The Thermohaline Cycle

The Thermohaline circulation is a global current that is the major driver of mixing in our oceans. It is often referred to as the ocean conveyor system.

Winds drive ocean currents in the upper 100 meters of the ocean’s surface but ocean currents also flow thousands of meters below the surface. The deep-ocean currents are driven by differences in the water’s density, which is controlled by 1) temperature (thermo) and 2) salinity (haline) hence the name  thermohaline circulation. This cycle is initiated at the earth’s polar regions in the following sequence:

  1. In the Earth’s polar regions ocean water gets very cold, forming sea ice
  2. When sea ice forms, the salt is left behind
  3. As a consequence the surrounding seawater gets saltier
  4. The saltier seawater has increased density which causes it to sink
  5. Surface water is pulled in to replace the sinking water
  6. The new seawater in turn eventually becomes cold and salty enough to sink

Man-made global warming is projected to have the following impact:

  1. There is a high population density in the high Northern latttitudes of Europe due to the warm moderating effect of the thermohaline conveyor warming the waters off Europe
  2. Increases rainfall in the North Atlantic causes more warm water and decreased salinity in the North Atlantic
  3. Greenland glacial melt will also decrease salinity
  4. This will cause an increase in the influx of warm freshwater onto the sea surface which could block the formation of sea ice
  5. Melting of glaciers and sea ice causes warming of polar regions and decreased salinity and water density which will disrupt the sinking of cold, salty water
  6. This sequence of events could slow or even stop the conveyor belt which could result in potentially drastic temperature reduction in Europe

Figure 1: Thermohaline Cycle (SourceIPSO)


May 2012 CSIRO study shows 60% reduction in cold dense water in Thermohaline conveyor off Antarctica

The amount of dense Antarctic Bottom Water has contracted each time we’ve measured it since the 1970s. There is now only about 40 per cent as much dense water present as observed in 1970.It’s a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions. The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important

- Steve Rintoul, of CSIRO and the Antarctic Climate and Ecosystems CRC

In a landmark study called  the Australian Antarctic program’s 2012 Southern Ocean Marine Science Voyage, 50 scientists on the Australian Antarctic Division’s research and resupply vessel Aurora Australis collected temperature and salinity samples from  77 sites between Antarctica and Fremantle.

This data was once dependent on ships traveling this route but holes in the data resulted due to reliance on ships not sailing during the winter months. The use of Argo floats which can collect data all year round have revolutionized the ability to measure the ocean year round.  When scientists compared the data retrieved from the Argo floats to historical data dating back to 1970, they found as much as a 60 per cent reduction in the volume of Antarctic Bottom Water, the cold dense water that drives global ocean currents. This suggests the densest waters in the world ocean are gradually disappearing and being replaced by less dense waters.

The ocean profiles also show that the dense water formed around Antarctica has become less saline since 1970. The Australian Antarctic Division’s Chief Scientist, Dr Nick Gales, corroborated Rintoul’s statement by saying that the findings of the oceanographic study are profoundly important.

Study finds hidden source of Antarctica Bottom Current

Antarctic bottom water current found by use of seals outfitted with sensor

Peter Ward’s Medea Hypothesis: an Alternative Theory for the Sixth Mass Extinction  caused by Slowing of the Thermohaline Circulation to Zero

Paleontologist Peter Ward has proposed the Medea Hypothesis and argues that we are headed for a global mass extinction event far sooner than anyone ever imagined – as early as a century away.

Ward’s book, The Medea Hypothesis is a critical response to James Lovelock’s Gaia concept, which contends that homeostatic physical and chemical interactions work to maintain Earth’s habitability. Ward argues that the opposite is in fact true–living organisms decrease Earth’s habitability, hastening its end by perhaps a billion years. The mechanism for all this to happen? – Aneorobic microbes and H2S. 

Paleontologist Peter Ward talks about the Medea Hypothesis on Radio Ecoshock

Peter Ward on TED talking about the Medea Hypothesis

Ward’s Medea Hypothesis posits that as the temperature differential between the Poles and the Equator equalize due to global warming, the thermohaline currents will grind to a halt,  causing H2S to be produced instead of oxygen.

Using the latest discoveries from the geological record, Ward argues that life might just be its own worst enemy. This is in stark contrast to James Lovelock’s Gaia hypothesis–the idea that life sustains habitable conditions on Earth. In fact, Ward entitled his book The Medea Hypothesis in response to Lovelock’s “Gaia” hypothesis. Ward chose to name his theory after Medea,  the mythical wife of Jason the Argonaut, who swiped the Golden Fleece. In a fit of rage against her husband, Medea killed her own children. According to the Medea hypothesis, life evolves to become its own worse enemy.  Using his aneorobic microbe / H2S theory, Ward demonstrates that all but one of the mass extinctions that have struck Earth were caused by life itself, not by meteors or volcanos.

Ward interprets the alarming decline of diversity and biomass as one that is naturally brought about by  life’s own “bio-suicidal” tendencies. The Medea hypothesis applies not just to our planet, but extends to all potential life in the universe. While life on Earth doesn’t have to become extinct, Ward warns that our time window to prevent extinction is quickly running out.

Here is how Ward explains the previous  mass extinction events:

  1. About  2.4 billion years ago, microscopic cyanobacteria emerged newly equipped with photosynthesis and the massive amounts of oxygen produced as the product of photosynthesis triggered the Great Oxygenation Event
  2.  Subsequently, aerobic organisms thrived and anaerobic organisms, the dominant life form until then began dying out in massive numbers
  3. The sudden release of oxygen is also likely what set off the Huronian Glaciation, a deadly “snowball earth” that kept the planet locked in ice for 300 million years
  4. As atmospheric carbon rose above 1,000 parts per million (ppm), a super-charged greenhouse effect dramatically weakens the temperature differential between the poles and the tropics
  5. Without pronounced temperature gradients to drive ocean mixing, only the top layer of the sea remains oxygenated
  6. Anaerobic bacteria populations began to explode because they thrive below this zone, producing massive quantities of hydrogen sulfide gas to poison the entire planet
  7. This poisoning may be to blame for the End-Permian event, “the mother of all extinctions“, when 96% of all marine organisms disappeared

 The Sixth Mass Extinction

Now it is simple matter to explain how humans may be the trigger for the Sixth Mass Extinction. Humans are also an element of the earth’s self-destructive tendency. If our Co2 emissions go above the tipping point of 1,000 ppm, as predicted by some upper-end IPCC estimates for 2100, we may trip an event identical to the one that wiped out the trilobites. Half the oxygen in the planet is produced by the ocean. By going above 1,000 ppm’s, we may reduce the temperature gradient between the Poles and the Equator. While temperatures at the equator cannot increase any further, at the poles they certainly can and in fact, are the regions currently experiencing the highest increase in temperature on the planet. As the average temperature at the poles increase, this will have the effect of slowing down the Ocean’s current. When the current reaches zero, no more oxygen will be produced and aneorobic microbes will create massive quantities of H2S which will kill all aerobic lifeforms, including us, of course.

Ironically, while CO2 may be the threat which can cause mass extinction in the short term, in the long run, it’s a carbon shortage that may spell the end of life, long before the sun vaporizes the oceans. It is predicted that in 500 million years, life’s insatiable need for carbon – the basic building block of every organism – could mean that atmospheric carbon might eventually drop below 10 ppm, the amount needed to sustain grasses. This is because as organisms die, they deposit their carbon rich bodies in layers and are covered over, making that carbon inaccessible to the atmosphere. Hence, concentrations of CO2 will continually diminish until it reaches a lethal 10 ppm. After grasses have all died out, then all animal life forms will soon follow. In terms of total biomass, evidence indicates that earth is already in its “old age”.

Life bubbled up from a happy coincidence of molecules. According to Ward, it will stick around for about 4 billion years, and extinguish itself after that. Hence, the universe’s great experiment with life will be finished for another cycle.


The Medea Hypothesis has been presented in the form of a popular book instead of a peer reviewed paper. There does not appear to be much research dedicated to either supporting or refuting the theory.

Dr. Andrew Glikson’s critique

Dr. Andrew Glikson, a visiting fellow of the Australia National University critiqued it in The Journal of Cosmology ,Oct 18,  2009, Vol 2, pages 230-234. in an article entitled: Mass Extinction of Species: The Role of External Forcing, Comment on “The Cronus hypothesis: Extinction as a necessary and dynamic balance to evolutionary diversification.”


he terrestrial biosphere, constituting a less than 20 km-thick zone constrained by the crustal depth at which bacteria occur and the atmospheric level to which birds can fly, has been repeatedly affected by external forcing, including deep Earth-derived volcanic events, asteroid and comet impacts, solar insolation and orbital forcing cycles, and likely supernovae. Intrinsic biological evolution and diversification through natural selection and adaptation has been repeatedly overprinted, and in some instances almost obliterated, by these events. Both, the ‘Medea hypothesis’ and the ‘Cronus hypothesis’, hinging on metaphors focused on biological self-destruction, appear to underestimate the role of externally forced destruction. The search for unifying principles and for a ‘dynamic balance’ in biological evolution must not overlook the unique origin and consequences of each of the mass extinction events.

Glikson believes that both the Medea Hypothesis and the Cronus Hypothesis (both propose a dominant role of biological self-destruction in mass extinction events) underestimate the role of externally forced destruction. The conclusions of his comment are:

1. Inherent in metaphors such as ‘Medea hypothesis’ or ‘Cronus hypothesis’ is a focus on intra-biosphere self-destructive processes, which overlook the major to critical role of extra-biosphere forcing events, including asteroid/comet impacts and volcanic eruptions.

2. Attempts at arriving at common intrinsic principles with regard to the respective roles of gradual evolutionary processes vis-a-vis mass extinction events must not overlook the unique nature and consequences of each extra-biosphere forcing event and related mass extinction.

3. The anthropogenic “Sixth mass extinction” appears to be unique in terrestrial history. The question remains subject to philosophical notions, for example in terms of the vulnerability of extreme complexity in nature to self-destruction and the “price” in terms of information entropy of the achievement of deep insights into nature. Homo sapiens may never know the answer to the deepest questions.

Dragon’s Tale (DT) Critique

The Dragon’s Tale website also contains a detailed critique of Ward’s hypothesis. The website author is a computer modeller working with scientists on a variety of computer models so is in an insider position to critique the finer points of Ward’s hypothesis. He breaks his critique into 5 parts:


Misuse of Models – DT’s Summary
  • Ward relies on Franck et al’s model for carbon cycling over multiple billions of years
  • Franck’s model is very simple: the model itself describes how carbon is cycled and sequestered in the earth system
  • Franck’s model takes into account continental growth, some biological activity, and even weathering of kerogen, Biomass is represented as a function of carbon dioxide and temperature
  • Franck’s model is a nice, simple, useful tool, but it has limitations:
  • Some examples of limitations of Franck’s model:
    • it cannot take into account the Snowball Earth episodes; nowhere can the model fit these important planet wide episodes in their model’s data
    • it cannot account for the temperature fluctuations of the Pharenozoic much at all: the ice ages that have come and gone are not represented almost at all
    • The temperature spikes (during Permian, Eocene) cannot be included
    • The increase in vulcanism attributed to supercontinental fragmentation is not included
    • Mass extinction,  important to biodiversity and biomass cannot be included or represented in this data
  • Franck et al. acknowledge these limitations.  Unfortunately, Ward uses this simple model and extrapolates results that are beyond its capability. DT claims that the model is far too simple and Ward, being familiar with the observational/paleontological data should know better.
  • When a model cannot take into account some very fundamental and important events as they pertain to the question of what you are trying to answer, it should not be used.
  • When a model contradicts key observational data, it should not be used.
Misleading Statements

DT raises some serious issues with at least 3 major claims made by Ward:

The Permian Extinction: The Great Dying

The Permian Extinction is considered the greatest of all mass extinctions. In the marine realm, potentially as much as 95% of everything died. and on land,  as high as 75% species may have died. This extinction event is very well researched and the mechanisms and causes are fairly well understood.

In DT’s opinion, Ward distorts this to fit the Medea Hypothesis. He claims the main kill mechanism of the mass extinction was  the excretions of anaerobic bacteria, hydrogen sulfide (H2S), which is highly toxic to most aerobic life.

DT’s claim is that this is highly distorted because there were many other equally likely kill mechanisms proven to coexist at that time:

  • The Siberian Traps were erupting; the single biggest eruption in the whole of the past 650 million years and possibly as much as the past billion years
  • Because of these volcanic eruptions, the world radically warmed: well over 10 C in very short time periods
  • The warming caused the oceans to turn very anoxic and subsequently, the methane calthrates erupted from the oceans
  • The ozone layer was destroyed: unblocked UV scorched the land and even the surface of the sea
  • The vast majority of land desertified
  • Hypersaline lakes released lethal halogen gases
  • The atmospheric oxygen levels crashed
  • Precipitation was probably extremely seasonal and possibly even a megamonsoon model
  • Anaerobic bacteria released hydrogen sulfide

DT points out that Ward’s bacterial menace was but one of many killing mechanisms. While anaerobic bacteria may have played an important role, it was the vast eruptions that took place in Siberia which was the main driver. DT feels Ward has greatly misled the reader by distorting the importance of the anaerobic bacteria above all the other killing mechanisms.

The Snowball Earths

The Snowball Earth episodes are time periods in Deep Time where the world is suspected to have virtually iced over. There is very sketchy data available on this period, certainly not enough to substantiate Wards claim that life caused them. Ward asserts that life, bacteria and more advanced photosynthetic life drew down the carbon dioxide so much that it caused the earth to lose its greenhouse effect and ice over.

The Pleistocene Glaciations

Ward again asserts that life caused the carbon dioxide to drop sufficiently for glaciations to take place. He presents no evidence to backup his claim and in fact,  the  CO2 data was actually higher than it is now. The actual data does not match his constant downward trending plots.