"The Temperature of the Whole and the Parts", by Willis Eschenbach

 Source:

https://wattsupwiththat.com/2021/06/27/the-temperature-of-the-whole-and-the-parts/

"I’ve been pointing out for some time that the current warming of the globe started about the year 1700, as shown in the following graph from the work of Ljungqvist:

Figure 1. 2,000 years of temperatures in the land areas from 30°N to the North Pole, overlaid with ice core and instrumental CO2 data.

Data source: A New Reconstruction Of Temperature Variability In The Extra-Tropical Northern Hemisphere During The Last Two Millennia

However, some folks have been saying things like “Yeah, but that’s not global temperature, it’s just northern hemisphere extratropical temperature”. 

I hear the same thing whenever someone points out the Medieval Warm Period that peaked around the year 1000 AD. 

And they’re correct, the Ljungqvist data is just northern hemisphere. Here are the locations of the proxies he used:

Figure 2. Location of all of the proxies used by Ljungqvist to make his 2000-year temperature reconstruction. SOURCE: Op. Cit.

So I thought I’d look to see just how closely related the temperatures in various parts of the globe actually are. For this, I used decadal averages of the Berkeley Earth gridded temperature data, file name “Land_and_Ocean_LatLong1.nc”. I chose decadal averages because that is the time interval of the Ljungqvist data. Here is a graph showing how well various regions of the globe track each other.

Figure 3. Centered decadal average temperatures for the entire globe (red) as well as for various sub-regions of the globe.

As you can see, other than the slope, these all are in extremely good agreement with each other, with correlations as follows:

Figure 4. Correlations between the decadal average global temperatures and the decadal average global temperatures of various subregions. A correlation of “1” means that they move identically in lockstep. 

Note the excellent correlation of the extratropical northern hemisphere with the entire globe, 0.98.

This extremely good correlation is more visible in a graph like Figure 3 above if we simply adjust the slopes. Figure 5 shows that result.

Figure 5. As in Figure 3, but variance adjusted so that the slopes match

Conclusions:

... Here, we can say “As the northern hemisphere land 30°N-90°N goes, so goes the globe”.

Simply put, no major part of the globe wanders too far from the global average. And this is particularly true of large land subregions compared to global land temperatures, which is important since the land is where we live.

And this means that since per Ljungqvist the NH 30°N-90°N temperatures peaked in the year 1000 and bottomed out in the year 1700, this would be true for the globe as well."