Sounds like a stupidly easy question to find out with a quick internet search, but it’s not.
I don’t want to know the average surface temperature, or the average ocean surface water temperature, or read another article about climate change.
But that’s all I found in the past hour.
I’d like to know the average temperature of all molecules that comprise earth, or a best guess scientific estimate.
Counting by volume would get tricky with the atmosphere. Where do you draw the line of where the atmosphere ends? Even thousands of miles from Earth there is very thin atmosphere.
Karman line?
That’s reasonable, but it means that whether you count by volume or mass the atmosphere is a negligible contribution to the average.