In 1959, the US Government drilled some ice cores in search of a location to put a missile base in Greenland. They found plants frozen into the ice, which indicates that Greenland was ice free recently, in climate terms. At some point within the last million years, possibly only several hundred thousand years ago, Greenland was ice free.
Now, to humans, a million years seems like a long time, but that is relatively recent in climate and geological terms. In fact, atmospheric CO2 was lower a million years ago than it is now. This indicates to me that climate is cyclical, and has little to do with humans adding CO2 to the atmosphere.