Why do some people say global warming is real and others say it is not?

There are multiple reasons why some people do not believe in global warming or climate change (to understand the difference and why the latter term is preferred, see this answer.

When you look at the people who actually study climate science and are actively publishing their results, 97% of them agree that climate change is caused by humans. Thus, those who "believe" in climate change may understand the science or may follow enough science to have repeatedly heard that climate change is real. Higher education in the US frequently gives students the opportunity to take classes either on climate change directly or that include climate change as part of the course. Thus, some young adults are exposed to the science through their education, and in some schools climate change may be taught as part of the K-12 education.

The main reasons people do not believe in climate change are:

1. Those contributing to climate change are invested in convincing the public that climate change is a controversial topic or that it is false
2. The media misrepresents climate change
3. Scientists are often poor at communicating climate change science to the public
4. In some parts of the U.S., there are cultural factors that collide with climate change

One of the less well-known reasons why many people do not believe in climate change is that there is an active campaign to distribute misinformation and create doubt surrounding the science. A very good movie/book worth looking into on this subject is Merchants of Doubt. Those who would benefit from climate change being false (fossil fuel companies for example) have reasons to spread false information.

The Heartland Institute is a powerful think tank in the US that does a brilliant job of creating fake science that looks like real science in varying forms. There's a good article about their work here.

Another problem is that many scientists do not do a good job in communicating their research to the public. Often they use jargon and do not represent themselves well when communicating with or through the media. By presenting themselves and their work in an manner that is inaccessible to most people, scientists can create more confusion about climate change and they may also cause people to stray away from reading about science.

The media also plays a critical role in why people do not believe in climate change. The media likes to portray climate change as an unresolved debate. They will put a climate scientist against a politician or someone who does not believe. However, this makes it seem as though the number of believers vs nonbelievers in science is equal. It's not. You may remember a petition where supposedly 30,000 scientists who have a problem with climate change. If you look at those names, very few are climate scientists and some of the names are even fake.

Mainstream media in the US is frequently inaccurate when reporting on climate change. They often get the science wrong or miss seemingly small but important details. A recent report found 72% of Fox News' reporting on climate change contain misleading statements.

Another reason why climate change is so controversial in the United States has to do with certain cultures and the authority that those cultures have. There are some who believe that god wouldn't allow such severe climatic catastrophes that are in our future, thus climate change must be false. There are those who believe scientists are corrupt and do not understand the process of science or the extensive review process it entails. There is a well-written article about this topic here.

Dr. Katharine Hayhoe has done some excellent work on science communication, religion, and climate change and has written a book on the topic.