There are 2 separate issues — between 1944 and 1998 there was a complete cooler regime and
a complete warmer regime.
Not exact matches
In this new
regime, with a
complete absence of sea ice and snow in the Northern Hemisphere, with rapid
warming of the arctic region due to increased solar absorption, a jump in regional temps will occur.
The 0.07 K / decade is the
warming across two
complete warming and cooling
regimes — 1944 to 1998.
The logical end points for trends is 1944 and 1998 — over the
complete record of a natural cooling and a natural
warming regime.
The last 2
complete warm and cool
regimes were from 1944 to 1998 — covering the period of most of the increase in CO2 in the atmosphere.
Let's take the period 1944 to 1998 — because that's the inflection points in HadCRUT4 that include the last 2
complete cooler and
warmer multidecadal
regimes.
But the simplest way is to take a period that includes both a
complete cool and
warm multi-decadal
regime — 1944 and 1998 — and divide it by the temperature rise.