The summer climate in the North Atlantic about the year 1000
The summer climate about the year 1000 reflected the fact that the low-pressure-paths had shifted approximately 7° to the north, almost as far as the 70th parallel but slightly further south in the western Atlantic. Temperatures were higher than in the present century (1900-1939), almost as in the hottest summers in the 1930'es and 1940'es (1933, 34, 35, 40, 42, 43, 45, 47, 49, 59). The climate had so to speak been warming up for the preceding 200 years. Westerly winds were weaker and less prevailing between the 40th and the 60th parallels. The influx of sunlight was greater and the Arctic pack ice covered a much smaller area and was confined north of latitude 80° North.
The climate along the sailing-route from Norway
Navigation over the North Atlantic was favoured at that period by the fact that violent storms were of rare occurrence and, as far as Greenland was concerned, there was a complete absence of Storis (Polar ice), which in our days disturbs the traffic along the east coast and the southern part of the west coast. The sailing-route to Greenland more or less followed the 61st parallel (cf. Søren Thirslund's article) The Nautical Part.
The surface temperature of the sea was a couple of degrees higher than today and the same applied to the temperature of the air.
On the first section of the voyage from Norway the prevailing wind direction would be westerly but in the neighbourhood of Iceland it would become more southerly, only to swing again to the west and even, near Greenland, to the northwest. Other wind directions would occur frequently, since the westerly wind, as named above, was less prevailing and also weaker than today. The cloud cover was less than today so that there would have been comparatively favourable conditions for employing the sun-compass described by Søren Thirslund and thereby of determining the deviation in the primitive compass, leiðarsteinn or loadstone, which was certainly known to the Norsemen in the 13th century (Hauksbók, but Abrahamsen (1985) has demonstrated the use of compasses in Denmark in the early 12th century and Scheen (1972) alleged that the Norwegians used leiðarsteinn as early as 868 on a voyage to Iceland). In Denmark the deviation around the year 1000 was approximately 27° east and it was considerably greater in the North Atlantic so here it was absolutely essential to be able to see the sun at regular intervals in order to check the deviation with the aid of the sun-compass.
The climate in Greenland
The gradual warming up during the two preceding centuries would have reduced the Arctic pack ice to such an extent that there would only have been permanent pack ice north of the 80th parallel and Storis (Polar ice) would not have been known. Storis (Polar ice) is the Arctic pack ice that is driven along the east coast of Greenland, round Cape Farewell and up along the west coast, sometimes far to the north of Nuuk. The Labrador current was also free of ice and weaker than now. It is also probable that there was no permanent ice in Canada's Arctic archipelago, at least to judge from studies of the Arctic ice-islands that once formed part of Ellesmere Island's ice-shelf (i.e. a glacier that projects out into the sea), for the age of the ice is estimated to be less than 620 years and it would seem that before that time the ice was melting. In the recent favourable climatic period in the 1930'es and 1940'es, the rise in the depression activity along the west coast of Greenland has been greatest around Upernavik, which was consequently the meteorological station with the greatest rise in temperature relative to the preceding cold period. The rise in temperature is greatest in winter, when the stronger winds mix warmer air with layers of cold air down near the surface of the earth (Frydendahl, 1989). Since the warm period at that time lasted longer and was more marked than the recent one, the depression activity must have extended much further north than to Upernavik and probably reached as far as the Thule region. The seas between Greenland and Ellesmere Island were ice-free, and if the Vinland map is genuine, there must have been so little ice that it was possible to circumnavigate Greenland. We do not now whether it was the Vikings themselves who did this, or the Inuit who told them about it.
Roots of plants and deep Viking graves found in South Greenland in soil that is now tjaele (permafrost or permanently frozen ground) indicate that the annual mean temperature must have been 2-4°C warmer than now. It is possible to estimate the summer temperature on the basis of the story in Landnámabók (985-1000) about Thorkel Farserk, who swam out to Hvalsey (in Hvalseyfiord) in order to fetch a sheep to make a feast for his cousin, Erik the Red. By way of comparison, Dr. Pugh from The Medical Research Laboratories in England has established on the basis of studies of Channel swimmers and the like, that 10°C would be the lowest temperature that a man who had not been in special training would be able to endure, even if he was fat. The average August temperature of the water in the fiords along this coast now rarely exceeds 6°C. The water in Thorkel's time must therefore have been at least 4° warmer and probably more than that. The summer temperatures (for the air) in the fiords in South Greenland would then have been 13-14°C (as compared with the present 8-10°C), and in Godthaab's fiord about 12°C, with a correspondingly shorter growth season. Further north around Melville Bay the summer temperatures would have been 9-10°C, as compared with the present 3-5°C.
The prevailing winds were north-easterly along the coast of South-East Greenland. In South-West Greenland there were south-easterly winds close to the coast but north-westerly winds further out in the Davis Strait. The very rich pasture that was found there around the year 1000 does suggest, however, that there must have been some westerly winds to bring in moisture as a result of the higher sea-temperatures. Further up along the west coast the winds came from a southerly direction, while on the Canadian side of the strait (Baffin Island etc.), the winds were north or northwesterly. By selecting routes at greater or lesser distances from the coast, it is therefore possible to find favourable wind- and current-conditions.
The climate along the Vinland route
On the sailing-route between Greenland and Newfoundland there were two significant climatic advantages in the Viking Age as compared with today. There was no drift ice and violent storms were very rare. In addition, there was the added advantage that both air and water were warmer than they are today.
On the Canadian side of Davis Strait the prevailing wind is north-westerly and the current (the Labrador Current) also comes from that direction. This wind-direction prevails to about halfway down the long north-eastern coast of Labrador, where the winds begin to veer towards the west. This direction is valid until in the neighbourhood of Belle Isle Strait, where the prevailing wind-direction is south-west and this is the most frequently occurring wind as far south along the east coast of North America as the Vikings can possibly have sailed. As mentioned above, however, westerly winds were fewer and weaker between the 40th and the 60th parallels, although they could still be called the prevailing ones. The Labrador Current was also at that period a cold ocean current and that may be the reason why Ungava Bay and North Labrador did not experience the warming that otherwise improved the climate not only in Greenland and North America but also in North Canada, where the remains of forests have been found 25-100 km north of the present timber line. The summer temperature was then as now (station Nain summer: 8.2°C) almost the same as Ivigtut (8.8°C) but the spring was much colder and August warmer.
The boundary between tundra and forest has therefore been relatively stable on this coast for the last 3,000 years. The Vikings from Greenland would have to have gone south of Nain to find forests that could yield satisfactory timber.
The climate in Vinland
The summer climate at the northern tip of Newfoundland reflected its situation in the westerly wind belt as well as the cold Labrador Current that flowed close to the northern and eastern sides of the island. The westerly winds between the 40th and 60th parallels were, however, not as prevailing and weaker then and the Labrador Current was warmer, less ice-filled and probably weaker. The Gulf Stream, which passes close by Newfoundland and exerts a strong influence on the climate, probably flowed closer to the island then than now. It would certainly have been warmer at L'Anse aux Meadows — Vinland -around the year 1000 than it is now. A cautious guess would be that the summer temperature would have been about 16°C on the average (almost as in Denmark) but with the significant difference that the summer there was shorter and that the spring was cold. The winter was probably a couple of degrees warmer than now.
On the average, then, it would have been rather warmer and less windy but the proximity of both a cold and a warm ocean current would have given great variations from year to year, depending on which of the currents came closer to land. The climate would, however, have been considerably more pleasant and advantageous for human beings, cattle and trees than it is today.[…]
Frydendahl, K.: Global og regional temperaturudvikling siden 1850. Danish Meteorological Institute. Scientific Reprt, 89-6. Københaven 1989.
Scheen, R.:Navigation og manøvrering. In Fra mast til køl, p. 224-225. Gøteborg 1963, Københaven 1972.
Thirslund, Søren & C.L. Vebæk: The Viking Compass, Skjern 1992.