat content of ocean water. Over millions of years in paleoclimate history, the rate of change doesn't mean much. But over centuries, it is an important variable.