Guest post from Ben Wouters
Geothermal flux and the deep oceans.
To appreciate how the small geothermal flux of ~100 mW/m2 can play a significant role in our climate we’ll take a look at a cross-section of the Pacific in Fig 1.
A typical temperature profile is given in Fig 2 below
First the profile below ~1000 m. Slowly decreasing temperature with depth, more or less the same for all latitudes. The dark blue layer (~30 C) can be regarded as the top of the cold deep oceans. From 1000 m. upward the temperature increases rapidly, warmest water at the surface in the (sub) tropics. The dark blue layer only reaches the surface at high latitudes (red arrows). All water above this dark blue layer is warmed from above by the sun, either directly or indirectly. This layer also loses its energy again at the surface to the atmosphere, and eventually to space. Solar energy only warms the upper ~1000 m. between ~50N and 55S. How high the surface temperatures will be, depends on the temperature of the deep oceans and how much the sun can warm the upper layer above the deep ocean temperature.