Dan,
Let me answer your question as well:
The theory is that the earth is warming due in large part to man's increased release of so-called greenhouse gases (CO2, methane, etc.) into the atmosphere.
In science, one cannot prove a theory, but can only provide data that supports it (e.g., the aforementioned polar ice loss, 1998 being the hottest year on record and the top 10 hottest years of the 20th century being after 1980).
So the simple answer is that nobody has provided much peer-reviewed data that refutes it. in fact, there is a scientific consensus (e.g., national academy of sciences 2000 report, 1995 UN sponsered effort of 2500 climate scientists, etc) that believes it will not be refuted.
the fossil fuel and nuclear industries among others have large financial interest in creating doubt about the global warming theory. the global penalty is much higher for waiting too long to do anything about it, than it is for potentially erring on the side of caution. besides, links golf is the best golf around.