While the fallout from such events can be devastating already – as shown by the cataclysmic eruption of Indonesia’s Mount Tambora in April 1815 – the effects of global warming could intensify the environmental impact of violent volcanic eruptions.
As Earth continues to warm, projected changes to the oceans could limit the planet’s natural response to a major eruption, which would cause global temperatures to plummet more deeply and reduce vital precipitation, scientists report Tuesday in the journal Nature Communications.
“We discovered that the oceans play a very large role in moderating, while also lengthening, the surface cooling induced by the 1815 eruption,” said lead author John Fasullo, a scientist at the National Center for Atmospheric Researcher (NCAR) in Boulder, Colorado.
“The volcanic kick is just that – it’s a cooling kick that lasts for a year or so. But the oceans change the timescale. They act to not only dampen the initial cooling but also to spread it out over several years.”
The 1815 eruption – the largest in the past several centuries – ejected a large volume of sulfur dioxide into the upper atmosphere, where it ultimately turned into sulfate particles called aerosols. The light-reflecting aerosols chilled the planet, producing a chain of reactions that resulted in an extremely cold summer in 1816, particularly in Europe and the northeast of North America.
The “year without a summer” caused widespread crop failure and disease, leading to more than 100,000 deaths worldwide.
To analyze the climate effects of Mount Tambora’s eruption, and to investigate how those results might differ for a future eruption should global warming continue on its current trajectory, the team used an advanced computer model developed by NCAR scientists and the broader scientific community.
The researchers used the Community Earth System Model (CESM) to create and examine two sets of simulations. The first set came from the CESM Last Millennium Ensemble Project, which simulates the planet’s climate from 850 through 2005, including known volcanic eruptions. The second set, which assumes that current greenhouse gas emission levels continue, was generated by extending CESM into the future and recreating a hypothetical Mount Tambora eruption in 2085.
Simulations of the historical model showed that two counterbalancing processes helped regulate Earth’s temperature after the 1815 eruption.
As aerosols in the stratosphere began reflecting heat from the sun, the cooling was magnified by an increase in the portion of land covered by ice and snow, which also redirected heat back to space.
The oceans, meanwhile, served as a critical equalizer. As ocean surface temperatures cooled, the colder water sank, enabling warmer water to rise and expel more heat into the atmosphere.
Once the oceans had cooled substantially, the aerosols began to dissipate – which led to more heat from the sun reaching the planet’s surface. At that point, the oceans assumed the opposite role of keeping the atmosphere cool, as the oceans warm much slower than land.
“In our model runs, we found that Earth actually reached its minimum temperature the following year, when the aerosols were almost gone,” Fasullo said. “It turns out the aerosols did not need to stick around for an entire year to still have a year without a summer in 1816, since by then the oceans had cooled substantially.”
Simulations of a hypothetical Mount Tambora eruption in 2085 revealed that Earth would undergo a similar increase in land area blanketed by ice and snow.
However, the ocean’s ability to limit the accompanying cooling would be hindered significantly under the projected scenario. As such, the degree of Earth’s surface cooling could be as much as 40 percent higher in the future, though the team cautions that the exact magnitude is tough to quantify given that they had only a relatively small number of simulations of the hypothetical eruption.
This change stems from a more stratified ocean, in which different layers serve as barriers to water mixing. This limits the ability of warmer water at the ocean’s surface to mix with the colder, denser water below.
In the simulations, this dynamic led to water cooled after the eruption becoming trapped at the surface, which reduced the volume of heat released into the atmosphere.
The researchers also discovered that the hypothetical eruption would have a larger impact on rainfall than the Mount Tambora eruption. Brisk sea surface temperatures limit the amount of water that evaporates into the atmosphere and decrease global average rainfall.
Though the findings show that the planet’s response to a Tambora-like eruption would produce a more acute cooling effect in the future, the team notes that the average surface cooling caused by the hypothetical eruption – about 2 degrees Fahrenheit – would not come close to offsetting the warming caused by climate change, which is roughly 7.6 degrees Fahrenheit.
“The response of the climate system to the 1815 eruption of Indonesia’s Mount Tambora gives us a perspective on potential surprises for the future, but with the twist that our climate system may respond much differently,” said co-author Bette Otto-Bliesner, a scientist at NCAR.