Science is anything that follows the scientific method.
One important step of the scientific method is "make testable predictions." Another step is "refine, alter, expand, or reject" hypotheses. Science rests on the possibility of falsifiability. If it can be tested, and found false, it is science. If there is no way to falsify it, it is not science.
For instance, evolution is falsifiable. We can expect that if we put certain organisms with rapid life cycles in a harsher environment, over a certain number of generations the population in the harsher environment will evolve to better suit that environment. If, say, 10,000 generations pass and the organisms in that environment are no better suited to it than a control population, we might conclude evolution has been falsified. Fortunately, we've done lots of tests like this, and evolution does indeed occur.
So I may better understand how this is science, can you tell me how the theory of manmade global warming can be disproved?