Water vapour released into the atmosphere adds one degree Celsius to global warming for every one contributed by humanity through greenhouse gas emissions.
The evidence for this phenomenon, long-debated among climate scientists, is now indisputable, according to a review of the evidence published in the latest issue of Science by Andrew Dessler of Texas A&M University and Steven Sherwood of the University of New South Wales.
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.
In their article, the authors note that doubts have long been held about whether the phenomenon was significant. They summarise the peer-reviewed evidence in support of water vapour feedback and conclude that the facts supporting it are now overwhelming.
Dessler and Sherwood base 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. These natural cycles of warming and cooling are very different to long-term global warming because they are not caused by human sources of greenhouse gases.
"It's a vicious cycle: warmer temperatures mean higher humidity, which in turn leads to even more warming," says Professor Dessler. "This process is always running, so some of the warming we have experienced over the last century is due to the water vapour feedback."
Scientific predictions of several degrees of Celsius warming over the next 100 years include warming by this water vapour feedback. Recent estimates suggest the earth will warm by between 2 to 4 degrees Celsius over the next century - a scenario that could have devastating long-term consequences.
"For many years there have been suggestions that we had this wrong," says Steven Sherwood, a professor at UNSW's Climate Change Research Centre. "However, there is so much evidence now that it is time to move on to other questions, such as the likely impacts of this warming and how best to minimise it."
Says Dessler: "Everything shows that the climate models are probably getting the water vapour feedback right, which means that unless we reduce emissions, it is going to get much, much warmer on our planet by the end of the century."
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