Discussion > Feedbacks and Forcings
A "positive feedback", by definition, amplifies a forcing, whether the forcing is large or small. So there is always more warming, in the presence of a positive feedback, than there would be in its absence. There can never be "less warming" (= "cooling") because of the presence of a positive feedback.Or have I missed a point?
Yes - that's why I gave the example of deglaciation above and specifically mentioned the role of ice albedo feedback (strongly positive) in amplifying cooling post-peak orbital forcing.
BBD
"The release of CO2 from geological sinks by the northward migration of the Indian plate and its eventual collision with Eurasia is the geological scale driver of climate change. It's the only reason we can discern why it got so hot during the Eocene."
From memory the temperature at the beginning of the PETM rose by 6C over a 20,000 years (Stanley, Stephen M forget the title, but you'll find it it used to be one of the text books on the Eocene but doesn''t give the right political message describing a fecund earth with tropics up to 45 degrees and mild weather. Having said that, 20,000 years isn't enough time for the temperature to be raised by India banging into Eurasia, so do you have a paper or citation where this assumption is made?
Still waiting to understand how the 1st Law dampens positive feedback. Have you got anything?
geronimo
dude, Mike's question relates only to whether a positive feeder-backer can turn negative. The question can be answered in the hypothetical..No one's asking for your pronouncement for what CO2 actually does in the climate.
Can an initial positive feedback turn negative or negate itself over a longer time period? That is all the question is.
shub
"Or have I missed a point?
Yes - that's why I gave the example of deglaciation above and specifically mentioned the role of ice albedo feedback (strongly positive) in amplifying cooling post-peak orbital forcing."
Aug 2, 2012 at 7:21 PM BBD
Thanks BBD.
I think the reason I seemed to miss a point is that you use a different definition of "postive feedback" from Mike Jackson.
You regard a feedback that magnifies something, whether it is a heating effect or a cooling effect, as a positive feedback. That point of view (that I would not argue against) agrees with what I said earlier - that it is arbitrary whether one chooses to regard a feedback as positive or negative (since your choice of calculating, say, energy removed vs. energy added is up to you).
Whereas Mike's view is that, if it is a positive feedback, it necessarily magnifies a heating effect.
BBD/Mike - have I understood your respective viewpoints?
Martin A
The point of my comment was that actual feedbacks are, inevitably, local. BUT, that does not mean one can simply add them up and come up with a figure that works globally over a period of time. There is no validity at all in saying that the feedbacks are positive or negative or that a doubling of CO2 must lead to 3 degrees of increase in the global average temp. It's nonsense. It just does not work like that. So many things going on, so many interactions interfering here and reinforcing there, depending on a list of factors nobody can even guess at. Humans can see patterns. We are programmed to do so, it is one of our distinguishing traits and a survival characteristic par excellence. BUT we see patterns where there are none. We want order when we can see chaos. Governments want simple answers, clear paths. Scientists provide them, and that is their survival characteristic. I wonder if we are not all trying to impose a simple view on a problem which could be viewed as intractable. Or maybe our only problem is to deal with whatever happens, and all this is pantomime.
Which is not the post I planned, my fingers done it. Might be something in it though.
rhoda
shub
Thankyou. I was afraid we were heading for a bit of displacement activity again.
I'm not about to get into a debate on whether or not clouds cool what's underneath them. As far as I know I'm not saying anything contentious. There are (supposedly) positive feedback effects with increased CO2. Unless I'm reading the accepted wisdom wrong, this feedback is the result of increased water vapour. Water vapour, unless my rather elementary knowledge of physics has gone agley somewhere, are what clouds are made of. Clouds have a net cooling effect according to my experience of clouds. (Having lived much of my life in Scotland I know about clouds.)
So please, BBD, don't try to shunt us all into the siding of "how do you know ..." and still less up the rack and pinion railway of geological time scales.
You assert that there is no difference between the forcings that start and end ice ages and those that start and end cold/warm periods during inter-glacials. I assert that since the scales are so different you might well be wrong and that is perfectly logical to hypothesise that it is totally different order of forcing that is needed to tip the earth into an ice age.
Perhaps some of our palaeo-climatologists could spend a little more time researching that instead of trying to convince us that well-recorded climate events like the MWP didn't happen!
But for the moment I see nothing contradictory in the proposition that sooner or later the particular feedback we are talking about can be overtutned by a negative one.
PS Martin A / Rhoda - I've just seen your posts. I'll be back in a minute or two (I'm watching the swimming finals!!)
Mike Jackson
This is the point I keep on trying to make: *if* feedbacks net neutral or negative *then* the kind of variable climate we know we have would be a physical impossibility. Simple deductive reasoning demonstrates that feedbacks must, therefore, net positive. We might not know the detail about cloud feedbacks, but we can infer that your assumption above is incorrect. Do you accept this?
I am not sure that I do accept that, no.
Are you saying that feedbacks can never net negative? It is my understanding that most dynamic systems have feedback systems that do net negative - or am I mistaken here as well? It is that fact that keeps them stable.
Or are you saying that if they cannot always net negative? (This may be self-evidently true.)
Or are you saying that they cannot net negative over "climatological" time periods i.e. because otherwise you would find it difficult to explain the swings in temperature that are known to have occurred?
matthu
geronimo
Having said that, 20,000 years isn't enough time for the temperature to be raised by India banging into Eurasia, so do you have a paper or citation where this assumption is made?
The PETM was an exceptional, relatively short-lived spike of super-warming on the up-slope leading to the peak of the Eocene hothouse climate (forced by the Indian/Eurasian collision). Argument as to what caused it continues, but nobody disagrees that a large carbon isotopic excursion is associated with the initial warming.Seafloor methane hydrades are a key suspect for the source, arguably warmed to instability by the general increase in GAT. So the PETM was was an indirect consequence of the ongoing tectonic forcing.
Still waiting to understand how the 1st Law dampens positive feedback. Have you got anything?
I think you may have misunderstood what I was trying to say. Perhaps this will help clarify.
BBD
shub
Can an initial positive feedback turn negative or negate itself over a longer time period? That is all the question is.
And the answer is still no.
BBD
Martin A
You regard a feedback that magnifies something, whether it is a heating effect or a cooling effect, as a positive feedback. That point of view (that I would not argue against) agrees with what I said earlier - that it is arbitrary whether one chooses to regard a feedback as positive or negative (since your choice of calculating, say, energy removed vs. energy added is up to you).Whereas Mike's view is that, if it is a positive feedback, it necessarily magnifies a heating effect.
BBD/Mike - have I understood your respective viewpoints?
Yes, but ;-). The essence is correct, but (especially given the confusion over terminology) I cannot go with the highlighted bit. I understand what you mean, but positive feedbacks are amplifiers of forcings and negative feedbacks counter forcings. The distinction is not arbitrary and for clarity, can we stick to the accepted formalisms?
BBD
Mike
Thankyou. I was afraid we were heading for a bit of displacement activity again.
I'm not about to get into a debate on whether or not clouds cool what's underneath them. As far as I know I'm not saying anything contentious.
What's with this talk about 'displacement activity'? You made a contentious statement based on demonstrably incorrect assumptions as part of your earlier comment. Am I supposed to just wave it by? The state of knowledge on cloud feedbacks (supported by the inference I described earlier) is that they are either neutral or weakly positive feedbacks to warming, while WV is a strong positive feedback. Do keep in mind that there is a *great deal* more atmospheric WV than cloud.
So please, BBD, don't try to shunt us all into the siding of "how do you know ..." and still less up the rack and pinion railway of geological time scales.
Please don't try to mischaracterise everything I say. It makes the discussion you are trying to have impossible. Resist the urge.
You assert that there is no difference between the forcings that start and end ice ages and those that start and end cold/warm periods during inter-glacials. I assert that since the scales are so different you might well be wrong and that is perfectly logical to hypothesise that it is totally different order of forcing that is needed to tip the earth into an ice age.
Why? What has time scale got to do with spatial/seasonal change in DSW (orbital forcing), the atmospheric physics of GHGs including CO2, CH4 and WV, and ice-albedo feedback? I'm arguing that basic physics remains unchanged, which is hardly a wild assertion.
But for the moment I see nothing contradictory in the proposition that sooner or later the particular feedback we are talking about can be overtutned by a negative one.
Then you haven't understood anything I've been saying here, which strongly suggests this conversation is a waste of our mutual time.
BBD
matthu
Are you saying that feedbacks can never net negative?
Not in the earth climate system, no. Obviously, or its known behaviour would be inexplicable.
BBD
Martin A
You have this wonderful knack of getting me confused!
What I think I mean is that positive feedbacks reinforce forcings (whether those forcings are themselves positive or negative) while negative feedbacks work against the forcings.
That's why I asked the question whether cooling is a positive (i.e. real) event or simply a negative warming.
A negative feedback would be one that serves to return the situation (whatever situation that is) to a neutral state.
Go ask an engineer ... I'm only a linguist, for heaven's sake!!
Rhoda
"My fingers done it" — what sort of an excuse is tha?. Even Jones managed a better excuse!
I wonder if we are not all trying to impose a simple view on a problem which could be viewed as intractable.I'm damn' sure we are, and I get more sure by the day.
There are more contradictory hypotheses flying around at the moment than most of us have had hot dinners. At least half of them must be wrong ... or must they?
We know the activist song book off by heart but we also know they have an agenda in which climate change/global warming is only a small part (ask Pachauri). They've been caught out so often that the claim "it doesn't affect the science" is starting to sound very weird.
BBD was trying to lead me down the IR route earlier.I know his views about back radiation (I think). I have also read others who agree with him about the existence of back radiation but disagree with the way he interprets its effects.
Then there are those who deny the existence of back radiation.
Then we get into the theory of black bodies. The earth isn't a black body so why do we pretend it is "for the sake of argument"?
And so on. And so on. And so on.
Mike Jackson
BBD
A quickie. It's getting late where I am!
Are you suggesting that what you call spatial/seasonal change — a phrase you use without explanation as so often — is connected with ice ages? If so you'll need to explain what you mean. In my book seasonal change is not relevant to this discussion at all.
Can you also explain the relevance of ice albedo feedback in relation to any of the warming/cooling periods we have been discussing?
And finally, I do not have a clue what DSW stands for and the one thing that pisses me off on these threads more than anything else is people trying assert their superiority by assuming a familiarity with the science that they do not actually possess and forcing others to make themselves look idiots by having to ask for explanations.
Have a nice evening!
Mike Jackson
Aug 2, 2012 at 8:42 PM BBD
I understand what you mean, but positive feedbacks are amplifiers of forcings and negative feedbacks counter forcings. The distinction is not arbitrary and for clarity, can we stick to the accepted formalisms?
"...can we stick to the accepted formalisms?" Certainly. As soon as I get myself clear on what are the accepted formalisms.
I'm assuming that "a forcing" the same as "radiative forcing". Please correct me if this is wrong.
So a forcing is always the response to something (eg to a change in atmos CO2) and is measured in units of power per unit area. It is therefore necessarily positive (or zero).
Radiative forcing is the change in the net, downward minus upward, irradiance (expressed in W m–2) at the tropopause due to a change in an external driver of climate change, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun. Radiative forcing is computed with all tropospheric properties held fixed at their unperturbed values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical equilibrium. (...)
AR4
Martin A
Aug 2, 2012 at 9:00 PM Mike Jackson
Then we get into the theory of black bodies. The earth isn't a black body so why do we pretend it is "for the sake of argument"?
I think for two reasons:
- A given bit of the earth's surface actually behaves pretty close to the behaviour of an ideal black body, at long infra-red wavelengths (according to what I have read).
- To do calculations about any physical system, you have replace all the detail you might have available with something simple enough to be manageable. A black body is very handy because its radiative behaviour is understood completely and the calculations are simple.
Having said that, I must go on to say that I find the normal black body model for the greenhouse effect as no better than, at very best, a plausibility argument (that the greenhouse effect exists) because of all the gross simplifications involved.
Martin A
"...why it got so hot during the Eocene."
Who's questioning the why. The question is about the the time scale.
If you can't grasp your mind around long timescales, it is no excuse to accelerate phenomena that occur over long periods so you can deal with them, and then draw conclusions. Slow is slow.
Secondly, it is only sufficient that you understand the question - regarding feedbacks, that Mike asked. You can attempt to answer them, but it is clear that you are biased and wont offer a straight answer because your ego will get in the way.
shub
Mike
Are you suggesting that what you call spatial/seasonal change — a phrase you use without explanation as so often — is connected with ice ages? If so you'll need to explain what you mean. In my book seasonal change is not relevant to this discussion at all.
Milankovitch theory (not BBD) explains the roughly ~100ka spacing of glacial terminations in terms of orbital mechanics and the effect they have on the intensity of summer sunlight at high latitude in the Northern Hemisphere. Roughly every 100ka, variations in the earth's orbit (eccentricity) and in its axial tile (obliquity) intensify summer insolation in the high NH latitudes for thousands of years. This 'orbital forcing' is a seasonal and regional (spatial) effect.
Can you also explain the relevance of ice albedo feedback in relation to any of the warming/cooling periods we have been discussing?
Any significant change in global ice cover (to be complete, include global snow cover) will change the planetary albedo and affect how much solar energy is reflected rather than absorbed by the surface. This is a strong postive feedback (amplifying warming *and* cooling in response to a change in forcing). It plays its part - large in glacial terminations and the approach to fully glacial conditions; smaller during interglacial climates.
And finally, I do not have a clue what DSW stands for and the one thing that pisses me off on these threads more than anything else is people trying assert their superiority by assuming a familiarity with the science that they do not actually possess and forcing others to make themselves look idiots by having to ask for explanations.
First, I'm sorry you reacted this way. I wasn't trying to 'assert superiority'; I was assuming that you were familiar with the routinely used acronyms within the climate 'debate'. DSW = downward short wave radiation, aka sunlight. As distinct from DLR (downward long wave radiation) aka downward infra-red and OLR, aka outgoing IR. When this happens to me, as it does, I google. It's useful to learn new stuff, and more productive than assuming that the other chap is trying to **** with my head.
BBD
shub
If you can't grasp your mind around long timescales, it is no excuse to accelerate phenomena that occur over long periods so you can deal with them, and then draw conclusions. Slow is slow.
Slow change in forcing = slow change in T. Fast change in forcing = fast change in T. The physics remains unchanged on all time scales.
BBD
Martin A
So a forcing is always the response to something (eg to a change in atmos CO2) and is measured in units of power per unit area. It is therefore necessarily positive (or zero).
Not necessarily. As I said to Mike upthread, things like aerosols are treated as forcings. Volcanic and anthropogenic sulphate aerosols are a forcing of negative sign as they reflect DSW back into space.
BBD
Aug 2, 2012 at 10:37 PM BBD
BBD - thank you. I've got it (I think).
You yourself often say things like "...sulphate aerosols are a forcing...". I'm assuming this is the normal manner of speaking.
But I'm also assuming that, to be strictly correct, the phrase should be something like "...sulphate aerosols produce a forcing..." since the forcing is the change in (downward W/m2 - upward W/m2) resulting from the sulphate aerosols.
Not being picky - just trying to pin down what is what - to be able to "stick to the accepted formalisms".
Martin A
Martin
You yourself often say things like "...sulphate aerosols are a forcing...". I'm assuming this is the normal manner of speaking.
Yes, but you are strictly correct :-) I'm not quite sure if/why this matters though... ?
BBD
BBD @ Aug 2, 2012 at 7:09 PM
This is the point I keep on trying to make: *if* feedbacks net neutral or negative *then* the kind of variable climate we know we have would be a physical impossibility.
And again, this argument remains incorrect. See my earlier comment for some reasons why.
BBD @ Aug 2, 2012 at 8:58 PM
Not in the earth climate system, no. Obviously, or its known behaviour would be inexplicable.
See above.
BBD @ Aug 2, 2012 at 10:27 PM
Milankovitch theory ... explains the roughly ~100ka spacing of glacial terminations in terms of orbital mechanics...
This is overstating the case. The 100 ka orbital forcing is weaker then either of the 23 ka or 41 ka orbital forcings, both of which are scarcely visible above the spectral background. If I take seriously the idea that orbital forcings dominate variability, then the spectrum must in turn be dominated by peaks at the orbital frequencies. This is not the case: the dominant variability is in the continuum.
Aug 2, 2012 at 2:37 PM | Mike Jackson
there is a 60-year cycle of heating and non-heating; ... there is a ~600-700-year cycle of heating and cooling, at least over the last 2-3,000 years.
Probably best not to take such cycles too seriously (they seem to fade in and out with monotonous regularity!). Here is quite a nice general discussion about these issues, if you haven't already seen it.
Philip Richens
Yes, but you are strictly correct :-) I'm not quite sure if/why this matters though... ?
Thank you. No, it's not a big deal. But I'm glad that I now understand what it the correct terminology and what is, in effect, a commonly used shortcut. Previously, I was not certain they were the same thing.
From now on I'll attempt to "stick to the accepted formalisms".
Martin A
shub
Are you just going to troll this discussion? It's boring.
As for this remark:
If you would take the trouble actually to read H&S12, you would understand that the relationship between CO2 and T over the Cenozoic. The release of CO2 from geological sinks by the northward migration of the Indian plate and its eventual collision with Eurasia is the geological scale driver of climate change. It's the only reason we can discern why it got so hot during the Eocene. At a fundamental level there is no disconnect between the geological and the decadal time-scales, except in your mind. Forcing is forcing.