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Climatology 3 pages: | 1 | 2 | 3 |
By David W. Stahle,University of Arkansas, Fayetteville
Dr. Stahle cores trees with Alan Alda in the segment,
"What Happened at Jamestown"

T
hanks to a number of natural phenomena, our knowledge of prehistoric climatic conditions is surprisingly detailed. Scientists have devised several ingenious methods to measure environmental conditions from the centuries- and even millennia- before modern meteorological instruments. Some of these methods are so accurate they reveal seasonal climate conditions with exact calendar year dating and can provide the environmental context for understanding human history.
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Photo of tree

Forest Archives

The annual growth rings of many tree species provide an excellent record of climatic variability. The rate of growth among certain trees is directly linked to moisture availability, so fat growth rings indicate wet years and skinny growth rings document drought years.

Photo of Jamie Sayen and Ed Cook
Conservationist Jamie Sayen (left) and dendrochronologist Ed Cook pose with a Sycamore.
 

Dendrochronologists, scientists who study tree-rings, take narrow core samples from living trees. We know the exact calendar date of the outside ring, so we can synchronize the patterns of wide and narrow rings back in time among hundreds of individual trees in a given climatic zone. By matching the rings among many trees, patterns emerge that reflect the unique history of favorable and unfavorable climate, giving dendrochronologists an accurate chronology of growth that corresponds exactly to each and every calendar year.

Some trees can live to an extraordinary age, allowing us to paint a clear picture of the climate over several hundred years. By examining moisture-sensitive baldcypress trees along the Blackwater River in southeastern Virginia, my colleagues and I were able to determine the environmental conditions during the early English colonization of North America.

Photo of tree ring
Growth rings from a baldcypress show the droughts that devastated the Jamestown and Roanoke settlements.
 

Core samples from these very old trees revealed that the first colonists at Jamestown Island had the misfortune to arrive during one of the worst extended droughts in centuries (lasting from 1606-1612). The three-year drought from 1587-1589 was in fact the most extreme drought in 800 years, and may have been an important factor in the disappearance of the Lost colony of Roanoke Island, one of the great mysteries of American history. Though the Jamestown and Roanoke colonists have been criticized for poor planning, the tree-ring data show that even well prepared settlers would have been seriously threatened by the climatic conditions they faced upon arrival in the New World.

Dendrochronologists have also used tree-ring data to reconstruct climate and crop yields of the Anazazi on the Colorado Plateau some 1000 years ago. The roof timbers found in these ancient ruins have been exactly dated, indicating the year the timbers were cut with stone axes for construction. By matching the construction dates with the climate and crop yield chronologies, we find that the remote cliff dwellings and other villages of the Anazazi were usually constructed during times of favorable climate and crop surplus. Unsurprisingly, village abandonment often appears to have occurred during drought and crop
shortfall.

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3 pages: | 1 | 2 | 3 |

Photos: University of Arkansas

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