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According to my science teacher. Yeah global warming is happening...but for the most part it's natural.
It's hard to tell exactly how much is natural. However, it appears that humans are at least having a significant effect on it. Remember, this isn't limited to cars and coal power plants; there are a wide range of other things we do that produce greenhouse gases, including hydroelectric dams (believe it or not they actually pollute almost as much for the electricity they make as fossil fuel plants) and, even more importantly, rice farms. In fact, the statistics show that our effect on global warming probably started not in the past 200 years but all of 8000 years ago, by the earliest asian farmers producing methane in their flooded rice paddies.
Some of it may be natural, but there's not really any room to argue that we're not having a significant effect.