Volcanic Activity

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On June 15, 1991, Mount Pinatubo in the Philippines erupted in one of the most powerful volcanic events in recorded human history. In fact, it was the second-largest eruption of that century. Fortunately, there were warning signs before the eruption, and thanks to accurate forecasts from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey, evacuation orders were issued in time. This likely saved more than 5,000 lives in the densely populated area around the volcano. When we think of volcanic eruptions, we often picture towering ash clouds, avalanches, and rivers of molten lava. But have you ever considered how such eruptions could affect the Earth's climate?

How Volcanos Impact the Earth's Climate

When a volcano erupts, it releases more than just dramatic ash clouds (see above) and lava flows—it also sends ash, dust, and gases high into the atmosphere. Heavier particles, like ash and fine rock, fall back to Earth relatively quickly, settling on the ground nearby. However, the smallest particles, especially those released in the most violent eruptions, can be injected high into the atmosphere and remain there for months, sometimes even longer. These fine particles, often in the form of aerosols, are carried by global winds, spreading around the Earth.

But that’s not all volcanoes release. Along with ash and dust, they emit a variety of gases, including water vapor, carbon dioxide (CO₂), and sulfur dioxide (SO₂). While water vapor and carbon dioxide are both well-known greenhouse gases that trap heat and contribute to warming the Earth’s surface, their effect from individual volcanic eruptions is actually quite small compared to human-driven emissions. Early in Earth’s history, when volcanic activity was much more frequent, these gases played a bigger role in shaping the planet’s climate. Today, however, the amount of carbon dioxide released during eruptions is minimal compared to the vast quantities emitted by human activities such as burning fossil fuels -- essentially just a rounding error in the carbon budget we'll talk about in the next lesson.

Sulfur Dioxide (S0₂)

Sulfur dioxide (SO₂), on the other hand, is particularly important. It plays a crucial role when it comes to volcanic impacts on the climate. When a volcano erupts, vast amounts of SO₂ can be ejected into the stratosphere. This is the atmospheric layer above the troposphere—well beyond where commercial airplanes fly. In the troposphere, rain can wash out particles fairly quickly, but in the stratosphere, where there is much (much) less condensed water, SO₂ can linger for months or even years. Once in the stratosphere, SO₂ reacts with the small amount of water vapor present to form sulfuric acid (H₂SO₄) aerosols. These tiny particles act like mirrors, scattering and reflecting incoming solar radiation away from the Earth’s surface, effectively increasing the planet’s albedo (I wasn't kidding when I said you'll see that word a lot in this class!). This reflection of sunlight causes a cooling effect on the Earth's surface, and after major volcanic eruptions, we can often observe noticeable dips in global temperatures. Scientists measure the amount of sunlight blocked by these aerosols using a metric called aerosol optical depth. This metric, commonly abbreviated AOD, is a gauge of how much sunlight is being scattered by particles in a given "column" of atmosphere.

The most violent eruptions can significantly impact the Earth’s climate, leading to rapid cooling followed by a slow recovery. The figure below shows notable volcanic eruptions over the past 150 years. What I've actually plotted is the stratospheric aerosol optical depth -- high values mean there are more particles in the stratosphere. Note the very strong linkage between when you see spikes in the data and volcanic eruptions, telling us that when we have lots of aerosols in the stratosphere, they were thrown up there by a big volcano.

Since these aerosols can increase planetary albedo, they can reduce sunlight reaching the surface and temporarily cool the planet. Historical temperature records reveal clear drops in global mean temperatures that align with major volcanic events. These cooling periods can last several years as the aerosols slowly dissipate from the atmosphere and the climate gradually returns to its previous state. The strongest volcanos may cool the planet up to 1°C, which is 10x more than the variability from sunspots we learned about! But I should emphasize that these effects are transient -- once all the aerosols "fall out" of the atmosphere, it's like the volcano no longer exists, so they don't do anything to stop or reverse long-term trends. While volcanic cooling is a natural process, it underscores the Earth’s sensitivity to changes in atmospheric composition, even for relatively short periods.