Most research focuses on varying the resonator and dielectric spacer to configure the absorption and resonant frequency of terahertz (THz) metamaterial absorbers (MAs), where a metal ground plane is used as a perfect reflector of incident THz waves. In this paper, we modulate the MAs absorption by varying the conductivity of the ground plane. Two THz MAs were fabricated by replacing the gold ground planes with cobalt silicide films, and the measured absorptivity decreases by 4% for the electric resonance and increases by 44% for the dipole resonance. Our quantitative analysis reveals that when the conductivity of the ground plane decreases from 1.8 x 10(7) S/m to 3.0 x 10(5) S/m, the absorptivity of the electric resonance decreases by 23%, while that of the dipole resonance increases by 62%. Our approach also provides a practical method to measure the electric conductivity of conductive films in the THz regime. (C) 2014 AIP Publishing LLC.