Pollution by vehicles, CFC, industries that let out smoke and the increase in Co2.
No single actor causes global warming.
Contributing factors are mostly around green house gases proportionaly increasing in the atmosphere as a whole.
Human contributions to this are important if for no other reason than that they tend to push the system toward an ever increasing temperature rise and increasing concentrations of greenhouse gasses.
It's a hoax
Human production of green-house gasses, and possibly natural environmental cycle.