Guest essay by Patrick J. Michaels and Paul C. Knappenberger –
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. Tinsley’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 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 reconstrictions from tree-ring densities overestimate volcanic cooling. Geophysical Research Letters, doi:10.1002/2014GL061268.
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.”
Mann seems to think that the only thing at influences tree growth is temperature. Never seems to consider the other things necessary for growth.
Anthony:
Great photo and an interesting pileus on the ash cloud. Looks like the cloud plowed right through it.
With normal volcanoes there is no such thing as volcanic cooling and no one knows what supervolcanos like Yellowstone will do. .What happens in an eruption is that hot volcanic gases ascend first into the stratosphere which they warm. This is followed by cooling a couple of tears later but it never reaches the ground level whose temperature remains unaffected. Most so-called “volcanic cooling” periods are nothing more than misidentified La Nina periods. That is because the ENSO oscillation that consists of alternating El Nino peaks and La Nina valleys is either poorly represented in global temperature charts or even wiped out by computer processing. The oscillation is not synchronous with the occurrence of volcanoes. As a result a volcanic eruption may coincide with an El Nino peak, a La Nina valley, or something in between. If it coincides with an El Nino peak it will be followed by a La Nina valley which is immediately appropriated for its volcanic cooling. This is the origin of the fake “volcanic cooling” that follows Pinatubo eruption. But if the eruption coincides with a La Nina valley it is followed immediately by an El Nino peak and the volcanologists are left scratching their head about the mysterious disappearance of that volcanic cooling they expect. That is what happened to El Chichon. But they are so insistent that cooling must be there that they have even written it into climate models. That is now the only place where El Chichon cooling exists today. If you look at some well-known eruptions like Krakatoa you find ninimal coolimng because it erupted just when an El Nino was almost over. And that despite the celestial displays reported the wprld over. Or take Katmai/Novarupta, the greatest eruption of the twentieth century, which left no trace of cooling because it coincided with the early beginning of an El Nino peak. All of which leads me to the conclusion that volcanic eruptions cannot possibly leave any trace of themselves in tree ring records. For more info read my book, pp. 17 – 21.
What about reduced solar insolation with big volcanic eruptions causing reduced chlorophyll activity in trees? I was amazed that this hadn’t been considered by dendrochronologists in looking for a fingerprint of vulcanism.
A possible article for someone with an academic background in botany (especially dendrology) to run with; I seriously question the qualifications of most of the climatologists doing tree-ring analyses. How many of them have degrees in botany with specialization in dendrology? What qualifies them to assert that tree-ring widths are influenced EXCLUSIVELY by temperatures. As other comments have pointed out, major volcanic eruptions can influence precipitation and atmospheric transparency. Reduced/changed precipitation patterns can influence tree-ring width. Reduced atmospheric transparency cuts down photosynthesis, which cuts down tree-ring growth.
The real showstopper, however, is that plant growth (and therefore tree-ring width) responds directly to CO2 not neccesarily to its side-effects. Let’s do a thought experiment (gedankene experiment) with two observers. A tree is grown in a greenhouse for several years, with the CO2 levels being monitored. They are adjusted up and down during the experiment. The temperature is not monitored. After several years, the tree is cut down, and the tree-rings are analyzed and compared with the CO2 corresponding levels. Tree rings are wider when CO2 is higher. We submit this data to 2 analysts…
a) Michael Mann will look at the data, and infer that increased CO2 caused higher temperatures, which caused the wider tree-rings.
b) An independant botanist/dendrologist will look at the data, and infer that the increased CO2 meant more plant food which caused more growth. No inference about higher temperatures.
To summarize, I question the qualifications of most climatologists doing dendroclimatology to claim that wider tree-rings are due to higher temperatures and only to higher temperatures. Even more so, when much of their funding depends on global warming hype.