Thinking about proxies

COMMENT ON DISCUSSION OF PROXIES IN HUNTLEY (2012)
All areas of research have strengths and limitations which are readily acknowledged by the scientists involved. The reconstruction of past climates (palaeoclimates) from biological indicators contained within the fossil record (proxies) presents some specific challenges; for example key limitations might be gaps in a sedimentary sequence or post-depositional degradation of samples. Understanding and interpreting data sets in the face of these challenges require the researcher to develop a wide range of skills. Huntley (2012) focuses upon the uncertainties within palaeoclimate reconstruction which he considers to be “frequently overlooked” (p. 2) by scientists making climate reconstructions from proxy records. Specifically Huntley urges researchers to consider carefully:

  • What a given proxy is actually capable of reconstructing, i.e. what climate variables controls its distribution?
  • What other variables might be influencing the proxy, i.e. could there be multiple influences, might these vary through time?
  • What is the spatial relevance of the proxy, i.e. macro versus micro scale?
  • Can multiple proxies be compared, either within or between sites?

In other words: which and how many climatic variables can be reconstructed form any one aspect of the fossil record?

Below I review and comment on some key arguments made by Huntley (2012) related to the use of proxies in reconstructing palaeoclimates.

REFERENCE
Huntley, B. (2012) Reconstructing palaeoclimates from biological proxies: Some often overlooked sources of uncertainty. Quaternary Science Reviews 31: 1-16.

SO, WHAT IS IMPORTANT?
There has been a long standing desire among researchers to reconstruct past temperature; understandably so because temperature can be: i) closely related to the ecology at a given location, ii) monitored by instruments on a day-to-day basis, and  iii) easily understood by a wider audience. Huntley emphasizes that, whilst many proxies are controlled by temperature in some way the relationship is often more complex than is often presented. Huntley expresses greatest concern on the building of proxy models to fit parameters determined by the meteorological data available, i.e. linking biological proxies to variables such as mean annual temperatures (MAT) primarily because the data are available. Modern biological and ecological studies tell us that MAT has little direct control over many organisms distribution (proxies) and that other factors, such as the temperature at a given time during the life cycle, are perhaps more critical. Huntley illustrates his point with discussion of the various temperature thresholds throughout the year which organisms respond to; for example many plants have “chilling requirements for seed germination, bud burst and/or flowering” (p.3). As there is not necessarily a simple relationships between the MAT and other temperature variables, such as coldest month or mean summer temperature, Huntley argues rightly that palaeoclimate reconstruction without consideration of the appropriate variables could lead to confused interpretations.

Within each of the identified issues Huntley makes strong cases for the climate limitations and consequent uncertainties within the palaeoclimate reconstructions. However, returning to the initial statement of “frequently overlooked” factors I feel this is perhaps slightly unfair on researchers working with biological proxy data. Firstly, with regard to the development of training datasets and models many studies consider multiple climate and environmental indicators, e.g. Brooks & Birks (2001) consider the importance of inter sample variation of environmental variables such as  pH, total phosphorus and lake depth when reconstructing training data sets for Chironomids. In addition, it is often not possible to parametrize all possible driving variables within an analysis due to a wider absence of data; a problem that is particularly acute when working outside the comfort of geographic regions with long research histories such as Europe or North America.  Furthermore, it is worth remembering that reconstructions from proxy records are usually not conducted as opposed to a much more accurate method, but instead they are often used in the absence of any alternative.

In presenting palaeoclimate reconstructions it is critical that scientists draw conclusions based upon the acknowledged constraints of understanding for the indicator concerned. Models of past environmental change continue to evolve as our understandings of the modern constraints on each proxy improve. Only as we continue to use and interpret proxies will our accuracy and understanding of the data develop. The paper presented by Huntley challenges researchers (perhaps unfairly) to reconsider which elements of past climate they are able to reconstruct and puts forward a good framework for thinking about how to infer past climates. This paper provides a good starting point for anyone new to proxy reconstructions and interested in the difficulties and limitations associated with this science.

REFERENCE
Brooks. J. S., Birks. H.J.B. (2001) Chironomid-inferred air temperature from Lateglacial and Holocene sites in north-west Europe: progress and problems. Quaternary Science Reviews 20: 1723-1741.

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