Volcanic winter: Extreme crop failures, famine and years without summer

A new scientific study has shown that the cooling effect of rare large eruptions increases as the atmosphere warms.

With a major eruption currently underway on La Palma, you probably want to know if it will have an effect on the climate. A new scientific study has shown that warming our atmosphere will increase the cooling effect of rare large eruptions, thereby making them even more apocalyptic.

As you already know, volcanic eruptions can have a huge impact on Earth’s climate. For example, volcanic ash and gases from the 1815 eruption of Tambora, Indonesia, contributed to making 1816 the “year without summer,” a year of crop failure and famine in the Northern Hemisphere. In 1991, the eruption of the Pinatubo volcano in the Philippines cooled the climate for about 3 years.

Large volcanic eruptions such as 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 a new study has shown that the current warming of the atmosphere may increase the cooling effect of such large eruptions, which typically occur a couple of times a century. But the study also showed that the cooling effect of smaller, more frequent eruptions could be significantly reduced.

“What really matters is whether these volcanic aerosols get into the stratosphere — that is, above 16 kilometers in the tropics under current climate conditions and closer to 10 kilometers at high latitudes,” Thomas Aubrey explains.

“If aerosols are injected at these altitudes, they can remain in the atmosphere for several years. If they are injected at lower altitudes, however, they will essentially be washed away by precipitation in the troposphere. The climatic effect would only last a few weeks.”

The strength of a volcanic eruption affects the altitude at which gases enter the atmosphere: more aerosols enter the stratosphere with stronger eruptions.

The buoyancy of gases also affects the height at which they settle in the atmosphere. Warming can affect this buoyancy: As the atmosphere warms, it becomes less dense, which increases the height 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.

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. “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%.”

The study is interesting because it makes us think about the processes involved between volcanic emissions and climate in a new way. However, the simulations limit 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.

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.

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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