Are humans definitely causing global warming?

Are humans definitely causing global warming?
This question and answer is part of the Guardian's ultimate climate change FAQ
The only way to prove with 100% certainty that humans are responsible for global warming would be to run an experiment with two identical Earths – one with human influence and one without. That obviously isn't possible, and so most scientists are careful not to state human influence as an absolute certainty. Nonetheless, the evidence is now extremely strong.