Despite the monotonic increase in green-house-gas (GHGs) concentrations, global mean sea surface temperature (GMSST) displays important fluctuations on decadal timescales that are often attributed to natural Tropical Pacific SST variability. Using several large initial-condition ensembles with Earth System Models (LENS), we show that a large fraction (~40-60%) of the decadal-scale variance in GMSST over past decades (1950-2010) is externally forced and largely linked to the representation of volcanic aerosols. This key role of volcanic forcing is indirectly confirmed from single fixed-forcing experiments of the Community Earth System Model, which reveal small and negligible roles for anthropogenic aerosol and GHG concentrations, respectively. Comparison of LENS past and future decades (2010-2070) suggests that the external forcing acts primarily by providing additional decadal scale variability. Given the prominent role of external forcing in GMSST variability, the unpredictable nature of future volcanic forcing and anthropogenic aerosols suggest that a large portion of decadal-scale GMSST variability is not predictable with long lead times. Further to this, the forced decadal variability in GMSST projects onto the Interdecadal Pacific Oscillation (IPO) pattern and its temporal variability is correlated with the unforced IPO index in most LENS simulations, both of which are suggestive of an externally forced component in the IPO variability that may act to synchronize the internal variability.