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Tree-Rings ‘Overestimate Early-19th Century Summer Cooling By 1.5 °C’

U Büntgen et al., Journal of Climate

Annually resolved and absolutely dated tree-ring chronologies are the most important proxy archives to reconstruct climate variability over centuries to millennia. However, the suitability of tree-ring chronologies to reflect the ‘true’ spectral properties of past changes in temperature and hydroclimate has recently been debated.

At issue is the accurate quantification of temperature differences between early-19th century cooling and recent warming. In this regard, central Europe (CEU) offers the unique opportunity to compare evidence from instrumental measurements, paleomodel simulations and proxy reconstructions covering both the exceptionally hot summer of 2003 and the year without summer in 1816. Here, we use 565 Stone pine (Pinus cembra) ring width samples from high-elevation sites in the Slovakian Tatra Mountains and Austrian Alps to reconstruct CEU summer temperatures over the past three centuries. This new temperature history is compared to different sets of instrumental measurements and state-of-the-art climate model simulations. All records independently reveal coolest conditions in the 1810’s and warmest after 1996, but the ring width-based reconstruction overestimates the intensity and duration of the early-19th century summer cooling by ~1.5 °C at decadal scales. This proxy-specific deviation is triggered by inflated biological memory in response to reduced warm season temperature, together with changes in radiation and precipitation following the Tambora eruption in April 1815. While suggesting there exists a specific limitation in ring width chronologies to capture abrupt climate perturbations with increased climate system inertia, our results underline the importance of alternative dendrochronological and wood anatomical parameters, including stable isotopes and maximum density, to assess the frequency and severity of climatic extremes.