[another_someone]

quote:Originally posted by crandles

See also:

What does the lag of CO2 behind temperature in ice cores tell us about global warming?

http://www.realclimate.org/index.php?p=13

This seems like a very selective interpretation of the data.  Ofcourse, being selective does not make it provably wrong, only that it makes it suspect.

There is a clear acceptance that the first 800 years of global warming (in the historic records of past warm periods) was not caused by CO2.  It then suggests we can ignore this 800 years when we look at the subsequent 4,200 years.  While it is true that we cannot prove that the subsequent 4,200 years have the same cause of warming as the preceding 800 years; but would not Occam's razor suggest that we should first look for one mechanism, rather than postulate two separate mechanisms.  Ofcourse, Occam's razor is not always right, but why assume it to be wrong before having shown it to be wrong?

Furthermore, since it is clear that historic CO2 levels have consistently risen after the initial rise in temperature, we may reasonably deduce that there are non-anthropogenic mechanisms within the biosphere that actually cause a rise in CO2 as a consequence of rising temperature.  To what extent can we even be sure that present rises in CO2 levels are actually caused by human activity, and to what extent are they themselves part of the mechanisms that have in the past been a response to increasing global temperatures (the fact that humans produce CO2 does not of itself prove that humans control the balance between CO2 and O2).

George

[another_someone]

quote:Originally posted by crandles
However, the totality of the evidence is much greater than the historical correlation. The science of how CO2 acts as a greenhouse gas is well understood.

There should be one very important piece of evidence that should be available to us to help indicate the magnitude of the greenhouse effect.  I have yet to find anywhere on the Internet any publication of this evidence.

If the current presumed rise in CO2 levels on Earth are reducing heat radiation into space, then we should actually see a significant reduction of emitted energy in the infra-red pictures of the Earth taken by weather satellites over the last few decades.  Is such a reduction in infra-red emission actually observed in satellite photos?

quote:
Without feedbacks, we are sure there would be a positive feedback of approx 1C for a doubling of CO2. The feedbacks are less certain but the main ones we know of increased water vapour and ice albedo are positive. There are several independant lines of research that suggest the climate sensitivity is about 3C

If you want to suggest there are negative feedbacks such that climate sensitivity might even be negative then you are going to have to come up with some important feedbacks that are currently unknown and provide reasons why all those different estimates are wrong. If there was no positive climate sensitivity, you would also have to explain the unpresidented rates of warming at coincidentally just the time we would expect rises if there was a positice climate sensitivity.

The climate is dynamic, but with multiple stable (transiently) states.  In order to be able to achieve such short term stability, there must be negative feedback loops.  In order to switch states, there must also be positive feedback loops.  I do not think one can discuss whether there are positive or negative feedback loops, but must inevitably accept that there are both, most of which we have not the slightest understanding of.

George

[crandles]

quote:The climate is dynamic, but with multiple stable (transiently) states. In order to be able to achieve such short term stability, there must be negative feedback loops. In order to switch states, there must also be positive feedback loops. I do not think one can discuss whether there are positive or negative feedback loops, but must inevitably accept that there are both, most of which we have not the slightest understanding of.

What makes you so sure of this? Why cannot the appearance of multiple stable states be a result of the forcings applied  eg Milankovitch cycles. To first order, the most important consideration is that the hotter the earth the more heat it ratiates. This creates the stability in the same manner as the leaky bucket analogy. A steady state can be reached. Reduce the size of the hole and the water level starts to rise but it doesn't necessarily overflow the bucket a new equilibrium level is likely to be reached because the flow out of the hole depends on the height of water.

Since there appear to be net positive feedbacks at more than one point in time (eg current, LGM, and other times from which estimates of climate sensititivity have been made), it is possible that at another point in time the net feedbacks could be different but that doesn't mean they are likely to become negative and I suggest that the experts know enough to say they are very unlikely to become negative soon.

[crandles]

quote:
This seems like a very selective interpretation of the data.  Ofcourse, being selective does not make it provably wrong, only that it makes it suspect.

There is a clear acceptance that the first 800 years of global warming (in the historic records of past warm periods) was not caused by CO2.  It then suggests we can ignore this 800 years when we look at the subsequent 4,200 years.  While it is true that we cannot prove that the subsequent 4,200 years have the same cause of warming as the preceding 800 years; but would not Occam's razor suggest that we should first look for one mechanism, rather than postulate two separate mechanisms.  Ofcourse, Occam's razor is not always right, but why assume it to be wrong before having shown it to be wrong?

George

This is actually funny. [)] Who is being selective? Is it the climatologists at realclimate for ignoring the other cause?

or could, just perhaps, it be the case that they are considering both causes of warming (one known to have a positive contribution and the other which is unknown whether it continues or not) and it is you being selective in wanting to consider only one cause of warming and deliberately ignoring the CO2 when that is the cause we know to have a greenhouse effect which will have a positive effect on temperature.

I don't think occam razor suggests ignoring a known cause in favour of an unknown one.

[crandles]

quote:Originally posted by another_someone

Furthermore, since it is clear that historic CO2 levels have consistently risen after the initial rise in temperature, we may reasonably deduce that there are non-anthropogenic mechanisms within the biosphere that actually cause a rise in CO2 as a consequence of rising temperature.  To what extent can we even be sure that present rises in CO2 levels are actually caused by human activity, and to what extent are they themselves part of the mechanisms that have in the past been a response to increasing global temperatures (the fact that humans produce CO2 does not of itself prove that humans control the balance between CO2 and O2).

George

We are pretty sure. We are putting 7 gigatones of carbon into the atmosphere each year through burning fossil fuels and when we measure the atmosphere 4 gigatones per year are accumulating in the atmosphere. The ocean is disolving most of the balance causing it to become more acidic. Prior to the industrial revolution CO2 had been pretty much stable in the atmosphere. So to raise uncertainty about us being the cause you are going to have to find some pretty big natural sources to have started at just the right time as well as methods for our CO2 to be dealt with naturally.

[another_someone]

quote:Originally posted by crandles
What makes you so sure of this? Why cannot the appearance of multiple stable states be a result of the forcings applied  eg Milankovitch cycles. To first order, the most important consideration is that the hotter the earth the more heat it ratiates. This creates the stability in the same manner as the leaky bucket analogy. A steady state can be reached. Reduce the size of the hole and the water level starts to rise but it doesn't necessarily overflow the bucket a new equilibrium level is likely to be reached because the flow out of the hole depends on the height of water.

Since there appear to be net positive feedbacks at more than one point in time (eg current, LGM, and other times from which estimates of climate sensititivity have been made), it is possible that at another point in time the net feedbacks could be different but that doesn't mean they are likely to become negative and I suggest that the experts know enough to say they are very unlikely to become negative soon.

A system that has pure positive feedback, and no negative feedback component, will always be in a runaway condition – this is not what the Earth has ever done historically.

Even with regard to Venus, which is often cited as a runaway condition that the Earth could fall in to – it is not at all in a runaway condition – Venus is relatively stable (possibly even more stable than the Earth), but is merely stable at a higher temperature.

Thus, to avoid runaway, you must either have absolutely no feedback, or a substantial component of negative feedback.  Since we know that we do not have a condition on Earth where there is no feedback, thus we may reasonably surmise that there are substantial negative feedback processes in place.

George

[another_someone]

quote:Originally posted by crandles
This is actually funny. [)] Who is being selective? Is it the climatologists at realclimate for ignoring the other cause?

or could, just perhaps, it be the case that they are considering both causes of warming (one known to have a positive contribution and the other which is unknown whether it continues or not) and it is you being selective in wanting to consider only one cause of warming and deliberately ignoring the CO2 when that is the cause we know to have a greenhouse effect which will have a positive effect on temperature.

I don't think occam razor suggests ignoring a known cause in favour of an unknown one.

We are not here looking at a known cause versus an unknown cause.

What we are talking about is an observed, but not understood, cause; and a hypothesised but not definitely observed cause.

What I am saying is when one compares hypothesis to observation, observation should always take precident.

We know there is a cause of global warming that does not include CO2, although we have no hypothesis that yet explains what it is.  We know that rises in CO2 seem to follow (not lead) these rises in temperature, and thus may infer (although cannot with certainty know) that CO2 rises are caused by rises in temperature.

We have a hypothesis that suggests that increased levels of CO2 should cause rises in temperature – yet we have no quantitative observations of such an effect that could unambiguously show this to happen.

A few things we do know:

• Temperature rises can occur even in the absence of rising CO2.


• Temperature falls can happen even after CO2 levels have risen.

We have a hypothesis that suggests that temperatures should rise after an increase in CO2 levels, and that temperatures will continue to remain high as long as CO2 levels remain high.  We know that this hypothesis conflicts with observation.  We can certainly suggest that there are unknown factors that conflict with the hypothetical case, but since we have not explained in any way what these unknown factors are, thus we have no way of knowing whether the hypothetical situation present any significant input into the real and observed results.

I accept that the hypothetical feedback cycle probably has some impact, but what I cannot accept is that it has any impact of significant quantitative levels that they may be observed against all the other (mostly unknown, although clearly present) factors that do affect global temperatures.

George

[crandles]

quote:
Originally posted by another_someone

A system that has pure positive feedback, and no negative feedback component, will always be in a runaway condition – this is not what the Earth has ever done historically.

Sorry but that is rubbish. Feedback can be positive but less than a factor of 1 and then you won't get a runaway condition.

Take something really simple like
xi=x(i-1)+forcing+feedback factor * ( x(i-1) - x(i-2) )

try it out with a feedback factor of greater than 1 and you do get a runaway condition. Try it with a feedback factor of between 0 and 1 and you do not get a runaway effect. The eventual change is greater than the forcing so the feedback is positive but there is no runaway effect.

The above alone defeats your "will always be"

With temperature it is much harder to get a runaway effect because of the first order effect that a hotter planet emits more radiation.

Sorry - I think I accidentally edited your post - hopefully back to where it should be.

[crandles]

quote:We have a hypothesis that suggests that temperatures should rise after an increase in CO2 levels, and that temperatures will continue to remain high as long as CO2 levels remain high. We know that this hypothesis conflicts with observation.

Do you want to try again on that one?

800 years is less than 20% of the 5000 year period. Given that there are lots of these periods and some show those 800 year periods and that the hypothesis is that CO2 (not alone but among other things affects temperature) then that is good observational evidence that CO2 does have the effect expected and that other causes do not dominate over the effect of CO2.

[another_someone]

quote:Originally posted by another_someone
Sorry but that is rubbish. Feedback can be positive but less than a factor of 1 and then you won't get a runaway condition.

Take something really simple like
xi=x(i-1)+forcing+feedback factor * ( x(i-1) - x(i-2) )

try it out with a feedback factor of greater than 1 and you do get a runaway condition. Try it with a feedback factor of between 0 and 1 and you do not get a runaway effect. The eventual change is greater than the forcing so the feedback is positive but there is no runaway effect.

The above alone defeats your "will always be"

Sorry, but how is the above a pure positive feedback mechanism – you have a term -x(i-2) – by my reckoning that amounts to a negative term, and thus a negative feedback component.

quote:
With temperature it is much harder to get a runaway effect because of the first order effect that a hotter planet emits more radiation.

But that too is a negative feedback, since it is a factor that increases the cooling as an increase in temperature occurs.

Ofcourse, as long as the effect is linear, then the effect will only reduce the rate of runaway, but not be sufficient to stop it.  If you can demonstrate that proportion of temperature radiated increases with increasing temperature, then you would maybe be able to show it was sufficient to stop runaway; but at present, the indications are actually contrary, that as temperature increases, so the amount of ice on our planet reduces, and so the albedo of our planet reduces, and so the negative feedback becomes less effective (i.e. we have a positive feedback cycle that dominates over the negative feedback cycles).  Ofcourse, there may be other planets, with different surface chemistries, that might have a different relationship between albedo and surface temperature.

George

[crandles]

quote:Sorry, but how is the above a pure positive feedback mechanism – you have a term -x(i-2) – by my reckoning that amounts to a negative term, and thus a negative feedback component.

Sorry not sure how to do subscripts on this forum yet. The i, i-1 and i-2 terms are all supposed to indicate time periods.

So start at a temperature of 10C. Apply a forcing that causes a 1C rise (This could be doubling of CO2 or something else. The temp initially goes to 11C then the feedback starts kicking in. If the feedback factor is 0.5 the temp would go to 11.5 then 11.75 then 11.875 and so on up to 11.999999. That is clearly a positive feedback that does not not lead to a runaway effect.  

[crandles]

quote:Ofcourse, as long as the effect is linear, then the effect will only reduce the rate of runaway, but not be sufficient to stop it. If you can demonstrate that proportion of temperature radiated increases with increasing temperature, then you would maybe be able to show it was sufficient to stop runaway; but at present, the indications are actually contrary, that as temperature increases, so the amount of ice on our planet reduces, and so the albedo of our planet reduces, and so the negative feedback becomes less effective (i.e. we have a positive feedback cycle that dominates over the negative feedback cycles). Ofcourse, there may be other planets, with different surface chemistries, that might have a different relationship between albedo and surface temperature.

No it is basic physics that a hotter body radiates more heat.

(Therefore the effects are much less than linear. This is why climate sensitivity is defined in terms of the effect of doubling CO2 you have to increase CO2 much more than linearly to get a constant response.)

The ice albedo effect is a positive feedback not a negative one. But is is not strong enough to overcome the first order effect that hotter bodies radiate more heat.

[crandles]

quote:Originally posted by another_someone

quote:
With temperature it is much harder to get a runaway effect because of the first order effect that a hotter planet emits more radiation.

But that too is a negative feedback, since it is a factor that increases the cooling as an increase in temperature occurs.

No that is part of the system not a feedback.

[another_someone]

quote:Originally posted by crandles
Do you want to try again on that one?

800 years is less than 20% of the 5000 year period. Given that there are lots of these periods and some show those 800 year periods and that the hypothesis is that CO2 (not alone but among other things affects temperature) then that is good observational evidence that CO2 does have the effect expected and that other causes do not dominate over the effect of CO2.

Sorry, but all that the coincidence between rises in CO2 and increased temperature shows is correlation, not causality.  The fact that increased temperature precedes increases in CO2 is strongly indicative (but does not prove) that CO2 responds to temperature.  Anything else is pure hypothesis.

Ofcourse, so long as there is observable coincidence, it is valid to ask whether there is a two way causality; but there is no necessity to assume this simply to explain the observed evidence.

George

[another_someone]

quote:Originally posted by crandles
Sorry not sure how to do subscripts on this forum yet. The i, i-1 and i-2 terms are all supposed to indicate time periods.

Neither do I, but I did understand what you were saying.

quote:
So start at a temperature of 10C. Apply a forcing that causes a 1C rise (This could be doubling of CO2 or something else. The temp initially goes to 11C then the feedback starts kicking in. If the feedback factor is 0.5 the temp would go to 11.5 then 11.75 then 11.875 and so on up to 11.999999. That is clearly a positive feedback that does not not lead to a runaway effect.  

The system as you described it was :

xi=x(i-1)+forcing+feedback factor * ( x(i-1) - x(i-2) )

this can be expanded to:

xi=x(i-1)+forcing + feedback factor * x(i – 1) - feedback factor * x(i – 2)

In other words, there is a component of the output (from two time slices earlier) that is subtracted from the input – this makes it negative feedback.  There is also a component (from the immediately preceding time slice) where the output is added to the input, and this is a positive feedback.  So your system contains both positive and negative feedback components.

George

[another_someone]

quote:Originally posted by crandles

quote:Originally posted by another_someone

quote:
With temperature it is much harder to get a runaway effect because of the first order effect that a hotter planet emits more radiation.

But that too is a negative feedback, since it is a factor that increases the cooling as an increase in temperature occurs.

No that is part of the system not a feedback.

I will probably grant you this (at least insofar as the effect is linear), because it is an effect that is purely dependent upon the inputs to the system, and not dependent upon any function or output of the system itself.

Nonetheless, any non-linearity (such as caused by changes in albedo, or changes in heat retention, must be regarded as feedback effects, since they are caused by changes in state of the system that is itself caused by previous inputs into the system).

George

[crandles]

The initial forcing pushes it up from 10 to 11. The new equilibrium level is 12. Therefore the effect of a feedback factor of 0.5 is to increase the movement of the system in the same direction therefore this feedback is positive.

To quote http://en.wikipedia.org/wiki/Positive_feedback

quote:When a change of variable occurs in a system, the system responds. In the case of positive feedback the response of the system is to change that variable even more in the same direction.

How is what I have shown not a positive feedback?

Artificially spliting my feedback funtion into two simply is not appropriate.

(edit spelling)

[another_someone]

quote:Originally posted by crandles
Artificially spliting my feedback funtion into two simply is not appropriate.

It is not an artificial splitting of your feedback function.

http://en.wikipedia.org/wiki/Feedback

quote:
In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. Often this is done intentionally, in order to control the dynamic behavior of the system. Feedback is observed or used in various areas dealing with complex systems, such as engineering, architecture, economics, and biology. Continuous feedback in a system is a feedback loop.

x(i-1) and x(i-2) are two different feedback paths (they must have different paths, because they have different delays), so they must be regarded as separate feedbacks loops.  You bundling them into one feedback path is artificial, because the fact that they have different time delays means they cannot have the same path.

George

[crandles]

( x(i-1) - x(i-2) ) could have been written as delta x (ie the change in the x variable) lagged by a timestep.

But suppose I describe another system which initially is in equilibrium at 10 we apply a forcing which moves the system to 11 then a feedback kicks in equal to 3 times the forcing (lagged by 1 timestep so the system moves to 14 and it then stays in equilibrium at 14.

Surely there is no negative feedback in that system and it does not go into a runaway effect.

[another_someone]

quote:Originally posted by crandles
But suppose I describe another system which initially is in equilibrium at 10 we apply a forcing which moves the system to 11 then a feedback kicks in equal to 3 times the forcing (lagged by 1 timestep so the system moves to 14 and it then stays in equilibrium at 14.

Surely there is no negative feedback in that system and it does not go into a runaway effect.

Sorry, I cannot see any description of what might be regarded as a complete system.

A system needs inputs and outputs (the closest to an input would be what you call a 'forcing'), and a complex system will normally have some feedback that moves some of the output back into the input.

You have said the system is initially at equilibrium at 10 – you have not told me how this equilibrium is maintained?  One assumes that in order to maintain an equilibrium, there must be negative feedback (one cannot otherwise maintain sustained equilibrium over a range of inputs, since the only way of offsetting changes in input is to feedback some of the output to cancel the input).  What is the nature of this feedback, and how does it alter with further changes in input?

You suggest that with a further increase in input (of some unknown amount), the negative feedback is sufficiently overwhelmed to allow the output to drift to 11 – but then there is a step change in the feedback that reduces the negative feedback (or possibly changes it to a net positive feedback) until the output climbs to 14, and then switches back to another negative feedback that holds it steady there.

What you have described is a system with multiple stable regions (an astable system), but you have in no way described mathematically (let alone physically) what the feedback mechanisms are that create this astable system.

George