The amount of CO2 in the atmosphere is well quantified and there are estimates regarding how much of this is caused by human activity that don't appear to be so

The amount of CO2 in the atmosphere is well quantified and there are estimates regarding how much of this is caused by human activity that don't appear to be source of too much controversy. The warming caused by CO2 is relatively clear, and not seriously disputed. But the direct warming due to CO2 is not what triggers all the public debate about climate change. That comes from the indirect warming associated with hypothesized positive feedback mechanism within the Earth's climate. There is no settled science on this indirect warming so the net affect of human-activity (the measured CO2 in the atmosphere contributed by human activity) is unclear. The climate models have historically over-estimated this net affect when their predictions are compared to the actual measurements.