  We consider periodic adiabatic processes of gapped many-body spinless
electrons. We find an additional contribution to the orbital magnetization due
to the adiabatic time evolution, dubbed \textit{geometric} orbital
magnetization, which can be expressed as derivative of the many-body Berry
phase with respect to an external magnetic field. For two-dimensional band
insulators, we show that the geometric orbital magnetization generally consists
of two pieces, the topological piece that is expressed as third Chern-Simons
form in $(t,k_x,k_y)$ space, and the non-topological piece that depends on
Bloch states and energies of both occupied and unoccupied bands.
