I still retain my antipathy to the idea. TL has now switched from "tipping points" to "tipping elements" to describe the components of the Earth system that can be switched - under particular conditions - into a qualitatively different state by small perturbations. This is broad and vague, but he produces a longer but also very broad and vague definition later in his article. He wants to include slow changes "analogous to passing the points on a railway track".
TL's first example is the Great Oxidation 2.4byr ago, and the "tipping element" is the entire earth system. Which seems to me to stretch things so far as to make the whole concept meaningless. The GO was - presumably - very slow by any kind of timescales we're used to - millions of years I'd guess at least. It only looks fast because it was so distant. If a "tipping point" really includes changes on this timescale, then saying "we are approaching a tipping point" should be met with yawns.
D-O events is a much better example, though unfortunately we still don't understand these so its not clear at all what the "tipper" was.
Probably the clearest example of a tipping element that has already been triggered by human activities is one that is not primarily related to climate change; the Antarctic ozone hole This isn't clear at all - because the CFC perturbation probably wasn't "small". Of course, since there is no meaningful definition of "small" there is no way to decide.
A case can be made that climate warming may have caused the Arctic sea-ice to pass a tipping point... analysis has shown that positive ice-albedo feedback (the warming due to changing from reflective ice to dark ocean surface) dominates over external forcing (the global warming signal) in causing the thinning and shrinkage since around 1988 so this at last may be a real tipping point? Possible, though I'm not sure what analysis he has in mind (my own work shows that, at least i a GCM, if you take the sea ic away it grows back again, so the ice-albedo doesn't dominate but needs external forcing).
And then, inevitably, we come to the Greenland ice sheet. Is it a tipping element? TL discusses what happens under a 3 oC warming. Is that, too, a "small" perturbation? Seems quite large to me.
Ha! It took a while, but I think I get the problem. There are 'tipping points/elements' within systems, and 'tipping points/elements' which knock on to all the other systems in the meta-system.
IOW: there may be a case for arguing that the Arctic ice/GIS already has or is on the verge of reaching a point of no return in itself - an in-system 'tipping point' beyond which recovery is impossible unless external conditions change - but I take note of your recent paper on this, which shows the sea ice recovering when suitably encouraged. Then there is the idea that an irreversible decline in the GIS mass balance (meltdown) must transform the global climate (this is the type of tipping point you seem to be uncertain of).
I don't think the confusion between the two is mine; it's in TL's article, too. Does this make sense?
[If our work is correct, then the albedo feedback is not very sig in maintaining the sea ice loss, becasue when you remove it all, the feedback doesn't prevent the ice regrowing. It doesn't have to be encouraged to re-grow.
Removing the GIS would raise sea level. Would it transform the global climate much? There would be a local albedo effect, obviously, and a local change in circulation sinc the orography would be missing. But globally?
But what I mean is, in what sense is Greenland a "tipping point" or "tipping element"? The changes needed to remove it are more reasonably described as "large" - +2-3 oC globally - rather than small -W]
I don't want to stir up anything, but recent posts suggest a slight disillusionment with the RC party line...
Getting back on topic, GIS is reasonably considered to have a hysteresis, and thus is a threshold at which it dies "irreversibly". But where this threshold is, and whether the time scale is rapid enough for it really to count (we are unlikely to stabilise CO2 for 1000 years, at any level), are still open questions.
I think there are two things being mushed together here.
The first is: Does a tipping point exist for the disintegration of the GIS. Hansen's argument is that there probably is such a thing, that it may be reached quickly (tens of years). This means that beyond that tipping point nothing could be done that would stop the disintegration, although it does not say specifically how FAST that loss would be.
[But without some idea of how fast, does it really mean anything? All this amounts to is trusting Hansens personal judgement - if there is anything in the science to strongly support his view (of rapid SLR) no-one else is piping up to support him -W]
The second is: Would the loss of the GIS affect climate in a step like way globally. This still looks open, although if the loss were within a hundred years or so the sea level rise would have very serious consequences. I guess if you consider a couple of meters sea level rise a serious climate change, the answer would be yes.
[2m of SRL would be a serious *consequence* of climate change; but its not really a change in climate itself -W]
I think Tim would probably argue that the changes in Greenland itself are sufficient to count as a "tip", even if there were no wider-scale effects. Of course the real issue here is the effect of sea level change on human society (rather than on climate or even ecosystems), so the whole idea rather loses its way at this stage IMO.
This isn't clear at all - because the CFC perturbation probably wasn't "small". Of course, since there is no meaningful definition of "small" there is no way to decide.
Relative terms are meaningful when it is clear what they are relative to. Is +3 C of global warming 'large'? If the context is climate changes during recorded human history, than the answer is clearly yes. If the context is resulting sea level rise, and the affected portion of our people, the answer is yes again. The same thinking could be applied to CFC levels. The 'perturbation' was large compared to any change in CFC levels in prior human history. It was large enough cause problems. A clear definition of context - what you might think of as a 'frame of reference' makes 'small' and 'large' meaningful.
I would be more critical here, definitely. Can we ever talk about possible Arctic ice and GIS disintegration and still to think, what the consequences would be??
Can anybody think (even if stretched over the period of 100s yrs), that we have the right to commit the Earth and (more importantly) our children to such an completely different world??
[I rather doubt that we should be trying to think beyond 2100. At best. The usual analogy is, would we have wanted people from 1900 to be thinking about now? -W]
Another point, does anybody really believe, that complete summer disappearance of Arctic floating ice will *not* have an significant effect on weather and climate on to much broader area in the Nothern Hemisphere?
"Another point, does anybody really believe, that complete summer disappearance of Arctic floating ice will *not* have an significant effect on weather and climate on to much broader area in the Nothern Hemisphere?"
I suppose my initial question is whether it would have any affect on NA depressions? Would it be safe to assume higher SSTs in the NA (or does the ice's influence not extend that far South - considering that some depressions come from around S Greenland) and thus would have some effect?
This isn't a feedback issue (or is it?) but more a question about whether it would affect the weather (and/or climate) of N Europe?
[I don't know, is the answer. I would expect it to have an effect, most certainly, but good/bad/mixed, I don't know -W]
2m of SLR is serious climate change for Bangladesh among other places. As RP Sr. points out all climate is local....
[Its a serious change all around the world. It just feels odd to call it a "climate" change -W]
However your points about rates and Hansen being out on a limb are not as strong as you think. First, given the existence of a point of no return system dynamics dictates that the descent is steep (if it were shallow, you would not be at a point of no return and there could be frequent recrossings. A lot of this comes from my world view of potential energy surfaces, and chaos in reaction dynamics, which appears to be a useful can opener for thinking about climate).
[Don't agree with that. I don't believe you can deduce anything useful about the rate from speculation about the existence of a tipping point. And (for Gr certainly) its all murky because the TP is probably in terms of CO2, which gives you T, which is lagged, and then the Gr response lags further -W]
Second, while the rush to the limb is not obvious, neither is there a chorus saying, can't happen. The response really has been that there be tygers out there, and we don't really understand what is happening under the GIC and in WA.
You have been pushing the "Hansen is not an ice expert" button a lot, but remember that Hansen has a long involvement with SLR issues dating back to the 80s and ice and water do mix.
[I've said Hansen isn't an ice expert but I haven't been pushing it. I've said, repeatedly, that there is little if any research to back up his claims. There is, perhaps, unease about the possibilities -W]
William, you said
[[I don't know, is the answer. I would expect it to have an effect, most certainly, but good/bad/mixed, I don't know -W]]
Isn't it time we tried to get a definitive understanding of the impact of Arctic sea ice metltback on temp and precip of Western North America.
The NSF spent a year and healthy budget on the Abrupt Climate Change Report. In my opinion, sea ice meltback (summer 'disappearance') is a much more immediate topic of concern.
James Annan, do you dream about replacing William in RC? Promise them that their party line will be a new Bible for you.
William, I am impressed how many things you got right. To see the right answers about the tipping points, click
[You are wrong about GR deglac being of known low probability. The probability is unknown. There is, in my opinion, little solid evidence for it being high. There is some evidence for it being low -W]
Think of it this way, the climate system is a multiparameter space. The current climate is the observed trajectory through that space which, as it happens is confined to a small region by the parameters which exist.
A tipping point is a place, where if the trajectory passes through, it will switch to another region of the parameter space. If the possibility of recrossing is high (equivalent to a very slow change), we would have already been in the second region. Since we are not and have not been, recrossing must be improbable. Since the second region is different from the first the change must, perforce, be rapid. This has to be the case for a noisy system, or more exactly a system which has random fluctuations.
[I don't think this logic really holds. How about expanding it onto a post on your blog if you really think its valid. Does your logic allow you to assign a value to "rapid"? -W]
Now no GIS is clearly another region of climate space, if for no other reason that we have not seen it, that sea level will be at levels not previously observed by civilisation, etc.