Seismic observations indicate that Earth's lower mantle is homogeneous as revealed by smooth depth variatons of the bulk sound velocity and Bullen's inhomogeneity parameter &eegr; 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 &eegr;~1.01 implying that the temperature drop across the Core Mantle Boundary exceeds 1000 degrees. ¿ 2001 American Geophysical Union