Volcanic Eruption in 1991 Sheds Light on Sensitivity of Climate to Water Vapor

Looking back to June 15, 1991: After being dormant for 500 years, Mount Pinatubo suddenly exploded with the second largest eruption of the century. 50,000 people were forced to evacuate, two military bases were effectively shut down, and hundreds of people lost their lives. The impact of this natural disaster continues even today.

Theory and computer models strongly suggest that water vapor amplifies the response of Earth’s atmosphere to any warming or cooling that may occur, including warming caused by greenhouse gases. While carbon dioxide is expected to cause a warming, volcanic eruptions cool the planet, because the global plumes of fine ash particles reduce the amount of visible sunlight that reaches the ground. In the presence of water vapor, the cooling is expected to be even greater. Water vapor amplifies a small warming or small cooling.

Brian J Soden and others at Princeton and Rutgers Universities investigated the cooling of Earth after the eruption of Mount Pinatubo in the Philippines in 1991. They compared the actual cooling with simulated coolings, both with and without the effect of water vapor. They conclude that the global plume of ash and aerosols plus the water vapor feedback cooled the Earth 60% more than the aerosol plume alone would have done. The observed cooling and its regional pattern resembled the cooling in a model simulation that included the water vapor effect. It was possible to remove the water vapor effect in the simulation by removing its influence on thermal radiation, and when the effect was removed, the cooling of the Earth was 60% less than it was in the standard simulation.

The effect of water vapor on the sensitivity of climate - even whether the feedback is positive or negative - is in dispute. Some have argued that water vapor or resulting clouds suppress any temperature changes and, thus, have a negative feedback on temperature changes. R. Lindzen takes this position when he proposed an Iris Effect, as described in our first issue of Climate Science Forum. Soden’s team believes it has made a strong case that water vapor amplifies any temperature change.