Yep that old warhorse about Vikings and Greenland has reared its ugly head again.
Let's get it straight. (Putting his trained geologist hard hat on)...
Greenland in the Viking Period was not a lush paradise, its coasts had isolated patches of marginal land, the interior was the same ice cap that's there today. Greenland was always colder than both Iceland and Norway; its winters were harsher and summers short and frost prone. The Greenland communities remained heavily reliant on imports from Iceland and Norway for even quite basic materials.
Greenland got its name through Erik the Red's nordic spin-doctory; as the Icelandic sources put it: "He named the land Greenland, saying that people would be eager to go there if it had a good name."
The whole situation ended with the passing of the Medieval Warm Period - a time of unusual warmth in the North Atlantic region. There is much less evidence of a global warm period at the time, especially in the Southern Hemisphere. It is therefore not analogous to the current warming which is not only global, but occurring much faster than the MWP.
Scientists are much less concerned about the absolute temperatures we're seeing (it's still relatively nippy compared to the Eocene about 50mya when London had a pleasantly African climate including hippos), than the rate of change which is unusual.
Admittedly it is not unprecedented; the aforementioned Eocene kicked off with the Initial Eocene Thermal Maximum which is associated with a major extinction event and massive changes in sea level; but it is one of the fastest we know about. At the moment, the best explanation we have is that our CO2 emissions are driving the warming.
Oh and before I go, the ice age fears of the 1970s were well founded considering the state of knowledge of the time. Although it is worth pointing out that even then, the majority of scientists predicted climate to either remain unchanged or warm slightly.
But back to the cooling hypothesis.
For the last 12,000 years we've been living in an interglacial (periods of relative warmth between glacial advances). Isotope work done on Pleistocene deposits showed repeated advances interleaved with relatively short interglacials of between 10,000 and 20,000 years. The present Flandrian interglacial is already 12,000 years old it seemed to be reasonable to assume the climate would be turning inevitably towards the next glacial advance.
We now know that interglacials can last in excess of 100,000 years and the switch between the two extremes takes much less time than we thought - ice ages don't take tens of thousands of years to develop or end, they appear quite abruptly.
There was some evidence to support the cooling theory. The climate between 1940(ish) and 1970(ish) had cooled somewhat, especially in the Northern Hemisphere. We know know this was mainly due to heavy industrialisation during the post-War economy and the newer economies of the Soviet Union, China and Japan producing huge amounts of particulates from coal and unchecked emissions of sulphur dioxide. Through the 1960s and 70s, the West rapidly switched to oil and gas and began to filter SO2 from its emissions, the skies cleared and the climate turned back to normal.