Anthropogenic influence on the climate system is manifest not only in the rise of near-surface tropospheric temperatures (the effect people experience most directly), but also in the hydrological cycle. Recent observational studies have shown that continental river runoff, zonal-mean rainfall, and surface humidity all display trends that can be ascribed to the results of human activity, primarily the temperature rise caused by increasing concentrations of atmospheric greenhouse gases. Another atmospheric attribute of great importance, the total amount of atmospheric water vapor, W, has been more difficult to study. Santer et al. use data from the satellite-based Special Sensor Microwave Imager (SSM/I) to show that the total atmospheric moisture content over the oceans has increased by 0.41 kg/m2 per decade since 1988. They then use results from 22 different climate models to show that the size of the observed increase in W, and the pattern of changes that it has displayed over that interval, can be explained only if the primary cause is the human-induced increases in greenhouse gases in the atmosphere. In this way, they show that the “fingerprint” of anthropogenic impact can be seen in the moisture content of Earth's atmosphere, and that the increase is consistent with theory, thereby strengthening confidence both in those models and in how well the mechanics of climate are understood. — HJS
Proc. Natl. Acad. Sci. U.S.A. 104, 15248 (2007).