Based on recent findings, it has been believed that Greenland has been covered by ice for at least the last 12 million years. However, fossils dating back to about two million years ago, previously only found in coastal sediments, were discovered beneath the ice sheet in northern Greenland. This suggests that the Greenland ice sheet must have temporarily melted around two million years ago. It is possible that either significant climate warming or volcanic eruptions in Greenland's highlands could have caused the ice sheet to melt, leading to rising sea levels and flooding the land. What is the main conclusion of this argument?