The most iconic aspect of a volcanic eruption is the massive plume of smog that emits from its crater. Many greenhouse gases, including carbon dioxide, are included in this chaotic haze that can sometimes trigger dazzling lightning storms. Not surprisingly, this has an outsized impact on our climate. However, while it's true that volcanoes contribute to climate change, that does not mean that our current climate change crisis — you know, the one that is driving a sixth mass extinction and shattering temperature records — is being caused by volcanoes alone. Indeed, experts strongly pin our out-of-control greenhouse gas problem on humans.
However, climate change deniers have spent years arguing the opposite, that human-caused climate change is, in fact, nothing more than the product of volcanic activity. In 2015, then-presidential candidate Mike Huckabee claimed that a single volcano puts out as much climate-altering gases as humans do in a century. In 2016, the climate scientist article aggregator RealClimate published a post comprehensively debunking the "volcanoes are to blame" denier myth. In 2020 a tweet went viral that falsely claimed the eruption of Mt. Merapi "spewed more CO2 than every car driven in history." By 2023, an underwater volcano near Tonga was being widely blamed as the supposed bad guy in any climate change.
The truth is that, while volcanoes can indeed alter the Earth's climate (more on that in a moment), there is no evidence whatsoever that current volcanic activity is responsible for this unprecedented global warming. In contrast, an abundance of evidence places the blame primarily at the feet of fossil fuel use.
"The burning of fossil fuels and the manufacture of cement releases 37 Gt (billion tones) of CO2 into the atmosphere per year," Yves Moussallam, Assistant Professor of Earth and Environmental Sciences and Geochemistry at Columbia Climate School, told Salon by email. "Volcanoes are estimated to supply globally 0.28–0.36 Gt CO2 per year to the atmosphere and ocean system." By contrast, "Volcanoes hence contribute about 100 times less CO2 than anthropogenic activities."
Flavio Lehner, an assistant professor of Earth and Atmospheric Sciences at Cornell University, was equally dismissive of the notion that volcanoes could explain the current level of global warming.
"Volcanoes only emit small amounts of CO2 relative to how much humans emit today," Lehner explained, before running through the list of other possible variables that deniers cite as causing climate change. "Another possible factor of 'natural' climate change are changes in solar radiation (energy we receive from the sun), but its fluctuations are too small to explain current climate change, plus it has been trending down since 1950, not up. Another cause of naturally-occurring climate change are changes in orbital parameters (distance to sun, tilt of Earth, etc), but those change on time scales of tens of thousands of years and more, so not applicable here. If anything we were on a slow walk into the next ice age over the next few thousand years, but that's averted (and then some) thanks to greenhouse gas emissions."
At the same time, this does not mean that volcanoes play no role in altering the climate. As one of Lehner's colleagues pointed out, "there are some volcanic eruptions like Hunga-Tonga recently [which] might even warm the planet temporarily by releasing large quantities of water vapor in the atmosphere." According to Daniele Visioni, who is also an assistant professor of Earth and Atmospheric Sciences at Cornell University, if that water vapor reaches the stratosphere it could "significantly increase the water vapor load for years. For Hunga-Tonga, it is estimated that the stratospheric water content has increased by 10%, which might result — based on current best estimates — in a further warming of 0.05-0.1 C up to 2030."
Lehner agreed with this, elaborating that "depending on the time scales, volcanoes can be very influential."
For example, one hypothesis among scientists holds that during the period when Earth was primarily covered with ice and CO2 was locked in the oceans, volcanoes were needed to break through the ice. Once that happened, they began emitting enough CO2 into the atmosphere that it could trap heat, melt the ice and ultimately create a planet capable of supporting life as we know it.
"Large volcanic eruptions can also lead to substantial cooling in the first couple of years after their eruption as they emit aerosols that reflect sunlight and thus cool the planet," Lehner added. It is speculated that the Little Ice Age, or a period of pronounced cooling during the Middle Ages, occurred after a cluster of large volcanic eruptions took place.
Indeed, as Moussallam observed, most volcanic eruptions do not wind up warming the planet — they wind up cooling it.
"At our human timescale (a few years to a few hundred thousand years), most volcanic eruptions have a net cooling effect on the climate," Moussallam wrote to Salon, citing global cooling periods that occurred after the eruption of Tambora in 1815 and Mt. Pinatubo in 1991. "This is because of another gas which is emitted by volcanoes, sulfur dioxide. In the atmosphere SO2 turns to H2SO4 (sulfuric acid) which condenses into little droplets (aerosols). If these are injected into the stratosphere, they can remain there for several months to years and have a net cooling effect on the surface as they reflect part of the Sun's radiation."
In the case of so-called "super-eruptions," the effect can be profound and lasting.
"The super-eruption of Toba volcano in Indonesia around 74,000 years ago occurred just before the onset of a cooler period when the Earth's climate abruptly shifted to particularly severe cold and dry conditions, lasting for several millennia," Moussallam wrote to Salon. "We don't know if the Toba eruption was the culprit, but it's possible and certainly didn't help."
Of course, even when one looks at some of Earth's most significant volcanic events, many still pale in comparison to what humans are currently doing to the atmosphere. Moussallam referred to how events like super-eruptions from 250 million years ago or 66 million years ago "had a strong impact on our atmosphere and caused mass extinctions, that is catastrophic loss in biodiversity and bio-density in a very short time," but nevertheless "emitted CO2 into the atmosphere at rates that was slower than current human emissions (the mass extinction rate was also slower than the current loss in biodiversity)."
In other words, experts agree that it is fundamentally irrational to blame volcanoes for the current phenomenon of climate change. If nothing else, humans have an ugly history when it comes to what we do when we get angry at volcanoes for the weather. As Francois Lapointe, a postdoctoral associate in the University of Massachusetts' Department of Earth, Geographic, and Climate Sciences, told Salon by email in August:
"It is noteworthy that periods of intensified volcanic eruptions, which contributed to the cooling during the Little Ice Age, coincided with peaks of persecution against individuals accused of witchcraft. These unfortunate 'witches' were often blamed for causing disastrous weather conditions and crop failures. The correlation between increased volcanic activity and the rise in witch persecutions suggests that climate-induced hardships during the Little Ice Age fueled fears and superstitions about supposed weather manipulation."
