Although global warming is expected to occur at approximately constant relative humidity, the latest IPCC report remains elusive about the magnitude of observed changes in global mean total precipitable water (GTPW) and their attribution. Here we use a novel quality-controlled dataset of in situ observations, global reanalyses, and a long record of global mean surface temperature (GMST) to constrain both recent and future changes in GTPW. Most state-of-the-art global climate models tend to exaggerate the projected atmospheric moistening, in line with their overestimation of global warming and of the GTPW sensitivity to both anthropogenic green-house gases and aerosols across the 20th century. A 39% narrowing in the range of GTPW projections is obtained after applying the observational constraints, with a best-guess estimate of 7% per °C of global warming. This finding provides further evidence of a substantial intensification of the global water cycle as long as GMST is not stabilized.