A processing aliasing artefact in the early Quelccaya ice core record

by Hans Erren, Posted 10 February 2004




Processing artifacts in the data of core 1 of the Quelccaya glacier as archived by NOAA, initiated a simulation of compaction and reconstruction of the accumulated ice of the Quelccaya glacier. It was demonstrated that the artifacts are caused by rounding the observed layer thickness to the nearest centimeter. The original observed layer thickness log is not archived with NOAA. The reconstructed dataset, including these artifacts, is used in climate reconstructions.


The Quelccaye ice core record cited (i.a.) in Thompson et al. [TMBK1985] [1] is a substantial element in the climate reconstruction of the last millennium by Mann et al. [MBH1999] [2]. I was directed by Steve McIntyre on the Climatesceptics web group to a strange pattern that is apparant in the unsmoothed data of the longest core at Quelccaya: Core 1 [3].

From TMBK1985 figures 1 and 3 it is clear that a significant amount of compaction - in the order of 95% - occurs in the early part of the core

The Simulation

For the compaction and inversion simulation we take a simple A/age compaction relationship, the actual formula doesn't matter really as will be clear further on. As input dataset we take an accumulation that varies in a cyclical way between 0.5 and 2.5 meters:

Using a compaction factor of 0.03*age yields:

Now If we would simply use the inverse function, the reconstruction would be perfect. However, if we round the compacted data to the nearest centimeter, a dramatic change gets visible in the inverted data.

Note that the early reconstruction even exceeds the input!


If we compare this simulation with the actual reconstructed data of core 1, the similarity is striking. (Black crosses: simulation, red crosses: Quelccaya core 1 reconstruction):


Rounding of layer thicknesses to the nearest centimeter leads to accumulation reconstruction artifacts. As the used compaction formula is not mentioned in TMBK1985 nor at the NOAA archive, Lonnie Thompson was asked for the original core log data. After a reminder he responded as follows on 10 March 2004:
Dear Hans: Just returned from China! Unfortunately, those logs are all 
hand done. These data where not put on electronic format.
We have just redrilled the Quelccaya ice cap in 2003 and brought back two 
frozen ice cores and will be producing a new log based on
this new data. Unfortunately, right we are processing Bona-Churchill 
ice cores and the new Quelccaya and Coropuna cores are
in the cue.

Sorry I can not be more helpful on these old data sets.

best wishes,

So we are are left with an interpreted accumulation log, without direct access to the core log data. The worrisome side is that this interpreted dataset is a cornerstone of the preferred millenium climate reconstruction of the IPCC.


zipped excel:


I thank Dr Lonnie Thompson for taking the time for replying to my email and Steve McIntyre for noting the artifact. No funding from any source was sought or received for this research.


[1] Thompson, L. G., E. Mosley-Thompson, J. F. Bolzan and B. R. Koci. 1985. A 1500 year record of tropical precipitation recorded in ice cores from the Quelccaya Ice Cap, Peru. Science, 229(4717), 971-973. Archived data at http://www.ngdc.noaa.gov/paleo/icecore/trop/quelccaya/quelccaya.html

[2] Mann, M.E., R.S. Bradley, and M.K. Hughes. 1999. Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations, Geophysical Research Letters 26:759-762. http://www.people.virginia.edu/~mem6u/mbh99.html

[3] Thompson, L., 1992, Quelccaya Ice Core Database. IGBP PAGES/World Data Center-A for Paleoclimatology Data Contribution Series # 92-008. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA. ftp://ftp.ngdc.noaa.gov/paleo/icecore/trop/quelccaya/q83cor1.txt

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