Guest Post by Willis Eschenbach
In three previous posts here, here, and here, I discussed problems with the paper by Shakun et al., “Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation” (PDF,hereinafter S2012)
Commenters said, and reasonably so, that I had not fully addressed their claim that warming progressed from south to north. Their Figure 5a shows the trends by latitude band. It purports to show that the further north, the later the warming.
Figure 1. Figure 5a from S2012. ORIGINAL CAPTION: Figure 5 | Temperature change before increase in CO2 concentration. a, Linear temperature trends in the proxy records from 21.5–19 kyr ago (red) and 19–17.5 kyr ago (blue) averaged in 10° latitude bins with 1 sigma uncertainties.
Now, that seems pretty clear. Less blue and more red as you go up towards the north pole. What could be wrong with that? Well, as usual, nature is not that neat. When you look at it closely, it’s nowhere near as clear as that chart seems to indicate.
The first thing that’s wrong is that out of the fourteen bands with data, only five of them show a significant difference between the early trends (21.5 to 19 thousand years ago [kyr BP]) and the late trends (19 to 17.5 thousand years ago [kyr BP]). In the other nine bands, the uncertainties overlap, so we can’t even say if they are different. (The uncertainties for each band are shown as red and blue long thin lines with short vertical ends.) As a result, they are meaningless, and should not be shown.
But that’s just a symptom of the real problem, which is that there is very little data in many latitude bands, and the proxies are very different from each other.
To investigate each of the bands, I started by expressing all of the temperatures as anomalies around the average temperature from 21.5 to 17.5 kyr BP. Then I divided them by bands and graphed them. Figure 2 shows the results for the Northern Hemisphere.
Figure 2. Trends by latitude band. The background colors correspond with Figure 1, with red for 21.5 to 19 kyr BP, and blue for 19 to 17.5 kyr BP. Dark red lines are centered Gaussian averages of the individual proxies, with the data shown by the green squares.
Let me discuss these panel by panel. First, let me note an oddity—why is the early period longer than the later period? But I digress …
Panel a: Only two proxies, and one of them has a hump right at 19 kyr BP.
Panel b: Five proxies. Three have a hump right at 19 kyr BP.
Panel c: Two proxies. One is dead level, one rises during the later (blue) period.
Panel d: Three proxies, but one of them starts just before 19 kyr BP. Seriously, folks, do you think an average of these is meaningful?
Panel e: Hard to tell what’s happening here. Several of the proxies go either up or down just after 19 kyr BP.
Panel f: All trends in the blue section are about the same, except the poor proxy taking a dive right after 19 kyr BP
Panel g: Another goofy one. Right after 19 kyr BP, two of the proxies head for the sky.
Panel h: No trend before, no trend after.
Again, by panel.
Panel a: Not much difference, red or blue period.
Panel b: Three proxies. Two go up at 19 kyr BP. One goes down at 19 kyr BP. Is this supposed to be meaningful?
Panel c: Three proxies. Neither the red period nor the blue period shows much.
Panel d: Two proxies. Two. One goes up after 19 kyr BP. So what?
Panel e: Here, a lot of the proxies have a low point at about 19 kyr BP … and they have a high point about 500 years before that.
Panel f: These four, all ice cores from Antarctica, agree pretty well. However, only one of them has a significant trend, and that only in the blue area.
Now, to me those results don’t mean much. Of the eighty proxies, only eight of them have a significant trend in both the red and blue periods … and that’s without adjusting for autocorrelation. The proxies show no clear pattern. They are too varied, and too few, to tell us much of anything.
Let me close with what may be a more revealing graph, dividing the globe up into 45° latitudinal bands.
Let me say that Panel a shows something very curious. The rise in temperature started quite early the two Greenland proxies … and timing of the others are all over the map. I can’t see how that supports any claim of late warming in the north.
BOTTOM LINE: I see no evidence in any these latitudinal bands of proxies to support the claim that the warming progressed northwards. It certainly may have done so … but these proxies are not useful for supporting that claim.
My goodness, I certainly hope that I’m done with these proxies.