"When you heat the planet, you increase the ability of the atmosphere to hold moisture," said Benjamin Santer of the Lawrence Livermore National Laboratory and the study's lead author. "The atmosphere’s water vapor content has increased by about 0.41 kilograms per cubic meter per decade since 1988 and natural variability in climate just can’t explain this moisture change.
"The most plausible explanation is that it’s due to the human-caused increase in greenhouse gases," he said.
The researchers noted more water vapor -- itself a greenhouse gas -- amplifies the warming effect of increased atmospheric levels of carbon dioxide.
"This is the first identification of a human fingerprint on the amount of water vapor in the atmosphere," Santer said.
The study that included scientists from Germany, Britain and Japan is reported in the Sept. 17 online edition of the Proceedings of the National Academy of Sciences.