When two objects at different temperatures touch, heat transfer occurs. This happens at the interface between the two objects, where the warmer object loses heat and the colder object gains heat. This transfer of heat can happen in a few ways:

* Conduction: Heat transfer through direct contact between objects. This is the most common way heat transfers when two objects touch.

* Convection: Heat transfer through the movement of fluids (liquids or gases). This can occur if one of the objects is a fluid, or if the heat transfer causes convection currents in the surrounding air.

* Radiation: Heat transfer through electromagnetic waves. This is less significant in direct contact, but can still play a role.

So, to answer your question directly, heat transfer occurs at the interface between two objects of different temperatures. This interface could be anywhere in the Earth system where objects of different temperatures come into contact, such as:

* The surface of the Earth: The ground and air

* The ocean: Water and air, or water and seafloor

* The atmosphere: Different layers of air

* Inside the Earth: The Earth's core and mantle

* Between living organisms: A human hand and a cold object

Let me know if you'd like more detail on any of these examples!