Greenhouse gas theory is necessary to believe in CAGW, but not sufficient.
Jere Krischel
2

And by the way, have you looked up the resolution of the ice core records? Edit: the resolution appears to be roughly annual. I read:

Ice cores have provided climate and ice dynamics information over many hundred thousand years in very high, sometimes seasonal, resolution. This information allows scientists to determine how and why climate changed in the past. By understanding how and why climate changed in the past, scientists are able to improve predictions of how climate will change in the future.
Because of their high time-resolution, the physical nature of their proxy records, and their ability to archive actual greenhouse (and non-greenhouse) gas concentrations from the past, ice cores have become one of the golden standards in paleoclimate research.
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