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We have studied the nonlinear conductivity of two-dimensional Coulomb glasses. We have used a Monte Carlo algorithm to simulate the dynamic of the system under an applied electric field $E$. We found that in the nonlinear regime the site occupancy in the Coulomb gap follows a Fermi-Dirac distribution with an effective temperature $T_{\rm eff}$, higher than the phonon bath temperature $T$. The value of the effective temperature is compatible with that obtained for slow modes from the generalized fluctuation-dissipation theorem. The nonlinear conductivity for a given electric field and $T$ is fairly similar to the linear conductivity at the corresponding $T_{\rm eff}$. We found that the dissipated power and the effective temperature are related by an expression of the form $(T_{\rm eff}^\alpha-T^\alpha)T_{\rm eff}^{\beta-\alpha}$., Comment: 9 pages, 9 figures