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The lag between temperature and CO2. (Gore’s got it right.)

Filed under: — eric @ 27 April 2007 - (Italian) (Español)

When I give talks about climate change, the question that comes up most frequently is this: “Doesn’t the relationship between CO2 and temperature in the ice core record show that temperature drives CO2, not the other way round?”

On the face of it, it sounds like a reasonable question. It is no surprise that it comes up because it is one of the most popular claims made by the global warming deniers. It got a particularly high profile airing a couple of weeks ago, when congressman Joe Barton brought it up to try to discredit Al Gore’s congressional testimony. Barton said:

    In your movie, you display a timeline of temperature and compared to CO2 levels over a 600,000-year period as reconstructed from ice core samples. You indicate that this is conclusive proof of the link of increased CO2 emissions and global warming. A closer examination of these facts reveals something entirely different. I have an article from Science magazine which I will put into the record at the appropriate time that explains that historically, a rise in CO2 concentrations did not precede a rise in temperatures, but actually lagged temperature by 200 to 1,000 years. CO2 levels went up after the temperature rose. The temperature appears to drive CO2, not vice versa. On this point, Mr. Vice President, you’re not just off a little. You’re totally wrong.

Of course, those who’ve been paying attention will recognize that Gore is not wrong at all. This subject has been very well addressed in numerous places. Indeed, guest contributor Jeff Severinghaus addressed this in one of our very first RealClimate posts, way back in 2004. Still, the question does keep coming up, and Jeff recently received a letter asking about this. His exchange with the letter writer is reproduced in full at the end of this post. Below is my own take on the subject.

First of all, saying “historically” is misleading, because Barton is actually talking about CO2 changes on very long (glacial-interglacial) timescales. On historical timescales, CO2 has definitely led, not lagged, temperature. But in any case, it doesn’t really matter for the problem at hand (global warming). We know why CO2 is increasing now, and the direct radiative effects of CO2 on climate have been known for more than 100 years. In the absence of human intervention CO2 does rise and fall over time, due to exchanges of carbon among the biosphere, atmosphere, and ocean and, on the very longest timescales, the lithosphere (i.e. rocks, oil reservoirs, coal, carbonate rocks). The rates of those exchanges are now being completely overwhelmed by the rate at which we are extracting carbon from the latter set of reservoirs and converting it to atmospheric CO2. No discovery made with ice cores is going to change those basic facts.

Second, the idea that there might be a lag of CO2 concentrations behind temperature change (during glacial-interglacial climate changes) is hardly new to the climate science community. Indeed, Claude Lorius, Jim Hansen and others essentially predicted this finding fully 17 years ago, in a landmark paper that addressed the cause of temperature change observed in Antarctic ice core records, well before the data showed that CO2 might lag temperature. In that paper (Lorius et al., 1990), they say that:

    changes in the CO2 and CH4 content have played a significant part in the glacial-interglacial climate changes by amplifying, together with the growth and decay of the Northern Hemisphere ice sheets, the relatively weak orbital forcing

What is being talked about here is influence of the seasonal radiative forcing change from the earth’s wobble around the sun (the well established Milankovitch theory of ice ages), combined with the positive feedback of ice sheet albedo (less ice = less reflection of sunlight = warmer temperatures) and greenhouse gas concentrations (higher temperatures lead to more CO2 leads to warmer temperatures). Thus, both CO2 and ice volume should lag temperature somewhat, depending on the characteristic response times of these different components of the climate system. Ice volume should lag temperature by about 10,000 years, due to the relatively long time period required to grow or shrink ice sheets. CO2 might well be expected to lag temperature by about 1000 years, which is the timescale we expect from changes in ocean circulation and the strength of the “carbon pump” (i.e. marine biological photosynthesis) that transfers carbon from the atmosphere to the deep ocean.

Several recent papers have indeed established that there is lag of CO2 behind temperature. We don’t really know the magnitude of that lag as well as Barton implies we do, because it is very challenging to put CO2 records from ice cores on the same timescale as temperature records from those same ice cores, due to the time delay in trapping the atmosphere as the snow is compressed into ice (the ice at any time will always be younger older than the gas bubbles it encloses, and the age difference is inherently uncertain). Still, the best published calculations do show values similar to those quoted by Barton (presumably, taken from this paper by Monnin et al. (2001), or this one by Caillon et al. (2003)). But the calculations can only be done well when the temperature change is large, notably at glacial terminations (the gradual change from cold glacial climate to warm interglacial climate). Importantly, it takes more than 5000 years for this change to occur, of which the lag is only a small fraction (indeed, one recently submitted paper I’m aware of suggests that the lag is even less than 200 years). So it is not as if the temperature increase has already ended when CO2 starts to rise. Rather, they go very much hand in hand, with the temperature continuing to rise as the the CO2 goes up. In other words, CO2 acts as an amplifier, just as Lorius, Hansen and colleagues suggested.

Now, it there is a minor criticism one might level at Gore for his treatment of this subject in the film (as we previously pointed out in our review). As it turns out though, correcting this would actually further strengthen Gore’s case, rather than weakening it. Here’s why:

The record of temperature shown in the ice core is not a global record. It is a record of local Antarctic temperature change. The rest of the globe does indeed parallel the polar changes closely, but the global mean temperature changes are smaller. While we don’t know precisely why the CO2 changes occur on long timescales, (the mechanisms are well understood; the details are not), we do know that explaining the magnitude of global temperature change requires including CO2. This is a critical point. We cannot explain the temperature observations without CO2. But CO2 does not explain all of the change, and the relationship between temperature and CO2 is therefore by no means linear. That is, a given amount of CO2 increase as measured in the ice cores need not necessarily correspond with a certain amount of temperature increase. Gore shows the strong parallel relationship between the temperature and CO2 data from the ice cores, and then illustrates where the CO2 is now (384 ppm), leaving the viewer’s eye to extrapolate the temperature curve upwards in parallel with the rising CO2. Gore doesn’t actually make the mistake of drawing the temperature curve, but the implication is obvious: temperatures are going to go up a lot. But as illustrated in the figure below, simply extrapolating this correlation forward in time puts the Antarctic temperature in the near future somewhere upwards of 10 degrees Celsius warmer than present — rather at the extreme end of the vast majority of projections (as we have discussed here).

Global average temperature is lower during glacial periods for two primary reasons:
1) there was only about 190 ppm CO2 in the atmosphere, and other major greenhouse gases (CH4 and N2O) were also lower
2) the earth surface was more reflective, due to the presence of lots of ice and snow on land, and lots more sea ice than today (that is, the albedo was higher).
As very nicely discussed by Jim Hansen in his recent Scientific American article, the second of these two influences is the larger, accounting for about 2/3 of the total radiative forcing. CO2 and other greenhouse gases account for the other 1/3. Again, this was all pretty well known in 1990, at the time of the Lorius et al. paper cited above.

What Gore should have done is extrapolated the temperature curve according this the appropriate scaling — with CO2 accounting for about 1/3 of the total change — instead of letting the audience do it by eye. Had he done so, he would have drawn a line that went up only 1/3 of the distance implied by the simple correlation with CO2 shown by the ice core record. This would have left the impression that equilibrium warming of Antarctica due to doubled CO2 concentrations should be about 3 °C, in very good agreement with what is predicted by the state-of-the-art climate models. (It is to be noted that the same models predict a significant delay until equilibrium is reached, due to the large heat capacity of the Southern ocean. This is in very good agreement with the data, which show very modest warming over Antarctica in the last 100 years). Then, if you scale the Antarctic temperature change to a global temperature change, then the global climate sensitivity to a doubling of CO2 becomes 2-3 degrees C, perfectly in line with the climate sensitivity given by IPCC (and known from Arrhenius’s calculations more than 100 years ago).

In summary, the ice core data in no way contradict our understanding of the relationship between CO2 and temperature, and there is nothing fundamentally wrong with what Gore says in the film. Indeed, Gore could have used the ice core data to make an additional and stronger point, which is that these data provide a nice independent test of climate sensitivity, which gives a result in excellent agreement with results from models.

A final point. In Barton’s criticism of Gore he also points out that CO2 has sometimes been much higher than it is at present. That is true. CO2 may have reached levels of 1000 parts per million (ppm) — perhaps much higher — at times in the distant geological past (e.g. the Eocene, about 55 million years ago). What Barton doesn’t bother to mention is that the earth was much much warmer at such times. In any case, more relevant is that CO2 has not gone above about 290 ppm any time in the last 650,000 years (at least), until the most recent increase, which is unequivocally due to human activities.

Below is the letter written to Jeff Severinghaus, and his response:

Dear Jeff,

I read your article “What does the lag of CO2 behind temperature in ice cores tell us about global warming?” You mention that CO2 does not initiate warmings, but may amplify warmings that are already underway. The obvious question comes up as to whether or not CO2 levels also lag periods when cooling begins after a warming cycle…even one of 5,000 years?

If CO2 levels on planet Earth also lag the cooling periods, then how can it be that CO2 levels are causally related to terrestrial heating periods at all? I am not sure what the ice core records are related the time response of CO2 to the cooling trends. If there is also a lag in CO2 levels behind a cooling period, then it appears that CO2 levels not only do not initiate warming periods but are also unrelated to the onset of cooling periods. It would appear that the actual CO2 levels are rather impotent as an amplifier either way…warming or cooling. We are talking about planet Earth after all and not Venus whose atmospheric pressure is many times larger than Earth’s.

If there is also a time lag upon the onset of cooling, then it appears that some other mechanism actually drives the temperature changes. So what is the time difference between CO2 levels during the onset of a cooling period at the end of a warming period and the time history of the temperature changes in the ice cores?

Dear John,

The coolings appear to be caused primarily and initially by increase in the Earth-Sun distance during northern hemisphere summer, due to changes in the Earth’s orbit. As the orbit is not round, but elliptical, sunshine is weaker during some parts of the year than others. This is the so-called Milankovitch hypothesis [this really should say “theory” — eric], which you may have heard about. Just as in the warmings, CO2 lags the coolings by a thousand years or so, in some cases as much as three thousand years.

But do not make the mistake of assuming that these warmings and coolings must have a single cause. It is well known that multiple factors are involved, including the change in planetary albedo, change in nitrous oxide concentration, change in methane concentration, and change in CO2 concentration. I know it is intellectually satisfying to identify a single cause for some observed phenomenon, but that unfortunately is not the way Nature works much of the time.

Nor is there any requirement that a single cause operate throughout the entire 5000 – year long warming trends, and the 70,000 year cooling trends.

Thus it is not logical to argue that, because CO2 does not cause the first thousand years or so of warming, nor the first thousand years of cooling, it cannot have caused part of the many thousands of years of warming in between.

Think of heart disease – one might be tempted to argue that a given heart patient’s condition was caused solely by the fact that he ate french fries for lunch every day for 30 years. But in fact his 10-year period of no exercise because of a desk job, in the middle of this interval, may have been a decisive influence. Just because a sedentary lifestyle did not cause the beginning of the plaque buildup, nor the end of the buildup, would you rule out a contributing causal role for sedentary lifestyle?

There is a rich literature on this topic. If you are truly interested, I urge you to read up.

The contribution of CO2 to the glacial-interglacial coolings and warmings amounts to about one-third of the full amplitude, about one-half if you include methane and nitrous oxide.

So one should not claim that greenhouse gases are the major cause of the ice ages. No credible scientist has argued that position (even though Al Gore implied as much in his movie). The fundamental driver has long been thought, and continues to be thought, to be the distribution of sunshine over the Earth’s surface as it is modified by orbital variations. This hypothesis was proposed by James Croll in the 19th century, mathematically refined by Milankovitch in the 1940s, and continues to pass numerous critical tests even today.

The greenhouse gases are best regarded as a biogeochemical feedback, initiated by the orbital variations, but then feeding back to amplify the warming once it is already underway. By the way, the lag of CO2 of about 1000 years corresponds rather closely to the expected time it takes to flush excess respiration-derived CO2 out of the deep ocean via natural ocean currents. So the lag is quite close to what would be expected, if CO2 were acting as a feedback.

The response time of methane and nitrous oxide to climate variations is measured in decades. So these feedbacks operate much faster.

The quantitative contribution of CO2 to the ice age cooling and warming is fully consistent with current understanding of CO2’s warming properties, as manifested in the IPCC’s projections of future warming of 3±1.5 C for a doubling of CO2 concentration. So there is no inconsistency between Milankovitch and current global warming.

Hope this is illuminating.


87 Responses to “The lag between temperature and CO2. (Gore’s got it right.)”

  1. 51
    Pierre Allemand says:

    Something bother me a little : ice cores analysis are the only and unique indicators of CO2 and methane and also nitrous oxide atmosphere concentrations for the last 600,000 years.
    The three curves follow perfectly the same path. Even so perfectly that one can suspect some kind of bias. When I was at work (I am retired now) in my lab, I should have been very very suspicious if somebody of my team had brought to me such set of curves… too perfect means “not perfect at all”
    Suppose that, for any reason an important bias occurs in these measurements (who knows ?) In that case, the entire construction would collapse.
    An other proxy, indicating precisely what were the CO2 or methane or nitrous oxide contents of the atmosphere 100, 200 or 300 years ago should be welcome…

    [Response: The curves do not perfectly correlate. CH4 for instance has a much stronger precessional component than CO2 or Antarctic temperature and has a much faster response time to climate changes – both completely in line with a shorter atmospheric residence time and important tropical wetland sources. The data for CH4 are also highly replicable from Greenland to Antarctica, and in Antarctica itself all the records – from very different depositional environments – show the same results. Where there are obvious signs of contamination (such as for CO2 in the Greenland cores due to too much organic material in the much higher dust levels there), those data have been discarded. – gavin]

  2. 52
    Kit Stolz says:

    Thanks for taking the time to clarify again this complicated issue. For laymen, I like the way Justice John Paul Stevens laid out the basic point in his recent Supreme Court opinion:

    “A well-documented rise in global temperatures has coincided with a significant increase in the concentration of carbon dioxide in the atmosphere. Respected scientists believe the two trends are related. For when carbon dioxide is released into the atmosphere, it acts like the ceiling of a greenhouse, trapping solar energy and retarding the escape of reflected heat.”

    Read this, and you have to ask yourself: Is it a coincidence? The answer is obvious.

  3. 53
    Timothy Chase says:

    The following would suggest that the Vatican believes there may be a genuine controversy regarding the threat of global warming.

    “The scientific community has been so divided and so bitter” over the climate-change debate that experts who disagree with each other don’t talk to each other, Silecchia told Catholic News Service.

    Mirroring wider debate, Vatican seminar on global warming gets heated
    By Carol Glatz

    However, all in all, they would seem to be giving greater weight to the issue, seeing it as something which must one of three essential goals which are to be balanced against one another: those of addressing poverty, economic inequality, and the environment. Thus, for example, when they speak of development, they speak of “sustainable development.”

    Unfortunately, I think they are still a bit behind the times, not simply in terms of their estimation of the division within the scientific community, but of cultural trends as well.

    From the same article:

    Silecchia said in some ways the environmental movement “has become its own new religion,” and this could be offset by a wider recognition of the church’s own tradition of God asking people to be stewards of creation.

    As near as I can tell, New Age environmentalism is next to non-existent and has been so for nearly two decades.

    Incidently, there were other, more professional contrarians there.

    “Not all the scientific world is crying disaster,” Cardinal Renato Martino, who heads the Pontifical Council for Justice and Peace, told Vatican Radio at the start of the two-day conference he hosted.

    “There are a good number of scientists who consistently don’t view these climactic changes in a negative light, and in fact say that these phenomena recur over the course of years and eras and sometimes they can have favorable results for agriculture and development.”

    Vatican Closes Climate Conference, Some Panelists Consider Warming Beneficial

    For example, representing the views of those who believe that global warming results in best of all possible worlds, we have Craig Idso from CO2Science. (See article above.)

    My own impression is that the Vatican isn’t necessarily interested in supporting the science or promoting the anti-science so much as in becoming a player, a power to be reconed with and courted. But that may be just my own cynicism.

  4. 54
    Alexi Tekhasski says:

    #36 Richard Ordway wrote:
    “1) The Earth’s temperatures increase a little due to Earth’s orbit changes.

    2) This little warming causes the oceans to warm up which releases huge amounts of CO2.”
    ==== end of quote ===

    Thank you Richard for this eloquent and precise explanation that huge amounts of CO2 could be releleased by oceans and cause global warming without any presence of humans! As you might be rightfully aware, according to Takahashi measurements of global CO2 fluxes from oceans, they outgas about 100GT of carbon per year in tropical areas. Thank you for boiling this post down to its bones. :-) :-)

  5. 55
    James says:

    Re #41: [Detroit would have produced a car thousands of miles long, probably with its own measurable gravitational pull and perhaps its own atmosphere.]

    Sshh! Don’t start giving them ideas!

  6. 56
    Nigel Williams says:

    Following on from Richard Ordway�s excellent 1-2-3 post (#36), we can take it then that 1 and 2 are already done.

    So in regard to our response to todays climate conditions cant we just ignore the arguments about the past, and simply start from today and say:

    a)The present temperatures are not in equilibrium with present-day CO2 levels.
    b)The climate will adjust the temperature to reach the correct equilibrium temperature. At 390ppmCO2 that temperature is about (say) 1K to 2K above today�s mean and the global effects identified in IPCC AR4 will prevail.
    c)If we keep adding CO2 (and with the third worlds fossil fuel power stations in the pipeline and unabated growth among *developed* nations, we will) then the equilibrium temperature will increase further.

    In essence the only equation that matters is the equilibrium equation, and the only variable input to that which matters (in the range we are interested in) is the concentration of GW gasses (CO2 among them). Put it on the front page of every newspaper, and let folk work it out for themselves!!!

    With all these discussions; Its as if our house is on fire; I have the impression that the head-in-the-sand folk are trying to identify and incarcerate the arsonist BEFORE they call the fire brigade! I feel you guys n gals at RC have done all you need to � the physics is simple. The hard question is What will humanity do about it?

  7. 57
    Bruce Tabor says:

    Hi Eric,

    Re – your response to my post about residence time:
    “The residence time calculation is confusing and indeed the concept of residence time is not very useful for a multicomopnent system. The average CO2 molecule does spend only a few years in the atmosphere, giving that 2-3 year residence time you cite. But effective residence time is much longer because of the equlibrium (or near equlibrium) between surface ocean and atmosphere and biosphere and atmosphere. The residence time of a CO2 molecule in the surface ocean is nearly as short as in the atmosphere, so many of the molecules that go in come right back out! So the effectie residence time of the combined system is what is important, and that is more onthe order of hundreds of years — or indeed, thousands, if we are talking about the residence time inthe combined surface ocean/atmophere/biosphere, vs. the deep ocean.–eric”

    I can see that “The residence time of a CO2 molecule in the surface ocean is nearly as short as in the atmosphere.” makes sense given the surface ocean (600Gt – 6 year residence), atmosphere (700 Gt – 3 year residence) and indeed the dead biomass (1100 Gt – 20 year residence) have similar carbon masses, and fluxes. What puzzles me is that a CO2 molecule released from the fossil fuel reservoir becomes indistinguishable from a CO2 molecule in the atmosphere and hence should be subject to the same residence time (3 years). In compartmental model terms, surely the atmosphere is ONE compartment WRT CO2 (I’m assuming it is well mixed). The equilibrium exchange rates with the surfact ocean and terrestrial biosphere sum to about 220 Gt/yr, which is consistent with an e-folding or half-life of CO2 of about 3 years (I’m not going to do the detailed sums but 220 is about a third of 700).

    The crux of the issue is this. If we completely stopped anthropogenic CO2 emissions, how long would it take to get CO2 down to say 330 ppm (half the anthropogenic addition). It might be helpful if you could refer me to a good paper on the topic. Since I’ve studied compartmental models I could handle a paper written in those terms.

    I’ve had a thought on this – the excess uptake over the equilibrium exchange rate for both the ocean and terrestrial biosphere are both about 2 Gt/yr, giving a total of 4 Gt/yr. Of the 700 Gt CO2 in the atmosphere we’ve added 100 ppm of the 380 pmm (26%), which is about 182 Gt (current net addition is about 2-3 Gt/yr). Relying on the excess capacity of the biosphere to take this up would take about 184 divided by 4 equals 46 years – sat 50 years. If I took into account the exponential decay, the half-time of anthropogenic CO2 would be about 50 years. Is this the correct track?


  8. 58
    Timothy Chase says:

    Re Richard Ordway #39, –

    With regard to the possible role of hydrogen sulfide in mass extinctions, this pop article might be of interest to those who aren’t familiar with it…

    Impact from the Deep
    Strangling heat and gases emanating from the earth and sea, not asteroids, most likely caused several ancient mass extinctions. Could the same killer-greenhouse conditions build once again?
    By Peter D. Ward
    Scienctific American October 2006 issue

    Fortunately, we are still a fairly good distance from that sort of scenario – at least in terms of the current levels of CO2 are concerned: less than 400 ppm CO2 compared to the 1000 ppm they are talking about, and it has been rising recently somewhere around 2 ppm per year. However, I would be interested in seeing what role the methane which might be released by thawing Russian tundra or shallow arctic methane hydrate deposits might play in getting us closer to it to that limit.

    In any case, I don’t like games of chance, either. But if this is the right explanation for the extinctions, I think the scientific community will come to realize it sooner rather than later. The evidence will have a way convincing them at least.

  9. 59
    FurryCatHerder says:

    c)If we keep adding CO2 (and with the third worlds fossil fuel power stations in the pipeline and unabated growth among *developed* nations, we will) then the equilibrium temperature will increase further.

    My personal feelings are that the 3rd World will never be able to be a big enough player in adding CO2 to the environment because it simply lacks the wealth to do so.

    The real risk, in my mind, isn’t the 3rd World, it’s countries such as India and China trying to modernize in a hurry, and depending very heavily on fossil fuels for doing so. China has plans to dramatically increase nuclear power production. India has six plants under construction, but will need to dramatically increase that number. If the developed world is already being forced to move from fossil fuels due to cost, I don’t see how the 3rd world is going to grow its fossil fuel consumption to any appreciable level.

  10. 60
    Timothy Chase says:

    I finally got around to watching “An Inconvenient Truth.”

    I was impressed, particularly with how Gore managed to weave together the personal and the global – and to illustrate that the issue is ultimately a moral one. At a personal level, I think that all issues are moral issues, and that the deepest of all is our commitment to acknowledge reality for what it is – which we face at every moment of our lives – but that I suppose is a different issue.

    In any case, I believe it is a highly effective film, and I can understand why it resonates with people. However, there were at least three factual issues where Gore was somewhat off mark.

    One was in overplaying ice melt in Greenland and Antarctica. One would get the strong impression that ice in both these places is being lost on the whole. However, depending upon the year of the survey, the net amount of ice in either landmass may in fact be increasing. A warmer ocean means greater evaporation, more evaporation means greater precipatation, and greater precipatation in a land that is well enough below freezing and which is still below freezing throughout the year even after having its temperature raised by several degrees will in fact accumulate more ice.

    However, this is not something which will seem entirely straightforward when a “contrarian” first denies the existence of a general trend of global warming – and cites surveys showing increasing accumulation of ice in some parts of either landmass or in one of the landmasses as a whole. Nevertheless, I have to wonder whether given the complexity of ice dynamics (some of which he speaks of in terms of the “moulins”) might conceivably make possible the sort of collapse which he warns the audience against with regard to Greenland.

    The other was with regard to the sea level. The documentary is written as if the only factor affecting sea level is the melt of ice and glaciers. However, they are not even so much as the dominant factor at this point. Instead, it is the thermal expansion of sea water itself which is currently causing the greatest increase in sea level.

    So if so much of the emphasis of the film is upon how the rising sea levels will affect hundreds of millions of people within the next few decades, its case is unnecessarily weakened when it fosters the view that ice melt is “the cause” of sea level rise.

    Then of course this piece dealt with another one of the weaknesses of “An Inconvenient Truth” – the issue having to do with the fact that, in deep time, temperature rises have often lead carbon dioxide rises, which makes it easier for “contrarians” to deny that raising the level of carbon dioxide will raise the temperature. One has to recognise the existence of positive feedback, such that a rise in either quantity will lead to a rise in the other.

    In any case, such weaknesses, while small, should be acknowledged – as they are weaknesses which will be made use of by those who seek to deny the science. Likewise, uncertainties should be acknowledged where they exist – to the extent that they exist. For the most part, I believe Gore’s “An Inconvenient Truth” did quite well in this regard.

    I think things are shifting, partly as the result people becoming increasingly informed and partly given the increasingly dramatic developments. But for the time being, there will no doubt be contrarians, and we certainly don’t need to make their goals any easier for them to achieve. We can make things especially difficult for them if we keep things simple without oversimplifying.

  11. 61
    SCM says:

    I have a question about the earth reaching temperature equilibrium with different forcings and the speed of climatic changes that maybe someone here would be kind enough to answer.

    From reading here and elsewhere I know that earth takes some time to reach equilibrium temperature in response to the increase in CO2. Is this time lag similar for other kinds of forcings (solar, aerosol etc)?

    Volcanos seem to have very rapid effects which suggest there is variation in how long things take to unfold. Is this quick-acting nature purely because of the size of the forcing or is there more to it? On the face of it if it was just a matter of reaching thermal equilibrium one would imagine the time involved should be the same for a given size of forcing regardless of the forcing type.

    Here’s hoping someone is prepared to satisfy my curiosity :-)

    [Response: The response time has to do mostly with the thermal intertia of the ocean mixed layer, and ultimately the mixing time of the deep ocean. The reason that one tends to think that volanos have “rapid” effects is that the volcanoes themselves are rapid events. The initial response is the only response you get, because just a couple of years later the volcanic aerosol has been cleansed out of the atmosphere. If you could keep Pinatubo going to hundreds of years, at the same rate, then the cooling would continue towards an equilibrium that was a greater response than the initial cooling actually seen for the real Pinatubo. So in general yes, your intuition is correct. The response time is the same for the same magnitude of radiative forcing, regardless of cause. Indeed, that is why the Pinatubo event is a good validation of climate models’ ability to accurately simulate other forcings. –eric]

  12. 62
    Alex Sen Gupta says:

    Sorry for the out of place post – but I just watched ‘ The Gulf Stream and the Next Ice Age’ on Australian (SBS) TV, featuring RC’s Stefan Rahmstorf. The programme was rather alarmist for my tastes, giving the impression that an ice age was just around the corner. Some words of wisdom from the RC team wouldn’t go amiss (I wasn’t able to locate any previous forums dedicated to the likelyhood and consequences of a THC shutdown).

  13. 63
    Rob Brookes says:

    Slightly of topic but similar, Although I think I understand and can explain what’s mistaken with most of the beliefs of Alex Cockburn in his article From Papal Indulgences to Carbon Credits
    Is Global Warming a Sin?
    I have not been able to find an explanation of this depite searching Real Climate.

    “”Now imagine two lines on a piece of graph paper. The first rises to a crest, then slopes sharply down, then levels off and rises slowly once more. The other has no undulations. It rises in a smooth, slowly increasing arc. The first, wavy line is the worldwide CO2 tonnage produced by humans burning coal, oil and natural gas. On this graph it starts in 1928, at 1.1 gigatons (i.e. 1.1 billion metric tons). It peaks in 1929 at 1.17 gigatons. The world, led by its mightiest power, the USA, plummets into the Great Depression, and by 1932 human CO2 production has fallen to 0.88 gigatons a year, a 30 per cent drop. Hard times drove a tougher bargain than all the counsels of Al Gore or the jeremiads of the IPCC (Inter-Governmental Panel on Climate Change). Then, in 1933 it began to climb slowly again, up to 0.9 gigatons.

    And the other line, the one ascending so evenly? That’s the concentration of CO2 in the atmosphere, parts per million (ppm) by volume, moving in 1928 from just under 306, hitting 306 in 1929, to 307 in 1932 and on up. Boom and bust, the line heads up steadily. These days it’s at 380.There are, to be sure, seasonal variations in CO2, as measured since 1958 by the instruments on Mauna Loa, Hawai’i. (Pre-1958 measurements are of air bubbles trapped in glacial ice.) Summer and winter vary steadily by about 5 ppm, reflecting photosynthesis cycles. The two lines on that graph proclaim that a whopping 30 per cent cut in man-made CO2 emissions didn’t even cause a 1 ppm drop in the atmosphere’s CO2. Thus it is impossible to assert that the increase in atmospheric CO2 stems from human burning of fossil fuels.””

    Could someone explain this or point me to a previous explanation thanks Rob

  14. 64
    tamino says:

    Re: #61 (Rob Brookes)

    The whole story by Alexander Cockburn is, to quote Macbeth, “a tale told by an idiot, full of sound and fury, signifying nothing.”

    1st: CO2 increase lasts a long time. So even if we completely stop emitting CO2 now, it’ll take hundreds of years for what we’ve already put there to be removed.

    2nd: He’s can’t even get the numbers right. Today, the CO2 emissions from the U.S. alone in only one year, is about 7 gigatonnes.

  15. 65
    Leonard Evens says:

    Re: Cockburn’s analysis.

    I don’t know the answer to your question, and I hope one of the RC experts will address it. One relevant fact is that accurate measurments of atmospheric CO_2 concentration weren’t done until Keeling’s work in the 50s. But the important thing to remember is that people who study atmospheric CO_2 are not idiots. They are not likely to have missed something like this, were it an issue. The link between the rise in atmospheric CO_2 concentration and human activities has been established beyond any doubt, by a variety of arguments. If Cockburn had been seriously concerned about the issue, he would have checked the IPCC publications to find a few Carbon budget experts and posed the question to them before trying to make a scientific argument about a subject he doesn’t have the qualifications to understand.

    [Response: Watch this space! We are working on a letter to the Nation regarding Cockburn’s rantings. It ought to please some of our readers that we’re no more tolerent of idiocy on the left than we are on the right. It does happen that the idiocy on cliamte issues has generally been fairly partisan, with the Right generally getting it more wrong! Perhaps Cockburn’s intent was restoring some balance…. –eric]

  16. 66
    Dave Dougherty says:

    Re #33 and #46

    Eric, I should have read those papers. They are the best I’ve seen on this topic, which has been bothering me. And they were free!

    I have some disagreement with the Monnin paper. They seem to focus ont he intitiation of the ramps at 17000 yrs, but ignore other breaks, which don’t show a lag. The second paper with the Argon ratio gets a temp from gases in the bubble itself, and is pretty convincing. Having more examples than this one transistion would be nice. Wow, they can measure isotope ratios very accurately.

    Still, the two papers quote ice – air age differences for Vostok of 2200 to 5000 and 4100 years. Muuuch longer than the CO2 lifetime (or trap time numbers that I have seen thrown around.) There should be a qualifier when people say that ice cores prove that CO2 is higher now than any time in last 420000yrs. You got to take that 4000 year (20 time constant) averaging into account. It may be true from other arguments, but you just can’t take the CO2 ice core data by itself.

    Thanks for the great post. I always learn a lot here.


    [Response: There are no records as long as Vostok and Dome C, which as you point out are low resolution. But there are shorter but still quite long (>20,000 years) records that are high enough resolution to rule out completely CO2 values as high as today for periods of more than about 40 years. So yes, strictly speaking one cannot prove that there wasn’t a year, or a day, when CO2 levels were higher. But the mechanism for accomplishing this would have to be rather strange, and is currently unknown. I would devote my own research career trying to find such an event, because the chances that it occurred are vanishingly small.–eric]

  17. 67
    Tim says:

    I continue to be amazed at the informative contributions here. I am light years ahead of where I was just a few weeks ago in terms of understanding the science of climatology as it relates to the climate change debate. So thank you, again.

    A few more questions:

    What was the trend of global temperature in the 250 years prior to the beginning of the Industrial Revolution (1750)? Has this been quantified with a high degree of confidence?

    What was the trend in CO2 levels during the same 250 year period?

    What was the trend in sea level rise in this period?

    What external solar forces are at work currently (orbit, wobbling, etc.)? Were the same forces largely at work in the 250 years prior to the Industrial Revolution?

  18. 68
  19. 69
    Allan Ames says:

    On the matter of ice core sample timing, it is clearly part of the debate and should not now be dismissed as irrelevant.

    My problem is that what I see in the data is not what is being discussed. I down loaded Vostok CO2, Me, and O18, dusted off my 3.5 in. disk of ACM algorithms, and got the following, generally the same for either parameteric or nonparametric analysis:

    1) CO2 and methane are highly correlated, 0.75 at the peak.

    2) Both CO2 and methane cross correlate with O18, such that a
    decline in O18 leads a rise in either methane or CO2 with a
    broad peak at around 3000 years, led by O18. Methane is much stronger (-0.6) than CO2 (-0.43). (+/- 0.1 would be “highly significant”).
    In any event the time is more than a few hundred years, more like a
    few thousand.

    3) The autocorrelation is quickest for O18, then methane, both around 500 y to half max, but much slower for CO2 at around three times as long.

    My understanding is that higher O18 is higher temperature proxy, is it not? Any explanation (except that I mucked up the numbers)?

  20. 70
    Alexander Ac says:

    Indeed, CO2 lags temperature increase if one of the most frequently used arguments by skeptics. One of my favorite skeptic, Lubos Motl says, that if the amplyfying effect of CO2 was to be true, then this positive feedback would end in the hot-house, i.e. there would be no other mechanism, which could stop the onset of this positive feedback. Obviously, this was not the case. I opposed to him, that at some stage, some (unknown?) mechanism will inhibit this positive feedback and will trigger the onset of cooling. For instance, take an example of THC, of which shut-down (and subsequent cooling) was triggered (in the past) by warming. Did I got it right? ;-)

  21. 71
    egbooth says:

    Eric wrote: “While we don’t know precisely why the CO2 changes occur on long timescales, (the mechanisms are well understood; the details are not), we do know that explaining the magnitude of global temperature change requires including CO2.”

    Do you have any references that discuss the mechanisms that are associated with the positive feedback effect of CO2? I understand the long-term chemical weathering negative feedback but I’m still a little unclear on the positive feedback. Thanks a lot.

  22. 72
    egbooth says:

    Scratch that last comment.

    Here are the positive feedbacks for CO2 that I have found:

    soil respiration, permafrost, and fire.

    Those all make sense. I would imagine that the tough part is to figure out the magnitudes of each. Am I missing any others?


  23. 73
    Tim says:

    Re: 66

    George Will’s point is very clear to the reasonable reader. To what part of his column do you object and why? How does this relate to the science of climate change discussed in this forum?

  24. 74
    Timothy Chase says:

    Alexander Ac #67 wrote:

    One of my favorite skeptic, Lubos Motl says, that if the amplyfying effect of CO2 was to be true, then this positive feedback would end in the hot-house, i.e. there would be no other mechanism, which could stop the onset of this positive feedback. Obviously, this was not the case.

    Agreed. The problem lies with the assumption that positive feedback necessitates runaway effects. This is a fallacy Gavin deal with in some detail a while ago. However, as he points out, there is no need for a special mechanism to bring positive feedback to an end in many cases:

    People often conclude that the existence of positive feedbacks must imply ‘runaway’ effects i.e. the system spiralling out of control. However, while positive feedbacks are obviously necessary for such an effect, they do not by any means force that to happen. Even in simple systems, small positive feedbacks can lead to stable situations as long as the ‘gain’ factor is less than one (i.e. for every initial change in the quantity, the feedback change is less than the original one). A simple example leads to a geometric series for instance; i.e. if an initial change to a parameter is D, and the feedback results in an additional rD then the final change will be the sum of D+rD+r2D…etc. ). This series converges if |r|<1, and diverges (‘runs away’) otherwise. You can think of the Earth’s climate (unlike Venus’) as having an ‘r’ less than one, i.e. no ‘runaway’ effects, but plenty of positive feedbacks.

    5 Jul 2006
    Runaway tipping points of no return
    Gavin Schmidt
    Climate modeller at the NASA Goddard Institute for Space Studies in New York

    But obviously once the system reaches equilibrium, in order to push it away from that equilibrium will require a change in one of the parameters maintaining that equilibrium – assuming it is a stable equilibrium, such as that which we have been blessed with for 10,000 years of recorded history prior to the twentieth century.

  25. 75
    tamino says:

    Re: #70 (Tim)

    Where shall I begin? How about with this:

    … the media-entertainment-environmental complex is warning about global warming.

    Will is implying — but doesn’t have the courage to say outright — that global warming science is trumped up by the “media-entertainment-environmental complex.” Detestable.

    Then there’s this:

    … require significant reduction of greenhouse gas emissions in America and some other developed nations but that would involve no “specific scheduled commitments” for 129 “developing” countries, including the second-, fourth-, 10th-, 11th-, 13th- and 15th-largest economies (China, India, Brazil, South Korea, Mexico and Indonesia).

    Now Will wants to fall back on the “why should we do anything when others won’t” argument. This is as morally bankrupt as claiming that I shouldn’t have to refrain from mass murder, because Ted Bundy didn’t.

    Then there’s this:

    … higher than the cost of providing the entire world with clean drinking water and sanitation, which would prevent 2 million deaths …

    For the simple-minded, this is the most persuasive argument. It’s not only wrong, it’s just plain unethical to imply that action on global warming will deprive the world of clean drinking water. The reason we don’t spend the money to do so now has nothing to do with diverting funds to fight global warming, it has everything to do with the developed world lacking the moral fortitude to do so.

  26. 76
    Timothy Chase says:

    eggbooth (#69) wrote:

    Here are the positive feedbacks for CO2 that I have found:

    soil respiration, permafrost, and fire.

    Those all make sense. I would imagine that the tough part is to figure out the magnitudes of each. Am I missing any others?

    What I would be interested in are the causal sequences myself, although in the process one is likely to see some double-counting.

    However, in a certain sense, there is already some double-counting. But lets do it anyway so that we can track the lines of causation…

    1. With permafrost melting, the soil which makes up the permafrost is finally having a chance to respire, with the organic material being metabolized by bacteria, releasing carbon dioxide and methane which eventually decays to carbon dioxide.

    2. Raising the carbon dioxide level raises the temperature. This causes ice to melt, exposing the permafrost and raising its temperature to the point at which organic decay sets in, raising the level of methane and carbon dioxide.

    3. However, the melting of ice decreases the albedo near the artic, resulting in less light being reflected into space and more light being absorbed by the ocean, further raising the temperature of the ocean which will feed back into the release of methane and carbon dioxide.

    4. Raising the temperature of the ocean will also result in more melting of ice, at least if the ice is in contact with the ocean.

    5. The formation of dark pools (moulins) on the surfaces of glaciers speeds up the absorbtion of light by those glaciers and likewise results in runoffs which lubricate their descent into the ocean and their melting – further diminishing the albedo, raising the global temperature and thus the release of carbon dioxide.

    6. Higher temperatures result in increased evaporation of moisture from the soil, making increasing the likelihood of forest fires which will add to the carbon dioxide in the atmosphere.

    7. Higher temperatures make more violent storms more likely, and with them the discharge of atmospheric electricity, lightening and thus triggers for such forest fires.

    8. Increased evaporation makes soil more dry, resulting in decreased plant growth, and thus results in the diminished sequestration of carbon by plants.

    9. Decreased foliage increases the rate of evaporation at the edges of such foliage, further decreasing plant growth and increasing the likelihood of forest fires.

    10. Increased atmospheric carbon dioxide raises the temperature of the ocean, reducing the amount of oxygen and carbon dioxide which it can absorb, thus reducing the amount of carbon dioxide which it has the capacity to sequester.

    11. Increasing atmospheric carbon dioxide raises the level of carbon dioxide absorbed by the ocean, increasing its acidity and further diminishing its capacity to sustain life through the destruction of coral.

    12. The warming of the ocean, particularly in the upper layers, raises the temperature of methane hydrate deposits in shallow waters, resulting in more methane being released to the atmosphere.

    13. An increased rate of evaporation makes it more likely that precipitation will occur nearer the source of the evaporation – over the ocean – making droughts more likely over land.

    14. An increased rate of evaporation means that when precipitation occurs over land, it is more likely to be infrequent, running off of impoverished soil rather than being absorbed by soil which is rich in nutrients, further reducing plant growth.


    There are multiple feedbacks, but there are also multiple feedbacks to the feedbacks. Incidently, while I focused on the positive feedbacks, there are also clearly some negative feedbacks, too. For example, exposed polar water is capable of radiating more heat back into space than polar water which is insulated by ice. I suspect that this effect is relatively small in comparison to other effects, such as the absorbtion of light by the very same waters, but it is an effect which we would want to take into account nevertheless. Likewise, the greater the amount of a given green house gas in the earth’s atmosphere, the less effective the gas is as a green house gas. This is largely why methane is twenty times more effective than carbon dioxide as a green house gas, and nitrous oxide (a product of biodiesel fuel) is twenty times more effective than methane.

  27. 77

    [[Thank you Richard for this eloquent and precise explanation that huge amounts of CO2 could be releleased by oceans and cause global warming without any presence of humans! As you might be rightfully aware, according to Takahashi measurements of global CO2 fluxes from oceans, they outgas about 100GT of carbon per year in tropical areas. Thank you for boiling this post down to its bones. ]]

    The oceans give off about 90 gigatons of carbon altogether per year, and absorb 92 gigatons. They are presently a sink for carbon dioxide, not a source. The recent increase in carbon dioxide has come about from fossil fuel burning and land-use changes.

  28. 78

    [[The two lines on that graph proclaim that a whopping 30 per cent cut in man-made CO2 emissions didn’t even cause a 1 ppm drop in the atmosphere’s CO2. Thus it is impossible to assert that the increase in atmospheric CO2 stems from human burning of fossil fuels.””
    Could someone explain this or point me to a previous explanation thanks Rob

    Sure. Cockburn is assuming the amount in the air is directly proportional to the input from the US. It isn’t. His 0.9 gigaton burn in 1933 was half absorbed by the ocean, and has to be compared to the 600 GT or so then in the air. To expect a 33% drop or increase is asinine. The man doesn’t understand the difference between total ambient CO2 and added CO2. Like saying if there’s 15 gallons in your car’s fuel tank, and you put in 2 extra gallons, then only one, the amount of gas in your car should suddenly be cut in half.

  29. 79

    [[Do you have any references that discuss the mechanisms that are associated with the positive feedback effect of CO2? I understand the long-term chemical weathering negative feedback but I’m still a little unclear on the positive feedback. Thanks a lot. ]]

    In the long run, the amount lost to weathering — about 330 million tons a year — is balanced by the amount added by volcanism and rock metamorphism. In the short run, these don’t always match and the amount in the atmosphere can go up or down.

    When it gets colder, ice covers the ground, so there’s less weathering from that cause, and weathering goes down with temperature anyway. So the atmosphere is losing less, but volcanoes and metamorphism are still pumping CO2 into the atmosphere.

  30. 80
    pete best says:

    Re #77, I have read that the Oceans are currently absorbing 45% of human carbon emissions but by 2030 it will be down to around 30% and in effect become a source?

    [Response: RealClimate contributor David Archer can give a better answer to this than I can, but in a nutshell yes. This is because as CO2 enters the ocean, the ocean becomes more acidic, and this shift the dissolved-CO2 / biocarbonate / carbonate ion balance in favor of CO2, which drives up the atmospheric CO2. –eric]

  31. 81
    Timothy Chase says:

    Tim (#67) wrote:

    A few more questions:

    What was the trend of global temperature in the 250 years prior to the beginning of the Industrial Revolution (1750)? Has this been quantified with a high degree of confidence?

    What was the trend in CO2 levels during the same 250 year period?

    What was the trend in sea level rise in this period?

    I found the following for temperature between 1850 and 2000:

    Figure 1. Global warming revealed . Air temperature measured at weather stations on continents and sea temperature measured along ship tracks on the oceans are combined to produce a global mean temperature each year. This 150-year time series constitutes the direct, instrumental record of global warming..

    … from :

    Global Warming – Just Hot Air?
    David S. Chapman

    The same article contains the following for sea levels from 1800-2000:
    Figure 2. Sea level is changing. Observing stations from around the world report year-to-year changes in sea level. The reports are combined to produce a global average time series. The year 1976 is arbitrarily chosen as zero for display purpose.

    Then it includes the famous deep-time diagram for CO2:
    Figure 6. Rise in atmospheric carbon dioxide. The concentration of carbon dioxide in Earth’s atmosphere has increased steadily from 270 to 370 ppm since 1700. Early data come from gas bubbles trapped in ice. Since 1959, carbon dioxide concentration has been measured at observatories in Hawaii and elsewhere. Recent measurements show uptake and release of carbon dioxide with seasons (inset) superimposed on the steady global increase.

    As for current orbit, wobbling and solar output, I don’t have the figures right off hand, but from what I understand, minus the increased carbon dioxide due to manmade emissions, we should actually be in a cooling period right now. Additionally, some of the global warming which would have taken place in past decades due to increased carbon dioxide levels has been masked through aerosols increasing the albedo of the earth, resulting in global dimming (e.g., jet entrails seeding the formation of clouds).

    Incidently, there is a great deal of information on all of this available on the web – so if you are really interested and people are a little busy you be able to find some of the stuff for yourself.

  32. 82
    David B. Benson says:

    Re #61: SCM — The brief answer is no. A good book with regard to the various time scales is

    W.F. Ruddiman
    Earth’s Climate: Past and Future
    W.H. Freeman & Co, 2001.

  33. 83
    Ike Solem says:

    RE#80, there’s a simple experiment that shows how warming a liquid reduces the capacity to hold dissolved CO2 – take two beers, open them, and warm one up, in a water bath perhaps, while placing the other in a freezer. After ten minutes, return both open bottles to the fridge and wait for them to cool. One will be flat, one will still have plenty of CO2. For demonstration purposes, it’d be best to get your hands on “Global Warming Beer”, produced from the melting Greenland icecap (Natl Geo)

  34. 84
    PHE says:

    I saw the film.

    The most dramatic part of the film was when Gore showed the temperature and CO2 graphs together, and then rose up on the mechanical lift to show that currently increasing CO2 concentrations are likely to lead to significant temperature increases. There is no question that his aim was for the audience to believe that CO2 rise drives temperature rise in both the past and present day. This was deliberate deception.

    [Response: There is nothing deceptive about this. Once a greenhouse gas, always a greenhouse gas. -gavin]

    To attempt to [inappropriate and inflamatory language edited]

  35. 85
    Philippe Chantreau says:

    #76. About methane hydrates. I read that massive release of oceanic methane hydrates was suspected to have played a significant role in the P/t extinction. Any take on that?

  36. 86
    Steve Bloom says:

    Re #61: Just to add that you can search this site for more detailed answers. Briefly, solar, volcanic and CO2 forcings of the same initial size will behave quite differently over time. Solar variations are to all intents and purposes instantaneous, volcanic dust has a short lag time since such emissions fall/rain out of the atmosphere within a few years, but CO2 tails off over centuries.

    [Response: This is right but doesn’t quite answer the quesiton posed. See my comments after #61 above.–eric]

  37. 87
    eric says:

    Comments are now off on this post. Thanks to all who commented in a constructive manner.