In a nutshell, absolutely nothing.
The climate denial community make a big deal about a spike in temperatures called the Medieval Warm Period which occurred somewhere between 1000 and 1270 AD. This argument is used to suggest that the current period of warming is nothing out of the ordinary. It is borne on a number of fallacies:
Fallacy 1 – Greenland used to be green
The derivation of the name Greenland may have nothing to do with the colour green. Early maps denote the name of the that the island as either Gruntland or Engronelant, both having something to do with ‘ground’, which refers to shallow bays, and that Green may have been an error in translation. The other popular theory is that this is the oldest known evidence of a real estate scam, with Erik the Red, a murderous exile from Iceland, using the name to try and attract settlers by making the place seem more hospitable than it is.
Either way, 81 percent of Greenland is covered in glaciers and ice-cap to a depth of 2-3km. Ice core studies have revealed that these are tens of thousands of years old and the last time the island held abundant greenery was perhaps 500,000 years ago. It is hard to believe that things would have been very different in Erik the Red’s day.
Fallacy 2 – Greenland was a thriving place in Medieval times
Greenland has been occupied since pre-historic times by a number of Paleo-Eskimo cultures. Around 986 AD, it was colonised by Icelandic nomads in two small settlements near the south-western tip of the island. As we have seen above, things are unlikely to have been significantly different on Greenland than they are today and these early settlements would have been small farming communities on the fringe of habitable land that enjoys the moderating influences of warmer, moist weather from the north Atlantic. Summers would have been far from tropical and very brief, with lengthy bitterly cold winters. Any cropping and livestock grazing would have been tenuous.
The native Inuit cultures survived on a diet comprised primarily of fish. The early Norse settlers resisted this and were initially dependent on their livestock and crops. Over time, reality set in and an isotope analysis of the bones of these settlers shows a trend of increasing proportions of marine material in their diet. This is direct evidence that the Greenland soils and climate were inadequate for raising cattle and food crops and essentially forced them to go against their cultural traditions and rely increasingly on fish and other marine creatures to survive.
Fallacy 3 – The Medieval Warm Period
The argument goes that somewhere between 1000 and 1270 AD, and there are some vagaries with the dates, there was a period of warming. It was initially believed that this was a global phenomenon, however in its 2001 report, the Intergovernmental Panel on Climate Change (IPCC) summarised the current research on this period and concluded that it was neither as warm, as long nor as widespread as scientists thought.
Temperature data taken from ice cores, tree rings and lake deposits showed that temperatures globally were cooler than in the last quarter of the 20th century and any period of warming was restricted to small parts of the northern hemisphere and sporadic in duration. This warming is likely to have been caused by land use changes. There is often the claim that “they used to grow grapes in Britain”. The evidence reveals that this was mainly confined to southern England, with Selley noting there were some 46 vineyards in medieval times. Unlike today, it was hardly a flourishing industry and mostly conducted in Benedictine monasteries for religious purposes. Now, there are some 400 commercial vineyards in Britain extending through to Lancashire and Yorkshire. Elsewhere in Europe, wine grapes are grown as far north as the island of Gotland in Sweden (58N latitude).
In summary, the medieval Greenland communities, founded by Erik the Red, can teach us nothing about climate change. That there were thriving Viking communities throughout Greenland, and that is was somehow both warmer and greener than it is now, is nothing more than a myth, and should be considered as realistic as the stories of Norse mythology.