New paper finds temperature reconstructions from tree-rings overestimate volcanic cooling

A paper published today in Geophysical Research Letters finds that tree-ring temperature reconstructions overestimate the cooling from large volcanic eruptions. According to the authors, 

"inferences on volcanic cooling from tree-ring density records has recently come into question, with competing claims that temperature reconstructions based on tree-ring records underestimate cooling due to an increased likelihood of missing rings, or overestimate cooling due to reduced light availability accentuating the response. 

We find that tree-ring densities overestimate post-volcanic cooling with respect to instrumental data 

...our results confirm that high-latitude tree-ring densities record not only temperature, but also variations in light availability [from volcanic aerosols]."

This has multiple implications including

1) Temperature reconstructions from tree-rings, such as Mann's bogus hockey sticks, may also be biased to overestimate cooling or underestimate warming during the past, e.g. during the Little Ice Age period of increased volcanic activity.

2) Other aerosols, such as clouds, can also affect light availability to trees and thus could affect climate reconstructions from tree-rings. Therefore, changes in cloud cover could instead be responsible for what was previously believed to be changes in temperature inferred from tree-rings. 

3) If tree-ring reconstructions overestimate cooling during the past from volcanic and cloud aerosols, then climate sensitivity to greenhouse gas warming must be correspondingly reduced to maintain Earth's energy balance. 

4) The fact that this "recent" major controversy about whether tree-ring reconstructions overestimate or underestimate cooling from aerosols is a stark reminder that very little about climate science is truly "settled science," despite the propaganda claiming otherwise. 

Related: Paper could explain why Mann had to "hide the decline" & relationship of solar activity to tree growth

Temperature reconstructions from tree-ring densities overestimate volcanic cooling

Martin P. Tingley, Alexander R. Stine and Peter Huybers

The fidelity of inferences on volcanic cooling from tree-ring density records has recently come into question, with competing claims that temperature reconstructions based on tree-ring records underestimate cooling due to an increased likelihood of missing rings, or overestimate cooling due to reduced light availability accentuating the response. Here we test these competing hypotheses in the latitudes poleward of 45° N, using the two eruptions occurring between 1850 and 1960 with large-scale Northern Hemisphere climatic effects: Novarupta (1912) and Krakatau (1883). We find that tree-ring densities overestimate post-volcanic cooling with respect to instrumental data (Prob. ≥ 0.99), with larger magnitudes of bias where growth is more limited by light availability (Prob. ≥ 0.95). Using a methodology that allows for direct comparisons with instrumental data, our results confirm that high-latitude tree-ring densities record not only temperature, but also variations in light availability.