The effect of sulfate aerosols in the earth’s atmosphere as from a volcanic eruption is to:

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.

“What really matters is whether these [volcanic aerosols] are injected into the stratosphere.”

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 in the United Kingdom 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.

Volcanic plumes will rise around 1.5 kilometers higher in the stratosphere in a warmer climate.

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 University of Bern 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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Karen Harpp, an assistant professor of geology at Colgate University, provides this explanation:

VOLCANIC ERUPTIONS inject ash and aerosol clouds into the atmosphere and produce more than 100 million tons of carbon dioxide each year.

In 1784, Benjamin Franklin made what may have been the first connection between volcanoes and global climate while stationed in Paris as the first diplomatic representative of the United States of America. He observed that during the summer of 1783, the climate was abnormally cold, both in Europe and back in the U.S. The ground froze early, the first snow stayed on the ground without melting, the winter was more severe than usual, and there seemed to be "a constant fog over all Europe, and [a] great part of North America."

What Benjamin Franklin observed was indeed the result of volcanic activity. An enormous eruption of the Laki fissure system (a chain of volcanoes in which the lava erupts through a crack in the ground instead of from a single point) in Iceland caused the disruptions. The Laki eruptions produced about 14 cubic kilometers of basalt (thin, black, fluid lava) during more than eight months of activity. More importantly in terms of global climate, however, the Laki event also produced an ash cloud that may have reached up into the stratosphere. This cloud caused a dense haze across Europe that dimmed the sun, perhaps as far west as Siberia. In addition to ash, the eruptive cloud consisted primarily of vast quantities of sulfur dioxide (SO2), hydrogen chloride (HCl), and hydrogen fluoride gases (HF). The gases combined with water in the atmosphere to produce acid rain, destroying crops and killing livestock. The effects, of course, were most severe in Iceland; ultimately, more than 75 percent of Icelands livestock and 25 percent of its human population died from famine or the toxic impact of the Laki eruption clouds. Consequences were also felt far beyond Iceland. Temperature data from the U.S. indicate that record lows occurred during the winter of 1783-1784. In fact, the temperature decreased about one degree Celsius in the Northern Hemisphere overall. That may not sound like much, but it had enormous effects in terms of food supplies and the survival of people across the Northern Hemisphere. For comparison, the global temperature of the most recent Ice Age was only about five degrees C below the current average.

There are many reasons that large volcanic eruptions have such far-reaching effects on global climate. First, volcanic eruptions produce major quantities of carbon dioxide (CO2), a gas known to contribute to the greenhouse effect. Such greenhouse gases trap heat radiated off of the surface of the earth forming a type of insulation around the planet. The greenhouse effect is essential for our survival because it maintains the temperature of our planet within a habitable range. Nevertheless, there is growing concern that our production of gases such as CO2 from the burning of fossil fuels may be pushing the system a little too far, resulting in excessive warming on a global scale. There is no doubt that volcanic eruptions add CO2 to the atmosphere, but compared to the quantity produced by human activities, their impact is virtually trivial: volcanic eruptions produce about 110 million tons of CO2 each year, whereas human activities contribute almost 10,000 times that quantity.

By far the more substantive climatic effect from volcanoes results from the production of atmospheric haze. Large eruption columns inject ash particles and sulfur-rich gases into the troposphere and stratosphere and these clouds can circle the globe within weeks of the volcanic activity. The small ash particles decrease the amount of sunlight reaching the surface of the earth and lower average global temperatures. The sulfurous gases combine with water in the atmosphere to form acidic aerosols that also absorb incoming solar radiation and scatter it back out into space.

The ash and aerosol clouds from large volcanic eruptions spread quickly through the atmosphere. On August 26 and 27, 1883, the volcano Krakatau erupted in a catastrophic event that ejected about 20 cubic kilometers of material in an eruption column almost 40 kilometers high. Darkness immediately enveloped the neighboring Indonesian islands of Java and Sumatra. Fine particles, however, rode atmospheric currents westward. By the afternoon of August 28th, haze from the Krakatau eruption had reached South Africa and by September 9th it had circled the globe, only to do so several more times before settling out of the atmosphere.

Initially, scientists believed that it was volcanoes' stratospheric ash clouds that had the dominant effect on global temperatures. The 1982 eruption of El Chichn in Mexico, however, altered that view. Only two years earlier, the major Mt. St. Helens eruption had lowered global temperatures by about 0.1 degree C. The much smaller eruption of El Chichn, in contrast, had three to five times the global cooling effect worldwide. Despite its smaller ash cloud, El Chichn emitted more than 40 times the volume of sulfur-rich gases produced by Mt. St. Helens, which revealed that the formation of atmospheric sulfur aerosols has a more substantial effect on global temperatures than simply the volume of ash produced during an eruption. Sulfate aerosols appear to take several years to settle out of the atmosphere, which is one of the reasons their effects are so widespread and long lasting.

The atmospheric effects of volcanic eruptions were confirmed by the 1991 eruption of Mount Pinatubo, in the Philippines. Pinatubos eruption cloud reached over 40 kilometers into the atmosphere and ejected about 17 million tons of SO2, just over two times that of El Chichn in 1982. The sulfur-rich aerosols circled the globe within three weeks and produced a global cooling effect approximately twice that of El Chichn. The Northern Hemisphere cooled by up to 0.6 degrees C during 1992 and 1993. Moreover, the aerosol particles may have contributed to an accelerated rate of ozone depletion during that same period. Interestingly, some scientists argue that without the cooling effect of major volcanic eruptions such as El Chichn and Mount Pinatubo, global warming effects caused by human activities would have been far more substantial.

Major volcanic eruptions have additional climatic effects beyond global temperature decreases and acid rain. Ash and aerosol particles suspended in the atmosphere scatter light of red wavelengths, often resulting in brilliantly colored sunsets and sunrises around the world. The spectacular optical effects of the 1883 Krakatau eruption cloud were observed across the globe, and may have inspired numerous artists and writers in their work. The luminous, vibrant renderings of the fiery late day skyline above the Thames River in London by the British painter William Ascroft, for instance, may be the result of the distant Krakatau eruption.

In 1815, the Indonesian volcano Tambora propelled more ash and volcanic gases into the atmosphere than any other eruption in history and resulted in significant atmospheric cooling on a global scale, much like Krakatau a few decades later. New England and Europe were particularly hard hit, with snowfalls as late as August and massive crop failures. The cold, wet, and unpleasant climatic effects of the eruption led 1816 to be known as "the year without a summer," and inspired Lord Byron to write:

"The bright Sun was extinguishd, and the stars Did wander darkling in the eternal space Rayless and pathless, and the icy earth Swung blind and blackening in the moonless air; Morn came and wentand came,

And brought no day"

There is a story that Byron invited some of his friends to his home in Switzerland that summer to relax by the shores of Lake Geneva. The lack of sun and warm summer weather led the group to hold a competition writing ghost stories to keep themselves entertained. One of the guests, Mary Shelley, wrote the famous novel Frankenstein for this contest, revealing that in addition to major climatic effects, volcanic eruptions can have some unexpectedly far-reaching results.

Answer originally published April 15, 2002.