This question (if you can call it like this) is not even wrong. Let's ignore the fact that the title has little to do with the actual content of the question.
Let's discuss your statement:
If this occurred 120,000 years ago and seems to be taking place again, Man is not the cause behind Greenland's glacial melting.
There is no logical part between the first part of the sentence to the second part. Let's change it a bit:
Forest fires occurred 120,000 years ago and seems to be taking place again, Man is not the cause behind forest fires.
Yes, forest fires happened 120000 years ago. Also a million years ago. Also 100 million years ago. And now. But now, forest fires are started by people throwing away cigarettes and not putting out their campfires properly. Does that mean that all modern forest fires are because of natural causes? No. I hope this analogy helps you understand the logical fallacy in what you wrote.
Now, to your question:
If global warming by man is not causing Greenland's ice sheets to melt, What natural source could be?
But global warming by humanity is causing ice sheets to melt. Therefore your question is invalid. I will not try to answer the hypothetical case that it is natural, because that will just be taken out of context and used as ammunition by climate change deniers, as always happens.