This may seem like an odd question, but I am beginning to wonder if there may be something interesting in the answer. Here is why I wonder. We are told that the average temperature of the surface of the planet is about 15 celsius. The oceans overlay the thinnest part of the earth’s crust. Hence they should be warm at the bottom too. Even if the geothermal forcing is relatively small (my estimate is about 0.2W/(m.k), it is still positive. So why is the average temperature of the ocean around 4 or 5 celsius? Given that the first 14.5 meters of the ocean (or 0.4% of the total mass of water on the surface of the planet) contains as much mass as the entire atmosphere, the average energy content of the fluid surface of the planet (oceans plus atmosphere) may be low enough that there is no need for a greenhouse effect to explain the temperature as it is very close to where it should be for a black body radiator. I am missing something simple I am sure, but can’t place it exactly.