How Do Volcanoes Contribute to Climate Change?

Volcanoes change Earth’s climate both by warming and cooling it. Their net effect on climate today is small compared to that of human-made pollutants.

Even so, the climate change caused in prehistoric times by near-constant eruptions and, over the last few centuries, by a handful of epic ones, provides a warning: It helps us imagine life on Earth if we let the environment be ruined by our negligence.

Volcanoes of Prehistory

The number of volcanic eruptions in recorded history pales in comparison with what scientists have discerned about volcanic activity in prehistoric times.

Roughly 252 million years ago, in a vast swath of what is now Siberia, volcanoes steadily erupted over about 100,000 years. (That may seem like a long time but, in geologic terms, it’s the blink of an eye.)

The volcanic gases and ash that the wind blew around the world triggered a cascade of climate changes. The result was a calamitous, worldwide biosphere collapse that killed off as much as 95% of all species on Earth. Geologists refer to this event as the Great Dying.

Volcanic Disasters During Historical Times

Before 1815, Mount Tambora on the Indonesian island of Sumbawa was thought to be an extinct volcano. In April of that year, it exploded—twice. Mt. Tambora was once about 14,000 feet high. After its explosions, it was only about two-thirds as tall.

Most life on the island was eradicated. Human death estimates vary widely, from the 10,000 killed instantly as reported in Smithsonian Magazine, to the 92,000 that the United States Geological Survey (USGS) suggests died mostly by starvation after volcanic gases and ash ruined the land and changed the climate. Except for four lucky people, the entire kingdom of Tambora (10,000 people strong) disappeared in the blasts. 

With the rapid injection of ash and gases into the atmosphere, the monsoons in Asia developed more slowly, resulting in droughts that led to famine. Drought was followed by floods that altered the microbial ecology of the Bay of Bengal. This seems to be what gave rise to a new cholera variant and global cholera pandemic. In the early nineteenth century, public health agencies were not in coordination, so the pandemic’s death toll is hard to pinpoint. Non-definitive estimates peg it in the tens of millions.

By the following year, Tambora-induced global cooling was so severe that 1816 is often remembered as the “year without summer” and as the “little ice age." Snowstorms swept North America and parts of Europe during the summer months, killing crops and livestock and creating famine, riots, and a refugee crisis. Paintings from the year show dark, weirdly colored skies.

Mount Tambora and a distressingly large handful of other volcanic disasters aside, matters have not been nearly as dramatic during historical times as they were during prehistory.

According to the USGS, along Earth’s oceanic ridges where tectonic plates slide past each other under deep water, molten rock from Earth’s superheated mantle constantly rises from deep inside Earth’s crust and creates new ocean floor. Technically, all of the places along the ridge where incoming molten rock meets ocean water are volcanoes. Aside from those places, there are about 1,350 potentially active volcanoes worldwide, and only about 500 of them have erupted in recorded history. Their effects on climate have been profound, but mostly short-lived.

Volcano Basics

The USGS defines volcanoes as openings in Earth’s crust through which ash, hot gases, and molten rock (aka “magma” and “lava”) escape when magma pushes up through Earth’s crust and out a mountain’s sides or top.

Some volcanoes discharge slowly, almost as though they are exhaling. For others, the eruption is explosive. With deadly force and temperature, lava, burning chunks of solid rock, and gases blow out. (As an example of how much material a volcano can spew, The National Oceanic and Atmospheric Administration (NOAA) estimates that Mount Tambora ejected 31 cubic miles of ash. Wired Magazine calculates that ash at that volume could “bury all of the playing surface of Fenway Park in Boston 81,544 miles (131,322 km) deep.”)

Mount Tambora was the biggest eruption in recorded history. Even so, volcanoes in general spit out a lot of ash. Gases, too. When a mountain “blows” at its top, the ejected gases can reach into the stratosphere, which is the layer of atmosphere that extends from about 6 miles to 31 miles above Earth’s surface.

Climate Effects of Volcanoes’ Ash and Gases

While volcanoes superheat the surrounding air and warm temperatures locally while the mountain and its lava remain red hot, global cooling is the more prolonged and profound effect.

Global Warming 

One of the primary gases that volcanoes discharge is carbon dioxide (CO2)—which is also the human-made greenhouse gas most responsible for heating Earth’s climate. CO2 warms the climate by trapping heat. It allows short-wavelength radiation from the sun in through the atmosphere, but it does so while blocking about half of the resulting heat energy (which is long-wavelength radiation) from escaping Earth’s atmosphere and moving back into space.

The USGS estimates that volcanoes contribute about 260 million tons of CO2 to the atmosphere each year. Even so, the CO2 emitted by volcanoes probably does not have a significant effect on climate.

NOAA estimates that humans poison Earth’s atmosphere with 60 times more CO2 than volcanoes do. The USGS suggests that the difference is even greater; it reports that volcanoes release less than 1% of the CO2 that humans release, and that “the carbon dioxide released in contemporary volcanic eruptions has never caused detectable global warming of the atmosphere.”

Global Cooling, Acid Rain, and Ozone

As the wintry aftermath of Mount Tambora’s explosions made evident, volcano-induced global cooling is a huge danger. Acid rain and the destruction of the ozone layer are other catastrophic effects of volcanoes.

Global Cooling

From gas: In addition to CO2, volcanic gases include sulfur dioxide (SO2). According to the USGS, SO2 is the most significant cause of volcanically induced global cooling. SO2 converts to sulfuric acid (H2SO4), which condenses into fine sulfate droplets that combine with volcanic steam and create a whitish haze that’s commonly called “vog.” Blown around the world by wind, vog reflects back into space nearly all of the incoming solar rays it encounters.

As much SO2 as volcanoes put into the stratosphere, the Environmental Protection Agency (EPA) tags the primary source of SO2 haze as “the burning of fossil fuels by power plants and other industrial facilities.” Hey, volcanoes. You’re relatively off the hook on this count. 

From ash: Volcanoes throw tons of tiny fragments of rock, minerals, and glass skyward. While the larger pieces of this “ash” fall out of the atmosphere fairly quickly, the smallest ones rise into the stratosphere and stay at extremely high altitudes, where wind buffets them. The millions or billions of minuscule ash particles reflect incoming solar rays away from Earth and back toward the sun, cooling Earth’s climate for as long as ash stays in the stratosphere.

From gas and ash working together: Geophysicists from several institutions in Boulder, Colorado, ran a climate simulation and compared their results with observations gathered by satellite and aircraft after the tropical Mt. Kelut eruption of February 2014. They found that the persistence of SO2 in the atmosphere depended significantly on whether it had coated ash particles. More SO2 on ash resulted in longer-lasting SO2 capable of cooling the climate.

Acid Rain

One might imagine that an easy solution to global warming would be to intentionally infuse the stratosphere with SO2 to create cooling. However, hydrochloric acid (HCl) is present in the stratosphere. It’s there because of industrial coal burning on Earth and also because volcanoes eject it.

When SO2, HCl, and water precipitate down to Earth, they do so as acid rain, which strips nutrients from the soil and leaches aluminum into waterways, killing many species of marine life. Were scientists to try to counter global warming with SO2, they might wreak havoc. 

Ozone

Aside from its potential to precipitate as acid rain, volcanic HCl presents another danger: It threatens Earth’s ozone layer, which protects the DNA of all plant and animal life from destruction by unfettered ultraviolet solar radiation. HCl breaks down quickly into chlorine (Cl) and chlorine monoxide (ClO). Cl destroys ozone. According to the EPA, “One chlorine atom can destroy over 100,000 ozone molecules.”

Satellite data after volcanic eruptions in the Philippines and Chile showed an up to 20% loss of ozone in the stratosphere over the volcanoes.

The Takeaway

Compared to human-caused pollution, the contribution that volcanoes make to climate change is small. The climate-ruining CO2, SO2, and HCl in Earth’s atmosphere are mostly the direct result of industrial processes. (Ash from coal burning is mostly a terrestrial and lower atmospheric pollutant, and so its contribution to climate change may be limited.)

Despite the relatively insignificant role that volcanoes typically play in climate change, the floods, droughts, starvation, and disease that have ensued after mega-volcanoes can stand as a warning. If man-made atmospheric pollution continues unabated, floods, droughts, famines, and disease, might become unstoppable.