How do we know there’s a scientific consensus on climate change?

How many times have we heard that 97 per cent of Scientists agree that global warming, errr I mean climate change is cause by humans. Therefore, if that's the case the consensus is in! There is no need for debate! If you don't agree you are a denier! You believe the holocaust didn't happen. Yes, we have heard it all. But, where did the 97 percent figure come from. Here is your answer.

– Gary