Researchers at the University of Miami Rosenstiel School of Marine and Atmospheric Science and colleagues compared the increase in upper-atmospheric moisture during that period with climate model predictions.
They determined that rising water vapor in the upper troposphere cannot be explained by natural causes, such as volcanoes or changes in solar radiation.
But they found it can be attributed to increases in other greenhouse gases, like CO2 from the burning of fossil fuels.
The increased moisture 3 to 7 miles above the surface causes the air to absorb even more heat, and is predicted to amplify global warming.
The study was published in the journal Proceedings of the National Academy of Sciences.