5 Volcanoes

At different points in history, volcanoes have influenced the Earth’s climate by heating or cooling it. Since the onset of industrialization, they have contributed to a cooling effect, just as  Mt. Pinatubo in the Philippines did when it erupted on June 15, 1991. ^[1] Its plume reached a staggering height of 40 kilometers (28 miles). ^[2] The massive eruption temporarily cooled the planet from 1991-1993 by about 1°F (0.5°C). ^[3]

Not every volcanic eruption impacts climate on a global scale. Only significant ones like Mt. Pinatubo – and usually only for a year or two. Whether an eruption creates a temporary cooling spike in the historical record depends on 1) the plume’s height and 2) the composition of ash and gases in the plume that shades incoming solar radiation. When Hawaii’s Kilauea volcano erupted on May 17, 2018, the plume only went up to 30,000 feet, which is roughly 5.6 miles or 9 kilometers. ^[4] So, unlike the 1991 Mt. Pinatubo eruption, it didn’t have the same effect of lowering global temperatures.

How do volcanic gases and ash cool the planet?

For an eruption to create a temporary cooling spike in Earth’s climate record, the plume of ash and volcanic gases must reach the stratosphere – the atmospheric layer that starts at approximately 10 kilometers high, or about 6.2 miles high. For reference, we live, breathe air, and experience weather in the troposphere. The layer above the troposphere is the stratosphere.

Shading occurs when significant amounts of ash (dust particles) and sulfur dioxide gas get ejected into the stratosphere. When sulfur dioxide gas reaches the stratosphere and combines with water, it produces sulfate aerosols. ^[5] In the stratosphere, there are no rain clouds to wash the aerosols and ash out quickly, unlike the troposphere. ^[6] As a result, winds in the stratosphere spread sulfate aerosols and ash around large areas. ^[7] The sulfate aerosols act like “billions of tiny mirrors” that reflect incoming sunlight, reducing the amount of solar radiation reaching the Earth, which results in surface cooling. ^{[8] [9]} In addition, volcanic ash (dust particles) blocks sunlight from reaching Earth, contributing to temporary cooling. ^[10]

Eventually, the ash and sulfate aerosols in the stratosphere fall back to the troposphere through gravitational settling and mixing that occurs at the tropopause (barrier layer between the troposphere and stratosphere). ^[11] When these particles reach the troposphere, they often get washed out by rain within a few days or weeks, making the effect temporary.

Volcanic Eruptions and Heat-Trapping CO₂ Emissions 

Along with ash and sulfur gases, volcanoes can emit large amounts of the heat-trapping greenhouse gas CO₂ into the atmosphere. For instance, when Mt Pinatubo erupted in 1991, it sent around 50 million tons of CO₂ (0.05 gigatons of CO₂) into the atmosphere. ^[12] Given this, you might question if volcanoes could be a pivotal contributor to the climate change we are experiencing. Could it be that the amount of carbon dioxide emitted by volcanoes adds more CO₂ than humans’ use of fossil fuels? The answer is no. The amount of CO₂ emitted from volcanoes, both above ground and underwater, is small compared to human sources of CO₂ emitted into the atmosphere. To understand why, it’s helpful to remember that our day-to-day activities heavily rely on fossil fuels. For example, these fuels power our cars and produce the electricity we use to heat and cool our homes. Every time we drive, flip on a light switch, or adjust the thermostat, we contribute to CO₂ emissions; this means we are continuously pumping CO₂ into the atmosphere. In contrast, volcanoes don’t erupt daily. Their eruptions are sporadic events. So, despite their dramatic displays, CO₂ emissions from volcanoes are significantly smaller than those generated by human activities.

Comparison of CO₂ Emissions: Volcanoes vs. Human Activities 

How small is the contribution of CO₂ from volcanoes? According to the U.S. Geological Survey, published scientific estimates of the global CO₂ emissions from underwater and above-ground volcanoes “lie in a range from 0.13 gigaton to 0.44 gigaton per year.” ^[13] In 2021 alone, humans emitted 36.3 gigatons of CO₂ from energy combustion (burning of fossil fuels) and industrial processes. ^[14] In other words, in 2021, these human activities added to the atmosphere more than 80 to 279 times the amount of CO₂ compared to volcanoes. Another way to think about it is that there would need to be roughly 724 Mt Pinatubo equivalent eruptions in a year to put into the atmosphere the same amount of CO₂ as humans emitted in 2021. This means the increase in temperature and the resulting changes in weather patterns can’t be blamed on the CO₂ emissions from volcanoes. These changes are primarily happening because of the increase in CO2 in the atmosphere, which mostly comes from human activities, not volcanoes.