The money quote in the PAGES2K abstract was that there wasn’t any worldwide Little Ice Age of Medieval Warm Period and that AD1971-2000 temperatures were the highest in nearly 1400 years, long before the Medieval Period:
There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age … during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.
In today’s post, I’ll show that the knock-on impact of changes to the Arctic reconstruction on the area-weighted average also make the latter claim untrue. Incorporating the revised Arctic reconstruction, one can however say that, during the period AD1971–2000, the area-weighted average reconstructed temperature was higher than any other time since, uh, the Medieval Warm Period.
Area Weighted Average
In the figure below, I’ve compared area weighted averages (both in SD Units as in PAGES2K) from PAGES-2103 to the corresponding series, replacing Arctic2K-2013 with Arctic2K-2014. PAGES2K converted each of the regional averages to SD units and then averaged in SD units (basis AD1200-1965) – an odd procedure given that each regional series was already in “deg C”. Rather than argue over the procedure, I’ve implemented their method in as reasonable a way as I could. (They did not archive their regional average, so I crosschecked by overplotting on figure and got a close but not perfect match.)
Figure 1. Area Weighted Averages. Black – PAGES-2013 version; red – replacing Arctic2K-2013 with Arctic2k_2014. Area weighted averages calculated using PAGES2K area weights. The two North American series were first converted to step functions and averaged; I don’t know exactly how PAGES handled this, but it’s not like there’s any “right” way and the way used here is reasonable.
One of the idiosyncrasies of multiproxy studies is that they agree on almost nothing except a very slight accounting advantage of the modern warm period over the medieval warm period. In this case, the narrowness of the accounting margin means that relative results for their “global” average are vulnerable to something as seemingly individual as their Hvitarvatn miscue (which is the main contributor to the Arctic2K changes).
In the present case, the Arctic2K changes result in a slight lowering (0.11 SD Units) of the AD1971-2000 area-weighted average and approximately 0.2 SD unit increase in the medieval period, including AD921-950, values of which now slightly exceed AD1971-2000. In the PAGES2-2013 version, the AD1971-2000 period was the warmest in “over 1400 years”. In the revised version, it is still the warmer than values in the Little Ice Age and indeed warmer than any period since, uh, the Medieval Warm Period.
By showing the above graphic, I do not imply any endorsement of other parts of PAGES2K. The most extreme closing value of the area-weighted reconstruction comes from the zombie Gergis reconstruction, rejected by the Journal of Climate, and occurring here as a rider to the omnibus reconstruction, an “earmark” for the paleoclimate community, so to speak.