Guest Post by Willis Eschenbach
There’s a new study entitled “Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation”, Shakun et al. (paywalled, hereinafter Shakun2012). The paper claims to show that in the warming since the last ice age, CO2 leads temperature. Anthony wrote about it in his post “A new paper in Nature suggests CO2 leads temperature, but has some serious problems“. The press release says (emphasis mine):
A new study, funded by the National Science Foundation and published in the journal Nature, identifies this relationship and provides compelling evidence that rising CO2 caused much of the global warming.
Lead author Jeremy Shakun, who conducted much of the research as a doctoral student at Oregon State University, said the key to understanding the role of CO2 is to reconstruct globally averaged temperature changes during the end of the last Ice Age, which contrasts with previous efforts that only compared local temperatures in Antarctica to carbon dioxide levels.
“Carbon dioxide has been suspected as an important factor in ending the last Ice Age, but its exact role has always been unclear because rising temperatures reflected in Antarctic ice cores came before rising levels of CO2,” said Shakun, who is a National Oceanic and Atmospheric Administration (NOAA) Post-doctoral Fellow at Harvard University and Columbia University.
“But if you reconstruct temperatures on a global scale – and not just examine Antarctic temperatures – it becomes apparent that the CO2 change slightly preceded much of the global warming, and this means the global greenhouse effect had an important role in driving up global temperatures and bringing the planet out of the last Ice Age,” Shakun added.
The good news about the paper is that they have provided the temperature records (Excel spreadsheet) for the 80 proxies used in the study. My compliments to them.
Me being a suspicious fellow, however, I figured “trust but verify”, so I plotted up the temperature records that they used. I always begin with the original data, without any additions or distractions. Figure 1 shows the data that they used.
As you can see, some of the ice core records are down where we’d expect them to be, well below zero. Those are the GRIP and NGRIP records from Greenland. But there are some oddities about these proxies.
One problem that is immediately obvious is the timing. The peaks for the previous interglacial period (the Eemian, about 130,000 BC) don’t line up. That may not be much of a problem, though, because the paper is about the warming from the most recent ice age.
One oddity is that there are ice core records that are right around freezing (0°C). In addition, there are pollen records around freezing as well. This shows that we actually have a mix of anomaly records and actual temperature records. This is not a problem, just an oddity.
Next, let’s take a look at the location of the proxies. Figure 2 is from their paper:
This looks good, it looks like there may be passable coverage. So let’s look at the last glacial transition, we’ll look at the time since 26,000 BC.
Here, you can see the Antarctic ice core records (yellow and green lines near 0°C) mentioned above that are shown as variations, with the modern value taken to be 0°C.
Some other observations. Greenland (yellow temperatures at bottom) seems to be an outlier in terms of change in temperature. The Antarctic ice cores and all of the rest of the records show much less warming since the ice age.
In order to compare these eighty proxies to each other, what we need to do is to “standardize” them. This means to first subtract the mean (average) of each proxy from the individual values. Then each of the individual values is divided by the standard deviation of the entire record for that proxy. The result will vary between about -3 and 3. Standardizing preserves the shape and timing of the data, it just makes all the proxies have a mean of 0 and a standard deviation of 1.
Next comes the part that the authors of these multi-proxy studies seem to have generally ignored. This is to look at each and every one of these proxy records and think about what they seem to mean. I’ll look at them sixteen at a time. Figure 3 shows the first sixteen of the Shakun2012 proxies.
Figure 4. Proxies from the Shakur2012 study. All of these cover the period from 26,000 BC to 1980 AD. Vertical dashed lines show the minimum (light blue) and maximum (dark red) values for the each proxy. Minimum and maximum times rounded to nearest 100 years. Colors as shown in Figure 1. Click for larger version.
NOTES BY NUMBER
1, 2: These are the Greenland ice cores. They show a warming of 32 and 27 degrees respectively, which is much more than any other proxy. Warming begins earlier than 20,000 BC.
4: The warmest date is at 1200 AD.
6: Warmest date is 1000 AD. Warming doesn’t start until 12,600 BC.
9: Maximum warmth is at 14,600 BC.
15: Very unusual shape, 11° warming.
Figure 5. Same as Figure 4, proxies from the Shakur2012 study. All of these cover the period from 26,000 BC to 1980 AD. Vertical dashed lines show the minimum (light blue) and maximum (dark red) values for the each proxy. Minimum and maximum times rounded to nearest 100 years. Click for larger version.
19: Warming doesn’t start until 10,800 BC
21: Maximum warmth precedes maximum cold.
28. Maximum doesn’t occur until 400 BC.
30. Maximum doesn’t occur until 1400 AD.
31. Maximum doesn’t occur until 2400 BC.
32. Maximum doesn’t occur until 1500 AD.
34: Maximum at 1600 AD
35: Maximum at 14,000 BC
36: Strange shape, constant warming until the present.
42. Maximum not until 400 AD.
44: Warming until the end of the record in 8200 BC.
50: Maximum not until 1100 AD.
51: Constant rise beginning to end.
52: Large drop and rise after maximum warmth.
53: Rises beginning to end.
54: Rises beginning to end.
58: Maximum not until 1300 AD.
59: Maximum not until 1600 AD.
60: Large rise in 1100-1200
67: Warming starts at 25,900 BC.
68: Warming only one tenth of a degree
76: Warming occurs almost instantaneously
The variety in the shapes of these graphs is quite surprising. Yes, they’re all vaguely alike … but that’s about all.
The main curiosity about these, other than the wide variety of amounts of warming, is the different timing of the warming. In some proxies it starts in 25,000 BC, in others it starts in 15,000 BC. Sometimes the warming peaks as early as 14,000 BC, and sometimes around 5,000 BC or later. Sometimes the warming continues right up to the present.
The problem becomes evident when we plot all of these 80 standardized proxies together. Figure 9 shows all of the standardized temperature traces.
Now, there’s plenty of things of interest in there. It’s clear that there is warming since the last ice age. The median value for the warming is 4.3°C, although the range is quite wide.
But if you want to make the claim that CO2 precedes the warming?
I fear that this set of proxies is perfectly useless for that. How on earth could you claim anything about the timing of the warming from this group of proxies? It’s all over the map.
The reviewers should have taken the time to plot the proxies … but then, the authors should have taken the time to plot the proxies.
[UPDATE] A hat tip to Jostein, who pointed in the comments to the Shakun Nature paper being available here.
[UPDATE] Some folks wanted to see the CO2 data they used on the same timescale. Other folks said the colors in Figure 9 were misleading, since ice cores were printed on top, obscuring others below. We’re a full-service website, so here’s both in one:
Figure 10. All proxies, along with CO2 record used in Shakun2012.
My best to all,
I decided to take a look at the various proxies by proxy type. There are ten different kinds of proxies.
Figure 11. Proxies averaged by type.
A few notes, in no particular order. The ice core records are similar, but the timing is different.
Foram assemblages seem to be useless. The same is true of the Tex86 proxies.
Pollen has a consistent signal, but the warming doesn’t start until about 10,000 BC.
MBT/CBT perfectly exemplifies the problems with this approach. Which one are we supposed to believe? Which one is it that is lagging the CO2?
Finally, the Mg/Ca and the UK’37 proxies kinda sorta have the same shape, but no uniformity at all regarding the timing of the rise.
Let me close with a black-and-white version of the above chart. This allows you to see where the denser areas are located.
Figure 12. Proxies by type. Blue line shows CO2 data as used in the study.
Note the difference in the underlying shapes of the different types of proxies, and the differences in their timing with respect to the rise of CO2.
Next, note that the CO2 record they are using is from Antarctica. That is the reason for the good fit with the single “ice core ∂18O and dD” proxy (left graph, second row) and the “ice core dD” (center graph, second row). Both of those are Antarctic records as well.
Also, as you can see, even within each proxy type there is no unanimity regarding the timing of either the onset or the end of the warming from the last ice age.
CO2 is the blue line … so was the warming before or after the blue line?
[UPDATE]—The discussion continues at Shakun Redux: Master tricksed us! I told you he was tricksy!