the deniers are saying "its not happening" when every piece of evidence in the world (literally, the world) says "yes it is".
in scientific debate, there are many places to have disagreement and debate. but reality itself isnt one of them.
No one denies it's not happening. What's being denied is whether humans are playing a part in it. We actually don't know regardless of how many times you take measurements and record data. We have no idea what the temperature was one thousand or a million years ago. Dinosaurs were thought to have lived in much hotter climate so post ice age (asteroid theory) we could very well be returning to the original climate of our planet. The only thing we can tell is the earth is slowly warming.