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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 76 to 100 out of 121:

  1. KR#75: "nobody has mapped the climate to your field of electronic circuitry" Well, dontcha know, somebody has. It's dated (2001) and doesn't seem to have much about co2, so I don't know (and don't much care) if it's any good, but jpat might study the lecture series and report back.
  2. The saturation I was referring to was the log dependency of forcing with CO2 concentrations. At the temperature extremes of the paleo record, the CO2 has been around 270 ppm which is substantially lower than today. My point being that if we haven't reached the point of diminishing returns at present, then neither had we back then.
  3. jpat, Your comment contains a number of errors:
    the difficulty is explaining the _other_ transition, when your at peak insolation.
    Actually, "peak insolation" happens relatively quickly. That's the kick start. Insolation does not slowly increase to keep temperatures and CO2 rising. It increases relatively abruptly, and in so doing starts the rise in CO2 that drags temperatures further upward.
    At that point dT/dt is at a minimum.
    No. dT/dt approaches a minimum as the effect of CO2 tapers off.
    CO2 is still rising rapidly due to the 800 year lag but no where near saturation.
    This is wrong. First, the term "saturation" is ill-defined. I don't think you mean "so much we can't add anymore." What we care about is the temperature-effect of CO2, which is strongest when CO2 levels are low, and decreases as CO2 levels rise. After a while, adding more CO2 just doesn't affect temperatures all that much, so it doesn't add much more in the way of CO2. The feedback dies and a new, stable temperature is achieved. This is because the temperature impact of CO2 is logarithmic, i.e. 1 unit of temperature change per doubling of CO2. So... 1x CO2 --> no change 2x CO2 --> 1 unit increase 4x CO2 --> 2 units increase 8x CO2 --> 3 units increase and so on.
    The CO2 is thus contributing an ever increasing radiant forcing.
    No, as explained above. It contributes an ever decreasing forcing (otherwise you'd have a runaway effect).
    How does the small insolation dT/dt overcome the CO2 forcing to turn the temperature back around
    At the end of the interglacial, when orbital forcings change, there is a decrease in summer time insolation (both in strength and the length of the summer). This allows ice sheets to advance (or, more appropriately, keeps them from melting back to their starting point each summer). The advancing ice reflects rather than absorbs more and more incoming radiation. Temperatures drop correspondingly. When temperatures drop, CO2 drops. When CO2 drops, temperatures drop. The entire cycle happens in reverse until much of the northern hemisphere is covered in ice.
  4. "You do realize I (and others) already provided the mathematical proof, right?" You do realize that a mathematical proof excludes all other possibilities, right? Do you really think you've accomplished that?
  5. Interesting, here's another electronic mapping: Schwartz 2010. Unfortunately, he's using a single compartment energy model, which is notably insufficient to capture climate behavior.
  6. 77, jpat,
    if we haven't reached the point of diminishing returns at present, then neither had we back then.
    Refer to my previous post. Note that the point at which the natural system stabilizes is always around 270-275 ppm. That was the point of diminishing returns as far as the natural climate goes. It wasn't going to add any more CO2 on it's own. Enter man. Add 115 ppm in just a hundred years (where the natural climb from 180 to 270 took thousands). This is a 0.5 fold increase. If we go all the way up to 540 ppm, double where we started, that is again another whole unit of temperature of increase. Consider that the change from 180 to 270 is only a 0.5 fold increase in CO2. That's what pulls us out of a glacial period, over the course of thousands of years, and we have already today duplicated that forcing in a mere 100 years. You are right in that the natural climate system had reached the point of diminishing returns. It was never going to push CO2 levels and temperatures above where they were at. But that doesn't mean that CO2 will now somehow magically fail to have the same, predictable effect.
  7. Sphaerica (at #78) et al. The plot below is what am using as a basis for my questions. It is a sum-of-sines best fit to the adjusted vostok data where the period and amplitude of the forcing components was extracted by fourier analysis (and match near perfectly to the known Milkanovitch periods.) Since we can not know how the phase of each component is affected by the climate we adjust the phase of each component for best fit (r^2=.6). Here's the Fourier analysis which was done with a technique called linear decomposition which avoids the spectral smearing one gets with windowing. The narrow line widths are strong indications that the forcings are astronomical as no natural terrestrial process could maintain this level of spectral purity over 500kYr. Finally here's the same temperature extraction plotted with the CO2 extraction and scaled to equal amplitude for easy comparison. In the first plot, note that the rapid decrease in temperature generally occurs near a dT/dt minima. In the last plot note that the CO2 is still rising at the temperature turn-around point.
    Response: [mc] Please restrict image width to 475
  8. jpat - are you saying that because CO2 is still rising, then temperature shouldnt be falling? But what is the strength of the forcing associated with delta-CO2 cf to strength of other forcings operating at the same time?
  9. scaddenp @83 I'm saying if one of my engineers brought me this plot and said that his system model indicated the red curve was the cause and the blue curve the effect I'd ask to see his equations :>) That's all I'm after here. No agenda, not trying to upset anyone or advocate a position. I just want to see the math for myself.
  10. jpat@84 I think this has been pointed out several times, but there are more variables in the model.
  11. Sphaerica @81 - Thanks for that explanation. It's a good point and clearly cause for concern. KR and muoncounter - Thanks for the papers. Schwartz talks about some of the same taxonomical issues I tried to with my divider + VCVS analogy. I think incorporating something like this into the header on this topic would be helpful. In reading through the comments I see others have fallen into the same trap I did.
  12. jpat#84: "I just want to see the math for myself. " Speaking of math, did you verify that the time resolution of the Vostok core was sufficient to identify such a short period lag as your 'extracted astronomical signals' graph illustrates?
  13. jpat - but there are no simple equations in the model. Just mighty complex ones. As I said earlier, if you want the equations then stick you head inside one the Earth System Models. The question for feedback is that for a given delta-T, what are the changes in the feedback? For many the feedbacks, then this also depends on what current T is. Only water vapour is straightforward in this. Albedo depends on cloud response plus elevation versus freezing level.Methane is particularly complex, with multiple sources, some having temperature-triggered stores; and CO2 depends on your full carbon-cycle model.
  14. Good point muoncounter - dating Antarctic core is not such a straightforward process and all of the date models have some assumptions built into them that making testing some hypotheses (eg delays between NH and SH responses) difficult.
  15. I was wondering what studies there had been of glacial onset using ESMs. Some papers - many, many more around. Meissnet & Weaver Calov et al Matthews et al At least examples of how complex the "equations" are. I'd say a lot more work is going to be done in this area though.
  16. "Speaking of math, did you verify that the time resolution of the Vostok core was sufficient to identify such a short period lag as your 'extracted astronomical signals' graph illustrates?" As you probably know, the Vostok data is not uniformly sampled. I resampled the data to a 5 year interval using standard interpolation and verified that the roll-off of the resampling was well above the signal band of interest. So yes, there's plenty of resolution.
  17. using which age model?
  18. Phil, Thanks for the links! I was thinking about looking up papers like this for my class in the spring. I need to update the mock global C model we use in the lab. These should give me some ideas. jpat...some of the feedbacks are not easy to model realistically outside of an ESM (as they are called these days). For example, one paper scaddenp points to looks at vegetation feedbacks. These cannot spin out of control interminably as there is only so much land than can be converted to forest and back again. Soil carbon has similar constraints. Ocean chemisty, circulation and carbon sequestration is not nearly so straightforward a function of temp as you'd think. Martin hypothesized that dust delivery to the Southern Ocean can alter CO2 storage by stimulating Fe limited phytoplankton. That effect is constrained by availability of other nutrients, though, and would not scale proportionately with climate change. Basically there are any number of ways to get an eventual damping of the CO2 -temp feedback. We're still trying to figure out which were really important.
  19. I think this point has laready been made, but jpat, you seem to be desiring a single, simple function to explain palaeoclimate and feedbacks. Except... climate variations depend on a series of interrelated systems, each operating at different rates, with different magnitdes at different phases of the climate history. Stephen Baines describes some of that complexity. As climate is forced into a cooling, we get ice sheet expansion over North America and northern Europe, increasing the albedo effect, and having knock-on effects for temperature, water vapour, biomass, CO2, sea ice and a whole lot of other things, each operating at a different rate, with different lags, and feeding back to temperature and drawing down a little more CO2. The process is self-limiting, because eventually the ice can't grow enough for albedo to overcome mid-latitude insolation and the forcings don't remain permanently low, and so with the present continental configuration, high-latitude glaciation is easy, but global glaciation is not so easy. Once it's in place, you need a sufficient forcing to drive the system in the other direction. When the forcing operates in the other direction, the now kilometres-thick ice sheets over North America and N Europe begin to melt, and can do so quite rapidly due to dynamical processes, especially when the height of the ice sheet begins to drop. That aids all the other feedbacks operating in a warming direction, but there is an element of self-limiting as eventually the big ice sheets have shrunk and so the albedo component can't drop quite so fast, and the forcing is no longer at a maximum. In order to continue the melting into the next vulnerable ice sheets - Greenland and West antarctica - you need an extra forcing kick. CO2 can operate as a forcing or a feedback (the molecules have no memory of how they got into the atmosphere, they just trap heat), and by releasing lots of CO2, we've provided the extra kick in forcing, which means that Arctic sea ice, Greenland and West Antarctica are vulnerable. There's nothing magical about why most interglacials appear to have approximately similar magnitudes, as that is a function of continental configuration and the length of the forcing. During the last interglacial, it's likely that parts of Greenland and West Antarctica melted as well. That's a very long way of saying that you cannot easily represent the full interrelationships of forcings and feedbacks with a simple function. In fact, the best way to capture the relationship is to build a model of all the relationships (a simple function is merely a simple model after all), incorporating all the radiative physics as best we can. You'll get even better results if your model is a spatial one that can capture trickier concepts like continental configurations and ocean circulations. This has been done, they are called GCMs. You didn't seriously think that the experts in this field who have worked on this for their whole careers hadn't thought of all this? You can't come at climate science from an unrelated field and completely grasp all the complexities without a great deal of effort. You seemed to be suggesting that you can, if your misconceptions about the palaeo record & CO2 and how much it's all 'figured out' in #58 and elsewhere are anything to go by. I can only recommend you re-read Sphaerica's advice in #51.
  20. We seem to be talking past each other so let me try one more time to illustrate the difficulty I'm having. Consider the following toy model of the climate. Forcing function F drives the input. It gets summed with the feedback signal and converted to temperature by Gain2. The feed back path encapsulates the functional relationship between Co2 and temperature. The transfer function models the time lag between a change in temperature and the corresponding change in CO2 concentration at node C. Gain1 handles the conversion from CO2 to radiant energy. The feedback is positive but low enough that the system is stable. The paleo temperature record corresponds to the signal at T, the CO2 record to the signal at C. We ask ourselves, what is the expected time relation between T and C under closed loop conditions? Answer: Same as under open loop conditions! I.e. feedback can not change the open-loop relationship between T and C. If the physical mechanism that produces CO2 when the temperature rises includes a lag (and it does), we expect to see that same lag under closed loop conditions. How can it be otherwise? The relationship between T and C is defined by the blocks between them. And note that relationship is completely independent of the complexity of the transfer function which could include other internal feedback loops, other gain paths etc. I can think of no system formulation that could possible convert a lagging signal to a leading signal. I hope this clears up my conundrum.

    [DB] Since you are fond of analogies, let me share this one with you: 

    Your conundrum, distilled, is that you are treating climate science as some that learn a foreign language:  you are insisting upon translating the words you hear into English before assembling them into sentences.  However, to truly learn a foreign language, one must learn enough vocabulary, sentence structure and syntax to understand the foreign language in your head without the need for translation.  In essence, you need to be able to think in that foreign language before true understanding of it is then reached.

    That is what is retarding your understanding.  As it has retarded the understanding of electrical engineer-types like RW1 and co2isnotevil before you.

  21. The lag between a positive temperature forcing and CO2 coming out of the oceans is relatively large, due to slow ocean circulation. Once the CO2 gets into the atmosphere, it operates immediately as a forcing on climate. In your electical system above, it gets there via the slow feedback, and so there would be a delay between the temperature change (which has to rise first) and the CO2 (which amplifies the temperature change, but rises later). Now consider what happens if you release a very large amount of CO2 directly into the atmosphere. We've done that, adding more that the entire CO2 difference between LGM and Holocene within about 100 years. Does the CO2 wait patiently for 800 years before operating? No, it starts working as soon as a suitable packet of longwave radiation passes by! How will your graph look now? Will there be much of a lag between temperature response and CO2? Which one will rise first, all other factors excluded?
  22. jpat - I dont know electrical circuits but one obvious thing you need is a second feedback circuit (albedo is of similar magnitude but much faster response). Second, can you build one so that feedback response is different when temperature is rising than when it is falling?
  23. Just to note: circuit theory is a particular case of network theory. Azimuth is the home of mathematicians who's are a bit good at the former and interested in climate science... The series on network theory have plenty, classical, feed back loop models which do not run away. If someone was really interested, they might ask there to analyse this.
  24. jpat, You are currently paralyzed by (a) restricting your thinking to circuit design and (b) trying to oversimplify the system. In particular, you need multiple feedback loops (short term CO2, long term CO2, H2O, low equatorial clouds, low NH clouds, high clouds, ice, CH4, etc.) which all interact with each other, some in non-linear ways, and with clock cycles that introduce varying delays. They are also bounded in unexpected ways. For example, the extent of ice cover can only advance and retreat so far, and ice further south is much more powerful since it covers a larger area, winter daylight hours are longer, and the sun strikes at a stronger angle of incidence. So the forcing-feedback effect Fice of the retreat of ice ∆I in response to a positive temperature change ∆T is also dependent on the actual ice extent I at the time of the change combined with the orbital configuration Xorbit. The ice extent I is itself dependent not only on temperature but also the current orbital configuration Xorbit. Fice is also moderated by the amount of low NH clouds Clow NH and aerosols A (since both of these block light and thereby negate any effects of ice on the surface). That's just a simplified set up for ice. The effects of CO2 and H2O are similarly complexly moderated (ice, aerosols and low clouds reflect visible light, resulting in less IR to feed the greenhouse effect, for example). The feedback loop for H2O is far, far faster than that for CO2. The different mechanisms for changing CO2 in the atmosphere (vegetation growth, vegetation decay, temperature-dependent ocean absorption/out-gassing, the very important biological pump, etc.) all work at different rates. In addition, the growth and decay of vegetation, like ice, is dependent on latitude... lower latitudes have more area, and are more hospitable to plant growth, so the initial effects of the retreat of the ice sheets on CO2 changes related to vegetation are far greater at lower latitudes than later retreat further north. The biological pump which is thought to cause the most abrupt and important changes in atmospheric CO2 in transitions between glacials and interglacials is, like all other factors in the system, bounded, non-linear and self-limiting. The entire system is just far, far, far more complicated than you are considering right now. A mechanic who tries to model the system in terms of an accelerator pedal, fuel line, fuel pump, etc. would have a better chance of producing an accurate analog of the system than an electrical engineer. I said it before: hammer/nail syndrome You are very strongly advised to stop trying to translate everything into your chosen profession and perspective, and instead to learn the science as it stands. You will be eternally stalled if you keep trying to hammer 27 round climate pegs into one, square EE hole.
  25. jpat, You should also consider that heat is not distributed evenly across the planet. A mean global temperature change of ∆T may be conceptually divided into at least 7 separate values by latitude (although the actual system is continuous in nature): equatorial, and NH and SH values for sub-tropical, temperate, and polar. These changes are non-linear (for instance, warming now is much greater at the equator than the south pole, and much greater at the north pole than the equator. As already explained, the strength of the effects of such temperature changes on feedback factors (ice, vegetation, ocean dynamics) vary by latitude as well, so now for each latitude you have a different ∆T, a different area, a different potential for impact (due to area, amount of insolation, and angle of incidence of the sun) and so an entirely different feedback value dependent upon absolute temperature and latitude.

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