Increase in atmospheric moisture tied to human activities

Human activities are directly responsible for rising levels of atmospheric water vapour, the most abundant greenhouse gas and a key driver of global warming, scientists say.
Scientists at the University of Miami Rosenstiel School of Marine and Atmospheric Science and colleagues have found that rising levels of water vapour in the upper troposphere will intensify climate change impacts over the next decades.
The new study is the first to show that increased water vapour concentrations in the atmosphere are a direct result of human activities.
"The study is the first to confirm that human activities have increased water vapour in the upper troposphere," said Brian Soden, professor of atmospheric sciences at the UM Rosenstiel School and co-author of the study.

To investigate the potential causes of a 30-year moistening trend in the upper troposphere, Soden, UM Rosenstiel School researcher Eui-Seok Chung and colleagues measured water vapour in the upper troposphere collected by National Oceanic and Atmospheric Administration satellites and compared them to climate model predictions of water circulation between the ocean and atmosphere.
They wanted to determine whether observed changes in atmospheric water vapour could be explained by natural or man-made causes.

Using the set of climate model experiments, the researchers showed that rising water vapour in the upper troposphere cannot be explained by natural forces, such as volcanoes and changes in solar activity, but can be explained by increased greenhouse gases, such as CO2.

Greenhouse gases raise temperatures by trapping the Earth's radiant heat inside the atmosphere. This warming also increases the accumulation of atmospheric water vapour, the most abundant greenhouse gas.
The atmospheric moistening traps additional radiant heat and further increases temperatures. Climate models predict that as the climate warms from the burning of fossil fuels, the concentrations of water vapour will also increase in response to that warming.
This moistening of the atmosphere, in turn, absorbs more heat and further raises the Earth's temperature.
The study was published in the journal Proceedings of the National Academy of Sciences (PNAS).