THE GERMANIC MIGRATIONS OF THE 3^rd and 4^th CENTURIES AD

            The region of central Europe, north of the Danube River and east of the Rhine River, remained unconquered by the Roman Empire.  This same region saw an influx in migration and aggression in the 3^rd and 4^th centuries AD that continued into the 9^th century.  These movements and invasions are well documented but are still scrutinized for their data integrity from historical perspectives.  These events, filtered from the errors of traditional history as the likes of Betsy Ross, emotional history as the likes of a young George Washington and the cherry tree, and errors of omission with the likes of Christopher Columbus’s second journey, have been chronicled to be as inclusive as possible.

These events have been compared to climate change, identified from tree ring chronologies, and examined for coincidence.  Further, it was necessary to evaluate the possibility of this coincidence of migration/ invasion and climate of these events being either a correlation, or causal in their relationship.  It was necessary to address this study from the methodologies of many disciplines such as history, paleoclimatology, archaeology, anthropology, geography, and communication.  These various methodologies each contributed a piece to the larger puzzle of examining the extent of the relationship between human migration events and climate change while focusing on the Germanic tribes in the 3^rd and 4^th centuries, a very turbulent time.

            Written documents tend to record the events that are either agriculturally strained or stressful to livestock as evident in the content analysis earlier in this document, but this does not limit this study to examining only drought periods.  The coincidences of historical events and climate change are examined from both prosperous and drought periods to hypothesize whether or not climate (good or poor) effected the migrations of the Germanic tribes.  These periods and climatic signals are identifiable through paleoclimatological proxies such as palynology, varved sediments, glaciology, and dendroclimatology. 

Dendroclimatology is an excellent paleoclimatological proxy with annual and interannual resolution.  This high resolution climate proxy indicator from over fourteen sites and compiled from four master chronologies in central Europe allows for accurate interpretation of climate changes over the same area in which the Germanic tribes were inhabiting.  The historic record reveals times, in which the Germanic tribes were moving or attacking their neighbors.  The palynological record helps support the interpretation of climate change from the tree ring record and, perhaps more importantly, identifies almost all of the plant pollen as being indicative of agriculture.  Identifying all potential agricultural crops in the pollen rain allows for the establishment of parameters of precipitation and temperature through an understanding of modern crop needs.  Any climate changes that move beyond these parameters become significant to the migration/ invasion study for correlative events.  Any prolonged period that remains outside the necessary growth conditions of the agricultural crops grown in ancient Germania would have substantially affected the indigenous tribes.

Meteorological data have shown that these periods exist in modern records.  They also calibrate the tree ring record and show the tree’s physiological response to changes in temperature and precipitation.  These changes in the tree rings’ annual structure reflect these same changes in the past, which would be directly relational to the agriculture of the same area, especially because there is no historical evidence of irrigation in Germania the 3^rd or 4^th centuries AD.  There would be a relationship between agricultural productivity and the sustainable population of the same area, albeit not as strong the tree-ring and agricultural relationship.  In Germania this would have been subsidized to some degree by the barter practices between the tribes and the Roman frontier (Wells, 2000).  Overall, there is an indirect relationship between tree rings and agricultural sustainability with full realization that the last relationship between agricultural product and human response is the weakest connection.

The historical data that are available, although not continuous through out the whole period, contains a Roman bias, but seems to cover most of the large migrations and attacks on or near the Roman frontiers along the rivers Rhine, Main, and Danube.  However, not all of these recorded events are relevant to this particular study on climate-migration.  These have been filtered using a few selected criteria to remove all continuation years and events with known political causes.  These filtered events are then compared to the tree ring-based climate reconstruction of the same period.

To objectively define these periods, a simple mathematical selection process was developed.  The terms “drought” and “prosperity” were avoided, although implied, to emphasize that climate severity is not defined simply as a series of tree ring measurements, but that these sequences of tree-ring measurements can be interpreted as “drought” or “prosperity”.  The method of selecting potential climate shifts was to categorize any sequence of similar tree-ring growth rates in one of three categories: above mean, below mean, and average.  This was based on the master chronology that was standardized by a 21-year running mean.  Any sequences of four or more years above or below the mean were classified as either “above mean” or “below mean”.  This sequence would continue to be classified in the same category as long as the tree-ring chronology anomaly did not change sign for two consecutive years.  All remaining years in the chronology remained in the “average” category if these years were not connected to a four-year sequence.  The rationale behind using four years was to filter out small changes in the growth rates in the tree-ring chronology and leave only the larger events.  It was also chosen because there is a process in trees that allows for changes in growth response in sustained periods, such as delayed recovery from a single drought year and delayed responses from a very wet year to dry year where the cambium develops in the late fall.  The use of a four-year sequence is a conservative approach that should only identify any long, sustained droughts and therefore be useful in comparing migration/ invasion events and larger climate changes.

The actual statistical analysis of these long periods both above and below mean and the first year of historic events that included either a large scale migration or invasion show that 81.4% of all 3^rd and 4^th century AD events occur in “below mean” periods and 65.2% occurring in “below mean” years.  The dynamics of the 3^rd century does differ from the 4^th century statistically with the 3^rd century having 73.9% of events in “below mean” periods and the 4^th century having 90% of events in “below mean” periods.  These statistics do not prove causation but do suggest that the Germanic tribes were responding in below mean periods.

There is a climate belt that covers most of ancient Germania with the exceptions of the high alpine region, the Black Sea coast, and the eastern portions of Germania near the Volga River.  With the archaeological evidence, palynological reconstruction, and an understanding of past and present climate, it is possible to identify these periods that would stress agricultural and change the sustainability of the region as a whole for habitation by agriculturalists.  The fact that 81.4% of all events occur in years that meet the profile of a sustained drought period is probably not just a coincidence.  These conclusions ultimately must be based on a tribe-by-tribe response, which is impossible with the scattered historical data on many of the individual tribes.  The tribes were quite diverse, but they were similar in their agrarian behavior and would have been affected to varying degrees by these long below mean periods.