Global Warming is a scenario that scientists are concerned may play out over the next century. There's strong support for it, but it's not a reality yet.
There's much evidence that man is having real impact on the atmosphere. But it's very uncertain as to how accurate the climate models are in predicting the average temperature 50 or 100 years from now. There are a lot of assumptions that go into these climate models, and historically, they have a poor track record with long-term forecasts (more than 10 years). There's general agreement that if these forecasts are true, then that would be a bad thing.
So the question is, given these forecasts, do you err on the side of caution by acting now to change the behavior of the entire industrialized world and prevent long-term calamity, or do you dismiss them since the cost to world is immeasurably large and the benefit is very uncertain?
The argument against the Global Warming camp points to the temperature data for the last 6000 years. The historical swings the Earth saw long before the industrialization of man are far greater than anything predicted over the next century. But that doesn't mean that man's activities can't change the environment now, or that climate change is no big deal. If global warming happens, it'll be a big deal.
On the other hand, some of the evidence that environmentalists point to amounts to a lot of hype. You can't look at short term swings in temperature in very local regions and draw conclusions about long-term trends. It's silly to point at glaciers melting or some severe weather and conclude that it's all man's fault. Local climates swing up and down over decades. It's normal. It happened all the time throughout the history of the Earth. And this talk about how we've had more hurricanes in the Atlantic Ocean than ever in history (for some reason, they never say "in recorded history" like they used to do before this global warming debate started) is misleading. The data is unreliable if you go even just a century back. Satellites that help us track storms, some of which go off harmlessly into the ocean, and which help us measure the speed of storms to classify them as hurricanes, didn't exist until several decades ago.