Acoustic data acquired during the 1995-2006 Acoustic Thermometry of Ocean Climate (ATOC) program were used to test the accuracy of ocean state estimates of the North Pacific obtained by various means: simple forward integration of a model, objective analysis of hydrographic and altimeter data, and data assimilation using general circulation models. The comparisons of computed and measured time series stringently tested the accuracy of the state estimates. The differences were substantial, indicating that acoustic thermometry provides unique information about the large-scale temperature. On some acoustic paths, changes in temperature occurring over time scales of weeks with magnitudes comparable to the seasonal cycle were observed. Acoustic thermometry offers valuable constraints on the large-scale thermal variability for the ocean observing system. Acoustic tomography was accepted as part of the Ocean Observing System during the OceanObs'99 and '09 international workshops. Sources and receivers of acoustic thermometry can serve multiple purposes. Hydrophone arrays are used to study a wide range of human, biological, and geological activity. Acoustic sources can transmit signals that can be used to track drifting instrumentation. A modest number of active and passive acoustic instruments deployed worldwide can form a general purpose global acoustic observing network.