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Author Topic: What will the Arctic resemble in 2050?  (Read 65726 times)

ChrisReynolds

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Re: What will the Arctic resemble in 2050?
« Reply #100 on: March 27, 2013, 08:38:43 PM »
Comment removed as I now get what Kevin was saying and agree.
« Last Edit: March 27, 2013, 08:55:15 PM by ChrisReynolds »

crandles

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Re: What will the Arctic resemble in 2050?
« Reply #101 on: March 27, 2013, 09:43:52 PM »
No, if feedbacks worked as you describe, negative feedbacks would be just as destabilizing as positive ones.

It's two sides of a coin; given an initial forcing in one direction (e.g. warming) a feedback may be positive and amplify the response (e.g. warmer air holds more water vapor which enhances greenhouse effect and further warms the air). If an opposite forcing happens (so, cooling in this example) the same feedback mechanism amplifies that response (cooler air, less water vapor, further cooling). Run away in either direction and you've got an uninhabitable rock.

A climate feedback mechanism is therefore only positive or negative by virtue of the context. It has no intrinsic directionality. Therefore talking about a long-term dominance of negative feedbacks is meaningless (or worse).

Either I don't follow or I don't agree. Surely the feedback is either in same direction as the forcing or in opposite direction to the forcing therefore does have directionality.

So 6 possible situations:
1. Positive feedback, gain of less than 1. The result is larger than the forcing in direction of forcing but situation doesn't run away.
2.  Positive feedback, gain of 1. Steady movement in direction of forcing. Can only have an unstable equilibrium.
3. Positive feedback, gain more than 1. Run away situation in direction of forcing. Can only have an unstable equilibrium.
4. Negative feedback, gain of less than 1. Moves in direction of forcing but result is a smaller move than the forcing. So it is hard to get large movements.
5. Negative feedback, gain of 1. Situation oscillates in direction.
6. Negative feedback, gain of more than 1. Oscillations become larger and larger so a run away situation of unclear direction.

When there are two or more feedbacks the situation becomes more complex.

In climate there is usually a weak negative feedback from warmer earth emits more radiation and we are thinking about feedbacks that hopefully have gain of less than one within that environment. Positive feedback with a gains of more than 1 can exist in some situations but such mechanism will usually run out of some necessary ingredient for the feedback before you get a Venus or complete iceball situation.

Jim Williams

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Re: What will the Arctic resemble in 2050?
« Reply #102 on: March 27, 2013, 10:54:55 PM »
A positive feedback is one which tends to increase a system behavior as the system changes.  A negative feedback is one which tends to decrease a system behavior as the system changes.  Feedbacks technically are not things, but rather instantaneous tendencies, so a force can be a positive feedback in one system configuration and a negative feedback in a slightly different system configuration.

The terms were originally used in cybernetics to describe working range-finding anti-aircraft guns near the end of WWII.  If the aim was drifting away from the moving target then the feedback applied would be positive in order to move the gun faster, but as the gun got closer to aiming at the target the feedback would become negative so as to slow down the turning of the gun so it would not overshoot the objective.  Before they figured out to use negative feedback the guns would go into wild oscillations back and forth.

kevin_s

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Re: What will the Arctic resemble in 2050?
« Reply #103 on: March 28, 2013, 02:09:35 PM »
I am very embarrassed. I realized after I went to work last night how badly I screwed up that last comment about feedbacks, when I wrote as though I thought a negative feedback was just a positive feedback backwards (i.e. when the forcing is going in the opposite direction). I do know the difference; I was having a brain malfunction for some reason.

It is especially humiliating to have based a criticism of someone else's statement on such a dumb mistake. So, I apologize to everyone for tossing that stinker into the discussion, and I apologize especially to Kim. I made a jackass of myself and I'm sorry.

With that said, there is a part of what I was trying to express that I stand by: positive feedbacks do not necessarily imply runaway effects, and the therefore the historical lack of runaway behavior in Earth's climate does not imply that we don't need to worry about positive feedback effects causing problematic amplification of current positive forcing from CO2.

I suppose my concern was that Kim was trying to imply that there is a widespread bias towards overestimating the risks of climate change, something with which I strongly disagree. If I'm wrong, I apologize for that as well  :)




ChrisReynolds

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Re: What will the Arctic resemble in 2050?
« Reply #104 on: March 28, 2013, 06:28:44 PM »
Kevin,

I think the problematic phrase is "negative feedbacks would be just as destabilizing as positive ones" that's flat out wrong. It's after that, in your following paragraphs, that you showed you understood this was wrong. So I took it as a slip, concentrated on the following two paragraphs and deleted my comment.

Although now I think about it again. It isn't meaningless to ask whether positive or negative feedbacks dominate.

As an aside, here's a problem I have with loose talk about feedbacks (not that I'm saying the talk here is loose). Lovelock has used the asymmetric profiles of cooling (long and drawn out) and warming (rapid), between glacial minima and interglacials to claim that the warming direction is highly amplified by positive feedbacks. However this asymmetry is a function of ice sheet response to forcing, ice sheets take many seasons of snowfall to grow but can lose volume/area rapidly. And crucially the big ice extra-polar sheets are gone, so can't play a role in further warming. So to carry an argument of positive feedback out the glacials and into the super-interglacial warming we face based on behaviour during those periods is very flawed.

frankendoodle

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Re: What will the Arctic resemble in 2050?
« Reply #105 on: April 04, 2013, 05:01:58 PM »
The one good thing about this melt season will be seeing what conditions are like when the sea ice is nearly all the first year as it would be in 2050. 80% of sea ice is less than a year old currently. We will get an idea of what a winter recovery looks like after an ice free summer and what a melt season looks like with ice that is only months old (not years). We will see how large a factor wind and sea currents play in transporting the ice, the major factor of Antarctic sea ice change.
So my guess for 2050 would be nothing. By that I mean the variables are too many and too interconnected to give a real answer. Take Greenland for example. By 2050 it's likely that Greenland will have given up a sizable percentage of its ice sheet. This alone will decrease the saliency of the North Atlantic, drive up sea levels, cause Greenland itself to rise in elevation, alter ocean currents, disrupt and possibly stop the Trans Oceanic Current, alter the ecology of nearby straights and that's just off the top of my head without delving into my Greenland file.

Jim Williams

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Re: What will the Arctic resemble in 2050?
« Reply #106 on: April 04, 2013, 05:23:33 PM »
The one good thing about this melt season will be seeing what conditions are like when the sea ice is nearly all the first year as it would be in 2050. 80% of sea ice is less than a year old currently. We will get an idea of what a winter recovery looks like after an ice free summer and what a melt season looks like with ice that is only months old (not years). We will see how large a factor wind and sea currents play in transporting the ice, the major factor of Antarctic sea ice change.

I don't think that the melt this year gives any information about the ice cover starting from a Summer during which most of it had little or no ice cover.  I think First Year Ice has something to do with melt out speed, but very little to do with turnover of the ocean surface.  The conditions at the end of this year, assuming the ice doesn't melt out during Spring, have nothing much to say about mid-winter after a Summer without ice.

ChrisReynolds

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Re: What will the Arctic resemble in 2050?
« Reply #107 on: April 04, 2013, 06:46:20 PM »
Frankendoodle,

I think you're correct here. We are already seeing the negative feedback of growth/thickness. i.e. ice grows by accretion to the base due to heat flux through the ice, thinner ice or open water allows more heat flux than thicker ice. This is why we've had a record growth, and why as the ice declines we'll see more of these. There is a clear negative relationship between the area or volume at minimum and the growth in the following autumn/winter.

Then we have this year's melt, in which the majority of the pack is FYI. This will give us a great deal of information about the role of dispersed MYI away from the CAA (there is none this year), and the behaviour of large swathes of FYI without MYI inclusion. In this sense 2013 is the most scientifically interesting year for many years, once the season is at an end it should give us some pointers towards progression over the coming years.

You're right: for practical purposes most of the pack will behave as if there had been no ice at the end of 2012.