The last article discussed negative and positive feedbacks in the climate. Now I want to see how these impact the stability of the climate and also how the stability of the climate can tell us what kinds of feedback are present.
- Criteria for Cycles
- Global warming and earthquakes
- Thermal crust expansion, decomposition and the Carbon cycle
- Overview of feedbacks
I introduced the idea in the overview of feedbacks that negative feedbacks are rather like a driver on a road constantly correcting any small veer to one side or the other. I then suggested that positive feedbacks produce the kind of effect that would occur if an ordinary driver got into a car where the steering wheel acted in reverse. So that as they turn to the right to correct a drift to the left, rather than the car going right as intended, the car goes further and further to the left. As a result almost all normal drivers getting into such a car will go off the road and so would a climate with large positive feedbacks.
In general climates with positive feedback tend to be unstable and the higher the feedback the more unstable they become.
To illustrate the idea of positive feedback I used the analogy of a driver in a dodgem car, which rather than behaving as normal, so that the driver would tend to act to reduce any error in the direction, the steering wheel is geared in reverse. This means that as they veer to one side, they turn the steering wheel so as to increase the error.
I showed in the last article that small levels of positive feedback can be stable. The example I used was a feedback of 1/10 so that a 1C rise in temperature resulted in 1.11111C.. total rise above what it would have been without the feedback.
But too large and always when the feedback is above 1, then the climate is unconditionally unstable and like the dodgem car with the reversed steering wheel, the climate tends to veer “off the road”.
In other words, simply having large positive feedbacks results in a situation that cannot possibly last because even a very small change will be amplified by the feedback and then as it causes more change (1c → >+1C = >2C) the increase due to feedbacks will be larger than the original change and so that in addition will cause even more change (1c → >+1C = >2C → >+2C = >3C). And so if the positive feedback introduces more change than the original change, the climate will just “go off the road”.
In contrast, if we have modest positive feedback such the increase is 1/10 of the initial change. Then we get: (1c → >+0.1C = >1.1C) then that additional 0.1 C will itself cause more change so (1c → >+0.1C = >1.1C → >+0.11C = >1.11C) and it can be shown that the total increase from something that tended to increase the temperature by 1C will be a totaly change after time from positive feedback of 1.11111111111… C
So, we can conclude as a matter of simple maths, that a climate that is stable has feedbacks which are either negative, or if they are positive, the feedback must be smaller than the original change.
So, no climate that has been relatively stable can have high levels of positive feedback. And if a climate did have very high levels of positive feedback it would have “gone off the rails” a very long time ago either leading to runaway warming or runaway cooling until the climate had so changed that the positive feedback mechanism disappeared and it became stable again but with an entirely new temperature or state.
However, whilst no climate can be stable over any length of period with high levels of positive feedback on their own, if the climate has some kind of “buffers” or threshold beyond which negative feedbacks dominate, then this is rather like the car I discribe above with the reverse geared steering wheel, but only this time they are not on an open road, but instead they are in a bumper car with large crash barrier to keep them on the road.
Here, almost all drivers initially heading down the track would tend to veer to one side. They would turn the wheel in the way they expect to self correct and find that instead they turn more and more sharply toward the side.
When the hit the side, (assuming it continues forward) the car would eventually end up heading slightly away from that side. Then because of the nature of the steering, the driver would tend to veer to the middle over-compensate and hit the other side. Then they would veer away from that side and hit the other. So, the driver would go down the track veering sharply from side to side and only stopping from going further by hitting the “buffer”.
So, climates can in principle have large positive feedbacks present. But in order to prevent them “going off the road”, there must be mechanisms that impose a threshold so that the effect of the positive feedback is eliminated and forms of negative feedback (buffers always push toward the middle) come into play.
So, we can also say, that unless there is an external driver such as the day length or yearly change in the climate where the sun imposes a daily or yearly cycle on the climate, if the climate is subject to oscillations, then at least within the range of the oscillation there must be high levels of positive feedback (such that a 1C changes leads to a further increase of >1C) AND there must be large negative feedbacks or very reduced levels of positive feedback at the limits of oscillation.