28.06.2021Can large volcanic eruptions make glaciers great again?

Large volcanic eruptions were thought to lead to a mass gain and an advance of glaciers worldwide. Whether this could happen again under current climatic changes was investigated by Michael Zemp, glaciologist at GIUZ, and Ben Marzeion, climate scientist at the University of Bremen.

The eruption of Mount Pinatubo, Philippines, in 1991 (left) and Oberaargletscher, Switzerland, in 2020 (right). How can a volcanic eruption influence the mass balance of glaciers around the world? Image sources: Pinatubo from http://volquake.weebly.com, Oberaargletscher from M. Zemp.

When a volcano erupts, enormous quantities of ash and sulfur gases are injected into the atmosphere. These influence the global climate due to a shading effect of the volcanic aerosols on the incoming sunlight and can lead to a mass gain and an advance of glaciers. Since the mid-20th century, however, the relevance of volcanic eruptions has become less important due to the sharp rise in greenhouse gas concentrations.

The present study examined the relationship between glacier changes from 1961 to 2005 and observations of human-released greenhouse gases, volcanic and sun activities, and a phenomenon related to ocean temperatures called the El Niño-Southern Oscillation. Global glacier mass loss was mainly caused by the increase in anthropogenic greenhouse gases, interrupted by only a few years of mass re-gain after large volcanic eruptions.

The impact of volcanic eruptions on climate and glaciers. Schematic diagram of volcanic inputs to the atmosphere and their effects on climatic and glacier mass changes in relation to the explosivity (y-axis) of and spatial distance (x-axis) to the eruption. At local to regional scales, glaciers react with opposed mass-balance anomalies: 1. Negative due to ice removal from the blast and melting by hot lava (first mountain in red sector), 2. Positive due to insulation under thick layers of ash (second mountain in blue sector), and 3. Negative due to reduced snow and ice albedo caused by thin ash layers (third mountain in red sector). Eruptions reaching the stratosphere result in 4. (fourth mountain below blue sector): positive mass-balance anomalies due to reduced solar flux and net cooling at the land surface. Figure designed by S. Steinbacher, UZH, modified after Robock (2000, Reviews of Geophysics).

If Mount Pinatubo were to erupt again in 2036

With increasing greenhouse gas concentrations, however, the relevance of volcanic eruptions is dwindling. In the paper, this illustrated at the example of Mount Pinatubo, a volcano in the Philippines. The eruption in 1991 was the largest volcanic eruption of the 20th century and managed to pause glacier melting at global level for one year. In a modelling experiment, the authors repeated the Pinatubo eruption at 45-years intervals in 1856, 1901, 1946, 1991, and 2036.

This experiment clearly demonstrates the dwindling relevance of volcanic eruptions for the global net balance of glaciers. While the hypothetic eruptions in the 19th century resulted in distinct periods of global mass gain over a few years, a repetition of the Pinatubo eruption in 2036 would be nowhere near able to compensate for the ice loss due to anthropogenic forcings. For this future scenario, an event with two to three times the radiative effect of the Pinatubo eruption would be required for a global net mass gain of glaciers.