The Earth receives virtually all of its heat energy from the Sun in the form of Electro-magnetic, EM, radiation (light). This is balanced by Earth (re)radiating heat back into space. What then is EM radiation and how does it depend on the temperature of the source (Sun or Earth)? Why does Earth’s atmosphere block some of the EM radiation? What are the roles of “Greenhouse gases” and how is this related to the Quantum Physics revolution of the early 1900s? As excess heat is mostly stored in the Oceans what are measurements telling us?