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

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.

As noted earlier, our main conclusions are insensitive to the precise details of the forcing estimates used, the volcanic scaling assumptions made, and the precise assumed climate sensitivity. They were also insensitive to the details of the biological tree growth model over a reasonable range of model assumptions. The conclusion that tree-ring temperature reconstructions might suffer from age model errors due to missing rings is bound to be controversial. A few points are worth making here. First of all, our conclusion is quite specific to temperature-sensitive trees at treeline, and it does not imply more general problems in the larger discipline of dendrochronology. Secondly, the conclusion at this stage simply a hypothesis, a hypothesis that can account for these key enigmatic features in the actual tree-ring hemisphere temperature reconstruction: the attenuation, and the increasing (back in time) delay and temporal smearing of the cooling response to past volcanic forcing. Were an equally successful and more parsimonious hypothesis to be provided for these observations, I would be the first to concede and defer to this alternative explanation.

One argument against the specific conclusion of missing growth rings is that trees are carefully cross-dated when forming regional chronologies, and this precludes the possibility of chronological errors. That, however, assumes that there are at least some trees within a particular region that will not suffer a missing ring during the years where our model predicts it. Yet our prediction is that all trees within a region of synoptic or lesser scale where growing season temperatures lie below the growth threshold will experience a missing ring. Thus, cross-dating within that region, regardless of how careful, cannot resolve the lost chronological information. It is my hope that dendroclimatologists will reassess raw chronologies more carefully and critically assess the extent to which the predicted features might indeed be present in the underlying tree-ring data. Again, this paper presents a hypothesis for explaining some enigmatic features of existing tree-ring temperature reconstructions. It is hardly the last word on the matter.

Finally it is worth discussing the potential wider implication of these findings. Climate scientists use the past response of the climate to natural factors like volcanoes to better understand how sensitive Earth’s climate might be to the human impact of increasing greenhouse gas concentrations, e.g. to estimate the equilibrium sensitivity of the climate to CO2 doubling i.e. the warming expected for an increase in radiative forcing equivalent to doubling of CO2 concentrations. Hegerl et al (2006) for example used comparisons during the pre-industrial of EBM simulations and proxy temperature reconstructions based entirely or partially on tree-ring data to estimate the equilibrium 2xCO2 climate sensitivity, arguing for a substantially lower 5%-95% range of 1.5–6.2C than found in several previous studies. The primary radiative forcing during the pre-industrial period, however, is that provided by volcanic forcing. Our findings therefore suggest that such studies, because of the underestimate of the response to volcanic forcing in the underlying data, may well have underestimated the true climate sensitivity.

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