Global Science Report is a weekly feature from the Center for the Study of Science, where we highlight one or two important new items in the scientific literature or the popular media. For broader and more technical perspectives, consult our monthly “Current Wisdom.”

A new paper overturns old suppositions regarding volcanoes, tree-rings, and climate sensitivity.

According to a 2012 press release accompanying a paper published in the journal Nature Geoscience, a research team led by Penn State’s Dr. Michael Mann concluded that the cooling influence of historical volcanic eruptions was underrepresented by tree-ring reconstructions of the earth’s temperature.

This, the press release went on to tell us, had potential implications when trying to determine the earth’s equilibrium climate sensitivity (ECS)—i.e., how much the global average surface temperature will rise as a result of a doubling of the atmosphere’s pre-industrial concentration of carbon dioxide. While most recent studies place the ECS noticeably less than earlier studies (including those most heavily relied upon by the U.N.’s Intergovernmental Panel on Climate Change (IPCC) and thus the U.S. Obama Administration), the 2012 Mann study was an exception. It implied that many existing determinations of the ESC were underestimates.

From the press release:

“Scientists look at 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,” said Mann. “Our findings suggest that past studies using tree-ring data to infer this sensitivity have likely underestimated it.”

Fast forward to today.

Appearing on-line in the journal Geophysical Research Letters (and sans press release) is a paper led by Penn State’s Martin Tingley that examined how the temperature response from volcanic inferred from tree-rings compared with that of observations. Tingley’s team concluded that tree-ring based temperature proxies overestimated the temperature response caused by large volcanic eruptions. Instead of responding only to the cooler temperatures, the tree rings also included signals from reduced light availability (from the shading effect of volcanic aerosols) and the two effects together produced a signal greater than what would have been produced by cooler temperatures alone. This is basically the opposite of what Mann and colleagues concluded.

In an article posted to the website RealClimate back in 2012 touting his team’s findings, Mann took time to point out the “wider implication” of his findings:

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.

It will be interesting to see if accounting for the potential biases identified in this study leads to an upward revision in the estimated sensitivity range. Our study, in this regard, once again only puts forward a hypothesis. It will be up to other researchers, in further work, to assess the validity and potential implications of this hypothesis.

Based on the new results of Team Tingley, it seems that Mann’s hypothesis is wrong. Using tree ring temperature proxies would overestimate the climate sensitivity.

“It will be interesting to see” if this is recognized over at RealClimate.

But regardless, there is no escaping the fact that the Tingley study provides additional evidence that the earth’s climate sensitivity to human greenhouse gas emissions is likely less than advertised by the UN IPCC and the Obama Administration. The direct result being that headlong pursuit of carbon dioxide emissions limits should be reconsidered in light of this and other scientific literature.

References:

Mann, M. E., Fuentes, J.D., and S. Rutherford, 2012. Underestimation of volcanic cooling in tree-ring-based reconstructions of hemispheric temperatures, Nature Geoscience,5, 202–205.

Tingley, M. P., Stine, A.R., and P. Huybers, 2014. Temperature reconstructions from tree-ring densities overestimate volcanic cooling. Geophysical Research Letters, doi:10.1002/2014GL061268.