What even is a global anoxic event? | Australian Greens

What even is a global anoxic event?

If you've heard the term "global anoxic event" but you're not sure what it means, why we need to avoid it, and what that would take, this is for you.

By Peter Everett
Friday, December 15, 2017

Discussions about the impacts of global warming are primarily about the effects of minor climate change, such as an increase in the frequency and intensity of severe weather events and forest fires. This happened during the last 2000 years with the warm period and the little ice age that followed it. Although relatively minor, these changes are thought to have led to the collapse of several marginal societies, including the Norse settlement on Greenland. 

During the last ice age (AKA glacial period), 1000 metre thick ice sheets covered large areas of the northern hemisphere. When the planet warmed, the melting of these glaciers caused sea levels to rise by 100 metres, and drastically changed the climate and geography of the planet (the Sahara became a desert; the UK, Tasmania and Papua New Guinea become islands, etc). Even this is only moderate compared to what will happen if we melt the polar icecaps, thereby bringing an end to the Ice Age conditions that have been with us for 2.6 million years (not long in geological terms – previous Ice Ages have lasted for 30-300 million years).

During an Ice Age (an era when the Earth has permanent ice caps at both poles), the main driver of ocean currents is the sinking of cold water in the north and south Polar Regions (the thermo part of the thermohaline ocean currents). Since gases dissolve better in cold water, these currents have a high oxygen content, and they provide a layer of oxygenated water to the ocean beds. If the sea ice at the poles melt, these ocean currents will fail, thereby cutting off the oxygen supply to the ocean depths, and resulting in a global anoxic event.1

This will lead to the death of oxygen dependent life on the ocean floor. If conditions are right (which they appear to be), this will then lead to the production of poisonous sulphide compounds. Eventually, the sulphide compounds reach levels that are toxic to life at the surface where oxygen still penetrates, and the result is an ocean full of dead fish. Hydrogen sulphide gas is then released into the atmosphere, poisoning terrestrial life forms. When the gas reaches the upper atmosphere it destroys the ozone layer, resulting in an increase in UV radiation reaching the Earth’s surface, which adds to the destruction of land-based life.

Deja vu... I feel like this happened before?

The last time polar ice caps melted was 250 million years ago, and this was instrumental in the Permian-Triassic extinction event (AKA the Great Dying), when around 95% of marine life became extinct. It also led to the extinction of 70% of terrestrial vertebrates (mammals, birds, reptiles and amphibians), the only known mass extinction of insects, and the destruction of all forests.2 Extinction rates remained high for 500,000 years, and it took 10 million years for the Earth’s biodiversity to return to pre-extinction levels.

How do we stop it?

The exact temperature increase that will trigger the melting of the ice caps is not known, but I don’t think it is relevant. What we need to know is the point at which global warming will run away despite our attempts to limit it due to feedback loops. The feedback loop of most concern is the methane trapped in the Arctic permafrost. Permafrost in the Arctic contains an estimated 1,850 gigatonnes of methane, which is already being released at a rapidly increasing rate.3 If 0.7% of this methane escaped each year, it would add the same amount of carbon to the atmosphere as the 35 gigatonnes of CO2 produced by human activity. It would, of course, be methane, not CO2, which would warm the planet 34 times more over the following century.

According to Professor Kevin Anderson, Deputy Director of the Tyndall Centre for Climate Change Research, 4°C would likely be an unstable state, leading to further warming regardless of mitigation efforts.4 If 4°C was our point of no return, we would have plenty of time to change, but unfortunately the point of no return appears to be 2°C. An assessment of the likely effects of global warming based on the 2007 IPCC Report found that 2-3°C of warming would see the melting of the Arctic and Greenland ice caps, and the loss of the Amazon rainforest, which in turn would lead to a further 1-2°C of warming.5 This would get us to Professor Anderson’s 4°C trigger point for runaway warming.

This prediction is supported by a 2005 joint report by the Institute for Public Policy Research in the UK, the Center for American Progress in the US, and The Australia Institute. They found that 2°C of warming would trigger an irreversible chain of climatic disasters. These climatic disasters included:

  • the loss of the West Antarctic and Greenland ice sheets (which could raise the sea level by more than 10 metres);
  • the shutdown of the thermohaline ocean circulation (the mechanism for an anoxic event); and
  • the transformation of the planet’s forests and soils from a net sink of carbon to a net source of carbon.

The report claimed that the point of no return could be reached as early as 2015.6 It seems, then, that we knew 2°C was the point of no return more than a decade ago.

Also in 2005, climate scientists working with Peter Wadhams, Professor of Ocean Physics at Cambridge University, found signs of a slowdown in the ocean currents. One of its driving forces, the sinking of supercooled water in the Greenland Sea, had “weakened to less than a quarter of its former strength.”7 This is disturbing, since it appears just a little warming could seriously deplete the oxygen supply to the ocean depths.

The economics of prevention

The problem of global warming is a two-sided coin, with the other side being the economics. Known carbon reserves are worth about $27 trillion, which is a lot of money for the fossil fuel industry to forgo. Unfortunately, these carbon reserves would produce 2,792 gigatonnes of CO2 if combusted.8 This is 5 times the 565 gigatonnes that we can produce before 2050 for an 80% chance of keeping global warming below 2°C, and yet the industry is still looking for more.9 We also have enough infrastructure in place to produce the 565 gigatonnes of CO2 and we’re continuing to build more.

Financial ties between the industry and world governments are strong, with the industry getting around $775 billion to $1 trillion a year in government subsidies.10 The money flows both ways: for example, the fossil fuel industry gave $73 million to political parties to fund the 2012 US federal election, and then spent a further $145 million on lobbying the US government during 2013.11

Campaign donations from multinationals are often seen as essential for a political party to win an election, but with fossil fuels it is bad business. According to the Stern Review on the Economics of Climate Change, spending 2% of global Gross Domestic Product on mitigation would prevent a loss of 20% of GDP, a tenfold return on the investment.12 Mitigation efforts invariably mean using less fossil fuels though, so the industry does whatever it can to minimize this. Personally, I don’t think 2% of GDP is enough, and we should be looking at 10-20%. Getting people to accept this will be problematic to say the least, but it’s not impossible, and I think there is a way to motivate people to do it.

The estimation that we can produce 565 gigatonnes of CO2 is not supported by Palaeoclimatic data. Climate modelling generally assumes planetary warming to be about 0.75°C per W/m2 of radiative forcing. Palaeoclimatic data, which, by its nature, takes into account the effect of feedback loops, suggests that the figure is more like 2°C per W/m2, which means that our current CO2 targets will lead to a temperature increase of about 5°C rather than 2°C.13


  1. Jeppsson, L. ‘An oceanic model for lithological and faunal changes tested on the Silurian record’ Journal of the Geological Society, 147 (4) 1990, p. 663–674.
  2. Ward, P. D. ‘Impact from the Deep’ Scientific American, October 2006, p. 64-71.
  3. Shakhova, N. Semiletov, I. Salyuk, A. Kosmach, D. ‘Anomalies of Methane in the Atmosphere over the East Siberian Shelf’ EGU, General Assembly 2008, Geophysical research abstracts, 10EGU2008-A-01526; Walter, K. M. Zimov, S. A. Chanton, J. P. Verbyla, D. Chapin F. S. ‘Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming’ Nature 443 (7107) September 7, 2009, p. 71-75; Shakhova et al. ‘Vast East Siberian Arctic Shelf methane stores destabilizing and venting’ Science, February 26, 2010. Shakhova, N. et al, ‘Methane release on the Arctic East Siberian shelf’ Geophysical Research Abstracts, 9:01071, 2007.
  4. Anderson, K., Bows, A. ‘Beyond Dangerous Climate Change; Emission Scenarios for a New World’ Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2011, 369:1934, p. 20-44.
  5. Lynas, M. ‘Six Degrees: Our Future in a Hotter Planet’ HarperCollins, London, 2007.
  6. Byers, S. Snowe, O. J. ‘Meeting the Climate Challenge: Recommendations of the International Climate Change Taskforce’ International Climate Change Taskforce, London, January 2005.
  7. Leake, J. ‘Britain Faces Big Chill as Ocean Current Slows’ Sunday Times, May 8, 2005.
  8. Leaton, J. ‘Unburnable Carbon: Are the World’s Financial Markets Carrying a Carbon Bubble?’ Carbon Tracker Initiative, 2011, p. 6-7.
  9. Meinshausen, M. et al ‘Greenhouse Gas Emission Targets for Limiting Global Warming to 2°C’ Nature 458:1161, 2009.
  10. Bast, E. et al ‘Low Hanging Fruit: Fossil Fuel Subsidies, Climate Finance, and Sustainable Development’ Oil Change International for the Heinrich Boll Stiftung North America, June 12, p. 16.
  11. ELECTION SPENDING: ‘Oil and Gas: Long-Term Contribution Trends’ The Centre for Responsive Politics, opensecrets.org
  12. Stern, N. ‘The Economics of Climate Change: The Stern Review’ Cambridge, Cambridge University Press, 2006.
  13. Hansen, J. et al. ‘Target Atmospheric CO2: Where Should Humanity Aim?’ Open Atmos. Sci. J., No 2, 2008, p. 217-231; Hansen, J. et al. Open Atmosphere; Science, vol. 2, 2008, p. 217; Royer, D. L. Geochim. Cosmochim. Acta 70, 5665 (2006); North, G. R. Journal of Atmospheric Science vol. 32, 1975, p. 2033; Schwartz, S. E. Climate Change; 10.1007/s10584-010-9903, 2010; Bijl P. K. et al. Science; vol. 330, 2010, p. 819.

Peter Everett is a Lismore Greens member.