windg,
first off, positive feedback means increasing the effect. That is if there is item X that is a positive feedback and it causes a forcing that incrases the temperature by 1 degree for every 1/10 degree rise, that is a positive feed that will result in runaway warming, as in ocean boiling away warming. What happens is some tiny tin pertubation occurs, maybe the earth's temperature rises by 1 e -19 degree K because you have a cold with a fever. That factor X of positive feedback then kicks in and all of a sudden it starts warming up the planet by 1e-18 degree. Since it'w 1e-18 degrees warmer, X continues and now it's 1e-17, til finally, if this feedback continues to exist, all of a sudden over some short period of time, the temperature has gone up 0.1 deg K - but then the feedback will cause it to rise another 1 degree, then 10, then 100, til finally some other factor stops it- like the earth totally evaporates. Positive feedbacks have to be swamped by an overall net negative feedback or there is no stability in the system.
On the other hand, a negative feedback might be one that changes forcings so that if the temperature rises by 1/10 degree, some forcing X will be affected and reduced that will drop the X forcing.
Now if co2 forcings increase by enough to raise temperature by 1 degree this negative feedback will start to reduce the X forcing as the temperature rises so that at some point below the 1 degree increase, things balance again.
An example of this X could be clouds. As temperature rises, more h2o enters the atmosphere and ostensibly forms more clouds, increasing the total cloud cover. As temperatures decrease, less h2o vapor enters the atmosphere and less clouds cover forms, allowing more energy to reach the surface, warming it. Conceivably, this cloud cover may be effected by cosmic rays and the solar cycles, providing for the prospect of climate changing from external sources. Also, were things to go with large cloud covers for a while, blocking lots of incoming radiation, the resulting iceage might block or short circuit some of this cloud negative feedback, permitting a new stable point that is somewhat colder.
There are numerous other potential X feedbacks, like aerosols and particulates, malinkovich cycles where the earth's orbit varies and changes the incoming energy.
As for speed of change, that factor is not something that is known. As for it having any effect, that's highly suspect. CO2 and other GHGs as do volcanic erruptions have extremely quick response times and there's nothing new about severe impacts from erruptions, far more so than is witnessed in recorded history. Co2 levels have been much higher in the past, it's been warmer in the past, it's been colder in the past. One could state that there's not really anything new under the sun, other than a bunch of two legged creatures wondering about it.
Hopefully now you realize that if clouds nullify or substantially nullify GW, it's because they cause negative feedback.
My little 0 dimension model indicates clouds themselves do affect the temperatures and that temperatures drop as more clouds cover the skies. That is a negative feedback. However, it seems the entire climate scientific world has yet to figure out just how clouds forms, how cosmic rays enter in versus how aerosols and particulates enter in, despite a hundred years of study, from Wilson to Svensmark.
My little simple model (and some thought) also indicate that it would appear that there may be strong net negative feedbacks but that no strong positive feedbacks appear to be involved in the current atmosphere - a pretty fair argument that net positive feedbacks don't occur.
You might want to take a look at this site,
http://geosci.uchicago.edu/~archer/c...radiation.html which is a modtran calculation model.
I can't say what's in it as I don't know at present. For a web page, it's a rather sophisticated calculation and it seems to be fairly accurate from what I've seen with rough comparisons, at least for the basics.