Consensus is really the only way science can move forward on anything. The thing with science, is nothing is (or ever will be) 100% "proven". There's always going to be an element of doubt. All we can do is find a pile of evidence that points us in one direction, and hope that we don't (at a later date) find a bigger pile of evidence that points us in another direction. That might not be comfortable for some people, but it's the only way we have of gaining knowledge, and it's worked pretty well for a couple of hundred years.
When a new theory is proposed, much debate takes place, and various experiments and studies are carried out to attempt to gain evidence for whether it appears to be true or not. Much debate ensues. Eventually, there'll be a consensus amongst most of the scientists in the field (although this doesn't always happen... physics has become hampered with our inability to gain more evidence to support theories, which is why they're trying to build massive and very expensive particle accelerators, telecopes, space probes, etc). The process of reaching consensus often takes a couple of decades, and is not an easy process. Scientists often have a "gut feel" for what they think is the truth, and will persue evidence that supports their point of view. Of course, gut feels are often wrong, but fortunately, this happens on both sides of the debate, and one side eventually wins out. There were several (some notable) scientists who strongly opposed the Big Bang Theory, but later had to concede they were wrong when the evidence became undeniable.
As for the Global Warming debate, this finished some time ago as far as most scientists are concerned (too lazy to find a link to support this, but it's easy enough to google). But it did go on for at least a decade. As with any consensus, not everyone agrees. Mostly these are crackpots or (sadly) shills, altho there are certainly some respected scientists who disagree. But there always will be on virtually any debate in science. That's not a reason to think it's likely to be false. We'd need some evidence (e.g. repeatable experiments) before the debate truly re-opened. If we re-opened the debate whenever someone objected, we'd never make any progress.
The way the consensus for Global Warming was reached was more or less the same as the way consensus has been reached on every scientific theory ever. If we suddenly decide that that process is wrong, we will have to throw out a lot more than just the theory of Global Warming.