I guess the questions for those who don't believe in anthropogenic climate change are;
- Is the climate changing?
- Does CO2 cause a 'greenhouse effect' in a closed system?
- Is the amount of CO2 rising?
- Is there a link between rising CO2 and rising global temperatures?
- If no, what is causing the climate to change?
- Is the rise in CO2 due to human factors?
- If no, why is the level of atmospheric CO2 rising?
Due to the scientific method, you cannot say to 100% certainty that there is a specific cause to a certain effect. However, you can start to put percentage likelihoods that something will happen. Climate science is continuously trying to firm up those percentages but, as I said before, trying to model an entire planet is hard!