Dendrochronological Modeling and Reconstruction of Large-Scale Climate Variability in Recent Centuries and its Relation to Atmospheric Forcing Functions.
Tree-ring chronologies from boreal treeline and other sites have been used to reconstruct patterns of climate variability, their relationship to known forcing functions, and climate as modeled using these forcings. Northern Hemisphere temperatures reconstructed for the past three hundred years agree with other proxy data and with temperature derived from a radiative-convective model incorporating volcanic, solar and CO_2 forcings. Superposed epoch analysis shows the effects of volcanism on tree growth and spectral analysis indicated periodicities which might be related to solar or other cycles. Comparison of the reconstructed temperatures with recent instrumental records reveals that the temperature departures within the past decade of elevated atmospheric trace gases levels exceed the "natural" variations in the tree-ring data in past centuries. A CO_2 fertilization effect is not detected in this data through 1973. This issue is further investigated for a high-elevation lodgepole pine site from California. Climate response models indicate that a recent growth increase cannot be completely explained by past climate-growth relationships. The contribution of atmosphere-biosphere CO_2 exchange of boreal forests to Pt. Barrow, Alaska CO_2 amplitudes is found to be significant using a 3 -D tracer model which employs an exchange function based on remote sensing photosynthetic indices. Positive correlations between variations in these amplitudes and tree-ring data suggest that tree-rings may be used as indicators of CO _2 uptake and remote sensing estimates of photosynthetic activity. The northern chronologies show patterns of variation which have climatic implications. Their coefficient of variation reveals periods of agreement/disagreement among the sites which in turn indicate varying periods of spatial coherence in atmospheric circulation patterns. Included among the years of highest variation is 1816, one year following the Tambora eruption. The tree growth anomalies support the hypothesis of a shift in the atmospheric long wave pattern with enhanced meridional Arctic airflow at this time. Internal variations of the climate system include the global-scale interactions associated with the El Nino -Southern Oscillation and variations in the monsoon cycle. A Java teak tree-ring series shows a response to ENSO and monsoon precipitation.
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- Geology; Physical Geography; Physics: Atmospheric Science