Water vapour released into the air adds one degree Celsius to global warming for every one contributed by humanity through other greenhouse gas emissions.
The evidence for this phenomenon, long-debated among climate scientists, is now indisputable, according to a review of the evidence by Andrew Dessler of Texas A&M University and Steven Sherwood of the University of New South Wales (UNSW).
Known to science as water vapour feedback, it is responsible for a significant portion of the warming predicted to occur over the next century, according to Dessler and Sherwood. This is because water vapour itself is a greenhouse gas.
As a result of burning fossil fuels and other human activities, rising temperatures are increasing the volume of naturally occurring evaporation on our warm, watery planet, they point out. In turn, that evaporated water contributes additional global warming.
Dessler and Sherwood based their statement on studies of how atmospheric water vapour varies in line with the natural cycles that warm and cool the planet, such as the cyclical temperature change caused by the seasons and El Nino events.
'It's a vicious cycle: warmer temperatures mean higher humidity, which in turn leads to even more warming,' says Dessler, a professor.
Recent estimates suggest the earth will warm by between two to four degrees Celsius over the next century - a scenario that could have devastating long-term consequences, said an UNSW release.
The study was published in the latest issue of Science.