According to the Online Writing Lab at Purdue University, your thesis statement should be a debatable one which any reasonable person could agree or disagree with. Concerning global warming, whether the stance is for or against, making a claim about its impact or the policy decisions made by government backed by evidence is a much stronger thesis then a generalized statement about whether global warming is good or bad.
The main body of your paper will then be used to argue this point or persuade your reader of the truth in its central claim. Also, as the Online Writing Lab at Purdue suggests, narrowing the focus of any evidence-backed claims makes for a stronger thesis, as opposed to a broad sweeping claim which is harder to argue for or against.
Instead of making a claim on the impact (or lack thereof) of global warming on the planet, keep the focus on a single geographic region, industry, sector or group. This will allow you to easily target any claims and build evidence throughout your paper without losing focus on the central argument and allowing criticism from areas outside of the paper's original scope, explains the Online Writing Lab at Purdue University.