Personally, I haven't looked at that much data on the issue but there is a pretty strong concensus among scientists that humans are having an effect on the environment which will result in warming. I generally trust the scientific community (for example they say atoms are made of protons, neutrons, and electrons but I have never verified this for myself) so therefor I believe humans are having an effect on the climate.

Obviously its pretty much impossible to know exactly how much is human caused and how much is natural so there is a certain amount of error no matter what is said.