For the first time, long-term climate changes in the atmospheric moisture over the global tropics are investigated using precipitable water (PW) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data sets for two different periods: 1979--1998 and 1948--1998. Dominant modes of PW variability estimated for the shorter period (1979--1998) are in an overall agreement with those of satellite-estimated precipitation in terms of both empirical orthogonal functions (EOF) spatial patterns and time-dependent coefficients. Encouraged by this result, the entire time series of PW for the period of 1948--1998 is subsequently examined. Climatology of annual mean PW agrees well with the radiosonde-based and satellite-based climatologies of atmospheric moisture. EOF analysis applied to the entire time series indicates that the two leading modes explain 37.3% of the total variance of PW. The first EOF mode reflects that the trend-like change is associated with evolutions in the observing systems of NCEP/NCAR reanalysis product. The second EOF has a spatial pattern, which is characterized by the zonal dipole structure. This mode is associated with the major climatic signal in the region, El Ni¿o-Southern Oscillation. Analysis of regional time series of annual mean PW has shown that, coincident with introduction of satellite data, step-like changes in the mid to late 1970s are present over Malaysia, central tropical Pacific, and central Brazil. Abrupt changes of PW over North Africa are noticed in the late 1960s, and the large positive PW anomalies observed during the 1950s and 1960s are suspected to have quality deficiencies.