Potholer54 is a rather famous old-time science communicator, and still among the best ones IMO. He has tackled misconceptions around evolution, GMOs and climate change. Last year he covered a recent film called “Climate - The Movie (The cold truth)”. However, though recent, the arguments are just rehashing PRATTs. Polishing a turd is still a turd. Potholer54 did his best to cover some of the claims in his video.
This got attention of Tom Nelson, one of the guys featured in the movie who wanted to debate. Potholer54 recently posted the first 30 minutes of the debate, which is how I got notified by this. However, Nelson apparently uploaded the whole debate on his channel. However, it’s 1 hour and 30 minutes in total. So, I will make it easier for myself and do the review in parts by first covering the parts in Potholer’s video.
I: Prologue
It starts out good, making promises to keep it cordial and let each other finish without interruption
and keep responses quick and to the point. Nelson even stated this:
[2:15] NELSON: Yeah, let’s do that. One thing I would like to do is to make sure there’s plenty of give and take here and I want to make sure that my audience hears everything that you have to say. So I’d hope that we don’t filibuster and try to keep our answers short and snappy. I think that would be great.
Well, guess what Nelson doesn’t do? When Potholer54 asks a simple question, Nelson quickly tries to move on the next argument, often tacking on several other points on top of one another, especially repeating the phrase “the spike of doom [i.e. the recent drastic increase in global mean temperatures] does not exist”. He is indeed filibustering.
II: The doctored Ljungqvist graph
The first issue raised by Potholer concerns a temperature vs. time graph that was included in the film (below this paragraph). The source is the study authored by Ljungqvist published in the year 2010. Although, the film provides the citation as ‘(Ljungqvist, 2000)’, but it’s from 2010.
However, the citation date error is not the main issue here. The issue is that the graph has been doctored. Here below is original from the paper.
The solid line represents Northern Hemisphere (90-30°N) [So, NOT exactly global. Not even hemispheric] mean temperature variation per decade in the period from year 1 to 1999 AD based on multi-proxy data. Temperature ‘0’ represents the mean of 1961 to 1990. The dotted line on the right shows instrumental data from 1850 to 1999. That dotted black line shows that temperatures of the late 1990s already exceeded the ones during the previous 2000 years. And that’s not even mentioning the warming seen after 1999. But that black dotted line has been removed in the graph shown in the film. Another issue, the end of the graph is labeled “current”. However, the graph ends in 1999, which is not exactly “current”, especially considering the fact that the average global temperature has increased by about 0.72°C from 1999 to 2024.
But this is the first question Potholer asks. Note how Nelson responds:
[7:33]
POTHOLER: Here’s the graph that you copied from the 2010 paper (showing the original). Here it is bit bigger now can you see the right hand side. That’s the Ljungqvist graph and and if we look at yours…
NELSON: So, the dotted part is not part of his graph. It’s added on and he does not even defend the dotted part. Marcott himself does not defend it.
POTHOLER: That’s what we’re here to talk about Tom. On on the right hand side there he’s got the instrumental temperature data marked in there. Now when you reproduce it you take that out
NELSON: Yes, because that’s not part of his graph and let me read you what he said about that part that’s been added on okay response by Marcott himself…
Nelson goes on to reference a quote by Marcott. At first I misheard “Markov”, I did not know what he was referring to. Well, he is referring to a different graph Potholer showed at 6:23 minutes in. That’s from a paper authored by Marcott et al. 2013. There is much to say about that (more on that later), but this is irrelevant when it comes to the issue Potholer is addressing here regarding the removal of the dotted black line on Ljungqvist graph. Potholer points this out after he politely let Nelson finish his ramble about Marcott. But Nelson doesn’t return the same courtesy.
[9:35]
POTHOLER: Okay, I didn’t want to interrupt you. We aren’t talking about this same graph. This isn’t Marcott. This is Ljungqvist.
NELSON: (Promptly interjects) It’s all of them. They all have the spike of doom. They all… You mentioned Marcott earlier. They all have the spike of doom. It’s all the same thing. They all do. The… Yeah, man’s hockey stick. They’re all the same.
Jeez. The man is a broken record.
Eventually, Potholer gets to the point he is making. The point being, removing the dotted line from the original WITHOUT informing the audience about this change is dishonest. You should inform your audience about any substantive change you make to graphics instead of just citing it as an original graph. Secondly, the label “current” is misleading since the graph ends at the year of 1999.
[9:55]
POTHOLER: But we are looking at a different graph. Whether or not you think it’s all the same thing doesn’t matter. You can’t look at this graph and say but Marcott said this wasn’t statistically relevant and so on. We’re not talking about the Marcott, we will come on to that so just give me be patient we’ll come in second.
NELSON: Our first topic was Marcott and Ljungqvist.
POTHOLER: First we’ll look at the Ljungqvist graph. Okay and what I want to point out is that when you recreated this and you claimed that this was Ljungqvist’s graph you took out the period of warming at the end.[skipping some of Potholer’s talk and an interruption by Nelson]
[11:25]
POTHOLER: You took that out. Whether or not you believe it or not and you can make all the arguments in the world that you don’t think it’s justified. But you didn’t say that in the movie. What you told people in the movie is that this is his reconstruction and you put in current temperatures when in fact the dotted line only goes up to 1999…
NELSON: No! We.. No, No! We showed the actual graph, we showed the actual reconstruction. That was the reconstruction without that dotted line of doom on the end. You can’t just add that on. We showed the actual thing and his proxies do not show the spike of doom. So, I know you want to believe in the spike of doom, but it it’s not real. So, we’re showing you the actual thing and if you’re saying “Oh we didn’t show his thing that’s not in his reconstruction.” Well we didn’t, because it’s not in there.
Nelson responds with denial. They did NOT “show the actual thing”. We can all see that the graph in Ljungqvist’s paper includes the dotted line representing instrumental data, which was removed in the graph that was shown in the film. Nelson may want to argue that the dotted line is wrong, that it shouldn’t be included in the graph (he is wrong about that), but that is irrelevant when it comes to the point Potholer tries to make here. Editing a graph such that it becomes substantially different while presenting it to an audience without informing them that you’ve made this change is dishonest.
III: Instrumental versus Proxy data
In an attempt to justify his rejection of the dotted line, Nelson makes some claims about the conflict between the instrumental and proxy data.
[12:10] NELSON: The reconstruction is without the dotted line. See, the [instrumental] dotted line goes higher than the [proxy] reconstruction. That’s a very important thing. His reconstruction shows a warm… medieval warm period. You can’t just add in this extra dotted line and say “Hey this is part of his” because it’s not. I don’t think he even… he doesn’t like the fact that the dotted line that his proxies didn’t show this dotted line… but they didn’t… and this whole idea that we can trust trees… or proxies… for all of time until 1960 or something and then suddenly we can’t trust them anymore and we have to use instrumental. It’s because the proxies don’t show what you want them to show. This is a very key point. They don’t.
Nelson claims that the - quote - “his proxies didn’t show this dotted line”. In other words, he claims that the proxy data does not match the instrumental data during the time period when they overlap. This is false. It is made clear in the paper that the two data lines match one-another remarkably well. Quoting from the study:
Ljungqvist 2010: The decadal correlation between proxy and instrumental temperature is very high (r. 0.95, r^2 0.90) and the 2 standard deviation error bars only amount to ±0.12°C in the calibration period AD 1850–1989.
You can even see this in the graphs. Note how the dotted line and the solid lines overlap:
So yes, they do match quite well. But Nelson cherry picks the part at the tail-end he doesn’t like, when the dotted line (instrumental data) goes much higher than the solid line (proxy data). This is the dreaded “spike of doom” Nelson is so terrified about. He rejects that because… well, he doesn’t really give reason as to why. He whinges about an imagined hypocrisy in trusting proxy data until 1960 when we suddenly can’t trust it anymore. Well, no. It turns out there is a good reason why (in this case) the proxy data at the right-end of the graph becomes less reliable. The data during this time is too limited. Again, the paper spells this out clearly (emphasis mine):
Ljungqvist 2010: The decadal mean temperatures in the extra-tropical Northern Hemisphere seem to have equalled or exceeded the AD 1961–1990 mean temperature level during much of the Roman Warm Period and the Medieval Warm Period. The temperature since AD 1990 is, however, possibly higher than during any previous time in the past two millennia if we look at the CRUTEM3+HadSST2 90–30°N instrumental temperature data (Brohan et al. 2006; Rayner et al. 2006) spliced to the proxy reconstruction. The proxy reconstruction itself does not show such an unprecedented warming but we must consider that only a few records used in the reconstruction extend into the 1990s. Nevertheless, a very cautious interpretation of the level of warmth since AD 1990 compared to that of the peak warming during the Roman Warm Period and the Medieval Warm Period is strongly suggested.
For the following minutes, this process repeats itself. Potholer tries to make the point about how you shouldn’t remove something from a graph without telling the audience that you have removed something from the origina graph. Nelson keeps yammering on about how the “spike of doom” is not in the proxy data.
IV: Why the… RATE… of global warming is an issue
Eventually, there is some common ground. Nelson does think the earth has warmed recently, but doesn’t think it is a big deal since the earth has been warmer in the past.
[15:21] NELSON: I think it has warmed since 1975. I think it has warmed since 1850. Uh nothing to be worried about, but it fluctuates a lot. There’s tons of warming and cooling periods. I think it’s it’s cooled since the peak of the medieval warm period. I think and it’s cooled since the Manoan warm period. It’s a lot cooler than it was 130,000 years ago when there were there were hippos in the temps. It goes up and down a lot right now. We are much cooler or we are cooler than most of the last 10,000 years. Most of the last 500 million years. We’re in ice age. The whole idea that the earth is too hot right now is preposterous. It’s absolutely not. As you know cold weather kills way more people than hot weather. So this whole idea we Should get all worked up by this fake spike of doom is it’s a non-starter.
And then Potholer tries to bring him back to the topic they were discussing. But this is also a thing that should be addressed. It’s true that the earth was warmer in the distant past, certain during the last 500 million years. But the warm temperature itself isn’t necessarily the issue (but although sometimes they are), the main issue is the RATE at which CO2 levels and the resulting radiative forcing are changing. If things changed very slowly over long time scales, we and all other organisms could manage to adapt, but sudden climatic changes can disrupt ecosystems.
Nelson’s mistake is to focus solely on the magnitude of the warming. However, it’s not just the magnitude that is a cause of concern. It is the rate of change and how short the time-frame is within which the warming occurs.
Changes of 5.2°C at a rate of 10°C per million years occurring within a time frame of 0.4 million years (or 400,000 years) are linked to the big five mass extinctions events. We are currently around 1.5°C warmer than pre-industrial times (mid 18th century), but the majority of the warming has occurred since the late 20th century at rates of 0.18°C per decade between 1970 and 2008. At that rate, we will reach the 5.2°C threshold in about 205 years, but probably much sooner since the rate of warming is also accelerating. Now it falls between 0.27-0.36°C per decade. With this we will reach the mass extinction threshold in 102-137 years assuming the rate does not accelerate again (but it likely will). But we shouldn’t wait until we reach the sixth big mass extinction event. We can expect plenty of issues long before we reach that point.
V: How ‘current’ is “current” ?
Later Potholer presses Nelson on the label “current” to describe data from the 1990s:
[17:30]
POTHOLER: You’re showing here is not current temperature.
NELSON: Yeah I mean any graph, if any graph you make as soon as uh the day later, it’s not current.
Sigh… deliberately being obtuse. When it come to climate data, current may mean something in the last 5 or 10 years, but definitely not from the 1990s. But Nelson knows that Potholer is not asking them to show data from yesterday. He is just being dumb on purpose. Or perhaps he is just dumb. I don’t know.
VI: Weather versus Climate (yup, this old PRATT)
Then Potholer shows a graph from NASA/GISS showing the annual mean global surface temperature compared to the long-term average from 1951 to 1980. This clearly shows that the earth during the last 2 decades has warmer since the previous 200+ years. The 10 most recent years are also the 10 warmest on record.
Nelson had a full-blown meltdown when Potholer showed this image:
[18:52] NELSON: This graph is fraudulent showing every year since whatever uh since the '90s every year was warmer than the warmest part of the 1940s. That… that’s fraudulent. No way. It was very warm in the 1940s. So… I’m not buying this whatsoever and you can look at uh at like the US hot temperature records. We have… uh… we have 50 states and the high temperature records, something like I don’t have it here something like 35 or 38 of those were set before 1940. If you look here in Minnesota, where I’m sitting, just as another example. It hit 100°F here in the 30s and 40s it hit 100°F. It hit it 38 times in those two decades. Since 1988 it has hit it eight times. So, this whole idea that it’s way warmer now it’s not. If you look at the record of heat waves. It’s it was hot back then. My dad is old enough to remember it. It was very hot back then. The whole idea that all the years that whole huge set of years is warmer than the 1940s it’s not. I’m not buying it. Totally not buying it.
So… when faced with global mean surface temperature averaged yearly… Nelson counters this by citing anecdotes of local instances of hot weather in the past. An all time classic. Confusing weather for climate. What’s next? Showing Potholer a snowball during your debate in December as proof that global warming is a hoax? The fact that one year or one decade may have had more heat waves does not mean it was warmer averaged out over each year. It definitely does not mean when the heatwaves are counted from a local area.
VII: Marcott’s graph
Okay, at 25:40 minutes in we come back to the Marcott et al. 2013 paper and this graph (below), but that’s where part 1 ends since zoom meetings have a time limit.
But I can quickly address what Nelson brought up before about the Marcott paper. Previously at 8:28 minutes, Nelson quoted a part (highlighted below) from a Q&A with Marcott. Please NOTE the part after the highlighted text, which Nelson does not mention:
Q: What do paleotemperature reconstructions show about the temperature of the last 100 years?
A: Our global paleotemperature reconstruction includes a so-called “uptick” in temperatures during the 20th-century. However, in the paper we make the point that this particular feature is of shorter duration than the inherent smoothing in our statistical averaging procedure, and that it is based on only a few available paleo-reconstructions of the type we used. Thus, the 20th century portion of our paleotemperature stack is not statistically robust, cannot be considered representative of global temperature changes, and therefore is not the basis of any of our conclusions. Our primary conclusions are based on a comparison of the longer term paleotemperature changes from our reconstruction with the well-documented temperature changes that have occurred over the last century, as documented by the instrumental record. Although not part of our study, high-resolution paleoclimate data from the past ~130 years have been compiled from various geological archives, and confirm the general features of warming trend over this time interval (Anderson, D.M. et al., 2013, Geophysical Research Letters, v. 40, p. 189-193; http://www.agu.org/journals/pip/gl/2012GL054271-pip.pdf).
Background: There are two key points one must understand:
- Marcott et al. used proxy data to reconstruct temperatures as far back as 11,000 years. However, the resolution of this dataset is a key issue. These range from 20 to 500 years, with a median resolution of 120 years. For this reason, they have ‘smoothed’ out the temperatures in 100-year bins. In other words, the temperature at… for example… 5000 years ago on the graph above is the mean temperature of the period from 5050 to 4950 years. What this means is that you can use this dataset to approximate the temperature changes over a long time period, e.g. over multiple 100s to 1000s of years. However, it’s not sufficient to talk about changes that occurred within a 100 year interval. The resolution of the data is not sufficient enough for this purpose.
- Secondly, the paleo-temperature record doesn’t cover the last 60 years very well. This is illustrated in graphs G and H of figure 1. Note how the orange line (denoting the number of records used to construct the Holocene global temperature stack through time) tapers off. There is not a lot of records covering the last 100 years.
For these reasons, their reconstruction (the ‘paleotemperature stack’) is not reliable regarding the 20th century portion. They made it clear in their paper that the 20th century portion is not used to draw any conclusion. What they do use is the long-term observations of their reconstruction with the recent instrumental record. As Marcott said in the Q&A, their “primary conclusions are based on a comparison of the longer term paleotemperature changes from our reconstruction with the well-documented temperature changes that have occurred over the last century, as documented by the instrumental record” …but Nelson’s does not mention this sentence when he quoted Marcott.
Nelson portrays Marcott as if he is “stepping away” from the claim that there was a recent increase in temperature during the 20th century… or the “spike of doom” as Nelson likes to call it. That’s not what Marcott says as you can see here. They say that their own long-term reconstruction based on proxy data should not be used to draw any conclusion about the 20th century for reasons just explained. Instead, they referenced other data (e.g. instrumental records) from other studies, which do address the 20th century. In fact, you should refer to studies that provide data which fill in the gaps in yours. Nelson got this completely backwards. He thinks the poor resolution of proxy data regarding the 20th century is a reason to exclude the ‘dotted line’ based on the instrumental data (although that refers to previous Ljungqvist’s graph, but the same principle applies here). However, this is actually a good reason to include instrumental data in order to complete the whole picture.
