What Does 2000 Years of Temperature Data Tell Us?

NOTE: This piece by Willis Eschenbach is reposted here with his permission.

I got to thinking about how little the atmospheric CO2 level has changed over most of the last two thousand years. Here is the CO2 data from ice cores (colored dots) and from the Mauna Loa CO2 measuring stations (red/black line).

Note that there is good agreement between the thirteen different ice cores, as well as good agreement during the period of the overlap between the ice core and the instrumental CO2 data.

Then I overlaid the CO2 record on the Ljundqvist reconstruction of the temperature variability of the extratropical northern hemisphere from 30°N to 90°N. It is a decadal average record that goes from the year 1 to the year 1999. I also added in the recent Berkeley Earth decadal averages for the period since the end of the study, for 2000–2009, and 2010–2019. (Because one is a proxy record and the other is an instrumental record, I have variance adjusted the Berkeley Earth record to match the variance of the proxy record over the period of their overlap, which is from 1850 to 1999. This is the same procedure that Ljundqvist used for the black dashed line in his reconstruction.)

Here is that graphic:

In Figure 2, we can see the early “Roman Warm Period”, which lasted up to about the year 200 AD. Then temperatures dropped until about the year 500 AD. After that, the world warmed up again to the “Medieval Warm Period”. Then the temperatures cooled to the “Little Ice Age” which ended about the year 1700 AD, and finally, temperatures have warmed in fits and starts for the three centuries since the Little Ice Age.

Depending on what we are doing, we can reduce the variance in the instrumental data or increase the variance of the proxy data until the two are in the best agreement. In the case of the Ljungqvist data, the modern data has had its standard deviation reduced by a factor of about five.

[Editor’s comment: Note that the scale indicates a difference between the Medieval Warm Period and the Little Ice Age of only one degree. This is an artifact of grafting together two datasets — in this case, ice-core proxies with instrumental measurements. When one is dealing with proxy data, one of the ongoing issues is that each proxy data point often represents an average of a number of years. As a result, the variations in proxy data are often smaller, and sometimes much smaller, than the corresponding modern instrumental data. Because of this, the Medieval Warm Period may have been as much as 4 degrees C warmer, and the last 20 years could be a few degrees cooler than what is shown on the above graph.]

Here are my questions about this historical temperature record:

  • Why did the temperature start dropping after the Roman Warm Period? Why didn’t it just stay warm?
  • Why did the cooling start in 200 AD, and not say in the year 600 AD?
  • Why did the temperatures start warming around 550 AD, and continue warming up to the Medieval Warm Period peak at around 1000 AD? It could have stayed cold … but it didn’t.
  • Why was that warming from 550 to 1000 AD, and not from say 800 to 1300 AD?
  • What caused the steady cooling from about 1000 AD to the depths of the Little Ice Age, where temperatures bottomed out around 1700 AD?
  • Why was that cooling from about 1000–1700 AD, and not e.g. 1250–1850 AD?
  • Instead of stopping at the year 1700 AD, why didn’t the world keep cooling down to real glaciation? Given the Milankovich cycles and the lengths of the other warm interglacial periods, we’re overdue for another real ice age.
  • Why did temperatures start warming again at the end of the Little Ice Age, instead of just staying at the 1700 AD temperature?
  • Why has it warmed, in fits and starts, from the Little Ice Age up to the present?

Here’s why all of that is important:

Not one.

And from Figure 2 above, it is quite clear that the answer is not “CO2” …

Without having enough understanding of the climate to know the answers to those questions, there is absolutely no way to say that the recent warming is not merely more natural fluctuations in the earth’s temperature.

Speaking of fluctuations, I note in Figure 2 that the Earth’s extratropical northern hemisphere surface temperature has wandered around in a range of about 1.5°C over the last 2000 years. The earth’s surface temperature is on the order of 288 kelvin, so including the variance adjustment, this represents a temperature variation on the order of one percent over a two-thousand-year period … to me, this is the most interesting thing about the climate — not how much it changes over time, but how little it changes.

This thermal stability is not from thermal inertia — the land in each hemisphere swings about 15°C over the course of every year, and the ocean in each hemisphere swings about 5°C every year.

I say that this surprising and exceptional stability is due to the thermoregulatory action of emergent climate phenomena including dust devils, tropical cumulus fields and thunderstorms, the El Nino/La Nina mechanism that pumps excess heat to the poles, tornadoes, and hurricanes. I describe this theory of emergent thermoregulation in a series of posts listed in my 2021 index of my posts here in the section called “Emergence”. These act to keep the earth from either excessive warming or cooling.

Anyhow, that’s where my mind wandered to most recently, to the question of two thousand years of temperature and how it’s not ruled by CO2.

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Provocateur, professional heretic, slayer of myths, speaker of truthiness to powerfulness, and defender of the Oxford comma.

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