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Does positive feedback necessarily mean runaway warming?

What the science says...

Select a level... Basic Intermediate Advanced

Positive feedback won't lead to runaway warming; diminishing returns on feedback cycles limit the amplification.

Climate Myth...

Positive feedback means runaway warming

"One of the oft-cited predictions of potential warming is that a doubling of atmospheric carbon dioxide levels from pre-industrial levels — from 280 to 560 parts per million — would alone cause average global temperature to increase by about 1.2 °C. Recognizing the ho-hum nature of such a temperature change, the alarmist camp moved on to hypothesize that even this slight warming will cause irreversible changes in the atmosphere that, in turn, will cause more warming. These alleged "positive feedback" cycles supposedly will build upon each other to cause runaway global warming, according to the alarmists." (Junk Science)

At a glance

Yet another climate change myth that has not aged well. As of early May 2024, all of the past 12 months had come in at more than 1.5°C above pre-industrial temperatures, so all of the first sentence is now tripe.

However, with regard to the rest of the myth, the evidence suggests it is extremely unlikely that Earth can enter a runaway greenhouse state.

Why is that? We have two good lines of evidence to support the contention. Firstly, we know an awful lot these days about the geography and climate of Earth in the past. Ancient geography can be determined by examining rock sequences on the continents and noting similarities in their fossil faunas, sedimentary environments and ancient magnetism.

So we know, for example, that around 55.8 million years ago, Ellesmere Island, off the NW coast of Greenland, was a lot warmer than it is today. The main geographical difference between then and now was that the Atlantic Ocean was narrower. The faunal difference was a lot more impressive. Where there are now glaciers and polar bears, back then tortoises, snakes and alligators thrived. Their fossils, along with those of redwood, ginkgo, elm and walnut, are to be found in Ellesmere Island's sedimentary rocks.

The time in question is known as the Palaeocene-Eocene Thermal Maximum. As the name suggests, it was probably the hottest climate experienced on Earth in the past 600 million years. To get temperate to subtropical temperatures in the Arctic is indeed impressive. But there was no runaway beyond that. Why?

Trapping of heat by CO2 and other greenhouse gases causes an energy imbalance on Earth. This imbalance gets amplified by positive feedbacks. A positive feedback happens when the planetary response to a change serves to amplify that change. For example, due to burning of fossil fuels, atmospheric CO2 has gone up by 50%. The resulting enhanced greenhouse effect is heating up the planet. The heating, among other things, melts arctic permafrost, releasing the CO2 and methane trapped within it. These gases amplify that initial change. The effect reinforces the cause, which will in turn further increase the effect, which in turn will reinforce the cause… and on and on.

So won't this spin out of control? The answer is almost certainly not. Feedbacks are not just positive. One very important one is that a warmer planet radiates more energy out to space than a cooler one. This feedback is not only negative but it is also strong.

Furthermore, positive feedback cycles will go on and on, but there will be a diminishing of returns, so that after a number of cycles the effects become insignificant. Thus, if we double the atmospheric concentration of CO2, the amount by which the response to that change - heating - can be amplified is approximately three times.

The creator and spreader of this particular myth is essentially putting words in people's mouths. No surprise there. But we do not need a runaway greenhouse effect to make life on Earth difficult. Just a few degrees of additional heating will do exactly that.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Some deniers ask, "If global warming has a positive feedback effect, then why don't we have runaway warming? The Earth has had high CO2 levels before: Why didn't it turn into an uninhabitable oven at that time?"

Positive feedback happens when the response to some change amplifies that change. For example: The Earth heats up, and some of the sea ice near the poles melts. Now bare water is exposed to the sun's rays, and absorbs more light than did the previous ice cover; so the planet heats up a little more.

Another mechanism for positive feedback: Atmospheric CO2 increases (due to burning of fossil fuels), so the enhanced greenhouse effect heats up the planet. The heating "bakes out" CO2 from the oceans and arctic tundras, so more CO2 is released.

In both of these cases, the effect reinforces the cause, which will increase the effect, which will reinforce the cause. So won't this spin out of control? The answer is, no, it will not, because each subsequent stage of reinforcement & increase will be weaker and weaker. The feedback cycles will go on and on, but there will be a diminishing of returns, so that after just a few cycles, it won't matter anymore. In addition, negative feedbacks also occur due to warming, of which the powerful Planck response is particularly important. Put simply, the Planck response is a feedback that makes a warmer planet radiate more energy from the top of its atmosphere to space than a cooler planet, thereby reducing the energy imbalance.

The plot below shows how the temperature increases, when started off by an initial dollop of CO2, followed by many cycles of feedback. We've plotted this with three values of the strength of the feedback, and you can see that in each case, the temperature levels off after several rounds.


So the climatologists are not crazy to say that the positive feedback in the global-warming dynamic can lead to a factor of 3 in the final increase of temperature: That can be true, even though this feedback wasn't able to cook the Earth during previous periods of high CO2.

One topic along this theme, that you may have heard of in the media, concerns Arctic Permafrost. This is important because large amounts of organic carbon are stored in the permafrost - ground that remains frozen throughout the year. If large areas of permafrost thaw out as the climate warms, some of that carbon will be released into the atmosphere in the form of carbon dioxide or methane. That will certainly result in additional warming. A serious enough threat, for sure, but projections based on models of permafrost ecosystems suggest that future permafrost thaw will not lead to a ‘runaway warming’ situation. That's the conclusion from the FAQ regarding permafrost in the IPCC's latest Sixth Assessment Report (AR6) (PDF).

A final point regarding runaway global warming involves deep time. On several occasions in the geological past, Earth has recovered from the 'icehouse' climate state, going back into a Hothouse regime. The implication is that if such profound changes happened before without Earth entering a runaway warming state, it's highly unlikely to occur this time. That is no reason for complacency, though. A few degrees will be bad enough.

Those of a mathematical disposition will find additional interest in the Intermediate version of this rebuttal.

Last updated on 2 June 2024 by John Mason. View Archives

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Argument Feedback

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Comments 51 to 75 out of 103:

  1. jpat, If I may interject with an observation, I think you are very in danger of succumbing to hammer/nail syndrome ("when all you have is a hammer, every problem becomes a nail"). You are trying to view everything in terms of your own area of expertise, circuitry. While this is easier for you, it is going to lead you into trouble. Your analogies are fine for understanding a problem initially, but you will lose track of the fact that they are only analogies, and necessarily flawed. To answer your questions and doubts I would very, very strongly suggest that you start by reading Spencer Weart's The Discovery of Global Warming -- A history. It's interesting, and you will learn a ton.
  2. jpat - the individual orbital cycles are very regular but the sum is not so much. Furthermore, they affect climate in slightly different ways. I would be extremely cautious about pushing this too far.
  3. KR #44/45 - I agree that analogies can only be taken so far. They can be useful though for bringing a foreign concept into a more familiar realm. That being said, I'm surprised that I can't seem to find a transfer function based model of the climate. Its a control system, surely someone as formulated it as such. There is a well developed discipline called system identification which can derive transfer functions from auto-correlation of sampled data which would seem to be useful in this application.
  4. Sphaerica/scaddenp - You both raised similar concerns. Believe me I have no illusions of building a circuit model of the climate! I'm simply trying to understand the science. I've read the links suggested, and am getting up the learning curve slowly. The feedback discussed here seemed counter-intuitive so I noodled it through until I understood the disconnect (f < 1 does not imply passive feedback as it does in control theory). Along the way I found an analogy I thought might be useful to others in similar straits so I posted it. It's not meant to be anything more than a tool to help engineers understand one tiny aspect of the puzzle. That being said, there is no reason the differential equations that describe the climate can't be reformulated as a non-linear control system problem. Such a system should be able to describe in broad strokes the major features seen in the Paleo record. The problem is they don't. They can not explain how CO2 lags temperature through the entire cycle (and please don't point me to the CO2 lags thread. Been there. I want equations not hand waving about feedback).
  5. Turns out the injection-lock analogy may not be so far fetched after all. I knew I couldn't have been the first to thought of this. Here is a peer reviewed paper on the subject. Comments?
  6. This is on the "100ky" problem - so far multiple theories on the subject but not enough data to constrain anything to everyone's satisfaction. Modelling is focused on representing the known physical system. A sweeping approach with simple equations doesnt tell us that much about what is really happening in the system. If you regard the "CO2 lags" as handwaving, then it because its not straightforward to put down a full physical model with coupled carbon-cycle model in a blog post. The point was explain, a/ considerable uncertainty remains in tying down CO2 feedbacks and b/ what we do know makes simplistic representation unlikely. These are the problems being tackled by AR5 models in paleoclimate. For details, go to the CMIP5 site and then look for ESM (Earth System Models).
  7. jpat wrote: "They can not explain how CO2 lags temperature through the entire cycle" I've never understood this objection. To me it has always seemed inescapable that, barring massive vulcanism or human injection of sequestered carbon into the atmosphere, simple cause and effect indicate that CO2 must lag temperature. Seriously... how could CO2 levels rise prior to the temperature increase which causes this CO2 to be released from the oceans and frozen biological material? How could they fall prior to the cooling which allows the oceans to absorb more CO2 and sequesters organic carbon in ice? Nor is the math required particularly complicated. The temperature swung by about 8 C during the glacial cycles. Offhand I don't know what the estimated factors are, but an 8 C swing could be produced from an orbital forcing of 0.8 C and total feedbacks (CO2, albedo, water vapor, et cetera) of f = 0.9; 0.8 C * [1/(1-0.9)] = 8 C If we change the forcing in the equation above from 0.8 to 0 then the feedback effect would also be zero. Ergo, the CO2 temperature feedback MUST lag the orbital temperature forcing. Again, how is this anything but obvious? How could CO2 lag temperature throughout the natural glaciation cycle? How could it NOT?
  8. CBDunkerson #57 - Do you really want to argue that f is a constant? If so you've eliminated any functional relationship between temperature and CO2. But of course that's not the argument. f is a function dependent on many factors and climate sensitivity depends on f. If one is to reject the null hypothesis inherent in the paleo record, name Co2 has a negligible effect on temp because effect can not precede cause, one must describe a plausible function f(C02,T,a,...) which under reasonable forcing can replicate the paleo record. I'm not arguing that it is physically impossible or that the CO2 lag proves anything about cause and effect. But by now I would have expected a plausible candidate function to have emerged. If it has, please point me to it so I can understand the dynamics involved. If not, let's stop pretending we have this all figured out and feedback explains everything.
  9. jpat wrote: "Do you really want to argue that f is a constant?" You asked for a formula showing how it could be possible that CO2 lags temperature throughout the glaciation cycle. I provided an example. In that example f was constant for simplicity. However, that does not mean I am claiming that f IS constant in the glaciation cycle... nor is f being constant a requirement for my example. Exactly the same conclusion would be reached with a value of f which changes over time... if the forcing effect is zero then, by definition, the feedback effect is zero - regardless of the value of f. Ergo, feedback MUST follow forcing. Thus, whether f is constant or not is completely irrelevant to the issue at hand. The simple forcing and feedback function in my prior message shows very clearly how a small forcing can produce a large change with a sufficiently large feedback factor. Exactly the same results could be achieved with an 'f' value which starts out at 0.95 and decreases down to 0.86 over time. Thus, we have a very simple mathematical model showing the possibility of CO2 (and other feedbacks) lagging temperature while still strongly influencing the total temperature change. This directly disproves your claim that CO2 must have "a negligible effect" if CO2 changes lag temperature changes. A realistic model would require multiple feedback factors with different signs, different rates of change, and complex interrelation between the factors... basically a full scale climate model. However, that wasn't your stated request. You were asking how it was possible for CO2 to lag temperature... that question has been answered. Simple logic and mathematics both make it obvious that CO2 MUST lag temperature in the glaciation cycle.
  10. jpat - Feedbacks have lags, that's part of physical nature. Water vapor has a lag of 5-10 days. CO2 solubility in the oceans has both a short term (months-years) and long term (~500-800 year) response times, based upon surface water adjustment and deep ocean circulation. Ice melt/accumulation and vegetative changes in albedo have their own response rates. The initial forcing is followed by an amplifying feedback, results continue to amplify (in decreasing amounts), a new stable state is reached (inter-glacial, for example). The initial forcing changes again, decreasing, allowing more CO2 to sequester in the oceans, hence another amplifying feedback until a new stable state is reached based upon the then current forcings (ice age). Rinse and repeat... In the electronic analogies you have used, you need to incorporate resistor/capacitor or resistor/inductor elements - nothing is instantaneous in climate.
  11. scaddenp #56 - Yes the phase-lock formulation is under-constrained and thus produces a functional (i.e a class of functions) solution. But these functions share common traits which may yield insights regardless of what the real underlying dynamics are. If in fact the climate dynamics are described by something akin to the Van der Pol equations synchronized to the Milankovitch cycles, we know quiet a lot about the dynamics of such systems which may provide insight into the question of primary import: "how hot will it get?" The paper linked above provides a plausible explanation for how small changes in insolation can result in large temperature swings even if the climate sensitivity is low. All that is required is a non-linearity in the feedback loop. The reason for this constraint is apparent. The equations define a limit cycle which precludes a constant forcing from increasing the maximum excursions. Instead, equilibrium is reached by translating the d.c. power into harmonics of the forcing function, hence the need for non-linearity. Now "non-linearity" handwaving is no better than feedback handwaving. But it seems to me that we should be putting more effort into understanding the nature of the limiting mechanisms. It just might save our bacon.
  12. jpat#61: "more effort into understanding the nature of the limiting mechanisms." Seems to me you are looking for a level of difficulty found on Isaac Held's blog.
  13. jpat, I really don't understand how people can have trouble with the CO2 lags stuff. It's simple. Increasing temperatures raise CO2 levels. Raising CO2 levels raise temperatures. Once something kicks the system up and raises temperatures initially (orbital forcings, for example), a feedback loop kicks in. Without CO2, only the initial forcing takes effect, and temperatures rise only a little. With CO2 (and other feedbacks), temperatures rise substantially further. If you look at the ticker tape, it looks like temperatures are slowly rising and CO2 is following, but that is not what is happening. What is happening is that temperature rose first as a result of a forcing, and then CO2 followed, pushing the temperature up, which pulled CO2 up, and so on. This is really not that hard to understand, and there is no reason to express it in equations until you get the basic concept down.
  14. Sphaerica - the difficulty is explaining the _other_ transition, when your at peak insolation. At that point dT/dt is at a minimum. CO2 is still rising rapidly due to the 800 year lag but no where near saturation. The CO2 is thus contributing an ever increasing radiant forcing. How does the small insolation dT/dt overcome the CO2 forcing to turn the temperature back around? It may well be that f(Co2,T,..) is such that this all makes sense. But it is not as intuitively obvious as the wand wavers would have us believe.
  15. CBDunkerson #59 I no where claimed that CO2 must have a negligible effect on temperature nor do I believe that to be true. I stated that as the null hypothesis that can not be rejected on the basis of a just so story. Short of a mathematical proof, one must show how the data doesn't support the null hypothesis.
  16. jpat - "But it is not as intuitively obvious as the wand wavers would have us believe." Wand wavers? Seriously? You are betraying quite a biased viewpoint there. This has been an interesting discussion, but that statement on your part indicates to me that you aren't very interested in answers. Do you understand the original post in this thread? That indicates the positive feedback response (with the delays necessary in a physical system) leads to a finite amplification? And that this amplification applies for forcing deltas both upwards and downwards, amplifying the magnitude of those +/- forcings changes? And hence that any changes in forcing are amplified to larger changes in temperature via feedback? Ice ages are initiated by decreasing insolation due to orbital mechanics. Interglacials are initiated by increasing insolation due (again) to orbital mechanics. Feedbacks turn a 1-3C forcing change into a 6-8C total change. CO2 as a feedback won't rise to 'saturation', and I would consider that a strawman argument - nobody has said it would. I would have to consider that statement either misdirection or a severe lack of understanding on your part.
  17. jpat - "just so" stories? No, you're aren't approaching this with any bias... Read the link Sphaerica provided - Spencer Weart's The Discovery of Global Warming -- A history. Based on the last 150+ years of physics, the radiative greenhouse effect and feedback via climate sensitivity are established science, poorly mapped electronics notwithstanding.
  18. KR @ 60:"eedbacks have lags..." Yes and lags (or more probably time constants) have associated eigenvalues which in a system of at least 2nd order with positive feedback would generally produce spectral peaking, if the system is stable, and oscillation if it is not. The problem is there is no evidence of this peaking in the PSD of the paleo record which instead has all the characteristics of a highly regulated, dominate pole compensated control system (or alternatively a system phase-locked to transcendental forcing function). One explanation would be that the system poles are highly dissipative and so slight peaking is masked by noise (but the positive feedback should provide Q multiplication). There are other possible explanations (like the one in the above paper) which fit the data. We can't of course jump to conclusions but neither should we adhere to orthodoxy and close our minds to alternative hypotheses.
  19. KR #67. That comment is unfair. The just so story I was referring to was in reference to the narrative provided by Sphaerica, not to AWG theory in general. I've perused the link you've provided (multiple times). I don't see a treatment of the subject at hand there.
  20. jpat wrote: "The CO2 is thus contributing an ever increasing radiant forcing." If you don't understand why the above is completely false, after having it explained to you several times, there really doesn't seem much point in continuing. The temperature forcing from CO2 feedback is finite. There is no need to "overcome" an "ever increasing" temperature rise from CO2. The warming from CO2 stops... all on its own. After the orbital forcing which caused it does. "Short of a mathematical proof, one must show how the data doesn't support the null hypothesis." You do realize I (and others) already provided the mathematical proof, right?
  21. KR #66. wand wavers should have read hand wavers but I don't think the distinction would change your accusation of bias, which is misdirected. I've asked to be pointed to a more formal description of the dynamics. In response I get various narratives (which if you'll read through this thread and the Wiki I was pointed to aren't even self-consistent), variations of "why is this so difficult for you to understand" and accusations of bias for simply pointing out that there are other possible explanations. If that's what passes for inquiry hereabouts I guess I'll move on.
  22. KR @66 "CO2 as a feedback won't rise to 'saturation', and I would consider that a strawman argument - nobody has said it would." CBDunkerson@70 "[condescending remarks deleted] The temperature forcing from CO2 feedback is finite. There is no need to "overcome" an "ever increasing" temperature rise from CO2. The warming from CO2 stops... all on its own. After the orbital forcing which caused it does." See what I mean about inconsistency?
  23. jpat, you need to stop assuming you already know the answers and start actually thinking about the things people are telling you rather than just dismissing them without understanding. There is a common (though false) claim that the warming effect due to CO2 is currently saturated and that increasing atmospheric CO2 levels will thus cause no further warming. I'd assume that is what KR was referring to when he said that CO2 feedbacks would not rise to 'saturation'. It is completely different from what I (and KR) said about temperature increases from CO2 being finite. So no... there is no inconsistency. You just do not understand. Read it again. Really think about it. Ask specific questions. As to people being 'condescending'... have you READ your own posts Mr. 'wand waving'?
  24. "how hot will it get?" jpat - the primary interest at the moment is how hot might it get in the next 100 years. The ice-age cycle is extremely interesting but carbon-cycle feedbacks are insignificant in that time period. We are providing all the CO2 at the moment, not nature. When the oceans start outgassing, it will get first. Also, low sensitivity is not supported by other evidence whereas 100ky problem has other solutions. Its very important to recognise that climate theory is based on physics not paleoclimate. Paleoclimate studies can provide some constraints but primarily are useful as testing ground. Does our theory work in former times? The puzzles are paleoclimate arent challenges to theory - they are challenges of finding ways to constrain multiple theories which account for different forcings. So to the question as to how hot could it get? Well paleoclimate does constrain that - as hot as the pliocene potentially.
  25. jpat - Your comments have repeated invoked unlimited runaway feedback, which as discussed in the header of this thread is both non-physical and simply not the case for the climate. Feedback amplification is limited, CO2 will not self-amplify past that, and a forcing change in either direction will be followed by feedback changes in the corresponding direction. CO2 solubility levels in the oceans are temperature dependent. Equations for the various sequestration pathways can be found in incredible detail in the Ocean acidification threads, but a 500-800 year lag time for a solubility response is supported by chemistry, ocean currents, and multiple observations. My interpretation of 'saturation' in your previous post was that CO2 would continue to rise past the initial amplification due to it's own effects - which again contradicts finite amplification of a forcing. There is no inconsistency between CB's and my comments in that regard. --- At this point it's unclear what your discussion is leading to. Climate is not a phase-locked oscillator, not an op-amp circuit. Your tone in this regard is rather remarkable - you seem to feel that because nobody has mapped the climate to your field of electronic circuitry that it's 'hand waving'. Climate the response of the global energy distribution and hence chemical and hydrological responses due to multiple forcings, multiple deltas, with a fair number of positive and negative feedbacks adding up to a net positive amplification of ~3x, over which is laid non-linear interactions leading to chaotic variations (ENSO, weather). Exact numbers for portions of this system have varying uncertainties, such as aerosol total effects, but the climate is surprisingly well understood as a boundary value driven physical system with chaotic variations.

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