I think it's quite well understood that GCC does in fact exist, just as it has for billions of years. The bigger argument is whether mankind's activities have significantly contributed to speeding up the current warming cycle.
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I don't think you find many that will argue against the greenhouse effect. It's just physics and I think Arrhenius may have been the first to discover it back in the 1700's. But the link between human activity and the current weather is very debatable. A better graph is:
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BTW what do you mean by GCC? Not the GNU Compiler Collection is my guess.