Seismic observations indicate that Earth's lower mantle is homogeneous as revealed by smooth depth variations of the bulk sound velocity and Bullen's inhomogeneity parameter η being close to one. Here we show with 3D spherical convection simulations that it should also be non‐adiabatic because a significant proportion of mantle heat sources is internal. The computer simulations predict non‐adiabaticity of 100–300 degrees for the lower mantle geotherm, with η ∼ 1.01 implying that the temperature drop across the Core Mantle Boundary exceeds 1000 degrees.