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Still another change with location that I couldn’t help noticing during my travels in Greenland is that some fjords have glaciers dumping into them, while others don’t. Those fjords with glaciers constantly receive icebergs of local origin, while those without glaciers only receive whatever icebergs drift in from the ocean. For example, in July I found Igaliku Fjord (on which lay Viking Greenland’s cathedral) free of icebergs, because no glacier flows into it; Eirik’s Fjord (on which lay Brattahlid) had scattered icebergs, because one glacier enters that fjord; and the next fjord north of Brattahlid, Sermilik Fjord, has many big glaciers and was solidly clogged with ice. (Those differences, and the great variations of size and shape among the icebergs, were one of the reasons why I found Greenland such a constantly interesting landscape, despite its few colors.) While Christian Keller was studying an isolated archaeological site on Eirik’s Fjord, he used to walk over the hill to visit some Swedish archaeologists excavating a site on Sermilik Fjord. The Swedes’ campsite was considerably colder than Christian’s campsite, and correspondingly the Viking farm that the unfortunate Swedes had chosen to study had been poorer than the farm that Christian was studying (because the Swedes’ site was colder and yielded less hay).

Weather changes from year to year are illustrated by recent experience of hay yields on sheep farms that resumed operation in Greenland beginning in the 1920s. Wetter years yield more growth of vegetation, which generally is good news to pastoralists because it means more hay to feed their sheep, and more grass to nourish the wild caribou (hence more caribou to hunt). However, if too much rain falls during the hay harvest season in August and September, hay yields decrease because the hay is hard to dry. A cold summer is bad because it decreases hay growth; a long winter is bad because it means that animals have to be kept indoors in barns for more months and require more hay; and a summer with much drift ice coming down from the north is bad because it results in dense summer fogs that are bad for hay growth. Year-to-year weather differences like those making life dicey for modern Greenland sheep farmers must have made it dicey for the medieval Norse as well.

Those are the climate changes that one can observe from year to year, or from decade to decade, in Greenland today. What about climate changes in the past? For instance, what was the weather like at the time that the Norse arrived in Greenland, and how did it change over the five centuries that they survived? How can one learn about past climate in Greenland? We have three main sources of information: written records, pollen, and ice cores.

First, because the Greenland Norse were literate and were visited by literate Icelanders and Norwegians, it would have been nice for those of us interested today in the Greenland Vikings’ fate if they had bothered to leave some accounts of Greenland’s weather then. Unfortunately for us, they didn’t. For Iceland, though, we have many accounts of weather in different years—including mentions of cold weather, rainfall, and sea ice—from incidental comments in diaries, letters, annals, and reports. That information about the climate in Iceland is of some use for understanding the climate in Greenland, because a cold decade in Iceland tends to be cold in Greenland as well, though the agreement isn’t perfect. We are on more secure ground in interpreting the significance for Greenland of comments about sea ice around Iceland, because that was the ice that made it difficult to sail to Greenland from Iceland or Norway.

Our second source of information about past Greenland climates consists of pollen samples from sediment cores drilled into Greenland lakes and bogs by palynologists, the scientists who study pollen and whose insights into the vegetational history of Easter Island and the Maya area we already