The latest paper by Roy Spencer claiming negative feedback from AGW really has the alarmists choking on their baguettes, so I thought I would try to explain it with an analogy.
Feedbacks represent a secondary effect, but its not that much harder to understand with the following analogy.
Imagine the atmosphere as a bucket. Short-wave solar radiation pours in the top (yellow arrow), some splashes out (orange arrow), and long wave radiation out a hole in the bottom (red arrow). The level of water in the bucket is determined by the relative sizes of the flows, in turn determined by the size of the holes.
The level of water is the observable global surface temperature (green).
The primary effect is this. If the radiation pouring in the top increases, the level of water (temperature) in the bucket must increase until the flow out the hole in the bottom matches the flow coming in the top. This relationship is rigorously determined by laws of conservation.
Feedback represents a tendency for the water (temperature) to depart from this relationship. We can analogise this by varying the size of the holes in the top or the bottom of the bucket.
Positive feedback can be imagined to occur in two ways. Either the size of the top of the bucket increases as the water level increases, so less splashes out, or the size of the hole in the bottom decreases, so less flows out. The latter mechanism is represented by such effects as greenhouse gasses holding back the outgoing radiation.
Negative feedback is when the water level increases less than expected, and could be due to the hole in the top of the bucket decreasing, due to increasing albedo (SW reflection from cloud tops) or increasing size of the hole in the bottom (decreasing low level cloud letting more LW radiation out, as in the Lindzen Iris effect).
Now Spencer makes at least two interesting claims. The first is that most previous studies have ‘blundered’ because feedbacks are essentially unobservable from temperature only. This can be seen from the analogy. If the level of the water increased by 10 units, but feedback caused it to reduce by 9 units, then the net effect of an increase of one unit is indistinguishable from all of the other possibilities.
The second claim is that the only significant observable feedback from examining the most accurate satellite temperature and outgoing radiation data is a strong negative feedback. That is, when the water level increases, the size of the hole in the top decreases or the size of the hole in the bottom of the bucket increases to let more radiation out.
Now if Spencer is correct the only argument that alarmists have left is that positive feedback (and correspondingly high climate sensitivity) is necessary to explain glacial-interglacial transitions of 5C every 100,000 years or so. Well there are so many unknowns that far back, and changes in surface albedo from changing ice sheets are a stronger effect than CO2 anyway, so to base your argument on somethng so vague, and distant, just sounds silly.
And silly is how the alarmists are looking more and more these days. First they put their faith in an IPCC that might be put-down due to misrepresentation. Then they rely on models that have been known to be ‘running hot’. Cite unprecedented temperatures based on 1000 year thermometers that don’t work. And now, blundering into false interpretations of feedbacks that they didn’t understand properly in the first place.