What causes climate change?
(Image: Sonny Whitelaw – Ambrym)
- 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.
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): C12, C13, and C14, 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).
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 dioxide—a gas that reflects sunlight—in 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 (although not as large 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.
- 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’. Some of these volcanoes include:
- 1257 Samalas (Indonesia)
- 1452 Kuwai (Vanuatu)
- 1589 Billy Mitchel (Papua New Guinea)
- 1600 Huaynaputina (Peru)
- 1783 Laki (Iceland)
- 1814 Mayon (Philippines)
- 1815 Tambora (Indonesia)
Video 5: The eruption of Toba led to intense glaciation.
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.
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 slowing of AMOC (Atlantic oceanic current) due to a sudden influx of freshwater following the ‘Medieval Warm Period’ 950-1250AD. This current is particularly crucial for keeping Europe warm, and slight changes have global impacts, which is of particular concern today.
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
- Carbon Brief: Why scientists think 100% of global warming is due to humans
- NASA: What do volcanoes have to do with climate change?
- PAGES: Past Global Changes; international coordination of past climate change research
- Accuweather: How massive volcano eruptions can alter global temperatures
- Bloomberg: What’s warming the world?
- 2020: Recent paleoclimate publications
- 2020: Stern et al; High mantle seismic P-wave speeds as a signature for gravitational spreading of superplumes, Science Advances 6/22
- 2020: Voosen; No asteroids needed: ancient mass extinction tied to ozone loss, warming climate, Science (editorial article) May 31, 2020
- 2019: Voosen; Did volcanic eruptions help kill off the dinosaurs? Science (editorial article) Feb 21, 2019
- 2017: Gutjahr et al, Very large release of mostly volcanic carbon during the Palaeocene–Eocene Thermal Maximum. Nature 548 (7669)
- 2016: Gernon et al; Snowball Earth ocean chemistry driven by extensive ridge volcanism during Rodinia breakup; Nature Geoscience 9, 242-248
- 2015: Rene et al; State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact: Science 350 (6256) 76-78
- 2013 IPPC: Chapter 8: Anthropogenic and Natural Radiative Forcing in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
- 2007: Storey et al; Paleocene–Eocene Thermal Maximum and the opening of the Northeast Atlantic; Science 316 587-589
- 2004 Pierrehumbert: High levels of atmospheric carbon dioxide necessary for the termination of global glaciation Nature 429, 646–649
- 2004: Svensen et al; Release of methane from a volcanic basin as a mechanism for initial Eocene global warming: Nature 429 542-545