Though water vapor concentrations in the stratosphere are several orders of magnitude smaller than those in the lower troposphere, water vapor in the stratosphere plays an important role in both the radiative and chemical control of the atmosphere. Several studies suggest that changes in stratospheric water vapor may have contributed, since 1980, a radiative forcing which enhances that due to carbon dioxide alone by 40%. This increase has also likely contributed to the observed cooling of the stratosphere. Long-term measurements of stratospheric water vapor are limited. Only one continuous time series of 20 years duration exists. The balloon-borne vertical profile measurements at Boulder, Colorado (40N) were begun in 1980 and continue to the present. These observations are made using a cryogenically cooled, chilled mirror instrument with soundings on an approximately monthly schedule consistently covering the altitude range from about 5-28 km.

The twenty-year record of water vapor in the stratosphere over Boulder shows a dramatic increase of approximately 1% per year. This increase has not been uniform over the data record but has shown periods of relatively small increases and several periods of more dramatic change. Both the long-term increase and the year-to-year changes are not well understood. The primary source of water vapor in the stratosphere is the oxidation of methane. Over the longer-term there has been a rise in methane in the atmosphere of about 10 parts per billion per year but this growth rate has been declining over most of the time period encompassed by the Boulder water vapor observations and is now less than 5 ppb/year. Increases in methane can account for at most half of the measured increases in stratospheric water vapor. The other major process responsible for controlling water vapor levels in the stratosphere is injection of air from the troposphere into the stratosphere. The dryness of the stratosphere implies that air is dehydrated in the vicinity of the tropical tropopause and transported into the stratosphere. The mechanisms for the dehydration and the transfer of air into the stratosphere are an active topic of research at the present time. A simple warming of tropical tropopause temperatures does not appear to be the cause for the increase in stratospheric water vapor since there is not evidence of a warming of the cold point temperature through which air passes into the stratosphere. Since the tropical cold point is only occasionally at saturation, mechanisms increasing the transfer of air from the troposphere can also lead to a moistening of the stratosphere. Additional measurements of water vapor in the stratosphere and the tropical tropopause region are required to understand the changes taking place in the water vapor content of the stratosphere.