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Causes: volcanoes

Causes: volcanoes

(Image: Sonny Whitelaw – Ambrym, Vanuatu)



  • Volcanoes are a natural climate forcing. In the last few hundred years, erupting volcanoes have caused global temperatures to drop slightly for short periods (Fig 1.).
  • This is because they erupted sulphur dioxide (SO₂), which reflects sunlight, cooling the atmosphere for up to 7 years.
  • Volcanoes also release carbon dioxide (CO2) into the atmosphere but in the last 200 years it was less than 2% of that from human activities. The evidence is the isotopic signature of CO2 from volcanoes, which differs to that of CO2 from burning fossils fuels.
  • Ancient mega-volcanoes and flood basalts including the biggest ever “volcanic outpouring” in the history of the planet right under New Zealand, caused catastrophic climate changes implicated in all five of Earth’s mass extinction events.
  • Volcanoes have also been implicated in short-lived climate change events, including the so-called Little Ice Age in Europe.
Fig. 1: The impact of volcanoes on climate from1880 to 2005. The grey line shows rise in global temperatures during that period. Compare it with the orange line (volcanoes) and it’s clear that while the cooling effect of volcanic eruptions during this period temporarily offset some of the warming, it wasn’t enough to offset the rise in temperatures (Image: composite of graphs from Carbon Brief).

But don’t volcanoes also release CO2?

Volcanoes make CO2 by melting rocks underground, not by burning fossil fuels (combustion), which requires oxygen. Because carbon comes in three forms (isotopes): 12C, 13C, and 14C, and the levels of oxygen in the atmosphere are decreasing (Fig. 2), the chemical and isotopic signature in the atmosphere unequivocally points to humans, not volcanoes (Video 1).

Fig. 2: Decline in oxygen.

Video 1: Professor Richard Alley explains how the isotopic signature of CO2 and also the declining O2 in the atmosphere clearly points to humans burning fossils fuels.

Volcanic activity has been a significant climate forcing in the past

Some (but certainly not all) key volcanic eruptions that changed or helped change the climate:

  • When Earth formed 4.6 billion years ago, volcanoes erupted greenhouse gasses methane and carbon dioxide. Along with other gasses (but no oxygen yet) this created an atmosphere, which stopped Earth from completely freezing over.
  •  2.4 billion years ago ‘Snowball Earth’ events—when ice covered most if not all of the planet some—ended when volcanoes erupted, increasing the amount of greenhouse gasses in the atmosphere (Video 2).

Video 2: How volcanoes took Earth from a ‘Snowball’ state into a warm state.

  • The ‘Great Dying’: the Permian-Triassic extinction event. Around 252 million years ago, a flood basalt eruption the size of Europe spewed lava over a vast area of Siberia in several waves for some 350,000 years (Video 3). Vast areas of carbon-rich rocks were melted and carbon dioxide (CO2) poured into the atmosphere, resulting in rapid and catastrophic global warming, plus the loss of the ozone layer that led to intense DNA-stripping UV radiation. The video explains how flood basalt eruptions have led to several mass extinction events over hundreds of millions of years, and the role played by methane clathrates.
  • 201.3 million years ago, a similar flood basalt eruption over ~11 million km2 at the Central Atlantic Magmatic Province in North America, is implicated in the Triassic-Jurastic Extinction Event.

Video 3: The role of flood basalt eruptions in climate change

  • Around 120 million years ago, volcanic rock found beneath New Zealand’s North Island, and beneath the ocean floor near American Samoa, Tokelau and the Solomon Islands, were once joined together and may have originated from an outpouring of volcanic rock covering some 2000km across.(Video 3 @13 mins).
  • Some 66 million years ago, India was an island heading north towards Eurasia. As it travelled over a hotspot in the planet’s crust, a 20,000-year-long eruption of flood basalt and sulphur dioxidea gas that reflects sunlightin an area called the Deccan Traps, cooled the planet around 2°C. The first eruptions happened shortly before the comet that struck Earth and may have contributed to the extinction of the dinosaurs: the Cretaceous–Paleogene (K–Pg) extinction event. The comet may also have triggered more long term eruptions (Video 4) both from the Deccan Traps and also along the edges of plate boundaries under the ocean.

Video 4: The comet blamed for killing the dinosaurs may have triggered more flood basalt eruptions that could may in fact have caused them to go extinct

  • Ten million years later, some 55 million years ago, the boundary between the North American and European tectonic plates opened, leading to large scale eruptions that lasted over 20,0000 years. When tectonic plates separate, lava and gasses erupt from chasms that can be hundreds of kilometres long, something that still happens in Iceland today (Fig. 3). So much CO2 entered the atmosphere that global temperatures rose some 6°C within 20,000 years—geologically a very short time. This period, the Palaeocene/Eocene Thermal Maximum (PETM), was the largest mass extinction event in the ocean in the past 93 million years—worse that the extinction event that wiped out the dinosaurs.
Fig. 3: (Photo: Reykjavik Helicopters)
  • Seventy-five thousand years ago the super-volcano Toba in Indonesia erupted. Earth was already in the current Ice Age. The eruption caused a ‘volcanic winter’ that lasted 6-10 years, followed by 1,000 years of colder weather and more intense glaciation (Video 5).
  • A series of volcanic eruptions from the 11th to 19th centuries were thought to have been a contributing climate forcing, but not solely responsible for the so-called ‘Little Ice Age’. These volcanoes include:

Video 5: The eruption of Toba led to intense glaciation.


Climate Forcing:

The term ‘climate forcing’ comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy reaching Earth’s atmosphere and the amount that escapes. If more solar energy escapes than arrives, the planet cools (negative RF). Conversely, if less energy escapes than gets in, the planet warms (positive RF). This is due to the The Law of Conservation of Energy, a basic law of thermodynamics, which states that: ‘Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.’

Different climate forcings each determine how much solar energy arrives and escapes.

  • Natural Forcings are those that happen through natural changes.
  • Anthropogenic Forcings are those due to human activities.

Click here to learn about the main forcings and how they work (links to page on this site).

Mass Extinction Events:

A sixth mass extinction is currently underway. The Anthropocene is a proposed geological epoch dating from the commencement of significant human (‘anthropogenic’) impact on Earth’s geology and ecosystems, including climate change.

The Little Ice Age:

This is generally regarded as the period 1300 – 1850AD. Several forcings appeared to have contributed, including the 1257 eruption of Samalas volcano.  The AMOC (Atlantic oceanic current) also seems to have slowed due to a sudden influx of freshwater following the ‘Medieval Warm Period’ 950-1250AD. This ocean current is crucial for keeping Europe warm. Slight changes have global impacts, which is why it’s a concern today, as the current is slowing down.

A very small contribution may have been from a slight decline solar radiation. However, the Maunder Minimum didn’t begin until 1645AD (Fig. 4; see Sunspots and Solar activity), three hundred years after cooling began in the Northern Hemisphere.


The year after Tambora erupted, 1816, was known in Europe as the ‘Year without summer’. Temperatures dropped 0.53°C in the Northern Hemisphere. However it wasn’t a global ‘climate change’ event; the effect on the Southern Hemisphere wasn’t as noticeable.

References and further reading