How much can a large volcanic eruption affect the climate?

The largest volcanic eruptions can significantly cool the Earth for years. In much rarer circumstances, a volcano could also temporarily warm the climate.

November 4, 2024

In the spring of 1991, Mt. Pinatubo sprang to life. The volcano in the Philippines sent an enormous cloud of gas and ash into the sky on June 12, followed by a massive eruption on June 15 that contained more than a cubic mile of ash and other material.¹ As the second-largest volcanic eruption of the 20^th century, the Pinatubo event didn’t just displace thousands and damage the surrounding farms and towns. It also noticeably affected the world’s climate, lowering global temperatures for a year or two, says Kerry Emanuel, MIT professor emeritus of atmospheric science.

“The main effect was from the ash and particles that stayed up in the stratosphere for a few years, reducing the amount of sunlight coming to the surface of the planet,” he says. That loss of sunlight temporarily cooled the Earth by about 1° F (0.6° C).² When you compare that to the 1.9° F (1.1° C) of warming humans have caused to date, which is already bringing us record-shattering heatwaves and extreme weather events,³ it becomes clear that volcanic cooling can be quite meaningful in the year or two before it fades away.

Not just any eruption can have this dramatic effect on our climate, Emanuel says. Pinatubo shot forth with enough force for its material to reach the stratosphere, the cloudless layer of the atmosphere where airplanes fly. There, ash and debris particles persist long enough to affect the climate. An eruption like the 1980 Mt. St. Helens event in Washington State didn’t reach above the troposphere, the lowest layer of the atmosphere where humans live and where weather happens. Weather events quickly wash volcanic detritus out of this layer, leaving the wider climate unchanged.

An eruption large enough to cool the planet happens only every few decades on average. Famous historical examples include the eruptions of Krakatoa in 1883 and Tambora in 1815, both in Indonesia. Tambora affected the short-term climate so much that 1816 was called “the year without a summer,” as temperatures halfway around the world in Europe were suppressed by nearly 1° F on average. 

“People say that if it weren't for Tambora, the novel Frankenstein would never have been written, because Mary Shelley wrote it holed up in a cabin with nothing to do because of the rotten weather,” Emanuel says.

Volcanoes don’t throw ash alone into the atmosphere. They also spew greenhouse gases like carbon dioxide (CO₂), which contribute to warming the planet. However, even large eruptions have not been shown to measurably warm the globe, because the amount of CO₂ they release is simply not that large on the scale of our atmosphere. Pinatubo, for instance, released an estimated 42 million tons of CO₂⁴—about what humanity emits every nine hours.⁵

A rare exception, Emanuel says, could be an event like Hunga Tonga, an underwater volcano in the Pacific Ocean, which erupted in 2022 with enough force to blast water vapor into the stratosphere. Water vapor is a greenhouse gas just like CO₂, but because it is so short-lived in the atmosphere, it typically doesn’t contribute much to climate change. If water vapor enters the stratosphere, however, it can linger long enough to induce some warming. 

“There are debates going on right now about whether that eruption might have led to a blip in the warming of the planet,” Emanuel says. Multiple studies have suggested that Hunga Tonga indeed caused some extra warming,^6,7 though other scientists have estimated that the aerosols from the eruption, which reflected away some sunlight, have more than canceled those warming effects.⁸  

Although human impacts on the climate today dwarf those of volcanoes, volcanic eruptions were one of the most important factors regulating the climate of ancient Earth. We know, for instance, that throughout history there have been eruptions of supervolcanoes such as the giant Yellowstone caldera in the U.S. Researchers debate how much the planet would be cooled by such an enormous volume of material filling the atmosphere, but it’s possible the effect could last for decades and reach up to about 7°F (4°C).⁹

Volcanoes contributed to warming periods in Earth history, too, but not because of any single eruption. Emanuel explains that long periods of higher-than-average volcanic activity have at times put CO₂ into the atmosphere faster than the planet’s oceans and plant life could remove it. Scientists who study ancient climates believe that these episodes of prolonged volcanism have been the main driver of past eras of extraordinarily warm temperatures.¹⁰

Thank you to Fred Pickhardt of Clearwater, Florida, for the question.

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