Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Donate

Twitter Facebook YouTube Pinterest

RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

Keep me logged in
New? Register here
Forgot your password?

Latest Posts


Why positive feedback doesn't necessarily lead to runaway warming

Posted on 13 September 2010 by nealjking

Some skeptics 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 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.

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.

Note: this is a new rebuttal written by Neal J. King to the skeptic argument "Positive feedback means runaway warming", a sentiment sometimes expressed in comments on this website. In a first for Skeptical Science, Neal actually wrote all 3 levels of this rebuttal in one fell swoop. So as well as the Basic Rebuttal (which is used in this blog post), those seeking a little more meat can opt for the Intermediate Rebuttal which goes into more detail about gain factor. For the climate tragics (you know who you are), the Advanced Rebuttal lets you dive into the equations Neal used to derive his results.

0 0

Bookmark and Share Printable Version  |  Link to this page | Repost this Article Repost This


1  2  Next

Comments 1 to 50 out of 73:

  1. So Dr. Hansen is an alarmist?
    0 0
  2. This is off-topic but, as usual, I don't know where to post it.

    For my book-in-progress on sea level rise, I'm making New Orleans one of my "poster children of sea level rise" and have just written a few pages on it. If anyone is interested in reading this draft, please contact me off-list at
    0 0
  3. re bmus:

    Has Hansen said there is no limit?

    If he says the limit is greater than what others are saying, it wouldn't suggest he has deviated from what this article has pointed out, since he would be suggesting a limit.

    'Alarmism' is meaningless in the context of this article.
    0 0
  4. beam me up scotty,
    he's not. Or at least, it depends on what you mean by alarmist. Hansen did not say that runaway warming is the most likely outcome. Here are his words (p. 24):

    "In my opinion, if we burn all the coal, there is a good chance that we will initiate the runaway
    greenhouse effect."

    His sentence begins with a caveat, a big if.
    0 0
  5. If the limit is above 100C, we will end up like Venus, because we will cook the oceans.
    0 0
  6. Let me speak for Neal and explain that this article is meant to address the sentiment often expressed in comments here on Skeptical Science: if our climate has net positive feedback, why haven't we experienced runaway warming? So this rebuttal establishes one very simple point - you can have net positive feedback without experiencing runaway warming.

    So the rebuttal is not saying runaway warming is impossible. Given a strong enough positive feedback or a strong enough warming, it may be possible. But that's not the point of this rebuttal.
    0 0
  7. What about the release of methane?
    0 0
  8. Needed to read more carefully. Sorry
    0 0
  9. Maybe the title needs to be changed.
    Something like 'Why positive feedback doesn't necessarily lead to runaway warming'

    And in the article, have something like 'although positive feedback can result in runaway..., it doesn't necessarily result in runaway..., here's why...'
    0 0
    Response: I'm having a deja vu from this morning. Tonight when I tweeted this blog post, I inserted a (necessarily) into the tweet. Again, thought briefly about updating the blog heading, decided against it (mainly due to laziness). I hope all the authors (who are doing a fantastic job writing all these rebuttals) aren't getting too fed up with me creating all the distractions from their articles :-(
  10. The Ville #9
    You beat me to it. I was going to comment the exact same thing, however, one could also question what is meant by "runaway". Doesnt the transitional state imply some degree of runaway?
    0 0
  11. A runaway greenhouse is one where the final temperature causes the ocean to boil away. At that point there is no way for a return to a lower temperature. Venus has a runaway greenhouse, so we know it is possible. Even there the temperature stabalizes at a high temperature. Dr. Hanson studied Venus and he feels that it could be a problem on earth if enough CO2 is emitted. The consensus is that it is unlikely to happen. Even without a runaway greenhouse it can get too hot for most of us.
    0 0
  12. In a first for Skeptical Science, Neal actually wrote all 3 levels of this rebuttal in one fell swoop.

    Nice work, Neal. Congratulations on being the first one to accomplish this! It will be interesting to see how different people are able to make use of the basic, intermediate, and advanced versions.
    0 0
  13. Nice job Neal! All levels are excellent.
    0 0
  14. The biggest problem with the IPCC predictions is this.

    The sensitivity for a CO2 doubling is somewhere around 1.1C.

    The IPCC suggests that is the lowest possible 21st century trend
    ( not too far from the observed thirty year trends from multiple temperature records - land, ocean, lower and middle trop from satellites ).

    So the IPCC indicates that net zero feedback is the LOWEST possible outcome.

    Since there are known negative and positive feedbacks,
    I would suggest that a balance of feedbacks for a net zero feedback
    would be the MOST LIKELY outcome,
    not the lowest possible as the IPCC suggests.

    That would mean the MOST LIKELY outcome a response somewhere about the 1.1C per century.

    The problems with claiming the water vapor feedback
    (which is modeled to be by far the greatest
    positive feedback, even exceeding the original forcing from GHGs) are:

    1. the actual observations (sonde data and ICCSP) indicate a negative feedback
    due to drier air aloft over lower level more humid air
    ( there, of course difficulties in measuring humidity, even greater than
    the difficulties measuring temperature, but that's what the data indicate).


    2. the models do not well accommodate the dynamics of moisture distribution.

    ( were the upper troposphere to become more humid,
    it would lead to increases cooling of the upper troposphere:

    which in turn leads to subsidence,

    which also leads to greater loss to space of IR)

    The global area of subsidence greatly dominates the global area of rising
    air ( convection takes place mostly at the ITCZ and
    along frontal boundaries, subsidence, largely caused by IR cooling aloft covers a much larger area even though the mass of air exchanged should balance. ):

    Finally, lots of papers demonstrate a seasonal temperature humidity correlation.

    But it's important to realize that a large
    contribution to this is dynamic ( dynamic migration of the ITCZ, subsidence and cold(dry) air mass migration in the winter hemisphere, etc. ).

    Observations do support some moistening of
    the surface, but since drier air over more humid air
    actually increases the IR cooling rate, it may well be that the water vapor feedback is actually negative.
    0 0
    Response: I'd really appreciate it if for future reference, you could add width=450 to your larger images so they don't break the website format. Thanks.
  15. ClimateWatcher, CO2 has increased from ~280 ppm to 390 ppm... about 40% of one 'doubling'. Temperatures have increased just over 0.8 C... about 73% of 1.1 C.

    Do you still think 1.1 C total warming for a doubling of CO2 is the most likely outcome?
    0 0
  16. "ClimateWatcher, CO2 has increased from ~280 ppm to 390 ppm... about 40% of one 'doubling'. Temperatures have increased just over 0.8 C... about 73% of 1.1 C.

    Do you still think 1.1 C total warming for a doubling of CO2 is the most likely outcome?"


    Somewhere around that.

    The century trends since the MSU era began look like:

    RSS MT 0.9K/century
    UAH MT 0.5K/century
    RSS LT 1.6K/century
    UAH LT 1.3K/century
    CRU 1.6K/century
    GISS 1.6K/century
    CRU SST 1.3K/century

    That yields a mean of 1.3K/century.

    Fairly consistent.

    Your look at long term variations is a worthy notion,
    but do recall:

    1. There is not a good yearly, or even decadal correlation of GHG forcings and temperature trends.

    2. Gasses other than CO2 (CH4, NO2, CFCs) accounted for a large but now declining portion of GHG forcing during the 70s and 80s:

    3. In fact, from 1910 through 1945, the CRU
    land/ocean index rose at a rate (1.7K/century) that
    is greater than the current rate (1.6K/century), even though GHGs were increasing at a rate of about one third of the recent rate.

    If one assumes that recent warming is due to GHGs,
    then something else must have contributed to
    the 1910-1945 warming to keep the scaling consistent.

    This period is certainly consistent with an increase in insolation:

    So too, of course, is a period of centuries long
    high temperatures commencing in the latter half of the twentieth century.

    From the CRU data set, the recent warming is not
    at a record rate, though it has taken us to higher
    absolute temperature levels.

    By the way, the IPCC says the
    "Best estimate for a 'low scenario' is 1.8 °C"

    They describe the 'low scenario' as the 'most optimistic'.

    When was the last time you heard the warming is
    taking place at a rate better than the 'most optimistic'?
    0 0
  17. though chlorophyll using organisms have been found quite high in the atmosphere (in storm whipped marine clouds), no one has proven they could live and proliferate there in the extreme heat, discussed in this thread, thus eventually decreasing the IR absorbance. in any case the temperature on the current ocean sea level would have to be higher than 100 since there are many mountainous areas which could become hot pools of photosynthetic bacteria. But, unlivable (meaning in case too hot) conditions for humans are quite possible in the tropics by human means AFAIK.
    0 0
  18. Thanks, this is a nice addition to the internet posts on feedbacks

    I would urge readers to be cautioned of a few subtleties in the post by neal which was not made explicit, and could lead to some confusion for first-time readers. For one thing, he was not explicitly talking about the radiative-based feedbacks (e.g., water vapor increase, albedo change in a warmer world) but rather carbon-cycle feedbacks (such as more CO2 ==> warming ==> more CO2) which involves complex interplay between the bio geochemistry of the ocean-land system. The notion of a converging series that can stabilize the system is valid in either case, but the carbon-based feedbacks are not that well understood and usually they become irrelevant if you choose to define a sensitivity metric in terms of a doubling of CO2 (as is commonly done).

    When we're talking about whether feedbacks are "positive" or "negative" in the context of global warming, we are *usually* talking about the radiative balance of the planet being further modified by the temperature change caused by some forcing (the most relevant forcing being CO2 in the modern day). This is also semantical to some extent, because the most dominant stabilizer of planetary temperature change is the Planck blackbody radiation law, which simply means hot stuff emits more radiation. This is not classically called a 'feedback' however, and so the notion that feedbacks are "positive" is a bit artificial given the lingo in the field. Rather, positive feedbacks should be thought of as modifying the Planck response, essentially smoothing out a plot of the outgoing longwave radiation vs temperature.

    I have been recently collaborating with Rasmus at RealClimate to put up a post about feedbacks, which hopefully will provide some clarity into this topic, including the runaway greenhouse. I'm hoping that can be available soon.

    The answer to the question about the runaway greenhouse effect is that CO2 is not a big part in that process. I'm not entirely sure Hansen understood this, but I've also not followed his public statements closely to know what caveats he might have made or what distortions other people may have spun on it. In fact, the runaway greenhouse process occurs when the solar radiation exceeds something known as the Simpson-Kombayashi-Ingersoll limit, and the runaway threshold is almost exclusively determined by the absorbed incoming solar radiation and a planet's gravity. The SKI limit is basically a 'maximum outgoing longwave radiation' threshold that occurs when water vapor can become a dominant constituent in the atmosphere (on Earth or Venus, it doesn't need to be water vapor though in general).

    Finally, ClimateWatcher is simply wrong on almost everything he said. I'd urge him to review the Dessler and Sherwood (2009) paper, or the observations of upper air moistening (e.g., see some of Brian Soden's work).
    0 0
  19. ClimateWatcher - minor point for readability, you should look at the Comments Policy for the instructions on posting large images, under "IMPORTANT: The image must be no wider than 450 pixels".

    The extremely large images you posted tend to break the site layout.

    Moderator - can these be reframed with the appropriate HTML to bring their size back down?
    0 0
  20. Gah - the link I put in my last post for the Comments Policy was poorly formed; my apologies.
    0 0
  21. "Finally, ClimateWatcher is simply wrong on almost everything he said. I'd urge him to review the Dessler and Sherwood (2009) paper, or the observations of upper air moistening (e.g., see some of Brian Soden's work)."
    I have read Dessler and Sherwood.

    Was there a particular passage that you found relevant?

    I am taken by the upper air decrease in both absolute humidity:

    and relative humidity:

    as outlined by Paltridge, Arking, and Pook:

    Particularly in the tropics, the region that Dessler and Sherwood assess to control water vapor feedback.

    This appears to be consistent with the ICCSP data as well:

    0 0
  22. Thanks for your comment Chris, that is really helpful.

    I have been wanting a public discussion of the idea of 'runaway' for ages- it seems very hard to get one.

    Mark Lynas in his book "6 degrees" created a scenario in which we pass 2 degrees C above pre-industrial and then things are taken out of our control by a sequence of events and we rise inexorably to 6.

    This did not seem to me like the picture I got from reading science, apart from as a tiny possibility, hardly one that is regarded as likely. But I have not heard any climate expert address it specifically, and many have been very positive about Lynas's book, which I found odd considering what they said elsewhere. What do you think about this?
    0 0
  23. The Paltridge paper is a re-analysis product, it's not observations. No one really thinks that these are appropriate to use to diagnose water vapor trends, which suffer from changes in instrumental over time. For instance, Soden et al (2005) quote:

    "“Although an international network of weather
    balloons has carried water vapor sensors for
    more than half a century, changes in instrumentation
    and poor calibration make such
    sensors unsuitable for detecting trends in
    upper tropospheric water vapor (27). Similarly,
    global reanalysis products also suffer
    from spurious variability and trends related to
    changes in data quality and data coverage (24).”

    A relevant reference to look at here is

    Trenberth, K. E., J. Fasullo, and L. Smith, 2005: Trends and variability in column-integrated atmospheric water vapor. Clim. Dyn., 24, 741-758

    Note also that other reanalysis products show different results.

    Josie-- my interests are mostly in the physical science rather than impacts, but I read Lynas' book a while ago. I think it was a good read. You need to be careful about extrapolating conditions in the past, such as "it was 3 degrees warmer before and this happened, therefore it will happen again" or that things which didn't happen before won't now. It's hard to look at other examples to use as a template for future impacts because we don't have any past analogs for human-induced climate change in the past, but again, this isn't my area of heavy interest.
    0 0
  24. "The Paltridge paper is a re-analysis product, it's not observations."

    The re-analysis is re-analysis of observations.

    No one really thinks that these are appropriate to use to diagnose water vapor trends, which suffer from changes in instrumental over time.


    But that is the only data set we've got, and it
    indicates drying aloft.

    To be sure there are differing hygristors and humidity is even
    more difficult to measure than temperature.

    But if it was instrumentation causing the trend, one would
    expect the trend to change in recent decades as sondes became
    more consistent and better. But the trends appear consistent,
    even through the 2000s.

    "Trenberth, K. E., J. Fasullo, and L. Smith, 2005: Trends and variability in column-integrated atmospheric water vapor. Clim. Dyn., 24, 741-758"

    In should be noted that drying aloft but increasing humidity
    near the surface is not inconsistent with a trend of increasing
    total column water vapor which this paper refers to.

    That is significant because, while CO2 is well mixed,
    water vapor decreases rapidly with height. CO2 forcing
    at a given level is then based on the temperature profile.
    H2O on the other hand causes forcing based on the temperature
    profile and the change of humidity with height.

    A steep lapse rate of humidity causes more cooling than
    a uniformly mixed amount of humidity for a normal temperature

    "Note also that other reanalysis products show different results."

    All the reanalysis I have seen indicates drying aloft.

    Do you have a reference to any which indicate otherwise?
    0 0
  25. All this analysis is very good, but I think the reason "positive feedback" is so widely misunderstood is still being overlooked.

    The reason people think that "positive feedback" always leads to runaway is because they are all familiar with one example where it DOES lead to runaway: the PA systems that squeal loudly instead of amplifying normally. This is because the feedback to the amplifier is positive, and positive feedback to an amplifier DOES lead to runaway (unless it is narrowband and at just the right phase); now if you dampen the runaway by tuning the phase, you can use this to turn an amplifier into a (crude) oscillator.

    As puts it, "Positive feedback often leads to exponential divergences or exponential growth of oscillations."

    But even here, they were careful to say 'often', not 'always'.

    So all we really need to point out is that the familiar example is a special case: only in such special cases does positive feedback necessarily lead to runaway. In general, it leads to large but not necessarily divergent values (usually of oscillations).
    0 0
  26. Could someone explain why we aren't seeing the warmest temperatures ever?
    0 0
  27. Re: cruzn246 (26, 27)
    "Could someone explain why we aren't seeing the warmest temperatures ever?"
    Sigh. You must really learn to be more patient.

    Here you go: "NASA reports hottest January to August on record; August tied for hottest in UAH satellite record".

    Do try to get more sleep. It's good for the body; and the soul.

    The Yooper
    0 0
  28. cruzn246 - are you try to say that temperature should increase linearly with CO2? (ie CO2 has gone higher, why hasnt temperature?). This might apply on a dry uniform planet, but the climate system here has a lot of internal variability and a vast ocean store to hide energy. If you look at individual model runs, you will notice that huge variability. You can periods of up to a decade with little happening - followed obviously by steep warming. On a decadal scale climate models dont have much skill because they are too sensitive to initialisation.
    0 0
  29. I've no argument agaist Ingersoll (SKI) limit, but want to point out that as long there's talk of CO2 various responses of biosphere should be taken in account. Clearly there are quite specific limits for a specific ecosystem to be located where they are, as the clines of various species are more an exception than the norm. These ecosystem survival limits could then be a deciding factor on where the temperature of the whole earth will eventually settle. As (generally) plants are more hardy than any insects (they were the first to colonize the land), the question becomes a one of mobility, that is the amount of dispersal of the species. It is the speed of change that does more damage than the (moderate amount) of warming, as insects and decayers(f.e.fungi) are faster in dispersal than the plants. Additionally the increased variability of the system with more energy creates problems, though on this point I'm of the (scientific) opinion that some of this increased variability stems from the location of the GIS, that may well affect the westerlies normally present on the southern Greenland latitudes.
    0 0
  30. Putting this up here as an example of what sort of research can be done on, of the heat stresses in natural systems, maybe someone will experiment with higher increases in surface absorption of heat (what was the greenhouse wattage on the surface again? (Further note: the experiment should include producing the higher humidity to the experiment location as well (not really easy to do)):
    0 0
  31. 6, John Cook: Thanks for stepping in. This is my first post in SkS, so I forgot that I have certain responsibilities for my "children".

    1 & 8, beam me up scotty: Yes, I was also surprised that Hansen had made that remark. Riccardo and Chris Colose have dealt with the substance of that question already. My impression (formed in large part by some explication, by Chris Colose, on this topic in the RealClimate site; and largely reproduced above at 18) is that Hansen was discussing what could happen if a lot of things go very bad; so, kind of a corner condition. On the other hand, Stephen Hawking and James Lovelock have taken a more apocalyptic stance: My impression is that the science is not with them on this point.

    25, MattJ: If you look at the Advanced version (link at the bottom of the posting), you can find an explicit discussion of the "classic" positive-feedback scenario (the squealing sound system), and the difference to the model of the carbon-cycle feedbacks.
    0 0
  32. ClimateWatcher: The evaluation of the overall feedback for global warming is an active area of research, and will certainly not be resolved here. The main intent of the post was to make the point that a system can have positive feedback without having an exponential blow-out.
    0 0
  33. 18, Chris Colose:
    - With regard to carbon-cycle sensitivity: I was thinking of fossil-fuel-based CO2 as an initial input, and the additional CO2 from out-gassing as being part of the system response. This was one reason I didn't discuss the case (in the Advanced version) of a 100% dollop of CO2, which would have invited direct comparison with the usual 2X-CO2 climate sensitivity.

    - In general, the calculation should not be taken too seriously, as it is intended as a demonstration of self-limiting positive feedback, and not really as a model for CO2 in the atmosphere. The main point was to show that the concept of diminishing returns, in the case of positive feedback, was not just a case of "special pleading".

    - I will be very interested to see the discussion you & Rasmus post on feedbacks.
    0 0
  34. 29, jyyh:

    I agree with you about the rate of temperature change and its ecological impact: I've done some simple estimates to show that an 0.1-deg-C increase in global average temperature would push ecosystems 15 km poleward; or about 10 m upward (geography & topography cooperating, of course). (This is another one of Hansen's favorite points, as well.) At current rates, that's quite a bit of distance for a tree to move in 10 years!

    A more refined look at just the borders of the tropical zone seems to lead to another factor of 3 or more for its rate of expansion.
    0 0
  35. Another article possible future scientists, doing research on plants' response on GW, will need:

    Note that the response curves end on 40 degrees Centigrade. I would have liked them to continue up to 70 degrees which is about the limit simpler plant enzymes will work. As I said above, bacteria can work at higher temperatures, which is (I guess) the reason it took so long for Eukarya to develop (if one takes evolution for granted).
    0 0
  36. jyyh,

    If you are talking about 70 degrees C, we are no longer dealing with climate change. We are talking about planet change.
    0 0
  37. nealjking, no argument there, 70 degrees C (global average) will change most of earth uninhabitable for all eukarya (higher life). I probably should try to track the whole RUBISCO response curve somewhere before continuing.
    0 0
  38. Found the following: "In the past, photosystem II (PSII) was considered a key weak link (Enami et al. 1994) but damage to PSII only occurs at high temperatures, often above 45 °C (Yamane et al. 1998)."

    So photosynthesis will weaken considerably in temperatures of over 45 degrees, just to set some limits to the tolerance of the ecosystems in the world. And of course if there's a cool season (like in India), annual plants will grow OK in the cool season.
    0 0
  39. Regarding : "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."

    Why is it that the negative feedback of the open ocean radiating heat is never mentioned? Ice makes a very effective blanket. If you're going to offer an "unbiased" opinion on feedbacks then at least mention them all for your examples.
    0 0
  40. I think this answer needs revising.

    The fundamental reason why positive feedbacks do not produce runaway warming is that the hotter the earth gets, the more heat it emits. Put another way, the hotter the earth gets, the faster it cools.

    The amount of infrared radiation emitted by Earth increases exponentially with temperature, so in simple terms, as long as some infrared can escape from the atmosphere, at some point heat loss overtakes heat gain.

    Yes, some positive feedbacks are diminishing - CO2 has less and less effect the more there is of it, for instance - but I don't think all positive feedbacks are diminishing.
    0 0
  41. TimTheToolMan, why exactly should an article explaining the mathematical proof against claims that 'positive feedback would lead to runaway warming' need to cite every imaginable feedback effect? They are immaterial to the issue at hand.

    Michael Le Page, when the article refers to diminishing returns it is in reference to a constant 'feedback multiplier' less than one being mathematically incapable of producing a runaway effect. You seem to instead be referring to cases where the 'feedback multiplier' itself decreases over time. That is an additional reason that positive feedback would not inherently lead to a runaway effect. Also, while heat output will increase to maintain equilibrium with heat input this doesn't really prevent runaway warming... look to Venus as an example.
    0 0
  42. 40, Michael Le Page:

    In the case you mention, the fact that heat radiation increases with temperature will put a lid on the temperature EVENTUALLY, but would not stop a runaway in all cases: As CBDunkerson pointed out, there actually has been a runaway in the case of Venus. Likewise, in the classic case of the sound amplifier, the output of which is fed into the input microphone, there is an ULTIMATE limit based on such constraints as maximum available power; however, these constraints don't have any effect until the system has gone into full squealing mode, due to the positive feedback. They then limit the volume of the squealing.

    This particular case, discussed in more detail in the Advanced version, shows that there can be a feedback that is ALWAYS positive, yet which is self-limiting. This case, as therein shown, also differs from that of the amplifier with constant gain less than 1: In that case, the output signal itself decreases to zero, rather than stabilizing.

    By the way: the power of a blackbody radiator goes like (Temperature)^4; it is not exponential in the Temperature.
    0 0
  43. 39, TimTheToolMan:

    - Case 1: Sea ice covers an area of ocean. The Sun's rays come down, bounce off into the sky. To excellent approximation, none of the energy is absorbed by the ice, nor is the reflected light absorbed by the atmosphere. Therefore, all that sunlight contributes essentially 0 to the radiant energy budget.

    - Case 2: Sea ice is gone, so that same area is now bare ocean. The Sun's rays come down, and, to excellent approximation, are completely absorbed by the water.

    a) This increases the water temperature a bit, causing it to radiate Infrared (IR) radiation a bit more than before. The radiation of the IR will bring the water's temperature down a bit - but not lower than what it was before the radiation hit: absorbing radiation cannot lower temperature. Therefore, some of that solar heat will be retained in the water, and some will be radiated as IR back to the atmosphere.

    b) The IR radiated back to the atmosphere will head upwards, but because of greenhouse gases, it will not stream outward as the visible light did, but be bounced around in the atmosphere. The rate of IR escape will not be as high as the rate of IR generation, so there will be a heat build-up.

    In summary:
    - When there is ice on the water, the sunlight makes no contribution to heating or cooling the ocean.
    - When there is no ice on the water, a portion of the sunlight's energy is absorbed and retained in the water, and the rest escapes to the atmosphere; of the escaped energy, some of it escapes to space and some of it is bounced back (resulting in heating).

    The "blanketing" capability of the ice becomes interesting if you think there is some reason that the water under the ice is going to be WARMER than the air, on the average.

    In either case, as CBDunkerson pointed out, this article is not a calculation of the actual climate sensitivity, but a demonstration of a mathematical principle: a system with positive feedback can demonstrate naturally self-limiting behavior, it can go for significant amplification without "running for the fences".
    0 0
  44. -------------------------------------------------------------------------------------------------
    "- Case 2: Sea ice is gone, so that same area is now bare ocean. The Sun's rays come down, and, to excellent approximation, are completely absorbed by the water."

    But only on the clear days:

    And only when ice has melted and the sun
    is more than ten degrees above the horizon.
    (melt season peaks nearer the autumnal equinox when the ceases to shine upon the pole, than the summer solstice when the sun rises to it's max of only 43 degrees or so above the horizon on the Arctic coast).

    "The "blanketing" capability of the ice becomes interesting if you think there is some reason that the water under the ice is going to be WARMER than the air, on the average."

    On annual average, of course, the Arctic waters
    are in fact warmer than the air temperatures above the ice. (and even at the same temperatures, the
    waters would still have a greater heat content).

    0 0
  45. ClimateWatcher,

    That cloudy days reduce the ocean's absorption of light is offset by the insulatory effect of the cloud cover. The ocean may not absorb as much, but it also can't radiate as much.
    0 0
  46. 44, ClimateWatcher:

    These points should be addressed/incorporated in a simulation that was intended to calculate climate trends (and thus climate sensitivity) by averaged-over dynamics.

    This article does not describe such a simulation.

    It's a simple analytical model intended to clarify an issue that has, according to John Cook, come up a number of times before: Yes, it is possible for a dynamical system to have a substantial positive feedback without blowing up.
    0 0
  47. ClimateWatcher... I have to say that I think your excessive use of charts and diagrams is not contributing to your arguments. It's more of a distraction. I'm personally curious about your perspective but find the charts break up the flow of the conversation.

    You might try imbedding links instead. There is a link at the bottom of the page for the proper technique for doing this.

    Just trying to be helpful.
    0 0
  48. nealjking @43

    Err yeah. Case three is that its Winter and there is no sun for 6 months. Scientists dont believe that we'll have an ice free Arctic all year around so its entirely reasonable to expect any heating that happened during the summer months will entirely be lost as the refreeze happens. Additionally there is now the possibility of a net heat loss from the oceans.

    I see you also believe the radiative heat loss is dependent on a difference of temperature between the atmosphere and the water. You should probably check your knowledge there :-P

    So I think the negative feedback of open oceans radiating heat is very relevent to an article that quotes it as an example and is considering runaway heating.
    0 0
  49. On the robustness of the Paltridge reanalysis paper, I will will only point to a coming in press paper by Dessler and Davis in JGR which address this issue explicity, although there are many places to find this sort of information. I really can't make sense of the rest of your argumentation, no one else seems to be paying much attention, and it clearly does not derive from the relevant scientific literature so I'm just going to take it as an ill-supported opinion and leave it at that.

    For those who can't access this in press article, this is the abstract:

    //"A recent paper [Paltridge et al., 2009] found that specific humidity in the NCEP/NCAR reanalysis declined between 1973 and 2007, particularly in the tropical mid and upper troposphere — the region that plays the key role in the water vapor feedback. If borne out, this result suggests potential problems in the consensus view of a positive water vapor feedback. Here we consider whether this result holds in other reanalyses and what time scale of climate fluctuation is associated with the negative specific humidity trends.
    The five reanalyses analyzed here (the older NCEP/NCAR and ERA40 reanalyses and the more modern JRA, MERRA, and ECMWF-interim reanalyses) unanimously agree that specific humidity generally increases in response to short-term climate variations (e.g., El Nino). In response to decadal climate fluctuations, the NCEP/NCAR reanalysis is unique in showing decreases in tropical mid and upper tropospheric specific humidity as the climate warms. All of the other reanalysis show that decadal warming is accompanied by a increases in mid and upper tropospheric specific humidity. We conclude from this that it is doubtful that these negative long-term specific humidity trends in the NCEP/NCAR reanalysis are realistic for several reasons. First, the newer reanalyses include improvements specifically designed to increase the fidelity of long-term trends in their parameters, so the positive trends found there are expected to be more reliable than in the older reanalyses. Second, all of the other reanalyses except the NCEP/NCAR assimilate satellite radiances rather than being solely dependent on radiosonde humidity measurements to constrain upper tropospheric humidity. Third, the NCEP/NCAR reanalysis exhibits a large bias in tropical upper tropospheric specific humidity. And finally, we point out that there exists no theoretical support for having a positive short-term water vapor feedback and a negative long-term one. "//
    0 0
  50. TimTheToolMan... Sorry, but last I read up on this, energy doesn't just disappear. So, 6 months of winter would not erase summer heating.

    Also, why is open ocean radiating heat a negative feedback? Can you explain this further?
    0 0

1  2  Next

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

The Consensus Project Website



(free to republish)



The Scientific Guide to
Global Warming Skepticism

Smartphone Apps


© Copyright 2014 John Cook
Home | Links | Translations | About Us | Contact Us