Volcanoes emit more CO2 than humans
Global volcanic CO2 emissions average roughly 0.15 to 0.26 billion metric tons per year, while human fossil fuel burning and cement production emit about 37 billion metric tons per year, making human output more than a hundred times larger.
What we know
The claim that volcanoes emit more carbon dioxide than human activity has circulated for decades, often used to argue that human contributions to atmospheric CO2 are trivial by comparison. Direct measurement of volcanic gas plumes, combined with submarine vent surveys and geochemical modeling, allows scientists to estimate global volcanic CO2 output with reasonable confidence. The United States Geological Survey and independent peer-reviewed work place total annual volcanic CO2 emissions, from both subaerial volcanoes and mid-ocean ridges, at between 0.15 and 0.44 billion metric tons, with a commonly cited central estimate near 0.26 billion metric tons per year.
Human emissions dwarf this figure. Global anthropogenic CO2 emissions from fossil fuel combustion, cement manufacturing, and related industrial processes reached approximately 36.8 billion metric tons in 2023, according to the Global Carbon Project. That places the ratio of human to volcanic emissions at roughly 135 to 1, a comparison made explicit by USGS geochemist Terry Gerlach in a widely cited 2011 paper published in EOS, the newsletter of the American Geophysical Union. Gerlach calculated that human activities generate the equivalent of one major volcanic eruption's worth of CO2 roughly every two and a half hours.
The misconception often stems from conflating the visible, dramatic nature of volcanic eruptions with their actual climate impact. A single eruption can release large amounts of ash and sulfur dioxide in a short period, and the sight of a smoking crater feels more consequential than an invisible gas seeping from a tailpipe. In reality, most volcanic CO2 escapes slowly through passive degassing rather than eruptions, and even the largest eruptions in recorded history, such as Mount Pinatubo in 1991, released only a small fraction of a single year's worth of human CO2 emissions. Pinatubo's dominant climate effect was actually cooling, because its sulfate aerosols reflected incoming sunlight, temporarily lowering global average temperatures by about half a degree Celsius for roughly two years.
Scientists also confirm the human origin of the recent CO2 rise through isotopic analysis. Fossil fuels contain carbon with a distinct isotopic signature depleted in carbon-13 relative to volcanic or atmospheric carbon, and the measured decline in atmospheric carbon-13 ratios since the Industrial Revolution matches the fossil fuel signature rather than a volcanic one. This isotopic fingerprint, along with consistent measurements from monitoring stations like Mauna Loa Observatory, leaves little room for volcanic sources to explain the observed increase from about 280 parts per million before industrialization to over 420 parts per million today.
Submarine volcanic activity along mid-ocean ridges, which is harder to observe directly than land-based eruptions, is included in the USGS and academic estimates through indirect methods such as measuring dissolved carbon in seawater near known vent systems, and even accounting for the uncertainty in these harder to measure sources, the upper bound of plausible volcanic CO2 output remains far below the well constrained figures for fossil fuel combustion tracked through international energy statistics.
The volcano comparison persists partly because it offers an intuitive, easily repeated counterargument to climate science, and partly because few people have reason to look up actual emissions inventories. Once the numbers are placed side by side, the disparity is large enough that even generous upper-bound estimates of volcanic output remain a small fraction of routine annual human emissions.
Common claims
- Volcanic eruptions release more CO2 than humansFalse - humans emit at least 135 times more CO2 annually
- We cannot measure volcanic CO2 accuratelyFalse - direct measurement and geochemistry give consistent estimates
- Major eruptions release huge amounts of CO2Partially true but small - a major eruption adds a fraction of one year's human output
Evidence hierarchy
All sources
- Which emits more carbon dioxide: volcanoes or human activities?NOAA Climate.gov · 2016
- Volcanic versus anthropogenic carbon dioxideUSGS / EOS Transactions AGU · 2011
- Volcano Watch: Which produces more CO2, volcanic or human activity?USGS Hawaiian Volcano Observatory · 2007
- Global Carbon BudgetGlobal Carbon Project · 2023