Surface air moisture content has increased since 1976, consistent with changes in atmospheric temperature and the Clausius-Clapeyron relationship, which states that the air holds about 7 percent more moisture per 1°C of warming.
According to Kevin Trenberth, a Distinguished Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research, “Water vapor has increased at rates consistent with Clausius-Clapeyron for the period 1987–2004 (1.3 percent per decades), and the relationship with changes in sea surface temperatures is sufficiently strong that it is possible to deduce an increase of about 4 percent in total column water vapor over the oceans since the 1970s.”
Observed moisture increases are largest in the tropics and in the extratropics during summer over both land and ocean.
The IPCC states that humans have contributed to observed increases in atmospheric moisture content since 1960. A 2007 study by Santer et al. modeled the response of atmospheric moisture to different external forcings and found, with high statistical confidence, that the large increase in water vapor over oceans is due primarily to human-caused greenhouse gas increases.
The fingerprint of climate change has been found in the increase of wet bulb temperature since 1973, driving heat stress globally and in most land regions analyzed. The northern hemisphere is tending toward increasingly warmer and more humid summers, and the global area covered by extreme water vapor is increasing significantly.
Recent study has confirmed these earlier results.