Drew Shindell, an atmospheric scientist from NASA's Goddard Institute for Space Studies (GISS) and Columbia University, NY, used the NASA/GISS global climate model with satellite and other remote sensing data to investigate long-term stratospheric cooling and ozone depletion. This study is the first to link greenhouse gases to increased ozone depletion over populated areas.

Shindell found that he was able to best simulate the behavior of temperature and ozone in the upper atmosphere when he added water vapor data into the climate model.

"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.

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

According to Shindell 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 noted that 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.

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

The impact on global warming comes about because both water vapor and ozone are greenhouse gases, which trap heat leaving the Earth. "When they change, the 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 stressed that although ozone depletion cools the 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 complete picture of the upper atmosphere. He also used 14 years of lower stratospheric measurements that show large increases in water vapor.

Though some studies conflict with lower stratospheric observations of water vapor trends, studies released since Shindell's paper was written, agree with the increases he used, and indicate that they have been taking place for more than four decades already.

Shindell's paper, "Climate and Ozone Response to Increased Stratospheric Water Vapor," appears in the April 15th issue of Geophysical Research Letters.

NASA's HALOE was launched on the UARS spacecraft September 12, 1991 as part of the Earth Science Enterprise Program. Its mission includes improvement of understanding stratospheric ozone depletion by analyzing vertical profiles of ozone, hydrogen chloride, hydrogen fluoride, methane, water vapor, nitric oxide, nitrogen dioxide, and aerosols.

Related Links
Ozone at Goddard
Halogen Occultation Experiment
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