Initialization based on data assimilations using historical observations possibly improves near-term climate predictions. Significant volcanic activity in the future is unpredictable and not assumed in future climate predictions. To examine the possible influence of unpredictable future volcanic activity on the decadal potential predictability of the natural variability, we performed a 2006–2035 climate prediction experiment with the assumption that the 1991？ Mt. Pinatubo eruption would take place again in 2010. The Pinatubo forcing induced not only significant cooling responses but also considerable noises in the natural variability. The errors due to the Pinatubo forcing grew faster than that arising from imperfect knowledge of the observed state, leading to a rapid reduction of the decadal potential predictability of the natural variability. 1. Introduction Near-term climate predictions up to the year 2035 are useful for adaptation to the possible consequences of rapid climate changes and will be highlighted in the next assessment report of the Intergovernmental Panel on Climate Change (IPCC) [1–5]. Near-term predictions could be significantly affected by the natural variability [3, 6–9]. Our previous studies [10, 11] addressed the issues of signal-to-noise ratios of temperature extremes, annual mean precipitation, and extreme precipitation in near-term predictions. Furthermore, climate modeling centers are now exploring the possibility that an initialization based on assimilations of observed historical data assists in the prediction of the decadal natural variability in near-term predictions [9, 12–21]. In general, climate predictions are performed under scenarios of greenhouse gas concentrations and anthropogenic aerosol emissions. Natural external forcing factors, such as changes in solar irradiance and volcanic activity, are not generally considered, since future changes in solar irradiance are unclear and volcanic activity is unpredictable. However, large volcanic activity may occur in the upcoming decades [22]. By including the stochastic fluctuations of natural external factors based on historical forcing, Bertrand et al. [23] examined the potential influences of natural external forcing factors. They suggested that natural external forcing might exert significant influences on anthropogenic global warming in the coming decades. Significant volcanic activity would reduce the global mean temperature for several years [24]. Volcanically induced cooling at the surface could penetrate into deeper oceans, where it might persist for decades
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