When it comes to greenhouse gases that contribute most to global warming, most people think of substances such as carbon dioxide, methane or even hydrofluorocarbons.

But did you know that, for a while now, scientists have considered the vapor of the most important ingredient in sustaining life on Earth – water – as one of the most abundant greenhouse gases in the atmosphere and a key driver of global warming?

A new study led by scientists from the University of Miami Rosenstiel School of Marine and Atmospheric Science confirmed that rising levels of water vapor in the troposphere – the layer of the atmosphere closest to the surface and rises to a height of between 5 to 20 km above Earth’s surface – will increasingly play an important role in climate change projections in the coming years.

The Florida researchers said their new study is also the first to demonstrate that the increasing amount of atmospheric water vapor is being caused by human activities.

The researchers wanted to find out what was causing a 30-year trend of increasing water vapor in the upper troposphere.

So they took data collected over the years by satellites from the US National Oceanic and Atmospheric Administration (NOAA) and compared it to climate models that predicted water circulation between the ocean and the atmosphere.

This allowed the team to determine whether or not the observed changes in atmospheric water vapor were the result of natural or human-made causes.

The experiments revealed that natural causes such as volcanic activity or variations in solar activity can’t explain the growing amounts of water vapor in the upper troposphere.

But their experiments did suggest that human activity can account for the increase.

In an email to Science World, Brian Soden, a co-author of the study and professor of atmospheric sciences at the University of Miami’s Rosenstiel School pointed out that those human-caused increases in water vapor in the upper troposphere are not the result of actual man-made water vapor emissions. Instead the water vapor is created as a response to man-made warming of the atmosphere, which he said is due primarily to the increase in CO2 from the burning of fossil fuels.

“Because the concentration of water vapor increases as the air temperature increases, this man-made warming triggers a natural “feedback” mechanism that causes the water vapor increase,” said Soden. “Because water vapor is itself a very powerful greenhouse gas, the increase in water vapor further amplifies the initial CO2 induced warming. Our study confirms the presence of this key feedback mechanism which is a crucial component of global warming projections in water vapor can be explained by a rise in the amount of other greenhouse gases such as carbon dioxide being fed into the atmosphere.”

If water vapor is plays a key role in global warming, which increases water evaporation, which in turn leads to even more atmospheric water vapor, could this feedback mechanism spin out of control?

“Fortunately the feedback is not strong enough to push the climate into a “runaway” mode which would cause indefinite warming,” Soden explains. “However, some hypothesize that such a runaway greenhouse effect may have occurred on Venus and led to its extremely large greenhouse effect and warm temperatures.”

The researcher’s study called “Upper Tropospheric Moistening in response to Anthropogenic Warming” was published today (July 28, 2014) in the early on-line edition of the Proceedings of the National Academy of Sciences (PNAS).