Guest contribution by Willis Eschenbach
I have been pointing out for some time that the current warming of the earth began around the year 1700, as the following graph from the work of Ljungqvist shows:
Figure 1. 2000 years of temperature in the land areas from 30 ° N to the North Pole, overlaid with ice cores and instrumental CO2 data. Data source: A new reconstruction of the temperature variability in the extratropical northern hemisphere over the past two millennia
However, some people have said things like “yes, but this is not global temperature, it is just the extra-tropical temperature of the northern hemisphere”. I hear the same thing when someone points out the Medieval Warm Period, which peaked around AD 1000. And you’re right, the Ljungqvist data is for the northern hemisphere only. Here are the locations of the proxies he uses:
Figure 2. Location of all proxies Ljungqvist used for his 2000 year temperature reconstruction. SOURCE: Op. Cit.
So I thought I’d see how closely the temperatures in different parts of the world are actually related. For this I used decadic mean values of the rasterized temperature data from Berkeley Earth, file name “Land_and_Ocean_LatLong1.nc”. I chose decadal averages because this is the time interval of the Ljungqvist data. Here is a graph that shows how well different regions of the globe are tracking each other.
Figure 3. Centered average decadal temperatures for the entire globe (red) and for various subregions of the globe.
As you can see, apart from the slope, these all agree very well with the following correlations:
Figure 4. Correlations between the global mean temperatures and the mean global temperatures of various subregions. A correlation of “1” means that they move identically in lockstep. Note the excellent correlation of the extra-tropical northern hemisphere with the entire globe, 0.98.
This extremely good correlation can be seen more clearly in a graph like Figure 3 above if we simply adjust the slopes. Figure 5 shows this result.
Figure 5. As in Figure 3, but adjusted the variance so that the slopes match
Conclusions? Well, in the US election it was said, “As Maine goes, so does the nation.” Here we can say: “As the land of the northern hemisphere goes 30 ° N-90 ° N, so does the globe”.
Put simply, no large part of the world is too far removed from the global average. And this is especially true for large land sub-regions compared to global land temperatures, which is important since we live in the countryside.
And that means that since, according to Ljungqvist, temperatures from NH 30 ° N-90 ° N peaked in 1000 and bottomed out in 1700, so does the globe.
As I mentioned in my last post, my beautiful ex-fiancée and I will be here from Tuesday 29th (who all stay with us all day) to enjoy the house without wrinkles.
So if you live in northern Florida and want to meet, drop me a line on the open thread on my blog. Just enter your city name without entering your phone or email. I’ll email you when we get there. No guarantees, but it’s always fun to speak to WUWT readers in person. I will likely be posting regular updates on our trip on my Skating Under The Ice blog for those interested.
The best of this wondrous planet to everyone