Of Warm Poles and Cold Tropics

Why are the poles warm and the tropics cold, and compared to what? An equivalent question is: what determines the difference in temperature between tropics and poles, and can we understand why that difference has the magnitude it has on the real planet Earth? Just for the record, there is not a single climate simulation model that tells us what that difference should be, let alone why its magnitude is what it is; it is one of the boundary conditions at the start of a climate ‘run’.