The myth probably arises from the very early days of equilibrium change runs, where the ocean heat sink effect did not apply. (realclimate.org)
First let's define the «equilibrium climate sensitivity» as the «equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. (thinkprogress.org)
In the real climate system, at any particular time the actual change in climate would lag behind the corresponding equilibrium change for any given CO2 level, largely because of the thermal inertia of the oceans. (usask.ca)