Water vapor plays a key role in regulating the Earth's climate. It is the dominant greenhouse gas and provides the largest known feedback mechanism for amplifying global warming. If the concentration of water vapor increases as the atmosphere warms, as is generally believed, the added radiative absorption will act to further amplify the initial warming. Current climate models suggest that this provides an important positive feedback, more than doubling the sensitivity of the surface temperature to an increase in anthropogenic greenhouse gases. Despite the importance of water vapor feedback in determining the sensitivity of the Earth's climate, the fidelity of its representation in climate models has remained a topic of debate for more than a decade. The difficulty in verifying models partly stems from the lack of observed climate variations that can provide suitable tests of the feedbacks in question.

The eruption of Mt. Pinatubo in June of 1991 provided unprecedented observations of both the radiative forcing from the volcanic aerosols as well as the climate system's response to this forcing. Satellite observations reveal that the Pinatubo aerosol plume decreased the amount of sunlight entering the Earth's atmosphere by ~5 Wm-2. This loss of solar energy led to a global cooling of the lower troposphere which peaked approximately 18 months after the eruption and lasted until the mid-1990s. Associated with this cooling, was a reduction in the global water vapor concentrations by ~0.7 mm (~3%) which closely followed the observed decrease in temperature. Thus Mt. Pinatubo provides a unique opportunity to not only study the sensitivity of the climate system but, more specfically, to examine the response of water vapor and evaluate its role in determining that sensitivity.

We use the global cooling and drying of the atmosphere that was observed following the eruption of Mt. Pinatubo to test model predictions of the climate feedback from water vapor. First, we show the success of the model in reproducing the observed drying following the volcanic eruption. Then, by comparing model simulations with and without water vapor feedback, we demonstrate the importance of the atmospheric drying in amplfying the temperature change, and show that without the strong positive feedback by water vapor the model is unable to reproduce the observed cooling.