Of course "We know that climate change has been going on some 4+B years, without human help."  But it is the RATE of climate change (temperature) and RATE of decrease in ocean pH, correlated to the RATE of increase of CO2 in the atmosphere, that is new, different, and very alarming for the future.  

Chem.E.'s are taught to pay very close attention to the rate of changes in chemical reactions which if not controlled can lead to an uncontrollable outcome - BOOM!  The continuous rate of change of temperature and CO2 in the atmosphere is clear evidence that photosynthesis from Earth's biota cannot keep up with the rate of  introduction of greenhouse gasses to the atmosphere. We are morally obliged to respond to these observations in accordance with the precautionary principle -"if there is a strong suspicion that a certain activity may have environmentally harmful consequences, it is better to control that activity now rather than to wait for incontrovertible scientific evidence."