There really isn't much debate in scientific circles that global average temperature has increased ~1 degree in the last century. There isn't even much debate that greenhouse gasses are the chief cause of the increased temperature. Even the sources for the greenhouse gasses aren't in much dispute in scientific circles. There is a consensus scientific view that global warming is happening because of man-made activities.
The contrary view also has scientists who dispute the evidence of global warming. With any scientific claim, there are always those who disagree. These scientists represent the minority scientific view.
Global warming is a scientific hypothesis with lots of evidence to support it. There is very little evidence to suggest that global temperatures aren't rising. Deniers focus on small contradictions, inconsistencies and anecdotes suggesting temperatures aren't rising, and that natural variation can account for the climactic changes observed.
Frankly, those holding the opposite view remind me of the scientists who worked for Big Tobacco to prove that to prove smoking caused any individual lung cancer is legally difficult -- since so many factors are variables in the development of any cancer.
It's a bit like that with global warming. Eventually Big Tobacco got the blame they so richly deserved and there isn't any doubt that smoking is a significant health risk factor.
So I think that an apt analogy is that global warming deniers are like those who denied smoking caused cancer. They have even adopted the same strategy.
