Burning fossil fuels has increased the concentration of CO2 in the atmosphere in a measurable way. By itself, this would lead to increased temperatures through the greenhouse effect. Unless other human activities or natural negative feedbacks have fully offset the effect, the bottom line is that the Earth is warmer now than it would have been otherwise. Increased CO2 is evidence of AGW. Sure, let’s discuss the amount of AGW and its impact, but let’s not question the reality.
Suppose T(t) represents the temperature T of the Earth at t years in the future. Suppose we have no way of accurately modeling or predicting T in future times; it may be warmer, it may be colder, suppose we just don’t and can’t know. That doesn’t preclude science from predicting that with increasing CO2, future temperatures will be something like T(t)+C(t), where C is some positive function of time. It is not necessary to have a fully working accurate predictive model of the Earth’s temperature in order to be able to predict that increased CO2 will mean an increase in temperature above what it would otherwise be.
It's often strange reading these comments. I wonder to myself "did I really write that?"