The importance of the geophysical context in statistical evaluations of climate reconstruction procedures |
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| Authors: | Caspar M Ammann Eugene R Wahl |
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| Institution: | (1) Climate and Global Dynamics Division, National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80305, USA;(2) Environmental Studies and Geology Division, Alfred University, One Saxon Drive, Alfred, NY 14802, USA |
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| Abstract: | A portion of the debate about climate reconstructions of the past millennium, and in particular about the well-known Mann-Bradley-Hughes
(“MBH” 1998, 1999) reconstructions, has become disconnected from the goal of understanding natural climate variability. Here,
we reflect on what can be learned from recent scientific exchanges and identify important challenges that remain to be addressed
openly and productively by the community. One challenge arises from the real, underlying trend in temperatures during the
instrumental period. This trend can affect regression-based reconstruction performance in cases where the calibration period
does not appropriately cover the range of conditions encountered during the reconstruction. However, because it is tied to
a unique spatial pattern driven by change in radiative balance, the trend cannot simply be removed in the method of climate
field reconstruction used by MBH on the statistical argument of preserving degrees of freedom. More appropriately, the influence
from the trend can be taken into account in some methods of significance testing. We illustrate these considerations as they
apply to the MBH reconstruction and show that it remains robust back to AD 1450, and given other empirical information also
back to AD 1000. However, there is now a need to move beyond hemispheric average temperatures and to focus instead on resolving
climate variability at the socially more relevant regional scale. |
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