Easy, the Earth goes through naturally occurring warm and cold cycles. Archeology has proven this.
The Vikings colonized Greenland during a warm cycle and at the time Greenland had a temperate climate suitable for growing crops to support a substantial population. That’s where it got its name. Greenland was abandoned when the climate changed and the ground froze permanently. It was also used as an example proving Global Cooling or the coming Ice Age scare. Now it's being used as an example of the Global Warming scare when a naturally occurring cycle allows Greenland to become green again.