Climate. The exact nature of the climate of medieval Scandinavia has long been debated, and remains controversial. The pioneers in the field of historical climatology, such as Lamb and Flohn, believed that a warm period occurred in northern Europe around 1000-1200, similar to the warmth of the early 20th century, followed by a decline in temperatures culminating in the so-called Little Ice Age from about 1450 to 1750. That there was a cold period or periods around this latter time is certain; only the details and timing within different geographical regions remain a matter for debate and further research. Changes in climate prior to about 1450 are, unfortunately, much harder to verify, because, although advances have occurred in the availability and methods of interpretation of indirect or "proxy" climate data, such data are generally available only for isolated locations within continental Scandinavia, and often only for short, discrete periods. More information exists for Iceland and Greenland, but, even where proxy data are available, they frequently pose problems of interpretation. The need for cautious and critical use of historical sources in particular has recently been emphasized. For these reasons, it is not possible to give a detailed description of medieval Scandinavian climate.
The available evidence can, however, give important clues as to how the climate may have varied. Because of lack of evidence for precipitation, the discussion will center on temperature variations. Over the entire period (en AD. 500 to ca. 1700), decadal averages of annual-mean temperatures, while appearing quite variable, probably deviated by at most a few degrees Celsius from those prevailing during the 20th century.
Different types of proxy data include tree-line fluctuations, pollen records, documentary evidence, glacier oscillations, and tree rings. For example, the widths of annual growth rings of certain trees in continental Scandinavia correlate well with summer temperature. One interesting long tree-ring series, spanning 436 to 1981 and based on pine (Pinus sylvestris), has been produced by Bartholin for the area around Lake Tornetrask in northern Sweden. This series indicates a period of cold temperatures around 550. A warmer period then lasted to about 780. From that time, temperatures remained cooler until about 930. A warm period set in then and lasted for twenty to thirty years. From around 950 to 970, it was cooler again. From then to around 1140, a distinct mild period is suggested, followed by a short cold spell lasting perhaps ten years. From about 1150 to about 1560, the climate appears rather variable, but tending toward the mild side. The coldest summers of this time were probably around 1350, 1400, and 1470. The most marked cold period of the entire series is from about 1560 to about 1740. If these climate variations in northern Sweden are correct, similar conditions are likely to have prevailed in northern Finland. A study by Siren, based on pines from a number of sites in northern Finland, concluded that the 1200s were colder than the 1300s, and the 1400s colder than the 1500s. An oak chronology from the Gotha River area in western Sweden suggests that temperatures between 1450 and 1700 were somewhat lower than those after 1700.
Indications of climate changes may also be drawn from studies of glacier moraine positions and mass balance estimates because glacial advance and growth generally signify a response to a colder climate. Conversely, a shrinking glacier suggests a time of warmer climate. There may, however, be a time lag of some years between a change in climate and a glacier's response. A number of methods are used to date glacier fluctuations, the most important of which are radiocarbon dating and lichenometry. Radiocarbon dating relies on measuring the proportion of the isotope C14 (radiocarbon) in the carbon contained in a particular material, for example, from buried soils or pieces of wood in glacier moraines. Lichenometry makes use of the fact that, in any given location, lichens, which can live for many centuries, grow at a rate that is roughly constant. Their size is therefore an indication of their age. Using these dating techniques, Karlén and other researchers have built up a guide to past glacier variations in Sweden and Norway. Several advances are indicated from about 550 to 950. Possible glacier advances between 800 to 900 would accord well with the dendrochronological evidence for a cooler climate in northern Sweden around 780 to 930. A radiocarbon date from Ritajekna in Sarek National Park points to another advance around 1290. It also seems likely that glaciers were relatively large during a period of cool summers from 1350 to 1400. Glaciers in Sweden probably reached their greatest extent in historical times in the 17th and early 18th centuries.
From 950 to 1590, glacial advances in Norway appear to have been better defined than in Sweden. Seven radiocarbon dates associated with moraines give ages around 1330 and 1490, and these data are believed to reflect periods of maximum glacier extent. The former of these dates is supported by a Norwegian document from 1340, which mentions a glacier advance of Jöstedalsbreen in southwestern Norway (see Eide 1955). Research by Karlén also shows evidence for an advance at this time from Vuolep Allakasjaure in northern Sweden. However, climatic changes in northern Sweden and southern Norway may not be synchronous. Following the advance of about 1490, it is possible that a warm spell occurred in southern Norway at the beginning of the 17th century, succeeded by a cold period. Another cold period is indicated from the late 17th century, with a warmer period separating the two.
Other documentary evidence from Norway includes sporadic references to weather in early sources, such as Heimskringla. This work refers to severe years around AD. 961 to 965, 974, 1020, 1030 -1035, 1047, 1207, and 1213. Good years are said to have occurred shortly after about 885 and from 1103 to 1130. These weather references cannot be regarded as reliable, however. A recent project led by Knud Frydendahi of the Danish Meteorological Office has attempted to gather all medieval historical references to climate in Denmark. Only isolated descriptions referring to individual years exist. Years that may have been severe in the Dano-Norwegian Baltic region include 1307, 1323, 1356, 1408, 1426, 1460, 1547, 1555, 1570, 1600, and 1608. However, the data sources used for this information have not all been historically verified.
Both Iceland and Greenland have high-resolution proxy-climate records that are among the longest in the world. The Iceland record is based on historical evidence, and Greenland has a number of records based on isotope data from deep ice cores. The evidence from Greenland will be considered first. The concentration of oxygen and deuterium isotopes in high-latitude precipitation is determined mainly by the temperature of its formation. This factor causes seasonal isotopic variations in snow and ice as well as long-term variations due to climatic changes. According to Dansgaard, ice-core data from Crête, central Greenland, suggests a considerable and rapid warming late in the 9th century. He argues that this rise may well have facilitated the Norse settlement of Iceland at this time. The name of Greenland, when this country was in turn settled by Eiríkr rauði ("the red") around 985, may have described reality. Dansgaard suggests that, when this happened, Greenland was at the end of a warm period longer than any that has occurred there since. A deterioration in the climate may have begun around 1030, with the climate remaining generally cold to about 1520. The coldest years of the period were around the late 1200s and the late 1300s. During this time, however, there were mild spells around the early 1100s and the early 1400s. From about 1520 to about 1740, temperatures remained on the cool side, but not markedly so. A cold period is seen from about 1740 to about 1900. Dansgaurd suggests that the cold climate of the 14th century played an important part in the tragic end of the Greenland Norse colony. McGovern, however, while conceding that climate may have played a certain part, emphasizes the importance of economic and social factors.
As documents referring to climate in Iceland are both sparse and noncontemporary until the late 12th century, evidence for a mild climate for the first few centuries of settlement (from ca. 870 to 1170) is circumstantial only. The fact that the settlers not only traveled to Iceland but also chose to remain, and that they were able to grow barley, are examples of this evidence. From 1180 to 1210, sources suggest a period of cold climate, and from 1212 to 1232 a milder one. Over the next few decades, there are sporadic references to severe seasons. The 1280s and 1290s were undoubtedly severe, while the years 1300 to around 1320 were comparatively mild. The early 1320s, on the other hand, were harsh, with sea ice off the coasts and severe weather. The 1330s and 1340s are likely to have been mild. From around 1350, the climate seems to have become colder, and the 1370s were particularly harsh. The frequent presence of the sea ice off the Icelandic coast during the 14th century may also be inferred from two geographical descriptions; one is in a version of Guðmundar saga, the other is attributed to ĺvarr Bárðarson. The period from about 1380 to 1430 was comparatively mild. There is some evidence for a mild climate between 1430 and 1560. For the latter part of the 16th century, sources suggest a comparatively harsh climate. The early and latter part of the 17th centuries were cold, but a mild spell occurred around 1640 to 1670.
From the research that has been done, it is evident that the climate of the Scandinavian regions varied considerably over the centuries from about 500 to 1700 and that these variations were spatial as well as temporal. It does seem likely, however, that there was a prolonged time of mild climate common to both the North Atlantic region (ca. 870 to 1050 or later) and to northern Scandinavia (ca. 950 to 1150). The available evidence is not yet sufficient for us to know whether southern Scandinavia was similarly affected. However, evidence for mild temperatures in places like England at this time suggests that this was possible. Subsequently, a number of alternating mild and colder intervals lasting for decades or longer occurred in the North Atlantic area. Over this period, the climate of northern Scandinavia also appears to have been variable, but may have remained milder than cooler. The "Little Ice Age" was undoubtedly a very real climatic event, but the time of its onset and duration appears to vary considerably in different geographical regions. According to Dansgaard's ice-core records from Crête, Greenland, the coldest years of the entire series (which runs from 600 to the present) occurred during 1150-1400. The period from about 1450 to 1700 was not particularly cold. The Iceland record suggests that the climate was always very variable on all time scales, and that cold periods from about 1610 to 1640, from about 1670 to 1700, and from about 1720 to about 1770 were interspersed with milder periods. In northern Sweden, however, a more protracted and very distinct cold period starts around 1570. In southern Norway, a cold period is indicated from around 1610, but it appears to have been relatively mild from the mid- to late-17th century, after which it became colder again.
Astrid E.J. Ogilvie