why does earth have a more complicated global circulation than a simple one-cell model (where air rises at the equator and sinks at the poles)?

Unipolar neurons

Explanation:

Invertebrate unipolar cells do not have dendrites. Human unipolar cells have an axon that emerges from the cell body, but it splits so that the axon can extend along a very long distance. At one end of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target.