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The marine boundary layer (MBL) is a key component of Earth's climate system, and its main characteristics (height, entrainment efficiency, energy and mass fluxes, cloud formation processes, etc.) are closely linked to the properties of the inversion layer, which generally determines its height. Furthermore, cloud response to a warmer climate, one of the main sources of uncertainty in future climate projections, is highly dependent on changes in the MBL and in the inversion-layer properties. Long-term trends of the time series of MBL parameters at 32 stations in the Atlantic Ocean have been analyzed using conveniently homogenized radiosonde profiles from 1981 to 2010. In general, decreasing trends are found in the strength and thickness of the inversion layer and in the difference between the precipitable water vapor (PWV) in the free troposphere and the MBL. In contrast, positive trends are found in the height of the bottom of the inversion layer, the lapse rates of virtual and equivalent potential temperatures, the PWV within the boundary layer, and the sea surface temperature (SST). The weakening trend of the inversion layer and the increasing desiccation of the free troposphere relative to the MBL could have important consequences for both the evolution of low cloud cover in a greenhouse-warming climate and the fragile local ecosystems, such as "cloud forests."