Critical to understanding the climate system is a technique by which the large-scale thermal field can be observed in the ocean. Acoustic measurements at basin scales do provide a sensitive means to recognize and map changes in temperature at large scales [J. L. Spiesberger and K. Metzger, J. Geophys. Res. 96, 4869–4889 (1991)]. These types of acoustic measurements provide a perspective of large-scale processes orthogonal to that provided by other data sets and are essential components of future ocean monitoring systems. Practical acoustic monitoring systems could be deployed within the next 5 yr. Economically feasible systems will be discussed which provide real-time monitoring at basic-scale resolution in the global oceans.