Global Temperatures, Volcanic Eruptions, and Trees that Didn’t Bark

Following the AD 1258 eruption, the climate model simulations predict a drop of 2C, but the tree ring-based reconstruction shows only about a 0.5C cooling. Equally vexing, the cooling in the reconstruction occurs several years late relative to what is predicted by the model. The other large eruptions showed similar discrepancies. An analysis using synthetic proxy data with spatial sampling density and proxy signal-to-noise ratios equivalent to those of the D’Arrigo et al (2006) tree-ring network suggest that these discrepancies cannot be explained in terms of either the spatial sampling/extent or the intrinsic “noisiness” of the network of proxy records.

However, using a tree growth model that accounts for the temperature growth thresholding effects discussed above, combined with the complicating effects of chronological errors due to potential missing growth rings, explains the observed features remarkably well.

Show in the above figure (Figure 2d from the article) is the D’Arrigo et al tree-ring based NH reconstruction (blue) along with the climate model (NCAR CSM 1.4) simulated NH mean temperatures (red) and the “simulated tree-ring” NH temperature series based on driving the biological growth model with the climate model simulated temperatures (green). The two insets focus on the response to the AD 1258 and AD 1809+1815 volcanic eruption sequences. The attenuation of the response is produced primarily by the loss of sensitivity to further cooling for eruptions that place growing season temperatures close to the lower threshold for growth. The smearing and delay of the cooling, however, arises from another effect: when growing season lengths approach zero, we assume that no growth ring will be detectable for that year. That means that an age model error of 1 year will be introduced in the chronology counting back in time. As multiple large eruptions are encountered further back in time, these age model errors accumulate. This factor would lead to a precise chronological error, rather than smearing of the chronology, if all treeline sites experienced the same cooling. However, stochastic weather variations will lead to differing amounts of cooling for synoptically distinct regions. That means that in any given year, some regions might fall below the “no ring” threshold, while other regions do not. That means that different chronological errors accumulate in synoptically-distinct regions of the Northern Hemisphere. In forming a hemispheric composite, these errors thus lead to a smearing out of the signal back in time as slightly different age model errors accumulate in the different regions contributing to the composite.

Including this effect, our model accounts not only for the level of attenuation of the signal, but the delayed and smeared out cooling as well. This is particularly striking in comparing the behavior following both the AD 1258 and AD 1809 eruptions (compare the green and blue curves in the insets of the figure). Our model, for example, predicts the magnitude of the reduction of cooling following the eruptions and the delay in the apparent cooling evidence in the tree-ring record (i.e. in AD 1262 rather than AD 1258). We have also included a minor additional effect in these simulations. While volcanic aerosols cause surface cooling due to decreased shortwave radiation at the surface, they also lead to increased indirect, scattered light at the surface. Plant growth benefits from indirect sunlight, and past studies show that e.g. a Pinatubo-sized eruption (roughly -2W/m^2 radiative forcing) can result in a 30% increase in carbon assimilation by plants. This effect turns out to be relatively small because it is proportional in nature, and thus results in a very small absolute increase when growth is suppressed i the first place by limited growing seasons. However, not including this effect results in a slightly less good reproduction (purple dashed curves in the two insets of the figure) of the observed behavior.

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