We know that the earth has undergone many climate changes without man's disturbance throughout its existence, correct? How do we know that today's climate change isn't a natural phenomenon?
I think it may be another 10 years before anyone can give you a 100% response to that question.
But what people tend to forget is that with challenges like understanding HIV, or linking smoking to cancer, it is an evolving field. More info appears regularly, and as it does, our understanding deepens and changes.
I've never understood people saying that some of the claims made 20 years ago about climate being inaccurate proves the field is a conspiracy - to me the fact that scientists are willing to admit error and critique each other shows the field is healthy and honest.
But to answer your question in short - the reason is the level of CO2 in the atmopshere. We know that mankind produces large amounts of CO2, and we know that the concentration of CO2 in the atmosphere is dramatically higher now than it was 50 years ago. We know that CO2 produced by nature seems to have been in balance, and most scientists conclude that rising levels of CO2 being released by man has, to coin a phrase, overloaded the system.
There are a dozen websites which can explain this better than I can: try the UK Met service.
Climate - Met Office