From Eos: “Climate Change Will Alter Cooling Effects of Volcanic Eruptions” 

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From Eos

9.20.21
Michael Allen

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The eruption of Mount Pinatubo, Philippines, in June 1991 was one of the most powerful of the 20th century. Credit: Dave Harlow, USGS.

Volcanic eruptions can have a massive effect on Earth’s climate. Volcanic ash and gases from the 1815 eruption of Mount Tambora, Indonesia, for example, contributed to 1816 being the “year without a summer,” with crop failures and famines across the Northern Hemisphere. In 1991, the eruption of Mount Pinatubo in the Philippines cooled the climate for around 3 years.

Large volcanic eruptions like Tambora and Pinatubo send plumes of ash and gas high into the atmosphere. Sulfate aerosols from these plumes scatter sunlight, reflecting some of it back into space. This scattering warms the stratosphere but cools the troposphere (the lowest layer of Earth’s atmosphere) and Earth’s surface.

Now new research published in Nature Communications has found that climate change could increase the cooling effect of large eruptions like these, which typically occur a couple of times every century. The study also found, however, that the cooling effects of smaller, more frequent eruptions could be reduced dramatically.

“What really matters is whether these [volcanic aerosols] are injected into the stratosphere—that is, above 16 kilometers in the tropics under current climate conditions and closer to 10 kilometers at high latitudes,” explained Thomas Aubry, a geophysicist at The University of Cambridge (UK) and lead author of the new study. “If [aerosols] are injected at these altitudes, they can stay in the atmosphere for a couple of years. If they are injected at lower altitudes, they are essentially going to be washed out by precipitation in the troposphere. The climatic effect will only last for a few weeks.”

The power of a volcanic eruption influences the elevation at which gases enter the atmosphere, with stronger eruptions injecting more aerosols into the stratosphere. The buoyancy of the gases also contributes to the elevation at which they settle in the atmosphere. Climate change could affect this buoyancy: As the atmosphere warms, it becomes less dense, increasing the elevation at which aerosols reach neutral buoyancy.

Modeling Mount Pinatubo

Aubry and his colleagues used models of both climate and volcanic plumes to simulate what happens to aerosols emitted by a volcanic eruption in the present climate and how that could change by the end of the century with continued global warming. In their models, all the eruptions occurred at Mount Pinatubo.

They found that for moderate-magnitude eruptions, the height at which sulfate aerosols settle in the atmosphere remained the same in a warmer climate. But the cooling effect of such eruptions was reduced by around 75%. This discrepancy has less to do with volcanic emissions and more to do with the atmosphere: The height of the stratosphere is predicted to increase with climate change. Aerosols from moderate volcanic eruptions will therefore be more likely to remain in the troposphere and be removed by rain, reducing their potency.

For large eruptions, models indicated that volcanic plumes will rise around 1.5 kilometers higher in the stratosphere in a warmer climate. This change in elevation will result in the aerosols spreading faster around the world. This increase in aerosol spread is mainly due to a predicted acceleration of the Brewer-Dobson circulation, which moves air in the troposphere upward into the stratosphere and then toward the poles. The change in Brewer-Dobson circulation is associated with climate change.

In addition to enhancing the global cooling effect of the aerosols, the increase in aerosol spread reduces the rate at which the sulfate particles bump into each other and grow. This further increases their cooling effect by allowing them to better reflect sunlight.

“There is a sweet spot in terms of the size of these tiny and shiny particles where they are very efficient at scattering back the sunlight,” explained Anja Schmidt, an atmospheric scientist at the University of Cambridge and coauthor of the paper. “It happens to be that in this global warming scenario that [we] simulated, these particles grow close to the size where they are very efficient in terms of scattering.”

“We find that the radiative forcing (the amount of energy removed from the planet system by the volcanic aerosol) would be 30% larger in the warm climate, compared to the present-day climate,” Aubry said. “Then we suggest that would amplify the surface cooling by 15%.”

Stefan Brönnimann, a climate scientist at the The University of Bern [Universität Bern](CH) who was not involved in the new research, said that the study is interesting because “it makes us think about the processes involved [between volcanic emissions and climate] in a new way.”

Brönnimann noted, however, that the simulations limited their models to eruptions of Mount Pinatubo in the summer. It would be interesting to see whether the conclusions still hold for eruptions at different latitudes and in different seasons, he said.

A Changing Stratosphere

It is difficult to say whether the amplified cooling from large volcanic eruptions or the decrease in cooling from smaller eruptions will have a net effect on climate, Aubry said.

Schmidt said that current increases in the frequency and intensity of forest fires could also alter the climatic effects of volcanic eruptions because they are affecting the composition of the stratosphere. “There is really a lot of aerosol pollution in the stratosphere, probably on a scale that we’ve never seen before.”

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