Causes: Volcanoes
Ambrym Island, Vanuatu image: Sonny Whitelaw
Summary
Volcanoes are a natural climate forcing. Over the last few hundred years, erupting volcanoes have caused global temperatures to cool for short periods (Figs. 1 & 2). This is because they erupt sulphur dioxide (SO₂), which reflects sunlight. Some eruptions have slightly cooled the atmosphere for up to 7 years.
Volcanoes do emit CO2 when they erupt, but they do so by melting rocks underground that contain carbon. Melting does not use oxygen. Burning, on the other hand, requires oxygen to combust fossil fuels.
Moreover, carbon comes in three forms (isotopes, i.e. the number of neutrons in each atom): 12C, 13C, and 14C. The chemical and isotopic signature of CO2 emissions from volcanoes is entirely different to that of emissions from burning fossil fuels and through wildfires. (Video 1).
Taken together, the changing chemistry of the atmosphere unequivocally points to humans, not volcanoes adding the staggering quantities of CO2 into the atmosphere while oxygen levels are simultaneously falling (Fig. 3).
What’s covered on this page:
Hunga Tonga-Hunga Ha’apai volcano
The eruption of Tonga’s Hunga Tonga-Hunga Ha’apai volcano in 2022 damaged the ozone layer and sent huge quantities of water vapour into the atmosphere. However the net overall effect was a short-term cooling of the atmosphere, not warming.
It is estimated that the Hunga eruption cooled the global average surface air temperature (SAT) by 0.05 K, but this reduction is so subtle as to be indistinguishable from the natural variability of the current climate. – Stenchikov & Schoeberl 2025
The ‘Little Ice Age’
This is generally regarded as the period 1300-1850 AD when average annual temperatures across parts of Europe and the UK declined by 1-2°C compared to the overall average temperature 1000-2000 AD. However, many part of the Northern Hemisphere such as China were hardly affected. Glaciers in the Southern Hemisphere, including Aotearoa and Patagonia advanced, but not at the same time as intense cold periods in parts of Europe. And, like China, the Andes experienced no changes to its climate.
Together this points to regional climate forcings that influenced some regional weather patterns; it wasn’t a global event. The onset may have been triggered by the 1257 eruption of Samalas volcano. Subsequent large volcanic eruptions may have contributed:
- 1452 Kuwai (Vanuatu)
- 1589 Billy Mitchel (Papua New Guinea)
- 1600 Huaynaputina (Peru)
- 1783 Laki (Iceland)
- 1814 Mayon (Philippines)
- 1815 Tambora (Indonesia)
- 1831 Zavaritskii (Kuril islands/Russia)
The year after Tambora erupted, 1816, was known in Europe as the ‘Year without summer’. Temperatures dropped on average 0.53°C in the Northern Hemisphere. But again there was considerable regional variations and the effect on the Southern Hemisphere wasn’t as noticeable.
The role of ocean currents
Prior to the so-called ‘Little Ice’ was the ‘Medieval Warm Period’ 950-1250AD. This warming resulted a sudden influx of freshwater flowing into the Atlantic Ocean, which caused the AMOC (Atlantic Meridional Oceanic Current) that normally keeps Europe far warmer than it should be given its high latitude, to slow down. This demonstrates how slight changes have global impacts, which is why it’s a concern today, as the AMOC is once again slowing down.
The sun
A very small contribution to the cooling may have been from a slight decline in solar radiation. However, the Maunder Minimum, the period when the sun’s output declined slightly, didn’t begin until 1645AD three hundred years after cooling began in the Northern Hemisphere.
Geological ‘Ice Ages’
How ancient volcanoes changed Earth’s climate over billions of years
Some (but certainly not all) changed or helped change the climate:
- 4.6 billion years ago volcanoes erupted the greenhouse gases methane and carbon dioxide. Along with other gases (but no oxygen yet) this created an atmosphere, which stopped Earth from completely freezing over for the next few billion years or so.
- 2.4 billion years ago ‘Snowball Earth’ events. There were several time when ice, or at least slushy ice and snow covered most if not all of the planet some. These ended when volcanoes erupted, increasing the amount of greenhouse gases in the atmosphere (Video 2). The breakup of the supercontinent Rodinia also played an important role.
- 252 million years ago the Permian-Triassic ‘Great Dying‘ extinction event was caused by a flood basalt eruption the size of Europe that spewed lava over a vast area of Siberia in several waves for ~350,000 years (Video 3). Vast areas of carbon-rich rocks were melted and carbon dioxide poured into the atmosphere, resulting in rapid and catastrophic global warming. The loss of the ozone layer also 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.
- ~120 million years ago volcanic rocks beneath New Zealand’s North Island, the ocean floor near American Samoa, Tokelau and the Solomon Islands, were once joined together. They may have originated from an outpouring of volcanic rock ~2,000km across (see also Video 3 @13 mins).
- ~66 million years ago: the Cretaceous–Paleogene (K–Pg) extinction event. 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 dioxide—a gas that reflects sunlight—in an area today called the Deccan Traps, cooled the planet around 2°C. The first eruptions happened shortly before two asteroid(s?) that struck Earth and may have contributed to, possibly even doomed the dinosaurs to extinction before the impacts arrived (Video 4)
An alternative earlier theory is that the comet/asteroid may have triggered more long term eruptions from the Deccan Traps and also along the edges of plate boundaries under the ocean (Video 5).
- ~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 gases erupt from chasms hundreds of kilometres long, melting carbon-rich rocks, something that still happens in Iceland today (Fig. 4). So much CO2 entered the atmosphere that global temperatures rose ~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 than the extinction event that wiped out the dinosaurs.
- 75,000 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 6).
