Buy

Books
Click images for more details

Support

 

Twitter
Recent comments
Recent posts
Currently discussing
Links

A few sites I've stumbled across recently....

Powered by Squarespace

Discussion > Feedbacks and Forcings

BBD

... if feedbacks net negative or even neutral, nothing much happens. Glacial terminations cannot happen under orbital forcing, you don't get warm periods... nor do you get cool fluctuations
OK. Let's take that at face value.
First question. Are you defining "positive feedback" as the same thing (or much the same thing) as a "forcing"? (No, I'm not being dim; I'm just aiming to make sure we agree on terminology!) And if you are can we agree that positive feedback can reinforce a cooling as well as a warming?
Once we 've agreed on the terms I can move on to the second question.

Aug 1, 2012 at 4:10 PM | Registered CommenterMike Jackson

Not meaning to interrupt Mike's question but I don't understand BBD's use of the term 'positive feedback' either. I would accept the wikipedia entry as how I think of it, with the statement "Positive feedback tends to cause system instability."

I would expect a strong positive albedo feedback at the end of glaciation due to all the ice melting. I would also expect this positive feedback to end eventually when most of it has melted. As Mike points out I would also expect a strong positive feedback when we enter the next glaciation as the build up of ice again increases the albedo leading more ice etc.. I'm not quite so sure what stops this feedback but I guess tropical heating of the ocean and maybe a lack of clouds helps to keep the the Earth at a certain temperature.

Aug 1, 2012 at 5:29 PM | Unregistered CommenterRob Burton

Rob,
Don't apologise. The more the merrier.
What I'm trying to get from BBD is some information on the question of positive vs negative feedback and how it affects the evident cycles that we've seen at least over the last two millennia — Roman Warm, Dark Ages, MWP, LIA, Modern Warm. At least as he interprets them.
I'm happy to accept:
- A doubling of CO2 results in a temperature increase of 1.2C
- That increase may be greater with the aid of positive feedback.
What I'm less sure about is:
- What initiated the previous warm periods.
- Was there a positive feedback effect and if so how did it come about?
- If the positive feedback effect which is causing the current warming is due to increased water vapour when does there come a point when that water vapour increases cloud cover thereby 'damping' down the feedback?
And most important of all
- What caused the system to go into reverse?
Since there is no evidence of previous CO2 levels causing the sort of unstoppable positive feedback that BBD appears to be arguing for I assume that the earth does have some sort of thermostat that over time balances the climate out within certain fairly limited parameters.
As you say, certainly in engineering terms — as we have heard from those of that discipline on here — positive feedback tends to cause system instability and since the climate appears to be fairly stable (except when it is heading into or out of an ice age) I am assuming that any feedback effect is weak. BBD appears not to agree.
The strong feedback that takes us into and out of ice ages would seem to me to be of a different order and to argue that those transitions are proof of positive feedback at work in every climatic variation doesn't strike me as plausible.
I'll also throw in something I've mentioned elsewhere which is that what we are discussing in general very much treats the earth as a "closed" system. There is no mention of any of the external forces (except solar insolation) which act upon the earth from the gravitational pull of the moon and the other planets to even wider events which (some argue) also have effects on the planet.
I just feel that a very new scientific discipline has latched onto something which it thinks might be nice and easy to deal with (relatively speaking) and is not looking beyond that for answers.

Aug 1, 2012 at 6:30 PM | Registered CommenterMike Jackson

Positive? Negative? It's arbitrary - take your pick.

http://www.ipcc.ch/publications_and_data/ar4/wg1/en/annexes.html
IPCC AR4 Annex I: Glossary

Climate feedback
An interaction mechanism between processes in the climate system is called a climate feedback when the result of an initial process triggers changes in a second process that in turn influences the initial one. A positive feedback intensifies the original process, and a negative feedback reduces it.

So a climate feedback is “an interaction mechanism”.

Note the gobbledegook:

(1) "A positive feedback intensifies the original process. "
You have two processes interacting. How do you tell which is the "initial" or “original” process? It's arbitrary.

(2) “Intensify a process”? “reduce a process”? It's arbitrary.

Suppose you have “a feedback” that causes more clouds (so less light reaching the surface). Is this a positive feedback (intensifying the production of clouds)?

Or a negative feedback (reducing the light reaching the surface?

Take your pick.

Aug 1, 2012 at 6:44 PM | Registered CommenterMartin A

I think, Martin, that in general parlance a positive feedback is one that augments the forcing. In the current situation, one that will serve to increase the temperature further, and in the instance that Rob was quoting one that will serve to reduce the temperature further as we head into an ice age (or conceivably just a cool period).
What I am trying to ascertain is what the forcings were that initiated the Roman and Mediaeval warm periods (and the Minoan if you want to go back that far) and what was the feedback process that allowed those periods to continue and what was the forcing that brought them to an end.

Aug 1, 2012 at 7:00 PM | Registered CommenterMike Jackson

Mike Jackson

Sorry for the delay - busy, busy...

First question. Are you defining "positive feedback" as the same thing (or much the same thing) as a "forcing"?

A feedback is usually considered to be an internal climate system process that modulates an external forcing, usually solar. A positive feedback amplifies the initial effect (so positive feedbacks can increase *cooling* as well as warming). Negative feedbacks damp down the initial effect.

Carbon removed from geological sinks and added to the atmosphere as CO2 by human or tectonic activity including volcanism is treated as an external forcing. Both volcanic and anthropogenic aerosols are normally classed as forcings.

CO2 and other GHGs emerging from 'shallow' non-geological sinks (clathrates, ocean sediments, permafrost, global ocean) under external forcing (eg orbital) are regarded as feedbacks to the orbital forcing. Water vapour is likewise treated as a feedback.

The strong feedback that takes us into and out of ice ages would seem to me to be of a different order and to argue that those transitions are proof of positive feedback at work in every climatic variation doesn't strike me as plausible.

Argument from incredulity is a logical fallacy Mike ;-)

The feedbacks that amplify high latitude orbital forcing and terminate glacials are the same as those that operate in the Holocene: water vapour, ice albedo, GHGs. Ocean heat transport by the AMOC from equator to Arctic appears to be vital too as does the release of CO2 from sedimentary sinks in the Southern Ocean triggered by the warming of Southern Ocean deep water (Shakun et al. 2012).

Aug 1, 2012 at 7:29 PM | Unregistered CommenterBBD

What I am trying to ascertain is what the forcings were that initiated the Roman and Mediaeval warm periods (and the Minoan if you want to go back that far) and what was the feedback process that allowed those periods to continue and what was the forcing that brought them to an end.

Dunno, but was under the impression that solar and volcanism were prime suspects. Sun up, volcanic aerosols down = warming. Sun down, aerosols up = cooling (you might only need to reduce solar flux to get the cooling). Also - big caveat - evidence for global and synchronous warming events is lacking. On a hemispherical scale, ocean heat transport and circulation change is very important too.

The essential point is that whatever the causes were, they are not demonstrably causing modern warming.

Aug 1, 2012 at 7:39 PM | Unregistered CommenterBBD

Dunno, but was under the impression that solar and volcanism were prime suspects.

The essential point is that whatever the causes were, they are not demonstrably causing modern warming.

Tut! This is what I was driving it and though I wasn't deliberately setting an elephant trap you've dug yourself one and fallen in.
You accuse me of an "argument from incredulity" and then proceed to string these two quotes together!
If you don't know what was the initiator for previous warmings then you cannot rule out the possibility that the recent warming was caused by the same forcing(s) or some other that we are not fully aware of.
However ...
What I'm really trying to establish is whether you can explain whether the positive feedback that you say is inevitable has a limiting factor. I don't (necessarily) dispute the warming effect of water vapour but since increased water vapour eventually leads to increased cloud cover I would contend that there is a limit to the warming.
Rob Burton would appear to agree.
On past history there would also seem to be an initiator that causes cooling and both these seem to work on a 60-year cycle overlaid by a ~700-year cycle. What would that initiator be?
There is something more at play in this than your — with respect, rather simplistic — sun+aerosols. CO2 may certainly be a part of it but since it is almost entirely agreed that CO2 did not play any part in previous warming and there is a strong body of opinion that CO2 had no significant effect on the current warming cycle (from ~1800) until about the 1940s, we all appear to be missing something.

Aug 1, 2012 at 8:18 PM | Registered CommenterMike Jackson

Hi Mike, I'm just going to throw out a few disconnected thoughts. Some or all of this stuff may be in the literature, for all I know, and some may be obvious.

The idea of averaging feedback is daft. As daft as averaging the global temperature. You think it is telling you something, but it is not. ALL feedback in the climate case is local and dependent on variable factors. Start temp, humidity, surface water, time of day, inclination of incoming IR, clouds and more. All these factors affect local feedback, which could quite easily be + or - according to their interaction. The assumption/assertion that there is a figure for CO2-related warming, so many watts/sqm for each doubling is therefore worthless. As is the idea that the figure will persist for the second or third doubling or that any temp-related feedback (there is no positive feedback for CO2 per se, only slightly negative because of partial pressure) will be consistent and not subject to all the above factors and to steps caused by state changes.


I don't know if the climate models work in the required way to take all this into account. Maybe they do. But maybe they work on averages, in which case they can only be wrong.

Aug 1, 2012 at 9:46 PM | Registered Commenterrhoda

BBD said, "The essential point is that whatever the causes were, they are not demonstrably causing modern warming."

Long term internal variability occurs irrespective of forcing and so could explain past warm and cool periods as well as not demonstrably not causing modern warming.

Aug 1, 2012 at 11:45 PM | Registered CommenterPhilip Richens

Philip Richens

Long term internal variability occurs irrespective of forcing and so could explain past warm and cool periods as well as not demonstrably not causing modern warming.

Does it? References for that? And what happened to the exhaustively calculated RF from increased CO2? Where did it go?

What about conservation of energy?

Is it not the case that internal variability can't produce a trend over longer periods because of conservation of energy?

Hearking back to shub's argument on the other thread, here's Kyle Swanson, co-author of a somewhat misunderstood study (Swanson & Tsionis 2009) discussing the conundrum at RC:

It first needs to be emphasized that natural variability and radiatively forced warming are not competing in some no-holds barred scientific smack down as explanations for the behavior of the global mean temperature over the past century. Both certainly played a role in the evolution of the temperature trajectory over the 20th century, and significant issues remain to be resolved about their relative importance. However, the salient point, one that is oftentimes not clear in arguments about variability in the climate system, is that all else being equal, climate variability and climate sensitivity are flip sides of the same coin. (see also the post Natural Variability and Climate Sensitivity)

A climate that is highly sensitive to radiative forcing (i.e., responds very strongly to increasing greenhouse gas forcing) by definition will be unable to quickly dissipate global mean temperature anomalies arising from either purely natural dynamical processes or stochastic radiative forcing, and hence will have significant internal variability. The opposite also holds. It’s painfully easy to paint oneself logically into a corner by arguing that either (i) vigorous natural variability caused 20th century climate change, but the climate is insensitive to radiative forcing by greenhouse gases; or (ii) the climate is very sensitive to greenhouse gases, but we still are able to attribute details of inter-decadal wiggles in the global mean temperature to a specific forcing cause. Of course, both could be wrong if the climate is not behaving as a linear forced (stochastic + GHG) system.

If you follow the link, note the last paragraph of Swanson's article.

Aug 2, 2012 at 12:44 AM | Unregistered CommenterBBD

Apologies to each of you for not pursuing this argument sooner. Being an hour ahead of the UK and with a tendency to shut down early I missed the later responses. This morning, unfortunately, has the makings of wall-to-wall domestic activity. I'll try to pick things up this afternoon.
Meanwhile ...
Rhoda
Thanks for that contribution. I think that, almost by definition, everything to do with climate is local. As I said a couple of days ago, when the CRU data was released at long last about one-third of the stations showed a declining trend. It may be that Global Mean is 0.X degrees warmer than last year or last century but it is arguable that there is no such thing as "global" warming and if that is the case how can a well-mixed gas be an integral part of what warming there is?

BBD
I agree with Swanson's first paragraph without hesitation. His last sentence is an interesting one: does rhe climate behave as a "linear forced system"? I don't think so.

More later.

Aug 2, 2012 at 8:39 AM | Registered CommenterMike Jackson

BBD: "And what happened to the exhaustively calculated RF from increased CO2? Where did it go?"

Never got off the page it was calculated on. What you need is measured radiation over time and area which will support the theory and show that the calculated effect works in the real climate. That is not a given, it has to be shown. When I examined the Harries paper which purported to show those effects, apart from evident difficulties in measurement I found that the effect found was not converted into heat units (why not?) and that one of the minor absorption bands of CO2 showed no resonse over time. If CO2 is saturated in this band, that has implications. It means that the 3.7 watts/sqm expected from CO2 changes as the concentration increases. One might wonder whether the 'wings' of the other band might also be eroded by H2O or further increases in CO2. This would be what Chiefio calls a 'dig here'. Nobody is digging. Why not, I ask myself. And the answer is that nobody wants to find the answer.

Aug 2, 2012 at 9:10 AM | Registered Commenterrhoda

Aug 2, 2012 at 12:44 AM | BBD

Hi BBD,

It's good to have an opportunity to discuss these issues with you again, and thanks also to Mike for raising the topic.

The question I keep banging my head against is the background spectrum, which as you will know extends well beyond the decadal time-scales discussed in Swanson’s RC article. As far as I am aware, there is no generally accepted explanation for it. Swanson does mention that the type of variability he has examined does occur in unforced GCM runs. So at least in principal, internal variability can occur in an unforced system. However, Swanson also implies that his type of variability cannot (at least not directly) cause longer term variability. It also seems that longer term variability is not the direct result of forcing either, because millenial reconstructions (solar, volcanic) do not permit a simple linear relationship between forcing and temperature. If the explanation for the background does not directly involve forcing and does not directly involve Swanson's kind of internal variability, then another explanation is required: either (or both) another kind of forcing or another kind of internal variability. Whatever the explanation is for the background, it is apparently not encoded into current GCMs.

You also mentioned conservation of energy. I’m not certain what you have in mind, but perhaps it is the requirement that on the average and for the climate system as a whole, “energy in = energy out”? If so, then on average and if the solar input is constant, the energy balance is determined by albedo and emissivity. Both can vary. Therefore, the issue appears to be whether and how these things will vary in the absence of forcing. The general behaviour of complex driven nonlinear systems suggests that such variation is at least possible. As far as I know, there is no reason in principle why it cannot occur. Therefore it is possible that the RF from increased CO2 may simply have escaped to space.

Swanson says at the close, "However, this apparent impulsive behavior explicitly highlights the fact that humanity is poking a complex, nonlinear system with GHG forcing – and that there are no guarantees to how the climate may respond." I've no doubt this is correct. The other side to the coin is that there are also no guarantees to how the climate may respond in the absence of any GHG poking. I think that both sides of the coin have implications for policy choices.

Here are some background references for the above,

Links between annual, Milankovitch and continuum temperature variability
Stochastic and scaling climate sensitivities: solar, volcanic and orbital forcings
The internal variability of HadCM3

Aug 2, 2012 at 9:53 AM | Registered CommenterPhilip Richens

Therefore it is possible that the RF from increased CO2 may simply have escaped to space.

This of course is supported by satellite observations which confirm that the earth gives off more heat when its surface is warmer directly contradicting what the climate models predict. This shows that the climate models trap heat too aggressively and that their assumed amplification is bogus.

Aug 2, 2012 at 10:13 AM | Registered Commentermatthu

Hi BBD, perhaps you can take the opportunity on this thread to explain to me why the positive feedbacks are limited by Conservation of Energy. I really don't have a clue where to start in using the 1st Law as a limiting factor for positive feedbacks in a system with a "generator", the Sun, positive feedback loops, water vapour and the natural "forcings" PDO, AMO, volcanoes etc.

Aug 2, 2012 at 11:30 AM | Unregistered Commentergeronimo

Aug 1, 2012 at 7:00 PM Mike Jackson

What I am trying to ascertain is what the forcings were that initiated the Roman and Mediaeval warm periods (and the Minoan if you want to go back that far) and what was the feedback process that allowed those periods to continue and what was the forcing that brought them to an end.


Mike - maybe you are hunting for something that was never there?

I know that even quite simple nonlinear oscillatory systems can go on in apparently stable oscillations for many, many cycles, and then quite abruptly switch into a different mode which then continues for a long time before swapping into yet another mode.

Is there any reason to think that the Earth's climate system, including circulating ocean currents, could not exhibit similar behaviour, swapping modes for no externally applied reason? Maybe you are hunting for a will-o'-the-wisp?

Aug 2, 2012 at 1:08 PM | Registered CommenterMartin A

Martin A
Let me pick up directly on your point because it goes to the heart of why I started this discussion.
I was, I admit it(!), trying to tease enough out of BBD to find out exactly how he views forcings and feedbacks because a lot of what he says in this area seems (to me) to be potentially contradictory or tending to circular argument.
So

... maybe you are hunting for something that was never there?
It's quite possible. It appears to be common ground that the previous warm periods were definitely not initiated by CO2. But when it comes to the question of what did set them going the answer from most of the experts, as succinctly stated for us by BBD (7.39 yesterday evening) is "Dunno". Followed by hand-waving and waffle about the sun and volcanoes.
[Sorry, BBD; I'm not really having a go at you, but this is the archetypal answer that the climate scientists give us: "we don't know, but it was probably ... and anyway this time it's different because it has to be."]
We seem to be quite happy to pay lip-service to the idea that the climate is a chaotic non-linear system which is why it so difficult (I submit in essence impossible) to model with any degree of reliability but refuse to accept the corollary that it is thereby unnecessary to identify any specific event or combination of circumstances as the initiator for any subsequent series of events.
And now I'm probably over-simplifying.
We know from historical records and current observations that
- there is a 60-year cycle of heating and non-heating;
- within that ENSO has its own influence on weather;
- there is a ~600-700-year cycle of heating and cooling, at least over the last 2-3,000 years.
EITHER each of these various changes to the planetary weather patterns is caused by a some combination of circumstances that we haven't yet identified (perhaps because those who ought to be looking for it aren't interested), which given the regularity of the events seems more likely,
OR they are happening totally at random.
If the former you cannot point to an anthropogenic source unless you are going to claim anthropogenicity (sorry, I've got an attack of verbiage today) for all the events.
If the latter then ..... well, nothing really. Shit happens!
But either way since all of these events are evidently self-limiting there has to be some sort of governor (like what you have on a lorry, guv!) that maintains the system within some fairly tight parameters. This says to me that while there may be positive feedbacks in the early stages of a climate event, sooner or later those feedbacks become negative reversing the previous warming/cooling and setting up the next 30-year cooling/warming, and so on.
Whether this is AMO or the PDO or solar activity I don't know but I'm certain it's there somewhere.
Is there any reason to think that the Earth's climate system, including circulating ocean currents, could not exhibit similar behaviour, swapping modes for no externally applied reason?
Not as far as I'm concerned, Martin!

Rhoda, While I take your point about forcings and feedbacks being local, there must be some linkage since the weather patterns world-wide are related and while it may be that one part warms as another part cools or one part becomes wet as another part becomes dry these phenomena are consistent across the globe. (As far as we know. I think) So if Antarctica cools as the Arctic warms it will do that in every climate event.
[Now I come to think of it I wonder if anyone has researched that because that would seem, if true, to provide evidence that the climate cycles follow similar patterns every time. Can I have my Nobel now or do I have to actually prove it? /sarc]

Aug 2, 2012 at 2:37 PM | Registered CommenterMike Jackson

Mike Jackson

This says to me that while there may be positive feedbacks in the early stages of a climate event, sooner or later those feedbacks become negative reversing the previous warming/cooling and setting up the next 30-year cooling/warming, and so on.

Terminology alert! Positive feedbacks cannot 'become negative'. That's unphysical. They stay positive. But... positive feedbacks amplify the original forcing and they can cool as well as warm if the forcing is reduced.

Think about deglaciation for example: small push (orbital forcing) = large climatic response (amplification by positive feedbacks). Then, post-peak orbital forcing, a slow cooling begins which becomes more rapid as ice albedo feedback (strongly positive) is increasingly engaged. End result is a tippy, unstable glacial dominated by ice albedo feedback from the NH ice sheet... until the next ~100ka just-so combination of eccentricity and obliquity triggers the next glacial termination.

Positive feedbacks remain positive throughout. It's the forcing that changes.

Aug 2, 2012 at 5:28 PM | Unregistered CommenterBBD

How sure are you about that?
CO2=increased heat=increased water vapour=increased cloud cover=less heat
The original forcing is still CO2.The feedback has turned negative. Why not?

Aug 2, 2012 at 5:50 PM | Registered CommenterMike Jackson

Looking again at one of BBD's graphs. It says at one place: "India collides with Eurasia". Heh.

Alarmism comes from having absolutely no sense of proportion or time whatsoever. Or temporarily becoming blind to it.

Aug 2, 2012 at 5:58 PM | Registered Commentershub

Aug 2, 2012 at 5:28 PM BBD

Terminology alert! Positive feedbacks cannot 'become negative'. That's unphysical. They stay positive. But... positive feedbacks amplify the original forcing and they can cool as well as warm if the forcing is reduced.

Hold on, hold on.

A "forcing" is necessarily a positive (or, at least, a non-negative) quantity, since it is measured in units of power per unit area.

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?

Aug 2, 2012 at 6:02 PM | Registered CommenterMartin A

"original forcing "


As though one has entered Bible class.

The climate system cares a whit for what was 'original' and what came later. Feedbacks and forcings are just a construct which are useful within their limits.

Aug 2, 2012 at 6:39 PM | Registered Commentershub

Oh, dear. This could be a long evening!
Let's get the terminology and the semantics out of the way first (this is what I was hoping to get sorted at the top of the thread!).
Are "warming" and "cooling" two separate processes or is cold simply an absence of heat?
Something (let us call it CO2; it's just a name) begins a forcing process which causes the atmosphere to warm by 1.2C each time the amount of CO2 in the atmosphere doubles. Let us further assume that the warming causes an increase in water vapour which itself further warms the system. This is a feedback process which is positive.
At some point the additional water vapour causes additional clouds — that being the sort of thing that atmospheric water vapour does. These clouds cool the system — that being the sort of thing clouds do.
This is also a feedback process which is negative.
What BBD appears to be saying is that cannot happen and that the only way for the climate to cool is to reduce the amount of CO2. Have I got that right, BBD?
And will somebody explain in that case where my process falls down?

Aug 2, 2012 at 6:54 PM | Registered CommenterMike Jackson

Mike

CO2=increased heat=increased water vapour=increased cloud cover=less heat
The original forcing is still CO2.The feedback has turned negative. Why not?

Too many assumptions: how do you know that the net cloud forcing is negative? Clouds trap and re-radiate IR down as well as reflecting DSW up. Clouds at different altitudes and of different types have different radiative properties. And - above all - what evidence is there that increased WV = increased cloud = less heat?

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?

Aug 2, 2012 at 7:09 PM | Unregistered CommenterBBD