Water vapor plays a fundamental role for chemistry, dynamics and the radiation budget of the atmosphere. It is the single most important greenhouse gas, contributing more than 80% to the total greenhouse effect. Yet the distribution of water vapor, its climatology and the short and long-term variability of the water vapor concentration in the upper troposphere (UT) and lower stratosphere (LS) are not known with sufficient accuracy to keep calculated radiative balance uncertainties on the same order as the radiative effect of doubling the CO2 concentration.

Because of its importance and the fact that there has never been an assessment of the state of knowledge of H2O vapor, the international SPARC Office sponsored a study called Water Vapor Assessment Study or WAVAS, to determine what is known about its distribution, variability and trends throughout the range from the troposphere to the mesopause. The emphasis of the study was on the upper troposphere and lower stratosphere region. Scientists from many countries participated in the effort.

The first chapter of the SPARC Water Vapor Assessment Report discusses ground-based, balloon, airborne and satellite measurement techniques, their spatial resolution, their accuracies and limitations. Chapter 2 focuses on the data quality, how well measurement systems intercompare with one another, limitations of the data sets and gives guidance on combining data sets e.g. for trend studies. The last chapter describes the H2O, variability and trends.

We will present the summary of the SPARC Water Vapor Assessment Report and discuss the findings in the context of the importance of water vapor as a climate gas.