"Climate models show cooler stratospheric temperatures happen when there is more water vapor present, and water vapor also leads to the breakdown of ozone molecules," Shindell said in a prepared statement.

The research was published in the latest issue of Geophysical Research Letters.

According to satellite data, upper atmospheric temperatures around the world (20-35 miles, or 32-56 kilometers high) have cooled between 5.4-10.8 degrees Fahrenheit in recent decades. The stratosphere is the typically dry layer of the atmosphere above the troposphere, where temperatures increase with altitude.

There are two driving forces behind the change in stratospheric moisture. "Increased emissions of the greenhouse gas, methane, are transformed into water in the stratosphere," Shindell said, "accounting for about a third of the observed increase in moisture there."

The second cause of change in the upper atmosphere is a greater transport of water from the lower atmosphere, which happens for several reasons. Warmer air holds more water vapor than colder air, so the amount of water vapor in the lower atmosphere increases as it is warmed by the greenhouse effect.

Climate models also indicate that greenhouse gases such as carbon dioxide and methane may enhance the transport of water into the stratosphere. Though not fully understood, the increased transport of water vapor to the stratosphere seems likely to have been induced by human activities.

"Rising greenhouse gas emissions account for all or part of the water vapor increase," said Shindell, "which causes stratospheric ozone destruction."

When more water vapor works its way into the stratosphere, the water molecules can be broken down, releasing reactive molecules that can destroy ozone.

Shindell said his global climate model agrees with satellite observations of the world's stratospheric ozone levels when the water vapor factor is increased in the stratosphere over time.

"If the trend of increasing stratospheric water vapor continues, it could increase future global warming and impede ozone stratospheric recovery," he said.

The impact on global warming comes about because both water vapor and ozone are greenhouse gases, which trap heat leaving Earth.

"When they change, Earth's energy balance changes too, altering the surface climate," said Shindell. Increased water vapor in the stratosphere makes it warmer on the ground by trapping heat, while the ozone loss makes it colder on the ground. Water vapor has a much larger effect, so that overall the changes increase global warming.

Shindell said that although ozone depletion cools Earth's surface, repairing stratospheric ozone is very important to block harmful ultraviolet radiation, and other greenhouse gas emissions need to be reduced.

Shindell used seven years of data from the Upper Atmosphere Research Satellite's (UARS) Halogen Occultation Experiment (HALOE) with ground-based data to paint a picture of the upper atmosphere. He also used 14 years of lower stratospheric measurements that show large increases in water vapor.

NASA's UARS was launched in 1991 to improve understanding of stratospheric ozone depletion by analyzing vertical profiles of ozone, hydrogen chloride, hydrogen fluoride, methane, water vapor, nitric oxide, nitrogen dioxide and aerosols.