A Hot Time In The Old Holocene Climatic Optimum transcript
Narrator:
Over the last couple of years there’s been much talk by journalists, politicians and activists about the Earth just experiencing its hottest day “ever” by a tiny fraction of a degree. These claims are implausible partly because there are so many defects and holes in the world’s temperature monitoring that we can’t confidently measure global temperatures down to hundredths of a degree even today, let alone get enough historical data, direct or proxy, to compare the current climate that exactly to, say, July 14th 1935, or August 2nd 1877 or March 3rd 1108. Or the Ides of March in 44 BC.
But leaving that recorded history aside, another crucial objection to such claims is the famous Holocene Climatic Optimum from about 9,500 to 5,500 years ago, or 7500 to 3500 BC, where prehistory meets history, a period long and uncontroversially believed to have been considerably warmer than today on the basis of a great deal of evidence.
John Robson:
So, there you have it. Or not, because recently we alluded to the Holocene Climatic Optimum, or HCO, in a video and someone challenged us, which was a bit unexpected given how well-documented it is. But it probably shouldn’t have been because it’s clearly critical to the whole notion of a man-made global warming crisis that such things not happen.
The narrative requires that there not be warmer periods in the past, especially not during times of lower atmospheric carbon dioxide, given the now-orthodox view that CO2 is the main driver of temperature.
So as part of the alarmist research project, they went after the Medieval Warm Period, most famously with Michael Mann’s “hockey stick”. And then they worked their way back to the year zero and beyond, as the conventional wisdom required them to do. But this kind revisionism is up against a large amount of evidence for the HCO. And by the way, the O stands for “Optimum” because that was when agriculture was invented, along with writing and metallurgy, and some areas now covered in desert, including much of the Sahara, were then lush and fertile.
So, did the HCO happen, and how do we know?
I’m John Robson and this is a CDN Fact Check video on the Holocene Climatic Optimum.
Narrator:
In their First Assessment Report in 1990, the Intergovernmental Panel on Climate Change presented a chart [WG I Fig. 7.1 on p. 202, p. 250 of the PDF] showing the standard view of temperature since the end of the last glaciation, with a long warm spell centered around 6,000 years ago.
Since then, many other similar reconstructions have been published. In this chart, geologist Andy May compares temperature reconstructions using ice core data from Greenland, shown in orange, and sediment layers from the tropical Makassar strait in Indonesia, shown in black, so it’s not a regional thing. The match is remarkably close and tells the same old story: rapid warming after the end of the last glaciation, a peak about 6,000 years ago, then an overall decline since.
The following chart, from the online materials for a 2016 University of Arizona course on “Weather, Climate and Society”, presents a standard reconstruction of Holocene temperatures, in this case showing the rapid increase in temperature at the end of the last glaciation followed by an undulating downward trend toward what appears to be an eventual next glaciation.
John Robson:
Now, we have taken issue in the past with these kinds of global reconstructions, particularly with their level of precision, because of all the uncertainties and limitations. But in this case, the idea of the HCO was based on an accumulation of evidence from many individual places around the world that seemed to leave little doubt about the broad outlines including warmth well above modern levels with atmospheric CO2 well below.
Narrator:
Earth scientists refer to the HCO interval as the “Hypsithermal”. And a search on Google Scholar shows over 6,600 studies on the topic. So yes, we’re talking about a lot of peer reviewed material here.
For example, in 1970 Canadian researchers found the fossil remains of a white pine forest in peat bogs in Northern Quebec, some 60 km beyond the current northern limit for such trees. They dated the forest remains to around 5,000 years ago, and noted that their findings exactly match similar fossil remains of forests beyond the current tree line in Scandinavia, indicating the existence of a long interval of mid-Holocene warmth in both North America and Europe.
Meanwhile, a 1985 study of ancient peat bogs in China showed it too experienced a lengthy warm interval between 9,000 and 5,000 years ago, reaching its peak around 7,500 years ago, once again illustrating that if this evidence holds up, it doesn’t just show the HCO, it shows it as global.
A 1991 study from Antarctica presented evidence that there was a lengthy warm interval there between 7,000 and 4,000 years ago which, rather counterintuitively, led to coastal glaciers advancing rather than retreating.
And in 2022 a comprehensive review examined the available evidence and concluded the hypsithermal occurred in Australia in the mid-Holocene and conditions then were warmer than today.
John Robson:
You get the picture. The HCO wasn’t pulled out of thin air. It’s a finding based on thousands of studies that kept coming to the same conclusion: it used to be warmer, and in many places much warmer than today, over a period of time stretching over thousands of years in some cases, all over the world.
That would be bad enough for the climate alarmists. But the picture gets worse because of something called the Holocene Temperature Conundrum.
Narrator:
While the proxy evidence shows the Earth has cooled over most of the past 10,000 years, climate models show it should have been warming. Indeed it must be, because atmospheric CO2 has been rising and if they are not essentially in lockstep, the whole theory and its whole research agenda fall to bits.
This Figure, from a 2014 paper, shows a proxy reconstruction in purple compared to average model simulations in black. The red line is the result of tweaking the models to try and improve the fit. But either way they can’t reproduce the HCO or the subsequent cooling.
John Robson:
And that brings us to the viewer comment that triggered this video. After an unnecessarily snide “I love it when you ask for real information!”, he cites a 2021 paper in Nature by Samantha Bova and coauthors that, he says, “resolves the Holocene Temperature Conundrum.”
Narrator:
In that paper, Bova and her coauthors argued that most proxies only pick up summertime conditions and yield a biased estimate of the annual average. They proposed a statistical correction called the SAT method that changed the cooling trend to a warming one, thus bringing the proxy data into agreement with climate models.
But far from settling the debate, it triggered strong objections from other scientists working in the field. A paper published a few months later pointed out that the way Bova et al. estimated the size of the bias depended on the assumption that the only thing driving sea surface temperature changes was changing solar output. But if they allow for other factors their results would change. They concluded that Bova’s “findings are effectively biased by overcorrecting insolation-induced seasonal bias in [Sea Surface Temperature] proxies.”
Another paper, also published in Nature the following year, argued that the assumptions behind Bova’s SAT method pre-determined the outcome, essentially guaranteeing removal of the HCO whether it happened or not.
“the [SAT] method by construction removes thermal maxima. Thus, the main findings of Bova et al. probably reflect peculiarities of the SAT method instead of shedding light on the so-called Holocene conundrum.”
John Robson:
Incidentally what you have there is an illustration of how real science works, with lively debate over the validity and meaning of evidence conducted without venom or anyone getting cancelled.
Our correspondent also mentioned a 2020 paper by D.S. Kaufmann et al. which he said “discusses the reconstruction of Holocene temperatures using paleoclimate data assimilation, showing that recent temperatures exceed those of the Holocene Optimum”.
And the Kaufmann paper is especially important because in their last report the IPCC based their conclusions on it. The Summary for Policymakers assigned “medium confidence” to the claim that current conditions are warmer than any time in the past 125,000 years:
Narrator:
“Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6500 years ago [0.2°C to 1°C relative to 1850–1900] (medium confidence). Prior to that, the next most recent warm period was about 125,000 years ago.” [para.A2.2]
John Robson:
Oh, and here let me remind you, when the IPCC says “medium confidence” it sounds as though they’re pretty certain. But actually, that terminology in their lexicon means 50-50. It’s a coin toss.
The first thing to note is that the Kaufmann reconstruction seems at first glance to reaffirm the HCO, despite the IPCC’s claim.
It also brings us back to the Holocene Temperature Conundrum, which is only a “conundrum” if you’re committed to the notion that CO2 drives temperature, since it clearly shows cooling over the 6,000 years that models say should have been warming as CO2 went up.
What the IPCC took notice of was that if you look closely at the end you can see a hockey stick blade jumping sharply upwards. And the authors conveniently include a blowup of that section to make sure you don’t miss it.
Narrator:
But the blow-up also shows that the Holocene reconstruction, shown as the coloured lines, is based on smooth long term proxies while the recent period, shown using black lines, uses annual temperature data.
These aren’t just different data sets spliced together. They’re sets using very different kinds of data. What if we stick to comparing apples to apples and only compare century or millennium-long intervals? On that basis, according to the authors, the hypsithermal is still the peak:
“On average, the warmest millennium of the Holocene was centered on 6.5 [thousand years ago] and was 0.6 °C warmer than the 1800–1900 reference period… The warmest 200-year-long interval was also centered on 6.5 [thousand years ago] and was 0.7 °C warmer than the 19th Century.”
John Robson:
To say more than that, you have to splice modern thermometer records on. But isn’t that problematic?
Narrator:
The authors themselves seem to think so.
“[Comparing] average temperatures between intervals of different durations can be problematic because shorter intervals tend to capture more variability (including maximum warmth) than when time series are averaged over longer intervals. In addition, age inaccuracies that exceed the scale of the sample binning can lead to smoothing when records are averaged. And even well-dated proxy time series based on marine and lake sediments are often smoothed by biological and physical processes that disturb the sediment-water interface, thereby time-averaging the paleo-environmental signal, which can further reduce the variability represented by the proxy record.”
John Robson:
So, the IPCC relied on a comparison made by splicing incompatible data sets together that don’t belong on the same graph. Now where have we heard that story before? Oh. Right. In our “Hide the Decline” video.
Finally, our correspondent mentioned a 2019 paper by I.I. Mokhov and A.V. Eliseev, called “Comparative analysis of climatic regimes during the Holocene optimum and at present” in something called Doklady Earth Sciences. We tried but we couldn’t find such a paper.
We did find a 2020 paper by these authors in that journal. But that one uses climate model reconstructions not observations, and we’ve already seen the models don’t get this interval right.
So we aren’t ready to get rid of the HCO. In fact in addition to the studies we’ve mentioned already, in the course of our blogging and videography we’ve described, and linked to, a huge number of studies on the HCO.
Narrator:
For instance a 2021 study in European Geosciences Union that found the Greenland Ice Sheet was smaller “Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE)” than it is now. Which is a fairly wide range, but they think it may well have reached its minimum as recently as 5,000 years ago and its maximum 300 years ago, a level at which, in many places, it still is.
John Robson:
Now, if you don’t like Greenland, we also referenced a 2012 study of phytolith records in the middle Yangtze valley to find, yes, low temperatures in the early Holocene and an HCO between 8,000 and 5,000 years ago.
Another study based on “Chironomid head capsules preserved in lake sediments”, which sounds disgusting and is, chironomids being midges and head capsules being head capsules, also found:
Narrator:
“the Holocene Thermal Maximum, which occurred between 8 and 5 [thousand calendar years ago], with temperatures generally higher than today's, maximum temperatures between 8 and 6.5 [thousand calendar years ago], and an average of + 0.9 °C.”.
Here’s another chart, from 2015, Fig. 2.6 from a study of the Baltic Sea, entitled Climate Change During the Holocene (Past 12,000 Years) and with the legend “Mean annual temperature reconstructed from pollen data from Lake Flarken (south-central Sweden) during the past 10,000 years (black line). Present-day mean annual temperature (5.9 °C) is marked by the point, modified from Seppä et al. (2005)”
From elsewhere in that paper, Fig. 2.9 shows reconstructed temperatures from pollen data from Lake Tsuolbmajavri in northern Finland (top panel) and from Lake Kurjanovas in south-eastern Latvia.
John Robson:
If you’re getting northern-hemisphered out, here’s a chart (Fig. 7) from a 2017 study of Antarctic ice cores, three central and three coastal).
You get the idea. If these various charts, and the proxy studies behind them, are even roughly right, the HCO is clearly real, global and warmer, possibly far warmer, than conditions today. It might have peaked at different times in different regions, climate is complex, but it lasted a long time and it left evidence around the world.
Is the matter settled? No, virtually nothing in science ever is. But the preponderance of evidence points one way, especially given what Anthropogenic Global Warming practitioners have to do in contravention of their own supposed data protocols to try to turn it around.
For instance, on our blog we also mentioned a 2023 study regarding Holocene warmth in Antarctica that conformed to the increasingly dogmatic requirements of its field by claiming to point directly to modern warming when it really did the opposite.
Narrator:
“The West Antarctic Ice Sheet is melting rapidly, raising concerns it could cross a tipping point of irreversible retreat in the next few decades if global temperatures rise 1.5 to 2.0 degrees Celsius (2.7 to 3.8 degrees Fahrenheit) above preindustrial levels. New research finds that 6,000 years ago, the grounded edge of the ice sheet may have been as far as 250 kilometers (160 miles) inland from its current location, suggesting the ice retreated deep into the continent after the end of the last glaciation and re-advanced before modern retreat began.”
John Robson:
So, what was really discovered is that the ice sheet was a lot smaller 6,000 years ago, which is a pretty strong indicator that it was a lot warmer back then over a long period time. And that no runaway disaster ensued. Yet they say it means the exact opposite.
For the Climate Discussion Nexus I’m John Robson, and I always say what I mean. Including the evidence for the HCO is very robust, and if climate alarmists can’t somehow make it go away, their whole project is in a heap of trouble.
Appreciate the presentation. It's main point seems to be that our weather isn't getting warmer than the ancient past but, in fact, cooler, and that the notion that CO2 causes warmer weather is twaddle. And that there is certainly no need for alarm or billions in taxes, government debt and spending (but there is in Canada a need to frankly "deal with it" - as many cultures in the past centuries have). It also continues to show that weather changes are (NOT) caused by human activity and the weather in general is (NOT) manipulated by it either.