Strong volcanic eruptions are known to cool Earth’s climate for as long as two years by pushing vast amounts of sulfate gases into the stratosphere. Now, scientists analyzing a 2011 eruption in Africa whose plume was too weak to reach such heights by itself have found a partner at work. Updrafts associated with monsoonal thunderstorms helped extend the plume’s reach, which resulted in the largest amount of airborne particles (aerosols) injected into the stratosphere in 20 years.
The analysis of the Nabro volcano in Eritrea, which erupted on June 13, 2011, appears in the July 6 issue of Science. Led by Adam Bourassa (University of Saskatchewan), the study was coauthored by NCAR’s William Randel and scientists from Rutgers University and the University of Wyoming.
Observations from a Canadian satellite show that the sulfate gases emitted by Nabro boosted stratospheric aerosols across the entire Northern Hemisphere. The volcano is located on the western edge of a large zone of high pressure that pushes moisture across Asia each summer and feeds the monsoon rains critical to life across the region. The authors show that the aerosol plume rapidly moved from northeast Africa to higher latitudes, following the track of the monsoonal flow. Thunderstorm-based updrafts along the plume’s path apparently helped the sulfur to reach its satellite-measured height of 13 miles (21 kilometers).
Once in the stratosphere, sulfate gases form tiny droplets of sulfuric acid. These aerosols can remain for a year or more, blocking a portion of the Sun’s rays, before they settle to the ground. Mount Pinatubo, in 1991, was the last volcanic eruption with a plume strong enough to push large amounts of sulfate gases directly into the stratosphere. The Pinatubo eruption cooled Earth’s lower atmosphere by as much as 0.9°F (0.5°C) for more than a year.
Since 2000, the global amount of stratospheric aerosol has increased by 5–7%. Possible factors include emissions from industry, transportation, and other human activities, and a series of relatively weak volcanic eruptions. The added aerosol has reduced incoming solar energy by only a small amount—less than 0.1%. However, the authors note, “the effects of these relatively minor eruptions are important to include for climate model predictions to avoid overestimation of radiative forcing [definition] and thus global warming.”
A.E. Bourassa, A. Robock, W.J. Randel, T. Deshler, L.A. Rieger, N.D. Lloyd, E.J. Llewellyn, D.A. Degenstein, "Large volcanic aerosol load in the stratosphere linked to Asian monsoon transport," Science 6 July 2012: 337(6090) DOI: 10.1126/science.1219371