Your thesis should be more specific than it currently is. Examples of good thesis statements about global warming would include the following:
1. Man-made CO2 emissions must be reduced by at least 80% by 2020 if we are to avert a global warming disaster.
2. Global warming is increasing the frequency and intensity of tropical storms.
3. Global warming is primarily a beneficial natural phenomenon that should not concern us.
4. A rational cost-benefit analysis of most global warming scenarios indicates that it would be cheaper and more effective to prepare to adapt to a warmer climate than it would be to try to prevent global warming.
Merely saying that it is "a big issue," and that it "has effects on the environment" is not enough. Saying the results would be catastrophic is a bit better, but still a bit vague. Plus, it smacks of hyperbole.
Note that the fact that someone could disagree with your thesis does not make it a bad one to write on. On the contrary, a good thesis should provoke strong disagreement from at least some quarters. That is, it should be debatable. "Water freezes at zero degrees Celsius under normal conditions" is perfectly true, but not a good thesis, because it is a fact that no one would seriously considering arguing against. All of the theses I've listed above could be the basis for a strong essay, because it would be possible to argue for or against them. I imagine Rich would disagree with 1 and 2, for instance, but he might accept either 3 or 4. No one could logically agree with all of them simultaneously.
Good luck with your essay.