Final answer:
The factor that increases as one moves poleward from the tropics is day length, as the Earth's axial tilt leads to significant variations in day and night length at higher latitudes.
Step-by-step explanation:
The factor that increases as one moves poleward from the tropics is day length.
Day length varies with latitude and is influenced by the tilt of the Earth's axis. Near the equator, day and night are nearly the same length year-round. However, as one moves towards the poles, the difference between the length of day and night becomes more pronounced due to the curvature of the Earth and its axial tilt.
This results in longer days during the summer and shorter days during the winter in these higher latitudes. Unlike solar radiation, temperature, and species diversity, which decrease away from the tropics, day length is the one factor that increases as one travels poleward.