They picked 1984 as the start date because it was a very cold period near the bottom of the Atlantic Multidecadal Oscillation (AMO.) The Arctic and the rest of the world warmed cyclically since 1984 – just as it did from 1910 to 1940.
The cyclical warming is particularly clear in the Arctic.
In 1947, scientists reported that the Arctic had warmed 10 degrees.
So how does NASA deal with past Arctic warming? They simply erase it.
From: Tom Wigley <firstname.lastname@example.org>
To: Phil Jones <email@example.com>
Date: Sun, 27 Sep 2009 23:25:38 -0600
Cc: Ben Santer <firstname.lastname@example.org>
So, if we could reduce the ocean blip by, say, 0.15 degC,
then this would be significant for the global mean — but
we’d still have to explain the land blip.
It would be good to remove at least part of the 1940s blip, but we are still left with “why the blip”.
Make no mistake about it, this is the biggest scam in science history.