What was done
Hundreds of well-preserved subfossil logs were discovered at the bottom of Heal Lake (48°32'N, 123°28'W) near the city of Victoria, Vancouver Island, Canada, in 1992.  Radiocarbon dating of selected logs enabled the conducting of dendroclimatological analyses of them from approximately 150 to 9000 radiocarbon years before present.  Based on data obtained from 121 subfossil logs and 29 living species of Douglas-fir, the authors were able to construct an ~ 4,000-year chronology that is sensitive to spring precipitation.

What was learned
Zhang and Hebda report that "the magnitude and duration of climatic variability during the past 4000 years are not well represented by the variation in the brief modern period."  Spring droughts, represented by ring-growth departures exceeding 2 standard deviations below the mean in at least 5 consecutive years, occurred in the late AD 1840s and mid 1460s, as well as the mid 1860s BC, and were more severe than any drought of the 20th century.  In addition, the most persistent drought occurred during the 120-year period between about AD 1440 and 1560.  Other severe droughts of multi-decadal duration occurred in the mid AD 760s-800s, the 540s-560s, the 150s-late 190s and around 800 BC.

Wavelet analyses of the tree-ring chronology revealed various decadal- to centennial-scale oscillations that have altered in frequency over the past 4000 years.  For instance, an oscillation of between 30-80 years prevailed in the 19th century BC, but drifted toward a centennial-scale oscillation in the mid 9th to mid 8th century BC, the mid 3rd to 4th century BC, the AD 2nd to mid 3rd century, and the late AD 6th to early 9th century, before reappearing as a pentadecadal oscillation in the AD 19th to 20th century.  Interannual (2-8-year) and decadal (10-30-year) oscillations existed intermittently with varying amplitude and persistence across the record.

What it means
This study documents some of the host of natural oscillations that permeate earth's climate on interannual to centennial time scales; and it indicates that although we may not understand all of the factors that cause these oscillations, the 20th century was in no way unusual in this regard, as there were many times throughout the prior 4000 years when it was both wetter and drier than it was during the last century of the past millennium.