(hattip to Nick at terry.ubc.ca)
The problem with the trick to hide the decline is that you can't separate the two.
Another meaning of trick apart from being clever is something designed to deceive.
How do you tell which is the meaning? You look at the word hide. Hiding something? Is that being clever or being deceiptful? It's being deceiptful.
Now, why is the last 50 years important and why should you switch from proxy to real? It's not the important point.
The critical point is that if the proxies do not show the recorded temperature, you can conclude one of two things.
1. The recorded temperatures are wrong.
2. The proxies don't work and show actual temperatures.
Take your pick
False dichotomy. You can also conclude that 3. The proxies don't work now due to recent anthropogenic, non-climatic influences, but work in the past when those influences were absent.
Not at all.
You have to assume that GW exists, in order to justify removing the proxies, in order to show that GW exists.
It's completely circular argument.
You can't haul yourself up by your own bootstraps.
Look, it's actually very simple.
We started getting actual instrumental records over 150 years ago.
We have proxies for that period and for earlier periods.
For 110 of those 150 years, the proxies and the instrumental records matched up well. Then, the tree ring proxies fell away and the instrumental records indicated real rises.
What this indicates is that for most of that time, the tree ring proxies are good indicators of real temperatures, but in the 1960s something happened to make them not good indicators -- could well have been impact by other types of pollution.
We KNOW that the temperature rise is real, and we know that the proxies USED to be good indicators for temperature but aren't any more. This isn't conspiracy, it is trying to be sane about data. And we need assume nothing.
If I had a boiling pan I'd know it was about water's boiling temperature.
But apparently if the pan got so a hole melted in it and all the water fell out or evaporated, it would be circular logic to suggest the pan is getting hotter?
My analogy is terrible, but I like it, so there.