The dendroclimatic divergence phenomenon: reassessment of causes and implications for climate reconstruction

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences


Palaeoclimate reconstructions extend our knowledge of how climate varied in times before expansive networks of measuring instruments became available. These reconstructions are founded on an understanding of theoretical and statistically-derived associations acquired by comparing the parallel behaviour of palaeoclimate proxies and measurements of varying climate. Inferences about variations in past climate, based on this understanding, necessarily assume that the associations we observe now hold true throughout the period for which reconstructions are made. This is the essence of the uniformitarian principle. In some northern areas of the world, recent observations of tree growth and measured temperature trends appear to have diverged in recent decades, the so called 'divergence' phenomenon. There has been much speculation, and numerous theories proposed, to explain why the previous temperature sensitivity of tree growth in these areas is apparently breaking down. The existence of divergence casts doubt on the uniformitarian assumption that underpins a number of important tree-ring based (dendroclimatic) reconstructions. It suggests that the degree of warmth in certain periods in the past, particularly in medieval times, may be underestimated or at least subject to greater uncertainty than is currently accepted. The lack of a clear overview of this phenomenon and the lack of a generally accepted cause had led some to challenge the current scientific consensus, represented in the 2007 report of the IPCC on the likely unprecedented nature of late 20th century average hemispheric warmth when viewed in the context of proxy evidence (mostly from trees) for the last 1300 years. This project will seek to systematically reassess and quantify the evidence for divergence in many tree-ring data sets around the Northern Hemisphere. It will establish a much clearer understanding of the nature of the divergence phenomenon, characterising the spatial patterns and temporal evolution. Based on recent published and unpublished work by the proposers, it has become apparent that foremost amongst the possible explanations is the need to account for systematic bias potentially inherent in the methods used to build many tree-ring chronologies including many that are believed to exhibit this phenomenon. This proposal is designed to build on recent innovations in tree-ring chronology production techniques, also developed by the proposers. These new methods will produce tree-ring chronologies whose variability is unbiased, either by temporal changes in the age structure of the constituent sample series, or by any distortion in the data that can arise when using the previously applied techniques. The extensive reprocessed and improved data sets will then form the basis for many detailed, site-by-site comparisons of local climate and various tree-growth parameters in order to re-characterise the nature, strength and temporal stability of the climate/growth associations. This will represent a systematic and objective re-assessment of the evidence for divergence in different forest contexts. The project will then explore all of the current theories for the cause(s) of divergence employing both statistical and process-modelling techniques. The project will go on to use the reprocessed tree-ring data sets to re-calibrate many important climate reconstructions, with varying levels of spatial detail, and carefully assess the implications of the divergence effect, as newly characterised, on reconstruction uncertainty. This project will provide results that will inform the international scientific debate and widespread public perception of the reliability of tree-ring-based climate reconstructions in particular, but also our current understanding of the reliability of current evidence for high-resolution temperature changes during the late Holocene.


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Anchukaitis K (2012) Tree rings and volcanic cooling in Nature Geoscience

Description The project has focussed on the updating and reprocessing of tree-ring records, and their use to infer past variations in climate over recent centuries and millennia. Some published records exhibit an apparent "divergence" from the instrumental climate record in recent decades, and this has to be carefully considered to try to identify possible explanations and to evaluate its effect on inferences about past climate variations and the reliability of tree-ring records. To date, we have focussed on millennial records from Europe (Tornetrask, Sweden) and northern Russia (Yamal and Polar Urals) and have produced new tree-ring records and summer temperature reconstructions that have used additional tree-ring data to earlier work and that have been processed using methods than can take into account systematic differences between subsets of the data. These records do not show the apparent divergence exhibited by some tree-ring records. A specific case study of the divergence phenomena as recognised in northern Canada has shown the role played by statistical processing methods that make it appear that tree growth has changed its direct response to temperature forcing. When using improved chronology production methods (developed as part of this project) and alternative temperature records, this interpretation is shown to be invalid. A reanalysis of a separate network of tree-ring data, comprising ring widths and ring densities from several hundred sites distributed around the Northern Hemisphere, has been partially completed. These data have previously been characterised as exhibiting recent divergence, specifically failing to exhibit growth increases that would be expected as a consequence of recent warming. The analysis to date reveals that this is also in large part a consequence of the data methods used to produce those tree-ring chronologies.
Exploitation Route The project findings support policy-making in the context of the current climate change problem. The link between the project findings and policy-making is indirect, taking place through, for example, the assessments of the Intergovernmental Panel on Climate Change (IPCC) that provide the scientific basis for international climate change negotiations. It is, nevertheless, a very important element of this, because the need to document and understand the background natural climate changes is essential for attributing recent changes and for making future projections. The recent slowdown in global warming is an excellent demonstration that natural climate variability must be considered more carefully when making short-term climate projections, and that this requires evidence from the real world to supplement and test the internal variability simulated by coupled computer-based climate models. The tree-ring chronologies and summer temperature reconstructions developed as part of this project are an improvement over earlier work, and can be exploited by including them in future attempts to make improved reconstructions of Northern Hemisphere temperature changes and patterns of temperature change over recent centuries and millennia. The improved tree-ring processing methods used within this study can be adopted by the dendroclimatological community, to support the development of more reliable tree-ring records and climate reconstructions in future. To facilitate this we are working towards making our tree-ring processing software available to the community.
Sectors Environment

Title Multiple Curve Signal Free Regional Curve Standardisation (SF-RCS) 
Description A new approach to tree-ring standardisation, that takes into account samples from different populations and simultaneously identifies the common tree-ring growth signal, the dependence of growth on tree age/size and the spread of values between trees/samples. 
Type Of Material Biological samples 
Year Produced 2013 
Provided To Others? Yes  
Impact The research method is already being used more widely in the dendroclimatological community due to its improved performance at (e.g.) addressing the modern sample bias sometimes seen in tree-ring chronologies. 
Title CRUST: Climatic Research Unit Standardisation of Tree-ring data 
Description CRUST (Climatic Research Unit Standardisation of Tree-ring data) is a computer program for the production of chronologies representing the variance of tree-growth over time. The innovative aspect of this program is the ability to apply various versions of ?Signal-Free Regional Chronology Standardisation? (SF RCS) to different types of tree-ring data. The program includes options to use some earlier standardisation methods and procedures, many of which were borrowed from ARSTAN (written and maintained by Ed Cook and Paul Krusic, available from the LDEO Tree Ring Research Laboratory software page). 
Type Of Technology Software 
Year Produced 2013 
Open Source License? Yes