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Ice-core dating corroborates tree ring chronologies

Filed under: — group @ 5 August 2015

Guest commentary from Jonny McAneney

You heard it here first

Back in February, we wrote a post suggesting that Greenland ice cores may have been incorrectly dated in prior to AD 1000. This was based on research by Baillie and McAneney (2015) which compared the spacing between frost ring events (physical scarring of living growth rings by prolonged sub-zero temperatures) in the bristlecone pine tree ring chronology, and spacing between prominent acids in a suite of ice cores from both Greenland and Antarctica. The main conclusion was that ice core dates, in particular those ice cores relied upon the Greenland Ice Core Chronology 2005 (GICC05), such as the NEEM S1 core, were too old by approximately seven years during the 6th and 7th centuries AD.

Last month, in an excellent piece of research (Sigl et al., 2015) by a collaboration including Earth scientists, dendrochonologists, and historians, the chronology of the Greenland North Eemian Ice Drilling core (NEEM) has been reassessed and re-dated, confirming that such an offset does indeed exist in the GICC05 timescale below AD 1000. The clinching evidence was provided by linking tree-ring chronologies to ice cores through two extraterrestrial events…
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  1. M.G.L. Baillie, and J. McAneney, "Tree ring effects and ice core acidities clarify the volcanic record of the first millennium", Climate of the Past, vol. 11, pp. 105-114, 2015.
  2. B.M. Vinther, H.B. Clausen, S.J. Johnsen, S.O. Rasmussen, K.K. Andersen, S.L. Buchardt, D. Dahl-Jensen, I.K. Seierstad, M. Siggaard-Andersen, J.P. Steffensen, A. Svensson, J. Olsen, and J. Heinemeier, "A synchronized dating of three Greenland ice cores throughout the Holocene", J. Geophys. Res., vol. 111, 2006.
  3. M. Sigl, J.R. McConnell, L. Layman, O. Maselli, K. McGwire, D. Pasteris, D. Dahl-Jensen, J.P. Steffensen, B. Vinther, R. Edwards, R. Mulvaney, and S. Kipfstuhl, "A new bipolar ice core record of volcanism from WAIS Divide and NEEM and implications for climate forcing of the last 2000 years", Journal of Geophysical Research: Atmospheres, vol. 118, pp. 1151-1169, 2013.
  4. M. Sigl, M. Winstrup, J.R. McConnell, K.C. Welten, G. Plunkett, F. Ludlow, U. Büntgen, M. Caffee, N. Chellman, D. Dahl-Jensen, H. Fischer, S. Kipfstuhl, C. Kostick, O.J. Maselli, F. Mekhaldi, R. Mulvaney, R. Muscheler, D.R. Pasteris, J.R. Pilcher, M. Salzer, S. Schüpbach, J.P. Steffensen, B.M. Vinther, and T.E. Woodruff, "Timing and climate forcing of volcanic eruptions for the past 2,500 years", Nature, vol. 523, pp. 543-549, 2015.

Reflections on Ringberg

As previewed last weekend, I spent most of last week at a workshop on Climate Sensitivity hosted by the Max Planck Institute at Schloss Ringberg. It was undoubtedly one of the better workshops I’ve attended – it was focussed, deep and with much new information to digest (some feel for the discussion can be seen from the #ringberg15 tweets). I’ll give a brief overview of my impressions below.

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Climate Sensitivity Week

Some of you will be aware that there is a workshop on Climate Sensitivity this week at Schloss Ringberg in southern Germany. The topics to be covered include how sensitivity is defined (and whether it is even meaningful (Spoiler, yes it is)), what it means, how it can be constrained, what the different flavours signify etc. There is an impressive list of attendees with a very diverse range of views on just about everything, and so I am looking forward to very stimulating discussions.

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The mystery of the offset chronologies: Tree rings and the volcanic record of the 1st millennium

Filed under: — group @ 19 February 2015

Guest commentary by Jonny McAneney

Volcanism can have an important impact on climate. When a large volcano erupts it can inject vast amounts of dust and sulphur compounds into the stratosphere, where they alter the radiation balance. While the suspended dust can temporarily block sunlight, the dominant effect in volcanic forcing is the sulphur, which combines with water to form sulphuric acid droplets. These stratospheric aerosols dramatically change the reflectivity, and absorption profile of the upper atmosphere, causing the stratosphere to heat, and the surface to cool; resulting in climatic changes on hemispheric and global scales.

Interrogating tree rings and ice cores

Annually-resolved ice core and tree-ring chronologies provide opportunities for understanding past volcanic forcing and the consequent climatic effects and impacts on human populations. It is common knowledge that you can tell the age of a tree by counting its rings, but it is also interesting to note that the size and physiology of each ring provides information on growing conditions when the ring formed. By constructing long tree ring chronologies, using suitable species of trees, it is possible to reconstruct a precisely-dated annual record of climatic conditions.

Ice cores can provide a similar annual record of the chemical and isotopic composition of the atmosphere, in particular volcanic markers such as layers of volcanic acid and tephra. However, ice cores can suffer from ambiguous layers that introduce errors into the dating of these layers of volcanic acid. To short-circuit this, attempts have been made to identify know historical eruptions within the ice records, such as Öraefajökull (1362) and Vesuvius (AD 79). This can become difficult since the ice chronologies can only be checked by finding and definitively identifying tephra (volcanic glass shards) that can be attributed to these key eruptions; sulphate peaks in the ice are not volcano specific.

Thus, it is fundamentally important to have chronological agreement between historical, tree-ring and ice core chronologies: The ice cores record the magnitude and frequency of volcanic eruptions, with the trees recording the climatic response, and historical records evidencing human responses to these events.

But they don’t quite line up…
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Can we make better graphs of global temperature history?

I’m writing this post to see if our audience can help out with a challenge: Can we collectively produce some coherent, properly referenced, open-source, scalable graphics of global temperature history that will be accessible and clear enough that we can effectively out-compete the myriad inaccurate and misleading pictures that continually do the rounds on social media?

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