They disagree in exactly the way gjm mentions below. Experts are climate scientists and scientists in related fields. Some politicians may be included as 'experts' in terms of solutions, too, I suppose. They disagree about the severity, cause, timeline and solution. And not by some trivial amount, but by enough to drastically shift priorities.

Also, while this is a reply to Eliezer's 2007 comment, I'm aware the situation has changed. I really just want to know how to begin to form a rational belief about climate change as of now.

I find climate change a strange issue. Not the situation itself but the public response and political tactics that are used.

On the surface, it looks like the vaccination controversies where one side goes "you guys are stupid for completely ignoring science". The difference is, the science for vaccines is rock solid. There is a negligible chance of vaccines hurting you. And we have an extremely large amount of evidence. Not evidence from computer models or something theoretical, but actual data from millions of people being vaccinated.

Climate change, no matter your opinion, cannot be said to be this sure of a thing. Yet the tactics used are the same. "If you don't take up our cause, you are the enemy of Science." Science isn't some deity. I don't obey out of some appeal to authority. It's useful because it can convince me with reason.

If the issue is trivial or unanimous, I may just accept the scientific consensus at face value. But for a possible existential threat that could either kill us or cost an unimaginable amount of money to prevent... And there are experts that disagree! And the consensus has changed several times in the last few decades! And politicians are pushing a certain direction! ...I'm not being ideological here, am I? This isn't a black and white issue is it?