Marcott et al 2013 has received lots of publicity, mainly because of its supposed vindication of the Stick. A number of commenters have observed that they are unable to figure out how Marcott got the Stick portion of his graph from his data set. Add me to that group.
The uptick occurs in the final plot-point of his graphic (1940) and is a singleton. I wrote to Marcott asking him for further details of how he actually obtained the uptick, noting that the enormous 1920-to-1940 uptick is not characteristic of the underlying data. Marcott’s response was unhelpful: instead of explaining how he got the result, Marcott stated that they had “clearly” stated that the 1890-on portion of their reconstruction was “not robust”. I agree that the 20th century portion of their reconstruction is “not robust”, but do not feel that merely describing the recent portion as “not robust” does full justice to the issues. Nor does it provide an explanation.
The uptick problems are even more pronounced in the zonal temperature reconstructions (NH and SH extratropics on which I will be posting) and in the reconstructions from individual proxies (alkenone, Mg/Ca). In today’s post, I’ll illustrate the uptick problem from the alkenone temperature stack on the premise that there are fewer moving parts in the alkenone reconstruction and therefore this will facilitate diagnosis of the uptick mystery.
The first figure below shows the Marcott alkenone reconstruction. Alkenones are the largest single class of Marcott proxies (31 of 73). According to the alkenone reconstruction, temperatures in 1920 were at their lowest value in 11,300 years (Marcott’s selection of 11,300 years begs the question of what happened just before the left frame of the graphic – an interesting issue that I’ll return to on another occasion.)
Between 1920 and 1940, Marcott’s alkenone reconstruction increased by a remarkable 1.4 deg C, returning to values of the start of the reconstruction as shown. Marcott’s 1940 value (0.39 deg C reference 1961-90) was considerably above the 1961-90 reference zero and higher than the 1990-2010 average, shown as red “+” for reference. Interesting as this figure may be, I don’t wish to dwell at present on what it might or might not show about temperature history (a topic that I’ll return to on another day.)
Today’s post is merely about the mysterious uptick from 1920 to 1940, an uptick that is not observed in any of the actual alkenone data.