Friday, February 13, 2015

Dendroclimatology: The divergence problem

I was reminded about dendroclimatology when reading a book about the geological evidence of climate change by E. Kirsten Peters. Dendroclimatology is the study of inferring past climactic conditions based upon tree ring width and/or density. The resulting data is high resolution since a tree ring is formed every year. Tree ring width and/or density correlates well with various climate parameters like sun, water, and temperature. Using techniques from dendrochronology (tree-ring dating), long climate records of thousands of years can be reconstructed using this technique. For example, summer temperature anomalies for the past 7000 years in Siberia were constructed using tree ring proxies in the figure below.

R.M.Hantemirov - Institute of Plant and Animal Ecology. Summer temperature anomalies of the Yamal Pennisula.

Dendroclimatologists have developed methods to ensure that the samples examined contain tree ring properties that best reflect only the climactic parameter of interest. Despite this there are confounding factors, as outlined in the dendroclimatology wikipedia entry, including nonlinear responses and environmental conditions and events that can otherwise affect tree ring width and density. The most interesting confounding effect has only been evident since the 1950s and is known as the divergence problem.

The divergence problem was first identified in Alaska by Taubes (1995)[1] and Jacoby & d'Arrigo (1995)[2]. The recognition that this problem was widespread in high northern latitudes was published in 1998 by Keith Briffa[3]. A study by Cook in 2004[4] demonstrated that the problem is unique in the past 1000 years, suggesting the possibility of an anthropogenic cause. The problem is that, in northern latitudes, tree ring proxy measurements have diverged from instrument-based temperature data since the 1950s (see figure below). Growth of trees at these latitudes is declining despite instrument-based temperature data that would normally correlate with increased tree-ring width. The cause is unknown but it is likely to be a combination of local and global factors such as global warming-induced drought and global dimming.[5]

Twenty-year smoothed plots of tree-ring width (dashed line) and tree-ring density (thick solid line), averaged across a network of mid-northern latitude boreal forest sites and compared with equivalent-area averages of mean April to September temperature anomalies (thin solid line). (Briffa 1998)[3] taken from wikipedia

Dendroclimatology seems like a fascinating field that, given some reasonably inexpensive equipment, could be done as an amateur. It would be a great excuse for a hike or backcountry ski while collecting data and learning about botany, local climate, local geography, statistical analysis, and sampling methods.


[1] Taubes, G. (17 March 1995), "Is a Warmer Climate Wilting the Forests of the North?", Science 267 (5204): 1595–1526.
[2] Jacoby, G. C.; d'Arrigo, R. D. (June 1995), "Tree ring width and density evidence of climatic and potential forest change in Alaska", Global Biogeochemical Cycles 9 (2): 227.
[3] Briffa, Keith R.; Schweingruber, F. H.; Jones, Phil D.; Osborn, Tim J.; Shiyatov, S. G.; Vaganov, E. A. (12 February 1998), "Reduced sensitivity of recent tree-growth to temperature at high northern latitudes", Nature 391 (6668): 678.
[4] Cook 2004
[5] d'Arrigo, R.; Wilson, R.; Liepert, B.; Cherubini, P. (February 2008), "On the 'Divergence Problem' in Northern Forests: A review of the tree-ring evidence and possible causes", Global and Planetary Change 60 (3–4): 289. 



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