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!
It is important to remember that just because you hope to be able to measure something, sometimes you just can’t.
Scientists may be very creative and inventive. But it requires a healthy dose of humility to seek the truth.
You wrack your brain and brainstorm to try to find clues in the physical record. But you have to bear in mind that sometimes it just isn’t there. You can want it. You can search for it. You can try to find a creative solution.
But you have to be ready to admit it if the clues just aren’t there.