Why do some people say global warming is real and others say it is not?
Because the rich want to get richer.
Any real credible scientist knows that global warming is real. The people that say it is not are those that stand to lose money if laws are enacted to control it, and also those that like to jump on conspiracy bandwagons.
One possible solution to the human impact of global warming is a carbon tax. Put simply, companies that put carbon into the air have to pay for the amount of carbon they put in the air, and that money would go to initiatives designed to lower atmospheric carbon, like planting trees for instance.
If a company is a big polluter, they may be forced to pay very large sums of money on carbon taxes. Therefore, if you are one of these companies it makes a lot of sense to put a lot of money into trying to discredit global warming instead of paying more in the long run in taxes.
The other people that refute global warming are those that want to draw attention to themselves. For example, take the late Michael Crichton. When he wrote State of Fear, he basically said that global warming was a conspiracy by the government to control people through fear.
The thing is Michael Crichton never really did as much research in his stories as people are led to believe. In Airframe, for example, he clearly knew nothing about air traffic control. Now, in State of Fear, he actually notes research at the end of his story supporting his claims. When you look at that research closely you see it is flawed or simply misunderstood by Crichton and there is not one argument presented that I cannot shred like tissue. Crichton was an awesome author, but everyone has to remember he wrote fiction.