By Andy May
Last week, I posted a global temperature reconstruction based mostly on Marcott, et al. 2013 proxies. The post can be found here. In the comments on the Wattsupwiththat post there was considerable discussion about the difference between my Northern Hemisphere mid-latitude (30°N to 60°N) and the GISP2 Richard Alley central Greenland temperature reconstruction (see here for the reference and data). See the comments by Dr. Don Easterbrook and Joachim Seifert (weltklima) here and here, as well as their earlier comments.
Richard Alley’s (Richard Alley, 2000) central Greenland reconstruction has become the de facto standard reconstruction and is displayed often in papers and posts. And, truth be told, I’ve often used it. See here for an example. But, it is a central Greenland reconstruction, uncorrected for elevation differences over time, and all of Greenland is north of 60°N. A better comparison is with my Arctic reconstruction that goes from 60°N to the North Pole. Continue reading
By Andy May
In previous posts (here and here), I’ve compared historical events to the Alley and Kobashi
GISP2 Central Greenland Temperature reconstructions for the past 4,000 years. Unfortunately, these two reconstructions are very different. Recently Steve McIntyre has suggested a third reconstruction by Bo Vinther. Vinther’s data can be found here. Unfortunately, Vinther is often significantly different from the other two. The Alley data has been smoothed, but the details of the smoothing algorithm are unknown. So the other datasets have been smoothed so they visually have the same resolution as the Alley dataset. Both datasets (Kobashi and Vinther) were first smoothed with a 100 year moving average filter. Then 20 year averages of the smoothed data were taken from the one year Kobashi dataset to match the Vinther 20 year samples. The Alley data is irregularly sampled, but I manually averaged 20 year averages where the data existed. If a gap greater than 20 years was found that sample was skipped (given a null value).
All three reconstructions are shown in Figure 1. There is no reason to prefer one of the three reconstructions over the other two, so I simply averaged them. The average is the blue line. I’m not presenting this average as a new or better reconstruction, it is merely a vehicle for comparing the three reconstructions to one another and to other temperature reconstructions. This is an attempt to display the variability in common temperature reconstructions for the past 2,000 to 4,000 years.
By Andy May
In 2000, Richard Alley released an ice core temperature reconstruction for Central Greenland using Oxygen isotope ratios. He describes the technique used here. I used this ice core proxy data in a previous post “Climate and Civilization for the past 4,000 years.” Since Alley’s data stops at 1855, I spliced the Greenland HadCRUT surface temperature data on the end to show the Modern Warm Period. In the previous post I erroneously thought Alley’s reconstruction stopped at 1905, not at 1855, so the plot in the earlier post is shifted 50 years. This problem is corrected in this post. I spent too little time on the splice, since my focus was on the effect of climate on civilization and several commenters rightly objected to the error. These included David Middleton, who offered up a better Central Greenland reconstruction by Kobashi, et al. as an alternative. A little research and I was able to find a 4,000 year reconstruction to 1993 by Kobashi et al.
Kobashi uses Argon and Nitrogen isotopic ratios from air bubbles in the ice to estimate paleo-temperatures. He claims that that his method is more accurate than the Oxygen isotope ratios Alley used. Details of Kobashi’s methods are in his PhD thesis, which can be seen here. His unsmoothed results from 2,000 BC to 1993 are shown in Figure 1.