This Scientific American article reports what it was like on earth, 15 million years ago, the last time atmospheric carbon dioxide levels were as high as they are now. Was there global warming back then? The article reports that, yes, there was: temperatures were about 3° hotter than they are today and sea levels were about 25 meters higher than they are today.  However, the article authors did not determine if the elevated atmospheric carbon dioxide levels 15 million years ago caused the global warming: their study could only find associations.

The article also reports that when atmospheric carbon dioxide levels started declining 14 to 11 million years ago, this coincided with major global cooling.  Again, the authors did not address causation: they only determined association.

Does higher atmospheric carbon dioxide cause global warming while lower carbon dioxide cause global cooling?  One can't tell for sure from this study, but if higher carbon dioxide does cause global warming, the earth should start heating up in the near future, speaking from a geological time perspective.

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