There are people who say it's not happening. Some of these are the same people who said it in the late 80s the first time global warming was an imminent crisis. Some of those are the same people who said that global cooling wasn't happening either the decade before that when an ice age was the imminent crisis.
The entire field of science is not convinced that global warming is happening, or that it is caused by humans, or that an increase in carbon dioxide causes an increase in average global temperature, or that an increase in global temperature leads to those images of terrible devestation you see on TV.