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.
Figure 1. Records and types of proxies used in the Shakun2012 study
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:
Figure 2. Location of the proxies used in the Shakun2012 study.
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.
Figure 3. Same data as in Figure 1, but showing the warming from the last ice age.
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.
Figure 6. Same as Figure 4. Click for larger version.
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.
Figure 7. Same as Figure 4. Click for larger version.
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
Figure 8. Same as Figure 4. Click for larger version.
67: Warming starts at 25,900 BC.
68: Warming only one tenth of a degree
76: Warming occurs almost instantaneously
Discussion
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.
Figure 9. All 80 temperature proxies from Shakun2012. Colors as shown in Figure 1.
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.
Final Conclusion
The reviewers should have taken the time to plot the proxies … but then, the authors should have taken the time to plot the proxies.
w.
[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,
w.
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?
w.
[UPDATE]—The discussion continues at Shakun Redux: Master tricksed us! I told you he was tricksy!
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One way to solve the matter of who is the best proxy, when you have that many data points, statistics can provide some relief.
I took the mean of all of the maximum times. I had to read from the graphs, so I am ± 500 years.
The mean ± sd is 6255 ± 3501
The median is 6000
So, one way to arrive at a better figure is to discard all values greater than 2 sd.
This is a good way to assess the good v. bad players on this value alone. I did not have time to do minimum time, but this could sort out a couple more outliers.
Let’s see if this passes some muster!
Thank you!! and nuts to the reviewers.
Those values should be -6255 and -6000 yrs BP, of course.
Ouch—bad grammar in my first sentence. Too much phone texting.
Obviously, all you need to do to arrive at an exact record of global temperatures is average all the proxy results. As the IPCC has demonstrated so well, no single result can be trusted, but all the untrustworthy results can be made trusty by taking an average. (sarc)
so the arctic warmed first and released the CO2.
no surprise the colder water and ice have the most dissolved gases, is it?
“Dr. Munchausen”. Hilarious!
At -12,500 am I seeing a sudden drop, then surge back up again?
Please note that my method is not fair for the most recent maxima. This is because ± 2 sd puts anyone below 6000 YBP into the future, so all 6000 BP or less values come out kosher by this quick and dirty test. This is because the distribution is heteroskedastic, meaning not a uniform Gaussian curve because of the time limit.
One thing is certain from the proxies. The peak temperature after the glaciation is quite some time ago (6000 YBP)—that is a surprise to me.
Steven Mosher says:
April 6, 2012 at 9:23 am
Say what? First, I’ve drawn lines through the maximums and minimums, so that you can use your eyeballs to determine the uncertainty.
Second, having no information about the errors in the datasets, it is not possible to put accurate error bars on the maximum or the minimum values.
Finally, my conclusions don’t depend in any way on where the maximums and the minimums are. I have plotted them for your information, so you can use your eyeballs to see the differences in the timing of the warming.
Seems like lately, sometimes you’re just objecting to object … hows about turning your fine analytical mind to objecting about the shabby kind of science represented by the Shakur paper?
w.
Did anybody actually read the article? I concluded from the abstract and the comments by the coauthor that they never actually say that the atmospheric increase in CO2 precedes the increase in T. Quite the opposite. They suggests that due to changes in the earth woble the oceans, in particular the southern oceans, warm up. Then, using their favourite feedback from increasing CO2, the rest of the major warming occurs. Nothing new here, warming occurs first, then CO2 increases, as anticipated. The only thing they might have tried to do is obfuscate the icecore data, spread out the warming over 7000 years rather then accepting a rather sudden increase as suggested by icecore data, and putting the amount of initial warming up for debate.
Looking at the wide spread of T increases from the proxy data, it is obvious that a precise timing using proxydata is impossible. The advantage of the icecore data is that even if one does believe that the timing might have an error bar, sequentially the CO2 follows the increse in T.
Downdraft says:
April 6, 2012 at 10:42 am
Obviously, all you need to do to arrive at an exact record of global temperatures is average all the proxy results.
Your point is not wrong, even if you sarced it!
Statistics says, that if you have enough sloppy measurers, the mean (not average) with the st.dev. is a more reliable result than the single number measured by the best measurer. Hard to conceptualize that, I know! Yet it is true science.
Interesting analysis, but another question occurs to me: Isn’t the very notion of reconstructing a “global average temperature” from such a disparate set of proxies (which, I suspect, have never been rigorously studied for their tendency to agree or disagree with each other) laughable?
Oakwood says:
April 6, 2012 at 9:42 am
I didn’t overlay it for several reasons.
One is that they have released their temperature data, but not their CO2 data, so I was unwilling to mix apples and oranges.
The second is that, no matter where you put the CO2 in that pile of nonsense, it won’t make a difference.
The third is that, as near as I can tell, they’ve looked at eighty temperature records and are comparing them to a single CO2 record. This is an ice core record from the EPICA ice core, which they say has “recently been placed on a more accurate timescale.” That seemed curious, they are averaging 80 proxies for historical temperature, but they’ve taken a single proxy for CO2.
In any case, I’ll do the overlay when I get a few moments. I’ll have to digitize the CO2 data they used, so it may be a day or so.
w.
Averroes says:
April 6, 2012 at 9:21 am
….
Summary: The WUWT/Eschenbach analysis of the temperature/CO2 data stands in good agreement with the Nature/Shakun article. In aggregate, the global data provide no evidence that temperature increases precede CO2 increases.
Not really. The data series which are usually referenced as showing that warming precedes CO2 increases are typically the ice core data from Antarctica – especially the Vostok core – and Greenland cores.
What Willis demonstrated is that by mixing apples and oranges, liverwurst and lettuce, sand and gravel, as the authors of the original article did, you don’t get fruit salad, which is what the original authors claim. At the most simplistic level, it is obvious to anyone who can tell a hawk from a handsaw that if you add two proxies together – say pollen and ice cores – with inferred maximum “temperatures” that fall within the last 2,000 years and in the early Holocene respectively, the mean date of the maximum temperature of the two proxies will be later than the earliest date, earlier than the latest date, and in fact, the new “maximum” tells us little other than the two proxies don’t behave the same. In fact, it seems obvious that one or both “proxies” may not be the useful proxies of temperature that one might hope.
What the authors seem to have done is broaden their selection of proxy types until the resolution of the resulting “time” series was hopelessly diluted. Then they claimed that they found no support for the idea that temperature increases preceded atmospheric-CO2 increases. In fact, (by tossing as many wildly varying forms of proxy in to the mix as they could find) they assert that CO2 increases preceded global warming.
Septic Matthew/Matthew R Marler says:
April 6, 2012 at 9:58 am
No, it’s not a “fair comment”. It’s the kind of crap I expect from Hansen and his ilk. It is peripheral nitpicking on a meaningless issue.
Next, I didn’t establish maxima and minima “by eyeball”. Is my writing that hard to fathom? Those are the years with the highest and lowest data points. That’s why they are called “maxima” and “minima”.
Finally, so what? How would establishing some kind of error bars on the maxima and minima advance our understanding in the slightest? The analysis works perfectly without even considering maxima and minima, it’s extra information.
Indeed, they have my appreciation for that.
I have a copy of the paper, and have had one since a day or so after it came out. What’s your point?
w.
Looks like one of those art works they make by loading a shotgun with paint.
Willis
Personally, I think you have ‘substantial evidence’ for your hypothesis and consequently you don’t need to provide any further information at all.
tonyb
Per previous requests, here is the Shakun temperature data, with the CO2 data overlaid on it …
I’ve added this to the head post.
w.
When I buy a jigsaw puzzle I digitally scan the pieces to produce a Cartesian coordinate readout for each on a mm scale. Then I make the necessary transformations to digitally compare the sides of each in all possible combinations, making fits accordingly. I find this saves much time over eyeballing it.
As most know, the great advantage of the ice cores is that multiple data are contained in close proximity in the ice. CO2 is locked in within a century or two of snowing, and the lag can be estimated and accounted for. So mixing in other proxies can only obfuscate. But the interesting thing is, a doctoral candidate already employed by the govt comes up with a study tailor made to counter scientific skepticism–by way of junk science, it seems. This shows how much progress you guys have brought about: the propaganda war is moving away from name calling toward spurious rebuttals of legitiate skepticism. Good work! –AGF
I see what you are trying to do, but is it really fair to directly compare different types of proxies and say that they can’t be used because they aren’t similar?
I really think you need to take into account that different proxies actually measure different things. Each of them can give you some indication of temperature, but each of them also has signals from other factors. Also, each of them actually measures diffent temperatures. For example, ocean sediments generally give upper ocean temperature. Some isotope proxies give upper troposphere temperatures (where rain forms) rather than surface temperature, etc.
A simple numerical evaluation, such as the one you have done here, has some value but don’t place too much importance in that type of oversimplified analysis.
Where does their data CO2 come from? It needs to come from an area representative of the temperature data. If they’re just using icecores, it takes quite some time for the co2 to mix and bring the measurement up.
How do CO2 levels measured at greenland and antarctica compare to RoW?
Off topic but I just saw this bumper sticker in town: “Everything I need to know I learned from Ranger Rick.”
Willis, that new data-plot is a considerable improvement!
Now if you color-coded the proxy data’s geographic origin … such that “more red” meant “farther north” … and “more blue” meant “farther south” … then you might find that the independent Willis/WUWT analysis pretty solidly confirms the Shakun et al analysis.
This would be a very substantial step toward reconciling skepticism with science … already your independent analysis has gone far in this direction … now is a good time to go further.
One location for CO2 = good.
One location for temperature = bad.