Stable isotope records from sclerosponges collected at 10--20 m depth in the Indonesian Seaway and Solomon Islands are particularly well suited for reconstructing century-scale trends in ambient temperature variability and the oceanic uptake of fossil fuel carbon. Basal U/Th dates demonstrate that the sclerosponges analyzed are 85--100 years old. Isotopic records from the Indonesian specimens suggest a strong subsurface cooling over the past 20 years that is not manifested in either surface instrumental or shallower coral proxy records. However, analysis of observed subsurface temperatures in Indonesia, observed winds in the west Pacific, and simulated subsurface temperatures from a steady state general circulation model hindcast forced by observed winds combine to suggest that thermocline adjustments could account for at least part of the recent cooling inferred from the Indonesian sclerosponges. If so, the sclerosponge data suggests that, on average, the west Pacific thermocline has shoaled significantly over at least the past 2 decades. ¿ 2000 American Geophysical Union