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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 101 to 121 out of 121:

  1. jpat#91: "resampled the data to a 5 year interval using standard interpolation" There's a problem: Standard interpolation or not, you can't claim higher temporal resolution in processed data than you have in the raw data (you can claim higher SNR; that's the point of processing). But to say you've squeezed out 5 year resolution from irregularly sampled ice cores is akin to creating something (supposed information) from nothing (gaps in the original data). At the very least, you should filter the sampled data back below the aliasing frequency of the original and then realize you might not be fully imaging these hundred year lags.
  2. I'm done here but one final note. The toy model presented above is not an electrical circuit, its a graphical representation of a differential equation. Last time I checked Poincare and LaPlace had not been overturned.
  3. 102, jpat, But you cannot accurately depict this system with such a simple differential equation, hence a graphical representation of such an equation is no more accurate or applicable than the equation itself. The last time I checked, using a hammer to propel a car forward was ineffective, even though the effort does not overturn belief in hammers as effective nail-driving instruments.
  4. Briago1, A response to the first part of your question here.
    1) ...as the atmosphere warms that will release more carbon dioxide from the oceans making the atmosphere even warmer. This can not be the case otherwise if the atmosphere ever got a little warm, it would be doomed to continue to heat up.
    You are confusing a runaway scenario with a simple positive feedback. If the CO2 released by the ocean warms the atmosphere less than the amount that caused the CO2 release, then as things warm, the ocean will release less CO2 in response. In math it is a simple convergent series, like 1 + 1/2 + 1/4 + 1/8 ... → 2.
  5. Nealjking, Thanks for your explanation, and especially thanks for the maths! I know you have stated that your mathematical analyses is overly simplified, but considering an increase in temperature can allow a rapid rise in absolute humidity, inducing an increase in radiative forcing which is not restricted to a logarithmic limiter, shouldn’t your advanced explanation at least take this into account. I still can’t see that earth could be subject to a Venus type runaway, but the argument doesn’t seem to be as obvious as you make out. To me the main danger is that the upper troposphere warms enough to allow a parcel of air to rise through it without dew point being reached. Is that so improbable at low temperatures and pressures?
  6. I think that modeling this situation as a linear feedback system is both improper and incomplete. I can understand the idea of the amplification. But as a linear feedback system it is unstable yet we do not actuall experience runnaway global warming.
  7. William Haas @106: Suppose that increasing the temperature by 1 degree C brings about effects that result in a further 0.6 C increase in temperature. That 0.6 C increase will as a result bring about a further 0.36 C increase, which in turn will bring about a further 0.216 C increase, and so on. As can be easily checked, this series converges on a total increase (ie, the sum of all individual increases) of 2.5 times the initial input. The formula is that the total response, f, equals 1 divided by (1 minus the gain at each step, g), ie: f= 1/(1-g) which for a gain of 0.6 times the initial stimulus gives a feedback of 1/0.4 = 2.5 times the initial stimulus. I note that Sphaerica has already covered this ground in his response to you at 104. I have chose a slightly different example simply because with a gain of 0.6 times initial stimulus, and an initial stimulus from doubling CO2 concentrations of 1.2 degrees C, the expected climate response from doubling CO2 would be 1.2*2.5 = 3 degrees C. There is nothing unstable about that. There is no possibility of a runaway effect. The only thing there is a possibility of here is your continuing to ignore simple mathematics in pursuit of an agenda. Ergo, if you do continue to ignore this simple mathematics, that will be proof that you are in fact simply trolling. Either learn, or leave.
  8. William Haas in climatology by runaway warming (or cooling) one does not mean an endless warming (cooling) like what you get mathematically from the feedback factor f=1/(1-g) for g~1. Usually it indicates that above a certain threshold the process will not end untill the system finds a new equilibrium in a radically different state. The feedback factor quoted above is just a first order aproximation which has, by definition, limited validity. When the response becomes large, higher order terms need to be taken into account. More details here.
  9. Re: the statement in the post that it is "virtually impossible to trigger a true runaway greenhouse in the modern day by any practical means" Hansen discusses the Venus syndrome in this AGU Bjerknes lecture. I don't know why people say he thinks this is a "very remote" possibility. The words he actually uses are he thinks it is a "dead certainty" if humans are actually stupid enough to try to burn all fossil fuels they can get their hands on. He noted that "our model blows up before the oceans boil", in other words he is unable to simulate the scenario, but he says the model "suggests that perhaps runaway conditions could occur with added forcing as small as 10-20 W/m2. When discussing feedbacks he points out that what caused the last ice age, the Milankovich forcing, was very small, somewhere around 0.25 W/m2, but the eventual forcing that resulted as the ice sheet and vegetation feedback and the greenhouse gas feedback kicked in was 6.5 W/m2.
  10. David Lewis @109, in that lecture, Hansen says,
    "Our model blows up before the oceans boil, but it suggests that perhaps runaway conditions could occur with added forcing as small as 10-20 W/m2."
    A 10 W/m^2 forcing, the lower limit of the range where a runaway greenhouse effect could be caused represents a 2.7 times doubling of CO2 concentration, or 1800 ppmv. To put that into perspective, the complete combustion of all currently known and speculated oil, gas and coal reserves would raise the CO2 concentration in the atmosphere to 1366 ppmv, or approx 8.5 W/m^2 forcing. Even where we foolish enough to do that, we could not realistically do it in a century, so there would be plenty of time to turn back from our folly. What concerns Hansen is the possible massive release of Methane from clathrates and from thawed tundra adding over 1000 ppmv of CO2 to the atmosphere; or if done quickly enough, acting as a direct forcing of much greater magnitude. Fortunately the probability of a methane release of that magnitude, or a rapid enough methane release to lift the forcing over the 10 W/m^2 level for the possibility of a runaway greenhouse effect is remote. I note that Hansen indicates that the run away effect is a "dead certainty" not only if we consume all coal, but also all tars (shale oil & tar sands). That indeed would lift CO2 concentrations above his lower limit for possibility, which together with a clathrate gun is would lift CO2 concentrations beyond the level where his model blows up. Again, however, this is not a prospect for this coming century because we cannot consume the coals fast enough on any likely economic scenario. More to the point, Hansen is simply wrong about this prospect, as is elegantly discussed by Chris Colose in his recent blog post on SkS. The upshot is, if we go for a suicide pact (burn all tars and coal), we can in fact make large portions of the planet literally uninhabitable by warm blooded creatures; but we cannot turn Earth into another Venus, or extinguish all life on Earth. (If nothing else, hypothermophiles will continue to hold on in geysers and ocean ridges. I'm sure that is a comforting thought.
  11. How exactly is this article "debunking" anything? Of course no one wants to indicate with "runaway feedback" that the warming/cooling will go on forever; Venus also stabilized at about 450°C. Remember, the claim by anthropogenic climate change enthusiasts is that the water vapor feedback only amplifies an initial temperature change by a specific factor of 1 or 2 and then mysteriously stops instead of going all the way to the maximum/minimum. I honestly fail to see how playing around with solar irradiance levels is going to show us how this feat is achieved.


    [DB]  "no one wants to indicate with "runaway feedback" that the warming/cooling will go on forever"

    No scientist says that it will.  Even Hansen has walked back from that:

    "With the more realistic physics in the Russell model the runaway water vapor feedback that exists with idealized concepts does not occur. However, the high climate sensitivity has implications for the habitability of the planet, should all fossil fuels actually be burned.

    Furthermore, we show that the calculated climate sensitivity is consistent with global temperature and CO2 amounts that are estimated to have existed at earlier times in Earth's history when the planet was ice-free.

    One implication is that if we should "succeed" in digging up and burning all fossil fuels, some parts of the planet would become literally uninhabitable, with some time in the year having wet bulb temperature exceeding 35°C.

    At such temperatures, for reasons of physiology and physics, humans cannot survive, because even under ideal conditions of rest and ventilation, it is physically impossible for the environment to carry away the 100 W of metabolic heat that a human body generates when it is at rest. Thus even a person lying quietly naked in hurricane force winds would be unable to survive.

    Temperatures even several degrees below this extreme limit would be sufficient to make a region practically uninhabitable for living and working.

    The picture that emerges for Earth sometime in the distant future, if we should dig up and burn every fossil fuel, is thus consistent with that depicted in "Storms" — an ice-free Antarctica and a desolate planet without human inhabitants"

    Beyond that, it is quite well-established that water vapor is a feedback to temperature changes, and not a driver of them.

  12. Hardly "mysteriously" stops. In a simply analogy water (and other climate) feedback has gain factor of less than one, so asymtophically reaches it limit.

    See comment 107 above.

  13. @scaddenp

    Actually, as a natural climate change proponent I wholeheartedly agree that the "gain factor" of the water feedback loop is smaller than 1 - so much so in fact, that the amplification in practice is indistinguishabble from 1. So I am a bit at a loss how to discuss a purely hypothetical situation that I don't believe to be true. The concrete assumptions ("model") going into it would need to be spelled out, the article(s) here quite tellingly avoid to discuss the actual water vapor feedback case.

    But there are clear indications that any attempt to model a water vapor feedback loop that results in any significant amplification at all, would find it very hard to avoid it being of the runaway type. Note, for instance, the highly nonlinear water carrying capacity of air at different temperatures. The (at least implicit) position of the greenhouse modeling industry is certainly that a runaway feedback loop will happen - they have proposed a hypothetical mechanism to counteract it (See answer to DB)


    I am sincerely puzzled how you managed to so completely misunderstand the issue. Of course Hansen has backed off his claim, because it is actually highly damaging to his case. The point is this: The anthropogenic climate change adherents claim that there are powerful positive feedback loops prevalent in the global climate system, in particularly water vapor feedback (In contrast to natural climate change proponents that maintain that negative feedbacks dominate).

    But if the claims of the anthropogenic climate change adherents are true, then we should have a bipolar climate: Any small initial temperature change would lead to a runaway feedback loop until some maximum (or minimum) ist reached. If the maximum is high enough to completely destroy the current atmospheric system in a venus like scenario it will stay there. If it is not so high, another induced small initial temperature fall would lead to a drop to the minimum. Depending on the exact assumptions, the climate would either be permanently stuck in either the minimum or the maximum or it would bounce between the two extremes. In any case, only the minimum or the maximum temperature would be stable (or both semistable).

    This is very obviously not how the climate on earth works. Therefore, climate change adherents need to find some explanation why the feedback loop should stop before reaching its conclusion. The explanations on this site are, in the famous words of Pauli, "not even wrong", they simply don't address the issue.

    I actually found it surprisingly difficult to find the "official" explanation of the greenhouse modeling industry for such an important issue. It is supposed to work somehow like this: the models can be made to move warm air in the tropics to higher tropospheric altitudes, where energy is more efficiently radiated into space. This can be made to be dependent on the rate of warming by the water vapor feedback loop, therefore providing a means to counteract and stop it. By turning the knobs of the models appropriately, about any cutoff point desired can be selected, leading to the wide range of assumed amplification factors from 1.5 to 4. As obviously arbitrary, self serving and lacking in evidence as this hypothesis is, it is still better than the attempts here which, as I wrote, don't address the issue really at all. I suggest therefore to delete the current answers here and replace them with the "official" narrative.


    [DB] Please keep in mind that the burden of proof is on the claimant, you, to provide source citations for claims.  Simply making unsupported assertions is not how dialogue is kept in this venue.

    Moderation complaints snipped.

  14. Menschmaschine @113,

    It was suggested that you read coment #107 above but you plainly have not.

    A feedback system with a gain of g=1 or g>1 will cause a runaway situation. The feedback from a climate forcing is expected to triple an initial pertubation (ECS=3 with feedback, =1 wiithout) and this would therefore correspond to a feedback gain of g=0.6667. This is what @113 you call "the (at least implicit) position of the greenhouse modeling industry." There is no "proposed ... hypothetical mechanism to counteract" runaway climate change as for today's climate g<1 and a runaway situation thus cannot happen.

  15. I apologize if this was addressed in the articles and/or comments and I missed it — given the limitations on feedbacks and the current rates of increasing CO2, what is the scientific consensus of where we "max out" on temperature increase from today's levels? As in, how much hotter will it get, by when? Thanks in advance for replies.

  16. AFT - if we keep adding CO2, then it keeps getting hotter.  Do you mean how much hotter would it get if we stopped adding CO2? There have been papers on this - see the articles at Realclimate that discuss them.  The CMIP5 model projections are the best guide to what temperatures will be under various emission scenarios. See below for summary but looking at the IPCC AR5 WG1 report would give you a lot more detail. The "RCPxxx" are the different emission scenarios considered. 2.6 is what happens with stringent mitigation of emissions, and 8.5 at the other extreme is a continue to burn all we can scenario.


  17. I (think I) get the concept of the feedback curve's shape. So my follow-up question is — how do we now what part of that curve we are on? I assume that the experts are concerned that we are still on an "earlier" part of that curve, where feedback effects are "significant" vs. a "later" part of that curve, where feedback effects are "not significant"?

    My layman's question comes from this — if we have already made large jumps in CO2 output, have we already experienced the "steeper" part of the curve, and any go-forward increases are out "in the flat tail" with much smaller effects?

    Any commentary would be appreciated.

  18. My latest comment posted simultaneously with scaddenp's reply, so I will look to see if you just answered my latest question too. Thanks.

  19. Climate feedback is not a simple curve. There are different feedbacks that work on different timescales. The water vapour feedback is more or less immediate while albedo feedback from melting ice and landcover changes is very slow. Again, the models are the best guide we have for forecasting the future. On the scale of centuries to millenia carbon feedbacks, (from reduced solubility of CO2 in the oceans, CH4 release from tundra) are also important (major components of the milankovitch-driven ice-age cycle) and not well-captured by models. However these are not likely to be much of a factor in next 100 years.

  20. This was useful.  Glad I read it.  I'm curios about where the curve for the "blue line" comes from.

  21. Please note: the basic version of this rebuttal was updated on June 2, 2024 and now includes an "at a glance“ section at the top. To learn more about these updates and how you can help with evaluating their effectiveness, please check out the accompanying blog post @ https://sks.to/at-a-glance

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