The climate of Greenland when Vikings were living there has been a long-term subject of scientific and popular debate, yet has only been tested very indirectly. The first study of proxy data closer to home confirms the southern region was warmer during the occupation than in centuries before and after, and about the same temperature as today.
Spend much time discussing climate change online and you'll encounter people attempting to disprove thousands of peer-reviewed studies with a handful of claims. A favorite is that Greenland was warmer 1,000 years ago than today. Therefore, the “logic” goes, the whole planet must have been warmer, therefore human-induced global warming can't be real.
The last part of this case is pure bunkum – for many reasons some places warm and cool out of step with the planet as a whole. Global climate assessments need to average a wide range of locations. Nevertheless, Greenland's climatic history remains interesting, and Northwestern University PhD student Everett Lasher has provided some answers.
Greenland is, of course, a rather large place – 10 times the size of Great Britain in area. A temperature record from one spot may not be as unrepresentative of the whole country as the island is of the world. However, the lakes near Narsaq are close to one of the main Viking colonies making it the location we need.
Lasher extracted oxygen from the exoskeletons of chironomid flies buried at the bottom of lakes. "The oxygen isotopes in precipitation are partly controlled by temperature, so we examined the change in oxygen isotopes through time to infer how temperature might have changed,” he said in a statement.
When Eric the Red established a colony there in 985 CE, Narsaq was 1.5ºC (2.7ºF) warmer in summer than it had been 100 years or so before, Lasher reports in Geology. Perhaps if this hadn't been the case, Eric and his followers wouldn't have even tried.
The area remained warm (relatively speaking) for four centuries, before experiencing wild swings in temperature around the time the colony was abandoned. After this, the cold era resumed until the last few decades when human-induced effects took hold.
Assertions of Greenland's past warmth often rest on flimsy claims, such as the Vikings (unlike native peoples) not being able to survive under modern conditions. The name they gave it supposedly indicates the place was lush, when it seems more likely the name Greenland testifies to Eric's marketing genius. Climatologists, on the other hand, noted Greenland's glaciers expanded at this time, and speculated it might have been experiencing a cool period.
Lasher's study ruled out fluctuations in the North Atlantic Oscillation, the previously favored theory, as the driver of the temporary warmth, leaving its cause a mystery.