How Volcanoes Change the Climate
April 6, 2026
A single large eruption can measurably cool the entire planet for a year or two. Over millions of years, volcanic gases have driven Earth’s climate to extremes in both directions. But on the timescale that matters for the present, one common claim is simply wrong: volcanoes are not a significant source of the carbon dioxide now warming the world.
The short-term effect: cooling
When an explosive eruption is powerful enough to punch sulfur dioxide into the stratosphere, that gas turns into a haze of sulfuric-acid droplets that spreads worldwide and stays aloft for one to three years. The haze reflects a small fraction of sunlight back to space, cooling the surface.
The two clearest modern examples are Pinatubo (1991), which lowered global average temperature by about 0.5 °C for a year, and Tambora (1815), whose roughly 0.5 °C of cooling produced the “year without a summer” in 1816. Ash is not the cause — it falls out within days. Sulfur is.
What it takes
Two things have to line up. The eruption must be explosive enough to loft gas above about 15 km — roughly VEI 5 and up — and the magma must be sulfur-rich. Some large eruptions have little climate effect because their magma is sulfur-poor; some more modest ones punch above their weight. A tropical location helps, because the resulting haze spreads into both hemispheres.
Effusive eruptions can do it too, without exploding, if they last long enough: the 1783–84 Laki fissure eruption in Iceland released so much sulfur over eight months that it cooled and disrupted Northern Hemisphere weather for the next year or two.
On the map, VEI 5 and 6 eruptions are the bright red and magenta circles. There are only a handful per century — and only some of those had a measurable climate effect.
The CO₂ question
Volcanoes do emit carbon dioxide. But the total from all of the world’s volcanoes, on land and under the sea, is estimated at roughly 0.3 to 0.4 billion tonnes of CO₂ per year. Human activity emits around 35 to 40 billion tonnes per year — about a hundred times as much. Even a large eruption like Pinatubo releases, in CO₂ terms, only a few days’ worth of human emissions, and its net effect is cooling, not warming.
Over tens of millions of years, sustained volcanic CO₂ from events like the flood-basalt eruptions has been a major climate driver, and the slow drawdown of that CO₂ by rock weathering is part of what keeps Earth habitable. On the timescale of a human life, though, the volcanic contribution to today’s warming is negligible.
Sources
- Robock, A. (2000). “Volcanic eruptions and climate.” Reviews of Geophysics 38(2), 191–219.
- U.S. Geological Survey — “Volcanic gases and climate change overview”; “Which produces more CO₂, volcanic or human activity?”
- Gerlach, T. (2011). “Volcanic versus anthropogenic carbon dioxide.” Eos 92(24), 201–202.
- Global Volcanism Program, Smithsonian Institution — Pinatubo; Laki (Grímsvötn).