[crandles]

I cannot believe you are not accepting the obvious that a positive feedback does not always cause a runaway effect.

Suppose:
A barrel contains 120 litres of water. The inflow appears fixed at 12 litres a minute. The outflow depends on the height of the water and so is equal to 10% of the volume in the barrel per minute.

This is currently in equilibrium. We now reduce the size of the hole so the the outflow is only .075 times the volume in the barrel (units still litres per minute). Given the current knowledge of the system we would expect a new equilibium to be reached with the barrel containing 160 litres and inflow and outflow equal to 12 litres per minute.

In fact we notice the quantity in the barrel goes above 160 lites so we investigate and discover that the inflow is not fixed at 12 litres a minute but there is a system feedback such that if the outflow in one minute is greater than 9% of the volume in the barrel then the inflow is fixed at 12 litres per minute but if the outflow is less than 9% of the volume then the inflow is set at 15 litres per minute.

This system reaches an equilibrium with 200 litres in the barrel.

This is a positive feedback because the feedback moves the system more in the same direction (increasing volume in the barrel in this example).

The system reaches an equilibrium and does not suffer a runaway effect.

Hopefully that is an adequate description of a system. Now will you accept the obvious?

[another_someone]

quote:Originally posted by crandles
Suppose:
A barrel contains 120 litres of water. The inflow appears fixed at 12 litres a minute. The outflow depends on the height of the water and so is equal to 10% of the volume in the barrel per minute.

This is currently in equilibrium.

 We now reduce the size of the hole so the the outflow is only .075 times the volume in the barrel (units still litres per minute). Given the current knowledge of the system we would expect a new equilibium to be reached with the barrel containing 160 litres and inflow and outflow equal to 12 litres per minute.

In fact we notice the quantity in the barrel goes above 160 lites so we investigate and discover that the inflow is not fixed at 12 litres a minute but there is a system feedback such that if the outflow in one minute is greater than 9% of the volume in the barrel then the inflow is fixed at 12 litres per minute but if the outflow is less than 9% of the volume then the inflow is set at 15 litres per minute.

Sorry, this statement does not make sense.

You cannot 'fix' the input of a system – the whole point about an input is that it is not controlled by the system itself.  The external inputs cannot be limited to either 12 or 15 litres/minute – they can be absolutely anything, since they cannot be controlled by the system – if they are controlled by the system, then they cease to be external inputs and must be regarded as part of the system.

Ofcourse, you can say that there is a negative feedback system that compensates for any external attempt to increase flow above either 12 or 15 litres/minute, and a positive feedback that allows a switch between the two stable states.

If I have a barrel of water, and a tap above that barrel of water – the barrel of water cannot control how much I turn on that tap.  Ofcourse, it can have a negative feedback system that causes that if the outflow each minute if less than 9% of the contents, then any water in excess of 15 litres/minute that I allow to flow out of the tap would be deflected so as not to enter the barrel – but that requires a negative feedback mechanism.  The barrel cannot cause me to switch off the tap, unless I become a part of the system (e.g. I look into the barrel, and make decisions according to what I see, and thus become a part of the negative feedback mechanism).

The system you describe is a bistable system, which is not at all an uncommon situation, but it requires a mix of positive and negative feedbacks to maintain it – a positive feedback to allow it to switch between stable states, and a negative feedback to maintain one or other stable state.

What you are suggesting is a system that creates a nett positive feedback over a very narrow range, and a nett negative feedback outside of that range.

George

[crandles]

The above system is an analogy to the climate system. The height of water in the barrel represents temperature. The volume of water is a quantity of heat energy and flows of water are flows of heat energy.

We can change the size of the holes which is analogous to increasing CO2 in the atmosphere to reduce the rate infrared outgoing radiation below what would otherwise be expected for the temperature. This causes temperature and heat in the system to rise. Temperature increasing melts ice causing a positive albedo feedback effect where the system accepts more incoming radiation from the sun rather than it being reflected.

Thus I am not only arguing that positive feedback does not lead to a runaway effect but I am also arguing about the particular case of the climate system and the ice albedo positive feedback is not expected to cause runaway climate change.

It probably wasn't a good system to use for the purpose of showing that positive feedback does not have to lead to a runaway effect because there is negative feedback on the outward energy flow but not on the height/volume of water.

Not really sure why you cannot accept the size of the hole as an input which is under my control. The output I am interested in is the height/volume of water. Nor do I understand you calling it a bistable system as I can change the input, the size of the hole and get equilibriums at an infinite number of different volumes of water in the barrel.

A new system comprises a tower of building blocks plus my neice.

The system is currently in equilibrium at 5 blocks high. I apply a forcing of adding a block. This in the absence of feedback would be expected to move the system to a new equilibrium of 6 blocks high. However there is a feedback in this system, my neice is playing copycat and does whatever I do to the tower. Thus the system actually moves to a new equilibrium level of 7. This is therefore a positive feedback as it moves the system in the same direction. The system does not result in a runaway high tower (or no tower). Therefore not all positive feedbacks lead to a runaway effect.

[Mjhavok]

A heat wave in the UK has been happening and I think temperatures have reached 37 degrees celsius.

[another_someone]

quote:Originally posted by Mjhavok

A heat wave in the UK has been happening and I think temperatures have reached 37 degrees celsius.

They did that a week or two back, but they are far more tollerable right now.

George

[Mjhavok]

I h8 it lol. Anything that makes me sweat more than normal (apart from the obvious fun thing) means I usually don't like it. I also  skin in the non cancer variety.