Several climate simulations, performed with an atmosphere-ocean coupled general circulation model, are made to evaluate the influences of anthropogenic and natural external forcing on the observed fluctuation of the Decadal El Ni¿o-Southern Oscillation (DENSO) during the second half of the 20th century. A comparison of DENSO in the model simulations and the observations suggests that the observed variability includes an unusually large trend relative to that expected from purely natural variations. Moreover, we show that there is a large probability that this trend is mainly attributable to anthropogenic factors.