I’m working towards a post on the effect of Marcott re-dating, but first I want to document some points on the methodology of Marcott et al 2013 and to remove some speculation on the Marcott upticks, which do not arise from any of the main speculations.
In the graphic below, I’ve plotted Marcott’s NHX reconstruction against an emulation (weighting by latitude and gridcell as described in script) using proxies with published dates rather than Marcott dates. (I am using this version because it illustrates the uptick using Marcott methodology. Marcott re-dating is an important issue that I will return to.) The uptick in the emulation occurs in 2000 rather than 1940; the slight offset makes it discernible for sharp eyes below.
Figure 1. Marcott NHX reconstruction (red) versus emulation with non-redated proxies (yellow). The dotted lines at the left show the Younger Dryas. Marcott began their reported results shortly after the rapid emergence from the Younger Dryas, which is not shown in the graphics.
I have consistently discouraged speculation that the Marcott uptick arose from splicing Mannian data or temperature data. I trust that the above demonstration showing a Marcottian uptick merely using proxy data will put an end to such speculation.
The other “explanation” is that the uptick results from high-frequency swings in individual proxies. Marcott’s email to me encouraged such speculation. However, this is NOT what causes the uptick. Below I show the series that contribute to the NHX weighted average before and after the uptick. The proxy values shown below have been re-centered to reflect Marcott recentering: (1) by -0.66 deg C to reflect the re-centering from mid-Holocene to 500-1450AD; (2) by -0.08 deg C to match the observed mean of Marcottian reconstructions in 500-1450 AD.
Readers will observe that there are 6 contributing series in the second-last step, of which 5 are negative, some strongly. Their weighted average is negative (not quite as negative as the penultimate Marcott value in 1920, but you see the effect.) Only one series is present in the final step, one that, after the two rescaling steps, is slightly positive. Thus, the uptick. None of the contributing series have sharp high-frequency: their changes are negligible. Ironically, the one continuing series (Lake 850) actually goes down a little in the period of the uptick.
uptricks upticks arise because of proxy inconsistency: one (or two) proxies have different signs or quantities than the larger population, but continue one step longer. This is also the reason why the effect is mitigated in the infilled variation. In principle, downticks can also occur – a matter that will be covered in my next post which will probably be on the relationship between Marcottian re-dating and upticks.
I have been unable to replicate some of the recent features of the Marcott zonal reconstructions. I think that there may be some differences in some series between the data as archived and as used in their reported calculations, though it may be a difference in methodology. More on this later.