Whenever images of a giant ash plume appear in the news, one of the first questions that arrives in my inbox is whether this eruption will affect Earth’s climate. It turns out that it takes more than a plume of ash rising 100,000 feet (30 kilometers) above a volcano to move the needle when it comes to our planet’s climate. What might be the most important factor in the impact of the eruption on climate?
First, some basics about volcanic climates. This is not ashes causing Earth’s climate to change after a massive eruption. Ash, which is volcanic glass, does not stay in the atmosphere for long, perhaps hours to weeks. It is the accompanying gases and aerosols (droplets) that come with the eruption that have a real impact on weather and climate.
The biggest culprit is sulfuric acid. This occurs when sulfur dioxide emitted by the volcano reacts with water in the atmosphere. Sulfuric acid droplets act as a veil over Earth’s lower atmosphere, absorbing sound high in the stratosphere. This leads to lower temperatures near the Earth’s surface — volcanic cooling. This process led to the “year without a summer” caused by 1815 eruption of Tambora in Indonesia (VEI 7 event).
Other gases and aerosols are released, such as carbon dioxide, water, chlorine, fluorine. However, they rarely have the same impact on climate as the sulfur released by large volcanic eruptions (so just stop there if you want to argue that volcanoes are the culprits of modern climate change).
The amount of sulfur released by giant volcanic eruptions is staggering. The The 1991 eruption of Pinatubo in the Philippines released 20 million tons of sulfur dioxide into the atmosphere. The Tambora eruption mentioned above may have been closer to 100 million tons. She was even bigger 1783-84 Lucky fissure eruptions in Iceland, which exceeded 120 million tons.
The impact of these volcanoes on the climate is roughly proportional to the amount of sulfur. Pinatubo saw global average atmospheric temperatures drop by about half a degree (C) and the troposphere (the lower 5-9 miles / 8-15 kilometers of the atmosphere) cool by 4 degrees (C)! Tambora was about a full degree (C) several years after the eruption. That doesn’t sound like a lot until you consider that it’s the average for the entire world. Consider how much change it might take to cool down the whole atmosphere by one degree!
However, we have had large eruptions that didn’t seem to leave such a dramatic trail. The 1902 eruption of Santa Maria was VEI 6 released 11 million tons of sulfur but no noticeable climate anomaly. This is about the same amount of (elemental) sulfur as Pinatubo, but no change.
Both patterns appear in the record of giant eruptions and their effects on climate. One is how high the ash pillar has reached. If you can eject volcanic aerosols high into the atmosphere, they will stay there longer, usually years! All these sulfur aerosols in the stratosphere will cause the troposphere to cool, so the longer they stick around (and the more of them), the deeper the cooling.
The other big factor is location. When a volcano in far northern latitudes like Alaska or Kamchatka peninsula erupts in eastern Russia, ash and aerosols tend to remain in the northern hemisphere. This means that sulfur compounds in the stratosphere are not as widespread, so less energy from the sun is also absorbed by the lower atmosphere it doesn’t get so cool.
However, if you put the same eruption closer to the equator—say Indonesia or Ecuador—and those aerosols can spread to both hemispheres, resulting in much more surface cooling. Therefore, when the atmospheric sulfur signal is picked up in the ice cores of both cores, we know it was a large eruption… and it probably happened near the equator.
The main message here is that we cannot assume that every major eruption will have the same effect. The amount of sulfur in the magma, the location of the volcano on the globe, and how high that eruptive plume reaches into the atmosphere will all play a role in whether we should expect cooler years or business as usual.