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.

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).

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.

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.

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.

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.

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?