CRITERIA FOR TIMELINE EVENTS

            As the annotated timeline presented in the last chapter shows, there were many human migration events in the 300-year period from AD 150 to 450.  There is an apparent relationship between the historical records and tree ring chronology (Figure 6.1).  Even though there is a discernable pattern, it may not be
a complete assessment.  It is necessary to set criteria for determining which human events could have been climate-based, politically motivated, an attack from a warrior club, or a random occurrence.  For example, the Marcomannic Wars are only significant in this study in the earliest years and the Alaman’s repeated attacks on Agri Decumates may have been more politically than climatically driven.  The historical events have been abbreviated from the annotated timeline from chapter five to eliminate any events that are a continuation from previous years, anecdotal information, or lack adequate historical validation.  Once the criteria have been established for the evaluation of the full historic timeline a simplified timeline can reveal the many coincidences between invasions/ movements and climate extremes.

            This analysis of historic events and the climate record includes evaluations of both long- and short-term phenomenon, the initial phases of long invasions, migrations, and climate documentation.  By eliminating some of the years from the comprehensive timeline discussed in chapter five, it should be possible to evaluate climate change in the context of the historical events.  The abridged timeline should allow for a more meaningful assessment of climate change and movements/ invasions of the Germanic tribes. 

            Even before the timeline is abridged, there is an apparent relationship between the tree-ring chronology and the historical events of the 3^rd and 4^th century AD (Figure 6.1).  Immediately following the long drought of AD 214 – 232 and during the droughts of 364-382, there is an increase in human activity both as invasions and migration.  These two long below mean periods are the primary concern, but there is a necessity to evaluate the entire chronology for potential shorter drought periods.

            The abridged timeline focuses on the southern and western portion of Germania and includes the initial stages of invasions, the beginning and ending of migrations periods, any climate references, and any other pertinent documentation that helps to validate the tree ring chronology.  Most of the detail from the timeline of chapter five has been removed leaving just the dates and a brief synopsis of how the climate apparently parallels these events.

There are comments inside the abridged timeline identifying periods as “above mean”, “average”, and “below mean” years.  An “above mean period” is defined as any period that has four or more years above the 21-year tree-ring running mean growth index that has been set to 100.  This “above mean period” continues until two consecutive years are below the mean.  A “below mean period” is defined similarly except that it is four or more years below the mean and truncated at two consecutive years above the mean.  All years that remain unclassified by this process are defined as “average years”.

For example, if there are four or more years on either side of the standardized tree ring chronology, it is defined as either an above mean or below mean period based on where it falls on the mean.  In table 6.1 the years between AD 158 to 170 are defined as a below mean period because the years AD 160 to 166 have seven years below the mean value of 100.  The two consecutive years above the mean in AD 156 and 157 and 171 and 172 define the “ends” of this period.  The period between AD 177 and 182 is also defined as a below mean period because all of these years are below the mean, but does not continue because of the two consecutive above mean years in AD 175 to 176 and 183 to 184.  The above mean periods in table 6.1 are from AD 183 to 200 because AD 183 to 186 is above the mean and there are not two consecutive years below the mean until AD 200.  The “average periods” in figure 6.1 are the remainder of the years not included in the above mean or below mean period or the years AD 150 to 157, 171 to 176, and AD 201 to 202.  This definition scheme is useful in generalizing the climate changes over a large data set.

The timeline discussion will include the general condition of the period and the year as evidenced by the tree ring chronology for comparison to the historical record.  Items in the abridged timeline that have been removed will be justified in the discussion.  The remaining events will be recorded as an abstract from the annotated timeline in chapter five.