Past reconstructions: problems, pitfalls and progress

For instance, South-Eastern Atlantic ocean sediment data from Farmer et al (2005) (Loehle data series #17) nominally goes up to the present 0 calendar years. This is really 1950 due to the convention that years “Before Present’ (BP) almost invariably begin then (some recent papers use BP(2000) to indicate a different convention, but that is always specifically pointed out). However, the earliest real date for that core is 1053 BP, with a 2-sigma range of 1303 to 946 BP – almost 400 years! That makes this data completely unsuitable for reconstructions of the last 2000 years – which in all fairness, was certainly not the focus of the original paper.

Similar issues arises with data from DeMenocal et al (2000) (Loehle #10) and SSDP-102 (Kim et al , 2004) (Loehle #18). In the the first record, the initial data point nominally comes from 88 BP (i.e. 1862 CE), but the earliest dated sample is around 500 BP. In the second, the initial date is closer to the present (1940), but the age model is constrained by only 3 ages over the whole Holocene (and it’s not clear that any are within the last two millennia. So while both records have more apparent resolution than Farmer et al, their use in a reconstruction of recent paleo-climate is dubious.

It should probably be pointed out that the Loehle reconstruction has mistakenly shifted all three of these records forward by 50 years (due to erroneously assuming a 2000 start date for the ‘BP’ time scale). Additionally, the series used by Loehle for the Farmer et al data is not the SST reconstruction at all, but the raw Mg/Ca measurements! Loehle #12 (Calvo et al, 2002) is also off by 50 years, but since it doesn’t start until 1440 CE, its presence in this collection is surprising in any case. The dates on two other ocean sediment cores (Stott et al 2004 – #14 and #15) are on the correct scale thankfully, but are still marginal in terms of resolution (29 and 44 years respectively, but effectively longer still due to bioturbation of the sediments). Neither of them however extend beyond the mid-20th Century (end points of 1936 CE and 1810 CE) and so aren’t much use for looking at medieval-vs-modern data.

Other dating issues arise if the age model was tuned for some purpose. For longer time scale records, the dates are often tuned to ‘orbital forcing’ periods based on the understanding that precession and obliquity do have strong imprints in many records. However, in doing so, you remove the ability to assess with that record whether the orbital expression is leading or lagging another record. Since reconstructions of recent centuries are often pored over for signs of solar or volcanic forcing, it is crucial not to use those signals to adjust the age model. Unfortunately, the Mangini et al (2005) speleothem record (Loehle #9) was tuned to a reconstruction of solar activity so that the warm periods lined up with solar peaks. This invalidates its use on that age model for any useful reconstruction, since it would be assuming a relationship one would like to demonstrate. If put on a less biased age model, it could be useful however (but see issue 3 as well).

Issue 2: Fidelity

This issue revolves around what the proxy records are really recording and whether it is constant in time. This is of course a ubiquitous problem with proxies, since it well known that no ‘perfect’ proxy exists i.e. there is no real world process that is known to lead to proxy records that are only controlled by temperature and no other effect. This leads to the problem that it is unclear whether the variability due to temperature has been constant through time, or whether the confounding factors (that may be climatic or not) have changed in importance. In the case where the other factors seem to be climatic (d18O in ice cores for instance), the data can sometimes be related to some other large scale pattern – such as ENSO and could thus be an indirect measure of temperature change.

In many cases, proxies such as Mg/Ca ratios in foraminifera have laboratory and in situ calibrations that demonstrate a fidelity to temperature. However, some proxies, like d18O which do have a temperature component, also have other factors that affect them. In forams, the other factors involve changes in water mass d18O (correlated to salinity), or changes in seasonality. In terrestrial d18O records, the precipitation patterns, timing and sources are important – more so in the tropics than at high latitudes though.

Page 2 of 4 | Previous page | Next page