20,000 years ago, much of the northern hemisphere was in the grip of the most recent ice age. As far south as Pennsylvania and Yosemite Valley, in California, the ice was a mile thick. At this point there were no humans in North or South America. The earth began to warm. It has been warming ever since.
As the ice cap began to melt, land passages opened up, and humans, coming in from Asia, migrated south, beginning in aprox 16,000 BC. Hence forth, those people were known as Native Americans. The point here is quite simple. The melting of glaciers and the shrinking of the polar ice cap is nothing more than a continuation of the steadily warming planet. It has nothing to do with man, or what man has done.
Whether man is on the planet earth, or if we became extinct, the planet earth will continue to go through never ending cycles of cooling followed by warming