The use of a global average surface temperature trend as a diagnostic to monitor global warming is, at best a crude approach, and at worst an erroneous tool for that purpose.
This post summarizes why.
First, to describe global warming, let’s use the seminal paper
Ellis et al. 1978: The annual variation in the global heat balance of the Earth. J. Geophys. Res., 83, 1958-1962
While the specific values they reported in their paper can be updated with the newer data since 1978, the framework and general conclusions are equally valid today. Excerpts read [highlight added] – with their figure 4 presented at the top of this post: