Excerpts from Evidence for Negative Water Feedback
by Clive Best
Abstract: Positive linear climate feedback for combined water effects is shown to be incompatible with the Faint Sun Paradox. In particular, feedback values of ~2.0 W/m2K-1 favored by current GCM models lead to non physical results at solar radiation levels present one billion years ago. A simple model is described whereby Earth like planets with large liquid water surfaces can self-regulate temperature for small changes in incident solar radiation. The model assumes that reflective cloud cover increases while normalized greenhouse effects decrease as the sun brightens. Net water feedback of the model is strongly negative. Direct evidence for negative water feedback is found in CRUTEM4 station data by comparing temperature anomalies for arid regions (deserts and polar regions) with those for humid regions (mainly saturated tropics). All 5600 weather stations were classified according to the Köppen-Geiger climatology [9]. Two separate temperature anomaly series from 1900 to 2011 were calculated for each region. A clear difference in temperature response is observed. Assuming the difference is due to atmospheric water content, a water feedback value of -1.5 +/- 0.8 W/m2K-1 can be derived.
I. INTRODUCTION
The Faint Sun Paradox was first proposed by Carl Sagan [1] who pointed out that the geological evidence that liquid oceans existed on Earth 4 billion years ago appears incompatible with a solar output 30% dimmer than today.
The sun is a main sequence star whose output is known to increase slowly with age. The total change in solar radiation over this long period turns out to be huge ~ 87 W/m2. It has been argued that an enhanced greenhouse effect due to very high CO2 and/or CH4 concentrations could resolve this paradox [2]. However, recent geological evidence does not support CO2 as being responsible but instead the authors propose a greater ocean surface leading lower albedo as a likely solution [3]. Others have suggested that high cirrus clouds effectively warmed the Earth [4]. Although the atmosphere must have been very different before photosynthesis began, the presence of large liquid oceans still implies that clouds and water vapor played a similar role in the Earth’s energy balance then, as they do today.
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Figure 1: Past temperatures extrapolating backwards from today (T=288ºK) assuming different linear feedback values.
It is apparent that a simple linear positive feedback of +2 leads to unphysical results. The basic problem is that if the temperature falls sufficiently so that 4σT3= F then a singularity occurs ~1.5 billion years ago. Instead a negative feedback value of -2 W/m2K-1 is more compatible both with current temperatures and with the Faint Sun Paradox..
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IV. CRUTEM4 ANALYSIS
Water vapor feedback in recent climate data have been investigated by studying differences between regions with very low atmospheric water vapor (Deserts and Polar) and those regions with very large water vapor content (Tropical Wet regions). The latest CRUTEM4 data [8] consisting of 5500 individual station data covering global land areas has been studied. Each station was classified by indexing its geographic location against the Köppen-Geiger climate classification [9].
“ARID” stations are defined as those with precipitation values ‘W’ or with climate ‘E’ in [9]. These are situated either in deserts or in polar areas having the lowest atmospheric water column on Earth [10]. “WET” stations are defined as those within fully humid Tropical areas – Climate ‘A’ and precipitation ‘f’ in [9]. These are situated in tropical rain forests or year-round humid climates having the highest atmospheric water column on Earth [10]. Global anomalies have been calculated for both stations ARID and WET stations independently using the same algorithm as used for CRUTEM4. The results are shown in Figure 4.
Figure 4: Temperature anomalies for ARID(DRY) stations in red and WET stations in blue. The smooth curves are FFT smoothed curves. The black dashed curve is an FFT smooth to the full CRUTEM4 global temperature anomalies.
There is a clear trend in the data that ARID stations warm faster and cool faster than WET stations. They respond stronger to changes in external forcing. The WET humid stations respond less than both the ARID stations and the global average.
Climate change is complex and global so it is reasonable to assume that both anthropogenic and natural forcing are reflected in the temperature anomaly data. For a given forcing DS the consequent change in temperature anomaly is gDT where g is a gain factor. The period between 1900 and 2005 is used to measure the temperature rise for each region DT1 and DT2 as given in Table 1. DS is assumed to be global in extent.
Table 1 : Temperature changes for ARID and WET regions and their ratio. Errors on DT are derived from differences between the FFT smooth and a linear fit.
| Period | DT1(DRY) | DT2(WET) | DT1/DT2 |
| 1900-2005 | 1.1 +/-0.1 ºK | 0.8+/- 0.1 ºK | 1.4 +/- 0.2 |
Heat inertia effects due to nearby oceans may cause tropical climates to react slower than desert regions, but not over such long periods. If positive feedbacks from increased water evaporation lead to enhance warming then this should be apparent in the tropics, and this is not observed. In fact the opposite is the case implying a negative feedback. Under the assumption that net water feedback F is present only for the WET stations (taking F=0 for ARID stations) then F can be measured from the data:
DT1/DT2 = 1 – G0F , where DT1 = G0DS and DT2 = G0 (DS+FDT2)
For G0-1 = 3.75W/m2K gives Water Feedback F = – 1.5 +/- 0.8 W/m2K-1
This is compatible with the value needed to resolve the Faint Sun Paradox. As has been pointed out by Lindzen [11] and others, much of the Earth’s heat is transported bodily through evaporation and convection to the upper atmosphere where IR opacity is low and can then escape to space. Therefore water feedback effects depend mostly on the water vapor content of the upper atmosphere. Increased evaporation, convection and consequent rain out could then result in lower humidity in the upper atmosphere. This is a possible mechanism for negative feedbacks in the tropics. Such effects would be largely absent in ARID areas, which have no local sources of evaporation.
Read the entire analysis here, it is well worth your time, well written, and easy to comprehend.
h/t to Scott Gates
At the risk of a sharp rebuke, how do the Faint Sun & a certain, somewhat obsessive scientist’s, Iron Sun theories, fit? If we do have this lump of Iron, as our primary heat source, will its output have varied over the Earth’s history?
SHARP REBUKE: We don’t discuss the “iron sun theory” here, to do so also opens the door to other way out there theory. Go to Tallbloke’s Talkshop for that sort of thing. – Anthony
“”””” Robert of Ottawa says:
May 27, 2012 at 6:07 am
Allan MacRae @ur momisugly May 27, 2012 at 3:39 am
if these feedbacks were strongly positive, life on Earth would be very different, if it existed at all
This cannot be emphasized enough. With positive feedback, any system will go to the extreme and stay there. Throughout its history, the Earth climate appears to have been remarkably stable, indicating negative feedbacks. “””””
Well the problem is that your assertion Robert, simply is NOT true…”””””With positive feedback, any system will go to the extreme and stay there. “””””
Positive feedback simply means, that a system output generates an additional input signal, that enhances the effect of the original INPUT signal. That additional signal is typically a function of the actual output; not necessarily a linear function.
Say we have a system (reasonably linear), such that an input signal of 1 apple yields an output of 5 oranges. So the gain is 5 oranges per apple. Now we connect up a positive feedback path, with a transfer function of 0.02 apples per orange.
So if my Son applies an input of 1 apple to the system, after some propagation delay time t, the output goes to 5 oranges, and a feedback of 5 x 0.02 (= 0.1) apple is generated and ADDED to the Son’s original apple, raising the system input to 1.1 apples. So at time 2t, the output now goes to
5 x 1.1 = 5.5 oranges, so the feedback amount now changes to 5.5 x 0.02 = 0.11 apples, and the total input now changes to 1.11 apples, yielding an output of 5 x 1.11 = 5.55 oranges at time 3t. well you can see what is happening; the total input will eventually reach 1.111111111111…..apples, and the output will settle down to 5.555555555555…..oranges.
The effect of the POSITIVE feedback of 5 x 0.02 = 0.1 apples per orange, is to change the system gain from 5 oranges per apple to 5.555555555555 oranges per apple, which is what you get from
5 / (1- 5 x 0.02). so the system gain is increased by the positive feedback; but it is not (as described) unstable. But if the positive feedback were increased to 0.2 apples per orange, the the gain would change to 5 / (1 – 5 x 0.2), and the gain would go to infinity.
If we changed the feedback to negative, so we got – 0.02 apples per orange added to the Son’s input of 1 apple, the new gain would be 5 / (1 + 5 x 0.02) = 4.54545454….oranges per apple. The ten times increase in negative feedback, would give 5 / (1 + 5 x 0.2) = 2. oranges per apple, which is quite stable.
The catastrophists are claiming that the feedback is positive, and also equal or greater than 0.2 apples per orange, so we get a runaway deluge of oranges.
I don’t know how many times I have to point out that H2O, O3, and CO2 ALL absorb solar energy from the Son, particularly the first two, and the result is that solar energy never gets to the deep ocean storage; and about half of it is lost to space as LWIR, while the other haf reaces the surface where most of it will prompt more H2O evaporation, thereby increasing the negative feedback. It is the stored ocean and rocks solar energy, that heats the earth; not the puny downdraft of LWIR from the (warm) atmosphere.
Notice I didn’t even have to invoke cloud albedo, to show the H2O negative feedback. A 25% reduction in apples from the sun, would lead to a drier planet, with less clouds, and less water vapor in the atmosphere; a lower albedo and solar energy loss to the atmosphere; but a frozen iceball earth couldn’t happen, because, there would be less clouds, and in the tropics even a 25% reduction in sunlight would be plenty to keep the oceans liquid.
Remember that the polar regions have ice because there is little solar energy there to stop it; that’s why it is cold. The ice is not making the earth cold.
Excellent!
Clive, thank you very much.
I’m linking to this article from my climate and weather pages.
that’s a 2.5 oranges per apple mod, not 2. for the big negative feedback.
Reblogged this on evilincandescentbulb and commented:
The water vapor mops up heat. As the vapor rises it leaves a cooler Earth behind. The water vapor rises and as it does the atmosphere becomes cooler and thinner and the water vapor eventually condenses. As it condenses the water vapor gives up its heat to the cold emptiness of space as the vapor returns to water and forms clouds or freezes and ultimately falls back to earth as rain, sleet, hail and snow.
Global Warming Vaporware and the Perpetual Motion Machine http://wp.me/p27eOk-nL
Really good paper Clive.
It should also be noted that the early Earth was not always warm. It was much colder than today for much of the past 2.4 billion years since the rise of oxygen.
It seems clear that cloudiness is a negative feedback – the climate models and the theory probably have it completely backwards. The water vapour feedback does not appear to be showing up either.
Phil Clarke: “the work appears to represent an incremental advance in our understanding of a problem that has already received attention in the peer-reviewed literature, and extends its conclusions beyond what is supported by the research methods and results“
Didn’t the author of that remark miss the point of the paper? The “standard theory” predicts that the dryer regions should have greater surface temperature increases than the wetter regions; analysis of the wet and dry regions shows that did not happen; ergo, the theory must be modified (at least if the result is replicated by others); the author of the paper presented a conjectured modification. Unless the reviewers are willing to criticize the soundness of the data that were used, the proper course of action would seem to be to publish the paper (perhaps with some changes to improve readability and remove a possible ambiguity, etc., etc., etc.)
richard verney says:
May 27, 2012 at 8:13 am
Some postulate that atmospheric pressure was double during the time of the dinosaurs.
=======
Everyone knows that the atmosphere itself contributes nothing to the warming of the planet. That a planet with an atmosphere would be exactly the same temperature as a planet without an atmosphere, unless the atmosphere has greenhouse gasses.
Ok, I’m just joking. However, I’ve heard this very argument made in lead posts on this site to rebut the notion that pressure as a result of gravity regulates atmospheric temperatures. Folks can’t have it both ways. If only GHG warms the planet, then pressure differences cannot make any difference.
In reality there is nothing unique about GHG in the atmosphere. N2 and O2 reduce outgoing radiation through conduction. Energy that would have escaped as radiation is instead captured as warming of the atmosphere.
N2 and O2 cannot radiate this energy. Instead they return this energy to the surface at night and at the poles through convection. This mimics the mechanism by which CO2 is assumed to warm the surface. Thus, if CO2 warms the planet, so does non GHG. The greater the pressure, the greater the density, the greater the effects of conduction and convection as compared to the effects of radiation.
Ah, nuts. I wrote: predicts that the dryer regions should have greater surface temperature increases than the wetter regions;
It’s the other way around. I wrote what the data showed, not what the “standard theory” predicted.
The water vapor mops up heat. As the vapor rises it leaves a cooler Earth behind. The water vapor rises and as it does the atmosphere becomes cooler and thinner and the water vapor eventually condenses. As it condenses the water vapor gives up its heat to the cold emptiness of space as the vapor returns to water and forms clouds or freezes and ultimately falls back to earth as rain, sleet, hail and snow.
Global Warming Vaporware and the Perpetual Motion Machine http://wp.me/p27eOk-nL
When climatists try to model earth climate as if it was analagous to an electronic feedback amplifier system, they don’t pay a lot of attention to the fit of their analog to reality. In a real electronic amplifier, you have an INPUT SIGNAL and a FORWARD GAIN or TRANSFER FUNCTION, and you get an OUTPUT RESPONSE. You also have another thing present that never shows up in any of the mathematics of the amplifier response including anyFEEDBACK PATHS. That missing element is A POWER SUPPLY OR SOUCE. Every thing an electronic amplifier does, is a consequence of that power supply or source. The rest of the paraphernalia simply controls the power source power, to determine where it goes. If you turn off the power the whole thing quits working; no matter what input signals or feedback loops you hook up.
With the earth system, the power supply is the Son. Well without the Son, we would have a different system, and maybe the heat of the earth core would be the power source for however long that would work.
So it is silly to talk about components of the system affecting each other, and simply ignoring their controlling effect on the power source.
Talking about how green house gases absorb some fraction of waste “heat” trying to escape the earth (it WILL escape), is simply re-rranging the deck chairs on the Titanic. No we all do understand how GHGs work; although some still trot out that silly “trace gas” notion. The trace boron, or phosphorous impurities in the silicon chips in this computer, are orders of magnitude less abundant (relatively), than any of the main GHGs in the atmosphere; and the computer won’t work without those trace impurities.
But meanwhile, there’s a big hole in the ship, and tons of ocean water is pouring in, and it is going to sink, no matter how the deck chairs are arranged. We should be looking to the power source, and how it is modified by controls, and not at the shape of the power or energy leaks in the sytem.
Some GHGs namely H2O, O3, and CO2 affect the power source much more than they affect the leakage.
(1) The climate models have a positive water vapor feedback and yet they don’t predict the climate to warm faster in the tropics than over drier regions. In fact, they predict faster warming in the arctic, as is observed. Hence, Clive’s hypthesis is clearly unwarranted.
(2) Since, over a large range of concentrations, forcing tends to be logarithmic in concentration, there is probably no reason to propose that the regions that are already more saturated would experience a higher enhancement of warming.
(3) The atmosphere is strongly coupled and one is limited in what one can conclude by making these sorts of local arguments.
In other words, Clive’s argument is just another example of why the “AGW skeptics” are not being taken seriously by the scientific community: They are not advancing serious scientific arguments, just floundering in desperation to prove what they want to believe for reasons that have nothing to do with science.
“”””” ferd berple says:
May 27, 2012 at 9:03 am “””
“”” N2 and O2 cannot radiate this energy. “”””
Of course they can; not as molecular resonance specral emissions, but certainly as thermal continuum spectra due to their local gas Temperature (being above zero K). They will stop emitting electro-magnetic radiation, as soon as you stop the N2 and O2 molecules from colliding with each other, or anything else. Under that condition the Temperature of those N2 and O2 molecules becomes zero Kelvins, and they stop radiating, since they do not accelerate, in collisions with other molecules
From Joeldshore.
“”””” (2) Since, over a large range of concentrations, forcing tends to be logarithmic in concentration, there is probably no reason to propose that the regions that are already more saturated would experience a higher enhancement of warming. “””””
You’re a scientist Joel. Please put some numbers to “a large range of concentrations”. Is that a million to one range, or is that a 10% range from 1.0 to 1.1
What does “tends to be” mean in scientific terms. The logarithmic function is a very well defined mathematical function whose properties are well understood. Things are either logarithmic, or they are not logarithmic. Nothing that is not logarithmic, tends to be logarithmic over any range.
So in amospheric CO2 since the Mauna Loa record began to be tabulated, the range of concentration of CO2 has gone from 315 ppm to 393 ppm which is a range of quite close to 1.25 to 1.0 Is that a large range of concentrations ? It’s the only reliably measured range we seem to have. So show us your preferred observed data of “forcing” that either is, or is not logarithmically related to that Mauna Loa data. Would your data (along with ML) be fit better by the functional form y = exp (-1/x^2), rather than y = log(x).
And try to be more scientific with your statements. For example, what means “probably no reason” ? Do you have some statistical analysis on that ?
joeldshore says:
May 27, 2012 at 11:38 am
(1) The climate models have a positive water vapor feedback and yet they don’t predict the climate to warm faster in the tropics than over drier regions. In fact, they predict faster warming in the arctic, as is observed. Hence, Clive’s hypthesis is clearly unwarranted.
>>>>>>>>>>>
Being a physicist, you know full well that SB Law dictates that a given level of forcing must produce a more pronounced temperature increase in the coldest areas of earth and the least in the warmest. BTW, the latitudes just below the arctic circles have actually warmed more than the (colder) arctic regions. The models are once again wrong on that point.
joeldshore;
(2) Since, over a large range of concentrations, forcing tends to be logarithmic in concentration, there is probably no reason to propose that the regions that are already more saturated would experience a higher enhancement of warming.
>>>>>>>>>>>>
Thank you for pointing that out. In other words, the tropics, which for the most part are very high humidity and thus saturated, are, accroding to you, likely to experience very little warming at all. It is good to keep in mind that, as you say, of any warming that does occurr, the least occurrs where it would do the most harm, and the most occurrs where it is most beneficial. Excellent point!
joeldshore;
(3) The atmosphere is strongly coupled and one is limited in what one can conclude by making these sorts of local arguments.
>>>>>>>>>>>>>
Oddly, I was given to understand that LW travels at the speed of light. One would think that areas with the most greenhouse gas presence would be early indicators of warming given that it would still take time for the absorbed LW to be redistributed even in a strongly coupled system.
joeldshore;
In other words, Clive’s argument is just another example of why the “AGW skeptics” are not being taken seriously by the scientific community:
>>>>>>>>>>>>>>>>>>>
That’s getting to be a tired argument Joel. How do you define the “scientific community”? What is “climate science” anyway? Allow me to answer that question as follows.
Climate is a highly complex and chaotic system that cannot be described by a single science. Understanding climate is understanding the intersection of physics, chemistry, biology and statistical analysis. Oddly, when most physicists look at the manner in which climate “scientists” have modeled the physics, they scoff. When chemists look at the manner in which climate “scientists” have modeled the chemistry, they scoff. When biologists look at the tree ring studies done by climate “scientists” they break up laughing. When statisticians look at the way climate “science” has handled statistical analysis of the data, they protest at the incompetance.
Sorry Joel, but the climate “science” community has nothing to do with science. They are a clique unto themselves that ignore the fundamentals of all the disciplines that must be taken into consideration when trying to understand how they interact with one another to result in the big picture we call climate. One might as well define literature as being exclusively comic books and that anything like books, magazines, and so on are all outside the domain of literature.
poppyc*ck!
@Larry Plume P.
Of course.
Let’s not forget that according to this study the earth has had much more water in the early days:
http://sciencenordic.com/earth-has-lost-quarter-its-water
+++++++++++++++++
The Mantle of the Earth is said to contain about twice as much water as the oceans. Could it be that it simply sunk into the mantle over time until it was ‘balanced’? It is very possible that we were ‘Water World’ in the early days.
I accept the thesis that the water feedback is negative. As it is a GHG, the obvious conclusion (more Occam) is that the temperature is controlled almost totally by water vapour as it is the primary GHG and its upper limit switch as well.
I am looking forward to hearing something new from Prof Lu at the Univ of Waterloo on the subject of Ozone, tropospheric and polar. There may be a strong mediating factor there.
Someone mentioned the Solar wind was much higher in the days of the Faint Sun. They there would be less cloudiness, right? It seems to me there is another completely separate argument which is that clouds largely controlled the temperature and compensated for the lower energy input. How does the Faint Sun power multiplied by the inverse of the 10Be record look?
Kev-in-Uk says:
May 27, 2012 at 3:02 am
Eric Simpson says:
May 26, 2012 at 10:09 pm
My understanding of the alarmist CAGW ‘argument’ is that the water vapour feedback is positive. Hence, the amplification of the alleged CO2 effect. As you say, a zero effect, or even negative effect of water vapour, completely destroys any CAGW position. It is quite noticeable how this is never really explained by the warmista!
Smarty Pants says:
May 27, 2012 at 6:00 am
The highest mean annual temperature anywhere on the planet is an equatorial low desert with less than 3 inches of annual rainfall. If water were a positive feedback then the place with the highest mean annual temperature would be a wet equatorial climate not a dry one. Quod erat demonstrandum.
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The basic physics used in these arguements, is junk, manufactured to promote AGW. The KT97 and ilk describes an impossible world – for one, it has completely excised The WATER CYCLE.
In the real world, the Water Cycle cools the Earth by around 52°C from the 67°C it would be without water – think deserts is how I stress this point..
Plus, Carbon Dioxide is fully part of the Water Cycle – all clean, pure rain is Carbonic Acid (as is fog, dew, etc.). In the real world carbon dioxide is not a “ideal gas as AGWScienceFiction teaches”, it is a real gas with volume, weight, subject to gravity, and has attraction.
There is no Greenhouse Effect of a rise from minus 18°C to 15°C by “greenhouse gases warming of 33°C” – it’s a sleight of hand created by distraction, by taking out the Water Cycle.
Think deserts.
If the greenhouse effect was so weak, we would be nearer 255 K than 288 K, so he goes wrong right there when he starts with 288 K for today’s temperature. He probably thought we would not notice this assumption.
Allan MacRae @ur momisugly May 27, 2012 at 3:39 am
if these feedbacks were strongly positive, life on Earth would be very different, if it existed at all
“”””” Robert of Ottawa says: May 27, 2012 at 6:07 am
This cannot be emphasized enough. With positive feedback, any system will go to the extreme and stay there. Throughout its history, the Earth climate appears to have been remarkably stable, indicating negative feedbacks. “””””
George E. Smith; says: May 27, 2012 at 8:36 am
Well the problem is that your assertion Robert, simply is NOT true…”””””
George – for clarity, can you please be more specific with your concern? For example:
You did not take exception to my statement, but you did to Robert’s – Correct?
So what wording would it take to change Robert’s statement such that you could agree with it?
For example: would adding the word “strong” before “positive” do the job?
Or other?
<i.Bill Illis says:
May 27, 2012 at 8:59 am
Really good paper Clive.
It seems clear that cloudiness is a negative feedback – the climate models and the theory probably have it completely backwards.
We know that anthropogenic aerosols and particulates both increase cloudiness (cloud persistence) and increase the reflectivity of clouds by the same mechanism of reduced droplet size.
It should be obvious what caused climate change over the 20th century.
Sorry to point this out, but the search for truth shouldn’t play favorites.
As to the rejection of Clive Best’s paper, could it be because he used degrees Kelvin? If I was quickly scanning a paper, glancing at the graphs and pics first, and came across that in Figure 1, perhaps I could excuse that as an accidental brain fart. But then it’s repeated in the figure’s description, Table 1, in the text…
If I was an editor, short on time, I’d write this off as amateur grade and reject it out of hand, no reviewers needed.
Well clouds don’t reflect, in the optical sense; well not more than the 2% of Fresnel reflection from water droplets. “aerosols” that reduce droplet size, may result in longer persistence times for clouds, before precipitation occurs. Optically, the water droplets of clouds, scatter rather than reflect. A simple spherical lens illuminated by a collimated beam or a half degree divergence sunbeam, ouputs a highly divergent beam (after first going through focus less than a droplet diameter away from the droplet). That output beam covers almost a full hemisphere, and multiply Fresnel reflected beams send some energy over the full 4 pi steradians. It only takes a few (3-5) sequential scatterings to completely homogenize the light so the cloud looks isotropic; but it is NOT reflecting (much). Those optical scattering properties are largely unaffected by droplet size, unless it gets down to where diffraction effects dominate, and all that does is scatter the light even more. But bigger droplets will rain out sooner.
“””””
Allan MacRae says:
May 27, 2012 at 4:05 pm “””””
Alan, you are correct my statement did NOT refer to you post; and I did specifically state that it was Robert’s assertion that was false (that all positive feedback systems will go to a limit and stay there)
As to your assertion Allan, “”””” Allan MacRae @ur momisugly May 27, 2012 at 3:39 am
if these feedbacks were strongly positive, life on Earth would be very different, if it existed at all “””””
Pretty inoccuous I would say; how could anyone disagree with that; I mean there could be unstable positive feedbacks that could blow the planet to smithereens. Can’t name any off hand; but as you say “if”.
I sometimes have difficulty in determining who said what, as some peole cite other without delimiters. That’s why I always use my patented delimiter; “”””” Balderdash @ur momisugly#!$% “””””
I don’t think I have seen it in the OED yet; or even Websters for us ‘Mercans ! Maybe before I die, I will get recognition.
George
On the base question, I demonstrated how positive feedback can increase “gain” , and negative feedback decrease it.
Two leading shoe drum brakes, have positive feedback; but that doesn’t mean that you tap on the brakes, and your car comes to a complete and sudden stop, with the brakes locked up. Regenerative radio receivers have positie feedback and have had for centuries; well one anyway.
Oh I didn’t address your question Allan; what would fix Robert’s statement. Yes large positive feedbacks would drive things towards a limit. You can’t say for sure that the system would stay there. Depends for example on whether the system is “DC coupled” or “AC coupled”. A DC coupled system does not indefinitely propagate the result of some “steady state”. An AC coupled system only propagates signals that are changing above some minimum rate; so a steady state; such as up against a brick wall, will stop generating a feedback signal if nothing is changing, so the output would eventually collapse, which likely would set off a mad dash for the brick wall on the other side of the road.
So Robert’s statement that the system would stay at the limit condition is only true, if that limit state is a stable state, that can persist indefinitely.
Feedback systems, have a “loop gain” which is the “forward gain”, say 5 oranges per apple, and a “feedback function” say 0.02 apples per orange. The loop gain is the product of those two; in this case 0.1 (no units) If this were a positive feedback the overall gain is 5 / (1 – 5 x 0.02), which is 5.5555555……. so it is stable. If the feedback were negative, the gain would be 5 / (1 + 5 x 0.02) = 4.54545454……..
If the feedback function increased to 0.2 apples per orange, then the positive feedback gain would go to infinity, and the negative feedback gain would drop to 2.5 which is stable.
If the system is AC coupled and excessive positive feedback ( loop gain >=1.0), then it will oscillate; but if DC coupled, it will jam up as Robert asserted.
There are more elaborate conditions for stability, specially ones that involve the time response of the system. Large propagation delays tend to result in oscillation, because an intended large negative feedback , in an AC or DC coupled system, is delayed so much, that by the time it gets to the input again, it is out of phase with the signal, and becomes a large positive feedback. Nobody ever talks about the time and frequency stability of the climate system.
kadaka (KD Knoebel) says:
May 27, 2012 at 10:25 pm
As to the rejection of Clive Best’s paper, could it be because he used degrees Kelvin?
What should he have used? Fahrenheit?