But what you are assuming here is the whole point:
What if the measurements were made on all different scales at different times and at different places?
What is some of the weights were measured on a home scale from K-Mart, some were measured on a doctor’s office scale by the football players doing it themselves, and some were performed by high-accuracy laboratory scales?
Now you try to combine different measurements from different sources that are not consistent.
That’s the problem.
Climateers want to combine guess-timates from counting the entrails of lizards (counting bacteria in fossils), tree rings, and ice core samples, and then combine those with modern thermometers.
The result must necessarily be wildly inaccurate.
As far as temperatures go, the world didn't have nearly enough thermometers of any kind to measure the whole globe in any sort of coherent fashion until just shortly before we started launching satellites.
The particular chemical the young man used for his proxies is rather more advanced than tree rings.
Speaking of Doctor Mann he used tree rings from down in a valley along the Lena river for part of his collection and then tree rings from high up in the mountains along the Lena river for another part of his collection.
It seems he would have used rings strictly from the top but the local climate had deteriorated and killed all the trees up there so he couldn't update his collection. That's when he got rings from downhill ~ there's a really big temperature difference between the mountain tops and the near sea level base along that river ~ probably 50 degrees C ~ easy!
He actually had to tone down his reported proxy temperatures for his famous hockey stick so he wouldn't look like a nutball.
He failed!