PP21B-2011: Excess volcanic cooling as recorded by tree ring densities

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Authors: Martin P Tingley, Alexander Stine, Peter Huybers

Author Institutions: Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA

Tree ring records are arguably the most important indicators of continental temperatures over the last centuries and millennia, though their fidelity has come into question on the basis of under-representing Arctic warming since approximately the 1960s. Here we explore divergences between tree ring densities and other markers of temperature over the last 600 years, using a recently developed Bayesian Hierarchical Model that explicitly models spatial and temporal covariance and thus allows us to compare inferences from spatially distinct data sets over common regions. We find that reconstructions based on tree ring densities alone consistently diverge from non-dendro-based temperature estimates after 1960, but that, at low frequencies, divergence occurred during no other interval since 1400. We additionally identify a novel type of divergence, wherein tree ring densities systematically overestimate the magnitude of cooling in the years immediately following volcanic eruptions. Comparing inferences from the tree ring density series and the instrumental record since 1850 reveals a statistically significant divergence in volcanic response, with the densities inferring, on average, 0.12C of excess cooling relative to the instrumental record. Comparisons of the volcanic responses inferred separately from the densities and from ice core records over longer time intervals likewise indicate that the densities are biased towards a colder response to volcanic eruptions. Our results demonstrate that divergence between temperatures and tree ring densities is not confined to the post-1960 interval, consistent with separate evidence (Stine and Huybers, AGU abstract 2012) that aerosol injection into the Arctic atmosphere from volcanism or anthropogenic sources leads to a reduction in tree ring density via a decrease in photosynthetically active radiation.

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