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Climate Change and the Nature of Science: The Carbon “Tipping Point” is Coming

Posted on 12 July 2013 by Micha Tomkiewicz

A guest post by Professor Micha Tomkiewicz, reposted with permission from Climate Change Fork.

The two attached pictures are schematic diagrams of the circulation of carbon on Earth (IPAA Report (2001) – the Carbon Cycle).  If I Google “Carbon Cycle Diagram” in the image mode, I get close to a million entries.  Most of these entries look like the second image – not the first.  What is the difference?  The second one doesn’t have numbers (photoshopping on my part).

The numbers in the arrows of the first image represent fluxes of carbon per year in units of billion tons of carbon.  The numbers outside the arrows represent quantities in the same units of billion tons of carbon. The man-made (anthropogenic) contributions are shown by the dashed red arrows.

Scientifically, it is very difficult to argue with the second diagram.  I have to make qualitative statements like, “I don’t believe that carbon is exchanging between the atmosphere and the oceans.”  It is much easier to argue – scientifically – with the first diagram.  If I have the background and tools, I can either try to follow the original measurements or to take the measurements myself.  It doesn’t really matter if the job is too big; the fact that, in principle, I can do it, makes the first diagram science, while
the second figure is obviously a good qualitative description but is not actually science.

Back to deniers and skeptics: I have been approached by friends (some of them with good science backgrounds) and students, who tell me (nicely) that since carbon dioxide is a “natural” product, it cannot be bad, so the EPA (Environmental Protection Agency) should not have to identify it as a pollutant.  Sometimes the conversations have gone on to suggest that if I want to avoid global warming, I should simply stop breathing (since they know that we exhale carbon dioxide).  They have asked me kindly to get off their backs and stay away from commenting upon their energy usage.  In the cases where conversations got more heated and evolved to include other greenhouse gases, the suggestions went as far as, “Well, why don’t we regulate cows so they’ll stop farting.”

Well, here is where science comes in.  When it comes to the carbon cycle, we can analyze the numbers.  We can add up the amount of carbon that is going from the earth to the atmosphere, and subtract that which is doing the reverse – entering the earth from the atmosphere.  (The carbon in theses fluxes mainly takes the form of carbon dioxide.)

The result? It shows that there are 3.1 billion tons of extra carbon being added to the atmosphere.  Since carbon dioxide is a very stable compound, it will stay in the atmosphere for many years.  If we assume that this same kind of flux will be more or less maintained from now until the end of the century (the “end of now” time-frame that I talk about in my book), the atmospheric concentrations of carbon will grow by close to 50%. This is a major difference that directly affects our energy balance with the sun.

3.1 billion tons is less than half of what we emit into the atmosphere (red broken arrows in the picture).  The difference means that both the earth and its oceans have now become net “sequesters,” or absorbers of the excess carbon dioxide that we produce.  Vegetation and soil, in the form of enhanced growth because of the carbon dioxide that fertilization contributes, and areas of the ocean that absorb carbon dioxide, contribute as well.  As the temperature rises, the capacity of these methods of compensation is expected to decrease, until they reach the point where both the earth and our oceans no longer absorb the carbon dioxide, but instead reverse themselves to be net emitters.  Some would call this a “tipping point.”

This makes us part of the physical system that we investigate, and negates, at least in my mind, the option of waiting with remedies until the consequences of these changes are absolutely certain.  Science tells us that the danger exists, so the remedies should be approached as an insurance premium.

We are now busy searching for planets outside the solar system.  We are particularly interested in finding planets in the habitable zone of stars- an area defined as a zone around a star, within which it is possible for a planet to maintain liquid water on its surface.  We have, up to now, been able to identify more than 700 exoplanets; last December, NASA announced the its discovery of the first exoplanet in a habitable zone of another star.  It is a narrow zone, but it offers the best chance so far to find life forms outside our own planet.  We are doing well, but we have a long way yet to go in that quest.  On a cosmological scale – destruction of a habitable zone is not very difficult – Venus can serve as a good example. The pace of the atmospheric changes that we are inducing, meanwhile, might lead to the first observable instance of the destruction of a habitable zone.  For a far away civilization, it will be scientific observation.  For us it will be a collective suicide.

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Comments 1 to 14:

  1. It has also recently been reported (Ramirez et. al. 2013) that the inner edge of the habitable zone for Earth is only about 0.99 AU, or just 99% of our current orbital distance. Any closer, and water begins to disassociate in the stratosphere, leading to hydrogen loss and eventual drying of the oceans. 

    We're already close to the edge.

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  2. I recall reading that the terrestrial carbon sink will fail (i.e. instead become a source) by 2050, and the ocean carbon sink by 2100 (presumably under a 'business as usual' scenario).  Also there was a study which concluded that 2013 is the last year in which we have any realistic chance of starting to reduce greenhouse gas emissions fast enough to avert a self-sustaining permafrost carbon feedback.  With decades of warming in the pipeline even from today's anthropogenic climate forcing, and CO2 emissions rising faster than ever, it seems to me that there's no chance of avoiding a 'tipping point' without geo-engineering.

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  3. An interesting article about climate modeling from

    short link

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  4. The link to Micha's book appears broken, pointing back to some unrelated ClimateChangeFork page. This is the correct link.

    ClimateChangeFork has lots of interesting articles dating back to April 2012. Well written in a very clear and simple language. I like that blog as the basic climate science teaching tool, worth following. E.g. the CC pictures above are brilliantly simple, down to the essence required.

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    [JC] Fixed, thanks for the heads up.

  5. That's a nice visual representation of the carbon-cycle, if a tiny bit old. The amount of carbon accumulating in the atmosphere today is actually closer to 4 gigatonnes/year - which works out at about 2ppmv/year. I don't think anyone knows for sure where the tipping-point is - lots of unanswered questions, but of course, the sinks are finite structures themselves and so they can't continue to absorb human CO2 at the current rate forever - something somewhere has to give eventually. Your article says that CO2 stays in the atmosphere for "many years", but of course the residence time for atmospheric CO2 is very short, what matters is the 'lifetime' or 'pertubation time' (the time it takes for atmospheric CO2 to return to its natural level). The Revelle factor, a chemical buffer, is probably one of the biggest factors in contributing to CO2's long lifetime, since it implies that the surface-ocean can only hold 10% of the anthropogenic CO2 that it dissolves (see IPCC's carbon-cycle) meaning that 90% of anthropogenic CO2 is simply swapping places with oceanic CO2, thereby providing a mechanism allowing CO2 to progressively build up in the atmosphere. So, despite CO2's very short residence, the lifetime is long, largely due to the Revelle Factor. I suspect (and it's nothing more than that) the tipping point for the oceans will be when the value of the Revelle Factor goes beyond 10.5. That's my prediction.

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  6. Susinct speech and thought are such a high art, achieved only with the greatest diligence and honesty. This simple argument is so powerful, yet it had never occurred to me before with such clarity.

    It is from such simple clarity that solutions to civilization's energy dilema will arise. It should not anger us when the answer springs from such simplicity. We must find an answer. When we do we must embrace it with open minds.


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  7. Professor Tomkiewicz, I do not understand the basis for predicting a reversal in the oceanic carbon sink.  With increased temperature, it is predicted that the Amazon may reduce from tropical rainforest to open woodland, or even savannah, which would account for the reversal of the land sink.  It is not so obvious in the oceanic case, however.

    Granted that increasing temperature reduces the ability of water to hold CO2.  Based on the CO2 difference between LGM and current, however, that reduction increases atmospheric CO2 in the order of 20 ppmv per degree C.  Allowing a 2 C increase by 2050, that still only represents a 40 ppmv increase in atmospheric CO2 ignoring human emissions.

    Human emissions represent the other half of the equation.  Surely with increased emissions, and hence increased CO2 concentration the equilibrium pCO2 in the ocean will increase.  Does that not, therefore, mean that the partial absorption of excess CO2 by the ocean will continue into the future, albeit at a reduced rate?

    What am I missing here?

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  8. Great article, but I think that the second sentence of the sixth paragraph should end with "Into the atmosphere" rather than "from the atmosphere".

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  9. The mechanisms for the reversal of the carbon absorption ability of the oceans are many.  1) we have made considerable areas of the oceans anaerobic, mainly where polluted rivers enter the sea.  Anaerobic parts of the ocean absorb no Carbon dioxide but instead give it out  2) We are fast approaching temperatures and acidity in the tropical oceans which will stop the growth of corals and we are polluting and mechanically destroying coral.  The skeletons of Coral and all other shell secreting organisms are 60.6 pecent Carbon dioxide. 3) We have destroyed the Whale pump which potentially could restore primary production in the oceans and absorb huge amounts of Carbon dioxide. 4) We have destroyed whole fisheries such as the Dogger Banks, the Grand Banks and the Tuna fisheries of the world -another place where carbon was once stored.  The list goes on and on.  The hope is that if we restored these systems, a great deal of carbon could be sucked out of the atmosphere.

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  10. Tom@7,

    Your consideration of CO2 as equilibrium temperature proxy (that I assume you've calculated using Charney sensitivity) during interglacials is very moot when talking about tipping points.

    You know that tipping points are defined as threasholds during the dynamic changes in forcings, that trigger positive feedbacks in Earth system (icesheets, permafrost, clathrates) leading to hyperthermal events such as PETM. Therefore, the delta forcings over time (currently the rate of CO2 change) should be your primary consideration.

    In all likelyhood, the PETM event was caused by the coincident maximum of Milankovic cycles, see for example (Lourens 2005). The trigger released ~2exagrams of C into the athmosphere but the realease lasted for millennia. In contrast, the current release of just half that amount: 1exagram of C from fossil fuels (the agreed limit the humanity may stiill overshoot) is happening in just a century or two - at least ten times faster.

    Even if you assume that ocean be the net C sink for the forseable future, e.g. according to (Archer 2008) model, arround 20% of that C will stay in the atmosphere for 2-3ky, see Fig.1 therein, I don't know how to paste it here. That's 200petagC; at 1 ppm  = 2.12 Gt C, that is equivalent to ~100ppm. That number is higher than your moot 40ppm. This number, over the preindustrial 280ppm constitutes 5.35*ln(380ppm/280ppm) = 1.6Wm-2 forcing. Coincidentaly, that forcing is identical to the best estimate of current total forcing since preinductrial, according to IPCC.

    Can such forcing, if applied for 2-3ky according to (Archer 2008) trigger the Earth system response such as complete icesheet melt or clathrate release in the amount of several exagram C, leading to PETM like event? Uncertainties are large, but my answer is: it certainly can! We are already witnessing the arctic ice melt in our 70-90y lifetimes that is the begining of albedo change which will amplify other systems. Therefore, the threat of tipping points as defined above is very real.

    On top of that, the diturbance of the CC due to FF burning is far greater than the PETM disturbance: back then just the several ky old permafrost was released, now the 300-400Ma fossil fuels are released, with greater unknown (=fear for bigger impact) associated.

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  11. When Dr. James Hansen spoke after the reading of my play "Extreme Whether" in April, he made a similar point about well-meaning, intellligent people who can't "see" global warming and suggest we wait until we can. Our climate crisis presents a problem with the imagination as much as  with conveying the science:  how can we imagine the future so we can act in our defense before it is too late.  In that regard, the play "Extreme Whether" will have another public reading, September 10 at 2pm at the Cherry Lane Theater in New York City, followed by a talk by Dr. Jennifer Francis--at that point, in Sept., this year's Arctic ice melt should be at its peak. 

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  12. kmalpede@11,
    Hi Karen,

    I'm interested in this play, however I have no idea if it ever comes to the theatre near me (Sydney AUS) let me know when. Maybe in the meantime, can you give us some info about it (review detailing the action) or perhaps some trailer or link to some online content so that we can watch and have a good feeling about its content. Thanks.

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  13. Is it not a error in the start of the 6th section? Where it should be to and not from?

    There is written:

    "... that there are 3.1 billion tons of extra carbon being added from the atmosphere ..."

    Should been: 

    "... that there are 3.1 billion tons of extra carbon being added to the atmosphere ..."

    ... and it could be also be added in the end of the sentence: ... every year.

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  14. A general problem far beyond this article, is that too often the horizon of problems into the future is set to be like 87 years or may be couple of hundred years (until 2100 or a couple of centuries more).
    Even quite a lot of science article fail to mention what David Archer told us in the The Long Thaw - it will last more than 100,000 years to reach back to preindustrial state of CO2 content in the air ...

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