"It is another piece of the puzzle that climate change is happening and we are influencing it," said Nathan Gillet, a climate researcher at the University of East Anglia.

Human emissions of gases such as methane and carbon dioxide that trap heat in the atmosphere are widely blamed for changes in the climate. Scientists say average global temperatures will rise by 2 to 6 degrees Celsius (4 to 11 Fahrenheit) by the end of the century, causing droughts, floods and violent storms.

Warmer air can hold more water vapour.

"It has been predicted for a long time that humidity would increase with greenhouse gas increases," said Gillet, who led the study.

"But this is the first study that shows a significant human impact on surface humidity," he said in a telephone interview.

The British team collected data from weather stations, buoys and ships across the world to measure the effect of rising greenhouse gases on humidity between 1973 and 1999.

A computer simulation showed that natural events such as volcanoes and variations in the sun's brightness could not alone have produced the increase in humidity, and pointed to greenhouse gases generated by humans, Gillet said.

"It is getting moister at the surface, so humidity is increasing," Gillet said. "You only see that in the model with the human effect."

The findings are especially important for tropical regions, which will see the largest increase in humidity because they are warm already, he said.