An international team of climatologists have conducted a study and have established that global warming owing to human interference has manifested itself as an increase in atmospheric moisture content.
"When you heat the planet, you increase the ability of the atmosphere to hold moisture. The atmosphere's water vapour content has increased by about 0.41 kilograms per cubic meter (kg/m²) per decade since 1988, and natural variability in climate just can't explain this moisture change," said lead author of the study, Benjamin Santer from the Lawrence Livermore National Laboratory, US.
"The most plausible explanation is that it's due to the human-caused increase in greenhouse gases," Santer added.
The study in the Sept. 17 online edition of the Proceedings of the National Academy of Sciences, says that more water vapour - which is itself a greenhouse gas - amplifies the warming effect of increased atmospheric levels of carbon dioxide.
Santer said this is a "positive feedback."
Santer said the water vapour feedback mechanism worked in the following way: As the atmosphere warmed due to human-caused increases in carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons, water vapour increased, trapping more heat in the atmosphere, which in turn caused a further increase in water vapour.
He said basic theory, observations and climate model results, all showed that the increase in water vapour was roughly six percent to 7.5 percent per degree Celsius warming of the lower atmosphere.
As part of the study, the researchers used 22 different computer models of the climate system and measurements from the satellite-based Special Sensor Microwave Imager (SSM/I).
The team found that the results, taken together with similar studies of continental-scale river runoff, zonal-mean rainfall, and surface specific humidity, all pointed towards an emerging human-caused signal in the cycling of moisture between the atmosphere, land and ocean.
Santer said this was the first identification of a human fingerprint on the amount of water vapour in the atmosphere.
"Fingerprint" studies seek to identify the causes of recent climate change and involve rigorous comparisons of modelled and observed climate change patterns.
To date, most fingerprint studies have focused on temperature changes at the Earth's surface, in the free atmosphere, or in the oceans, or have considered variables whose behaviour is directly related to changes in atmospheric temperature.
"This new work shows that the climate system is telling us a consistent story. The observed changes in temperature, moisture, and atmospheric circulation fit together in an internally- and physically-consistent way," Santer said.