Guest posting by (or rather, ripped from) Eric Wolff.
It is indeed a very fundamental question about whether the CO2 leads or lags the temperature. If there was somewhere in the ice core record where CO2 increases and temperature does not, then our understanding of the greenhouse effect must be faulty. However, so far we don't find such a place. [*]
[*] Eric is a scientist, not a lawyer. His words, whilst essentially still valid, were not carefully enough framed. He writes (2012/4): I should have carefully included the words "all othe things being equal" and "significantly" as in: "If there was somewhere in the ice core record where, all other things being equal, CO2 increases significantly and temperature does not, then our understanding of the greenhouse effect must be faulty. However, so far we don't find such a place". This is to cover the case of the last 6 kyr-to-preindustrial, where CO2 has risen a bit (though very little, by comparison with iceage-interglacial changes), and T has stayed more-or-less-flat.
Unfortunately in detail the phasing between CO2 and temperature rise is a difficult question to answer. Here is some technical stuff about that which you can ignore if you like, but explains why it is difficult. The reason is that at a given depth in the ice core, the gas has a different age from the ice. the reason for this is that the "temperature" signal is laid down at the surface of the ice sheet. The gas bubbles form only at depth (typically about 100 metres) when the weight of snow on top of a layer has compressed the snow into ice with trapped bubbles. The result is that at a depth (in out idealised ice sheet) of 100 metres: the air is very young (say 30 years, the time for air to diffuse through 100 metres of snowpack) the ice is rather old (if the snow accumulates at 3 cm per year, the ice could be about 3000 years old) and similarly at 200 metres, the air is 3030 years old and the ice 6000 years. This difference between the ice age and the air age can vary, and so it has to be calculated; and there is an uncertainty in that calculation. This all leads to the statement that there is an uncertainty of a few hundred years in the phasing between temperature and CO2.
After this caveat let's turn to the data, and to the place where we know best this age difference, coming out of the last cold period 20000 years ago. I attach a figure from a paper by Monnin et al that appeared in the journal Science. On the top you see the detail of Antarctic temperature (actually water isotope content representing temperature) and in the middle CO2. You can see that they move absolutely in parallel. A statistical analysis does suggest that temperature slightly leads by a few hundred years and you can see a hint of that, as the curve at the top starts to rise just before the vertical dashed line while the CO2 only starts at the line. However, the fact that they move in parallel for most of the 5000 year increase, even tracking each other in the hiatus period is the characteristic pattern of a chicken-and-egg positive feedback, where the temperature causes the CO2 to rise and the CO2 causes the temperature to rise further. Of course we cannot prove that is what is happening, but it is consistent with that, and the crucial point is that there is no period when CO2 is rising and temperature is not.
Just to be clear: no-one is claiming that temperature cannot change naturally, clearly it does. Thus there is no surprise in the idea that the end of the ice age was kicked off by something other than CO2. however once it started, CO2 appears to be involved in keeping it going. There is still some question whether there is any lag at all (because there is some evidence we may have calculated too large an age difference, think of it as 800 years +/- nearly 800), but if there is a lag what that means is that it takes time for the ocean and land to give up CO2 in response to an initial temperature warming. But that is not an issue for the future since we are actively adding the CO2 to the system ourselves, so that half of the feedback is taken care of.
[Update: its worth pointing out that the 800-y lag stuff is *not* definitively established. Loulergue et al argue otherwise (and since its an open-access journal you can go argue with them if you like!) -W]
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"But that is not an issue for the future since we are actively adding the CO2 to the system ourselves, so that half of the feedback is taken care of."
A good, useful, explanation. I'm not sure about the sentence I quote though. Does that mean that there will be less feedback, or that the affects of the feedback are lessened? Or does the fact that we've raised the temperature mean that sometime (within ~800 years) there will be a feedback like we see in these ice cores that will push the CO2 content up further. Or a combination of all three?
[It means that half the feedback - that of increasing CO2 - has been done, by us. So all we have to do is work out the other half - the T response -W]
As I see it, it means that what happened in the past is irrelevant because there never has been a situation when the amount of CO2 was being increased outside of the Earth's natural processes.
> a chicken-and-egg positive feedback
People laid the eggs; some of the eggs hatched; we've got hens and roosters now; expect additional eggs and chickens to follow.
As I see it, it means that what happened in the past is irrelevant because there never has been a situation when the amount of CO2 was being increased outside of the Earth's natural processes.
Well, C02 has been 1600% higher than now.
Question on the graph.
Why 10K years?
There are graphs going back 400K years?
[Indeed there are, but if you want to look at the details, a shorter scale is better. Also the lags are only really evident during terminations -W]
An intriguing idea...
if it is so, that increasing temperature will cause further increase in CO2 - from oceans, peatbogs etc. - whatever the time lag, clearly, without any *negative* feedback we will finish in the hothouse and we will burn up in the hell. This is not likely, therefore we should expect some negative forcing, in response to positive T/CO2 feedback.
As mentioned few times before, one of these feedback mechanism *was* the THC shutdown. But we also know, that present day situation is different from the past event. It could also happen, that the arctic ice and greenland will melt *gradually* and no treshhold needed for THC shutdown will be reached (it means sudden influx of fresh water into the ocean). Then of course (after melting of most of arctic ice) we would have a great imbalance between North Pole and South Pole - there is a reasonable question if such a state could be in (climatic) balance. Further, if current models predict ice free arctic in the summer by cca 2040, this means a pretty *fast* melt and I don't expect, this will be the final stage.
I.e. there would be a very strange world with a lot of ice in the South Pole and having much less ice in the North Pole... with currently unpredictable effects on the weather patterns...
"It means that half the feedback - that of increasing CO2 - has been done, by us. So all we have to do is work out the other half - the T response"
and
"As I see it, it means that what happened in the past is irrelevant because there never has been a situation when the amount of CO2 was being increased outside of the Earth's natural processes."
Thanks William & Karl. But I'm still a little confused (nothing new there), as if the CO2 rise is external to the Earth's natural CO2 feedback process(es), if there is one obviously, then wouldn't we expect that to kick in once the temperature rises beyond a certain threshold?
[Possibly. But since we don't know exactly what the mechanisms were, its hard to be sure. There may be one that "does" glacial-interglacial changes and is now dormant. But feedback *is* expected from permafrost etc -W]
Caillon et al 2003 got around the gas-ice age difference using Argon isotopes http://www.sciencemag.org/cgi/content/full/299/5613/1728
Another issue is that CO2 signals are essentially global, whereas temp is more local. http://www.brighton73.freeserve.co.uk/gw/paleo/400000yrfig.htm
Regarding CO2 feedback - it will kick in, but it'll look different. The thing is that 50% of CO2 emissions are currently being absorbed - due (primarily) to the negative balance between ocean emission and ocean absorbtion, driven by increasing atmospheric concs. As the oceans warm, they will 'emit' more CO2, but they will still absorb more than they emit because of the enormous amount of CO2 entering the atmosphere from us. So what we will see is not positive feedback but rather less negative feedback.
As the arctic ice disappears, the total surface area for this to happen increase, and this will actually offset (temporarily and partly) the fact that net absorbtion per unit area will decrease.
Incidentally, we know that GHG emissions cause temp rises - how else do you explain the paleocene-eocence thermal maximum?
Thanks again William, but that does seem to contradict the statement that "this is not an issue for the future"?
Thanks Tom, I did wonder if changing atmospheric concentrations had an effect on this. So the oceans will absorb more than in (some) previous times in total, because the atmospheric concentrations will be higher?
I've just read Chapter 10 of David Archer's book which touches on this (http://forecast.uchicago.edu/) and that helps. I think I'm still not quite there, but I know enough to look the rest up now, I think.
Hi William and Eric
and thanks for this excellent science blog. Just two comments.
1) I am not sure if this would be really aproblem for the CO2->Temprature part of the feedback:"If there was somewhere in the ice core record where CO2 increases and temperature does not, then our understanding of the greenhouse effect must be faulty." There might be other forcings just overwhelming the greenhouse forcing. I am not sure if one can make a strong argument that all the forcings have to have the same sign. The problem with the 800 year lag for me is in any case only in the T->CO2 part. What balances exactly the CO2 though the Southern Oceans are probably warming during the termination?
2)Eric you mentioned the uncertainties in the computation of the ice age/gas age difference. Have you seen http://www.copernicus.org/EGU/cp/cpd/3/435/cpd-3-435.htm? They come up with a possible 0yr lag. Already any comments on that one?
Best regards Georg
W.Connolly wrote when trying to explain the alleged mutual coupling between CO2 and T:
âthe crucial point is that there is no period when CO2 is rising and temperature is not.â
This is obviously a false statement. If you examine the Vostok data more carefully, you will see that after every rapid deglaciation there is a period of time (2000-3000 years) when CO2 keep rising while T goes down. Look at 212k-215ky range, or 231-233ky range, or 114-126 ky range where CO2 was flat to up, while T was gradually down. Or most recent chunk of data, 2.3k - 5ky. This clearly contradicts to your statement.
According to the autocorrelation function presented in Hansen et al publication (âClimate change and trace gasesâ, Phil. Trans. R. Soc. A (2007) 365, 1925â1954), the function has one peak with 92.5% correlation at the time shift of 700 years. It means that CO2 is nearly a shifted copy of T, and your statement is a topological impossibility.
Your 2nd sentence: "If there was somewhere in the ice core record where CO2 increases and temperature does not, then our understanding of the greenhouse effect must be faulty."
Willis Eschenbach ovelayed ice core CO2 data on Shakun et al. temperature proxy data (Shakun2000) that shows CO2 rising the last 6,000 years with temperature not rising (actually falling). From your statement it follows that "our understanding of the greenhouse effect must be faulty".
[Thanks for the comment. You're right about the apparent contradiction. I think the best answer I can give for the moment is to not get too hung up about the introductory words. If you do want to worry about it, then I think the answer would be that both the CO2 and temperature changes are relatively small - to make sense of it, you'd need to work out the radiative forcing from those CO2 changes (plus feedbacks per normal) and compare them to whatever else you were seeing at the same time - the Milakovitch forcings most obviously. You would also need to do a better job of reconstructing the temperature - the scatterplot WE shows isn't a temperature reconstruction -W]
This article is a little deceptive.
[Why thank you, you're too kind -W]
What is the depth at which 90% of air is preserved? My bet is this is at about 20 or 30 ft.
[If you actually want to know, you shouldn't be betting, you should be looking it up. Its a fairly intensively studied subject, because it matters. http://www.phys.uu.nl/~pelt0108/karthaus09/lecturenotes/HubertusFischer… might be a good place to start -W]
Other than barometric pressure changes there is nothing to cause diffusion and the diffusion isn't with "fresh" air but mostly with the surrounding ice. Which really just means if the source concentration of the air was plotted against time you would see a normal distribution with the peak at the date of the original snow and the 3 standard deviations point would represent about 100 years.
[How have you suddenly plucked 100 years out of thin air? -W]
There has to be data from near surface Antarctic ice cores that show how the CO2 concentration is evolving.