Though not fully understood, the increased transport of water vapor to the stratosphere seems likely to have been induced by human activities. 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. The impact on global warming comes about because both water vapor and ozone are greenhouse gases, which trap heat leaving the Earth. 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. 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. 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.