In the tropics, the absorbed solar energy is greater than the emitted thermal radiation lost to space. The reverse is true at the poles. What stops the tropics getting hotter and hotter and the poles getting colder and colder?

It is called Global atmospheric circulation

Explanation:

Though the earth receives virtually the same amount of energy from the sun but Due to the shape of the earth and some other factors like the surface albedo the tropics absorb more heat energy from the sun than the poles. What stops the tropics getting hotter and hotter or the poles getting colder and colder is the atmospheric circulation. It majorly follows the pattern that cold air from the poles are dense and heavier compared to warm air from the tropics which is warm and light. The great circulation ensures that the dense cold air from the poles moves to the tropics replacing the warm light air which has replaced the cold air at the poles. This cycle is repeated as soon as the new air at the poles becomes cold and the air at the at the tropics becomes warm.