There is no true global temperature possible because of the existence of the atmosphere and the oceans.
there is a global temperature in that there is an emmission of energy from the globe as a whole which can be measured from space.
temperature is the energy radiated by each component of the globe. If we start from a simple concept of a metal sphere and measure the energy being given off it is axiomatic that the size of the sphere determines the temperature at the surface.
secondly that the composition of the metal is immaterial??
thirdly that the mass of the sphere is unimportant???
how to put it The sphere must radiate back to space the energy that inputs eventually. But must a solid sphere heat up to the same surface temp as a hollow sphere to do so. This seems wrong it implies that the energy is going into and never coming out of te sphere until it reaches a magical emmision level so all of this sphere is suddenly holding all this energy that it is then releasing a bit like a dam filling up until it spills over the top whereas a smaller dam spills over much sooner
The sphere takes a heat amount and redistributes it such that each bit of the sphere that could radiate heat does so whereas the incoming heat has only been received by a certain number of bits
the problem for temp of any one bit of the sphere and atmosphere is that the atmosphere effectively rolls the surface area out like a giant radiator or moebious band such that the surface area is greatly increased.