Climate Science

Ringing in the Old Years

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Science  09 May 2008:
Vol. 320, Issue 5877, pp. 718
DOI: 10.1126/science.320.5877.718a

The annual rings of the bristlecone pine trees in eastern California, extending back in live trees about 8000 years, are a lynchpin for calibrating radiocarbon dates. They have also provided important local climate records, as ring width is thought to reflect temperature and moisture; the length of the record is particularly useful in applying the data toward long reconstructions of global climate. Berkelhammer and Stott have now obtained an annual resolved record, extending back to the year 1700, of the oxygen isotope composition of cellulose from the rings of two California bristlecone pines. Interpreting the record is somewhat complicated, as isotopes are fractionated within the tree and by transpiration from the leaves as relative humidity varies, but large changes in values probably reflect variation in the main sources of storms in the region. The record shows small oscillations every 20 years or so that probably reflect Pacific climate variations. More dramatic is a steep change in the mid-1800s that indicates a change from southern-derived moisture during the time of the Little Ice Age to more northern winter storms since. The effect of such changes on the overall growth of the trees, and thus the ring width used in longer reconstructions, need further study. — BH

Geochem. Geophys. Geosys. 9, 10.1029/2007GC001803 (2008).

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