Which occurs when a warm fluid cools down

The outside temperature increases.

Explanation:

The warm fluid cools down only when the outside temperature is lesser than the temperature of warm fluid.

According to the second law of thermodynamics, the heat always flows from higher temperature body to lower temperature body without being aided by any machine.

So, heat flows from the warm fluid into the surroundings and after some time the temperature of warm fluid cools down.

The energy is released into the environment.