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Target CO2

Filed under: — gavin @ 7 April 2008 - (Español)

What is the long term sensitivity to increasing CO2? What, indeed, does long term sensitivity even mean? Jim Hansen and some colleagues (not including me) have a preprint available that claims that it is around 6ºC based on paleo-climate evidence. Since that is significantly larger than the ‘standard’ climate sensitivity we’ve often talked about, it’s worth looking at in more detail.

We need to start with some definitions. Sensitivity is defined as the global mean surface temperature anomaly response to a doubling of CO2 with other boundary conditions staying the same. However, depending on what the boundary conditions include, you can get very different numbers. The standard definition (sometimes called the Charney sensitivity), assumes the land surface, ice sheets and atmospheric composition (chemistry and aerosols) stay the same. Hansen’s long term sensitivity (which might be better described as the Earth System sensitivity) allows all of these to vary and feed back on the temperature response. Indeed, one can imagine a whole range of different sensitivities that could be clearly defined by successively including additional feedbacks. The reason why the Earth System sensitivity might be more appropriate is because that determines the eventual consequences of any particular CO2 stabilization scenario.

Traditionally, the decision to include or exclude a feedback from consideration has been based on the relevant timescales and complexity. The faster a feedback is, the more usual it is to include. Thus, changes in clouds (~hours) or in water vapour (~10 days) are undoubtedly fast and get included as feedbacks in all definitions of the sensitivity. But changes in vegetation (decades to centuries) or in ice sheets (decades(?) to centuries to millennia) are slower and are usually left out. But there are other fast feedbacks that don’t get included in the standard definition for complexity reasons – such as the change in ozone or aerosols (dust and sulphates for instance) which are also affected by patterns of rainfall, water vapour, temperature, soli moisture, transport and clouds (etc.).

Not coincidentally, the Charney sensitivity corresponds exactly to the sensitivity one gets with a standard atmospheric GCM with a simple mixed-layer ocean, while the Earth System sensitivity would correspond to the response in a (as yet non-existent) model that included interactive components for the cryosphere, biosphere, ocean, atmospheric chemistry and aerosols. Intermediate sensitivities could however be assessed using the Earth System models that we do have.

In principal, many of these sensitivities can be deduced from paleo-climate records. What is required is a good enough estimate of the global temperature change and measures of the various forcings. However, there are a few twists in the tale. Firstly, getting ‘good enough’ estimates for global temperatures changes is hard – this has been done well for the last century or so, reasonably for a few centuries earlier, and potentially well enough for the really big changes associated with the glacial-interglacial cycle. While sufficient accuracy in the last few centuries is a couple of tenths of a degree, this is unobtainable for the last glacial maximum or the Pliocene (3 million years ago). However, since the signal is much larger in the earlier periods (many degrees), the signal to noise ratio is similar.

Secondly, although many forcings can be derived from paleo-records (long-lived greenhouse gases from bubbles in the ice cores most notably), many cannot. The distribution of sulphate aerosols even today is somewhat uncertain, and at the last glacial maximum, almost completely unconstrained. This is due in large part to the heterogenity of their distribution and there are similar problems for dust and vegetation. In some sense, it is the availability of suitable forcing records that suggests what kind of sensitivity one can define from the record. A more subtle point is that the ‘efficacy’ of different forcings might vary, especially ones that have very different regional signatures, making it more difficult to add up different terms that might be important at any one time.

Lastly, and by no means leastly, Earth System sensitivity is not stable over geologic time. How much it might vary is very difficult to tell, but for instance, it is clear that from the Pliocene to the Quaternary (the last ~2,5 million years of ice age cycles), the climate has become more sensitive to orbital forcing. It is therefore conceivable (but not proven) that any sensitivity derived from paleo-climate will not (in the end) apply to the future.

We’ve often gone over the Charney sensitivity constraint for the Last Glacial Maximum. There is information about the greenhouse gases (CO2, CH4 and N2O), reconstructions of the ice sheets and vegetation change, and estimates of the dust forcing. A recent estimate of the magnitude of these forcings is around 8 +/- 2 W/m2 (Schneider von Deimling et al, 2006). This implicitly includes other aerosol changes or atmospheric chemistry changes in with the sensitivity (or equivalently, assumes that their changes are negligible). So given a temperature change of about 5 to 6ºC, this gives a Charney sensitivity of around 3ºC (ranging from 1.5 to 6 if you do the uncertainty sums).

Hansen suggests that the dust changes should be considered a fast feedback as well (as could the CH4 changes?) and that certainly makes sense if vegetation changes are included on the feedback side of the equation. Since all of these LGM forcings are the same sign (i.e. they are all positive feedbacks for the long term temperature change), that implies that the Earth System sensitivity must be larger than the Charney sensitivity on these timescales (and for this current geologic period). So far so good.

Hansen’s first estimate of the Earth System sensitivity is based on an assumption that GHG changes over the long term control the amount of ice. That gives a scaling of 6ºC for a doubling of CO2. This is however problematic for two reasons; first most of the power of this relationship is derived from when there were large N. American and European ice sheets. It is quite conceivable that, now that we are left with only Greenland and Antarctica, the sensitivity of the temperature to the ice sheets is less. Secondly, it subsumes the very special nature of orbital forcing – extreme regional and seasonal impacts but very little impact on the global mean radiation. Hansen’s estimate assumes that an overall cooling of the same magnitude of the LGM would produce the same extent of ice sheets that was seen then. It may be the case, but it is not a priori obvious that it must be. Hansen rightly acknowledges these issues, and suggests a second constraint based on longer term changes.

Unfortunately, prior to the ice core record, our knowledge of CO2 changes is much poorer. Thus while it seems likely that CO2 decreased from the Eocene (~50 million years ago) to the Quaternary through variations related to tectonics, the exact magnitude is uncertain. For reasonable values based on the various estimates, Hansen estimates a ~10 W/m2 forcing change over the Cenozoic from this alone (including a temperature-related CH4 change). The calculation in the paper is however a little more subtle. Hansen posits that the long term trend in the deep ocean temperature in the early Cenozoic period (before there was substantial ice) was purely due to CO2 (using the Charney sensitivity). He then plays around with the value of the CO2 concentration at the initiation of the Antarctic ice sheets (around 34 million years ago) to get the best fit with the CO2 reconstructions over the whole period. What he ends up with is a critical value of ~425 ppm for initiation of glaciation. To be sure, this is fraught with uncertainties – in the temperature records, the CO2 reconstructions and the reasonable (but unproven) assumption concerning the dominance of CO2. However, bottom line is that you really don’t need a big change in CO2 to end up with a big change in ice sheet extent, and that hence the Earth System sensitivity is high.

So what does this mean for the future? In the short term, not much. Even if this is all correct, these effects are for eventual changes – that might take centuries or millennia to realise. However, even with the (substantial) uncertainties in the calculations and underlying assumptions, the conclusion that the Earth System sensitivity is greater than the Charney sensitivity is probably robust. And that is a concern for any policy based on a stabilization scenario significantly above where we are now.

157 Responses to “Target CO2

  1. 101

    To my mind good theory creates more good theories. It seams that our predictions of future climate changes more or less becomes truth. This is why it is of great importance that we start to understand the Universal Law of Attraction and start saying the good things that we want to happen to our planet.

    The We Project is a good start. They state: We can solve the climate crisis. Telling what we want make all the difference.

  2. 102
    Chris O\'Neill says:

    Ferdinand Engelbeen:

    CO2 trends follow the d18O (temperature proxy) trend in the LGM-Holocene transition, with no measurable feedback from increasing CO2 levels

    If it was the other way around, i.e. temperature lagged CO2 by several hundred years, that would actually be very strong evidence that sensitivity to CO2 was low (because the temperature delay should be less than 100 years). However, as the record stands, you cannot rule out CO2 causing warming because warming always occurs when the CO2 rises (with perhaps some short term variation).

  3. 103
    David B. Benson says:

    Khebab (87) — Already by 1862 CE Tyndall wrote regarding the amplification effect of global warming (so-called greenhouse) gases. The history is in “The Discovery of Global Warming”, linked here:

  4. 104
    Lawrence McLean says:

    Re #94, Jim, a simple household experiment can be carried out to show the significance (in terms of numbers only) of the levels of Carbon dioxide in the atmosphere that we are dealing with.

    Get 10 litres of water, then add 27 grams of Potassium permanganate. In terms of parts per million, this will give a solution that is of about the same number of parts per million as CO2 in the 1960’s. Then add 1 gram for each 10 year period up to the present. Quite rough, it is actually a little more than 1 gram per 10 years equivalent, however it does show that what we are dealing with is certainly not insignificant.

  5. 105
    Pekka Kostamo says:

    RE 91. I still think the main factor in hysteresis is the altitude change.

    The Antarktis is not commonly perceived as a high plateau. This is due to the map makers’ unfortunate break-down of logic in the area. Instead of the dark brown hues justified by the surface altitude readings, all maps show this continent as white, same as any sea level ice fields.

    To destabilize the Eastern Antarktis, extensive surface melt must start. This is critical as it changes the thermal processes radically. The surface albedo goes down and more solar energy is captured. Another very important fact is that the excess energy is stored in the meltwater as latent heat. Through cracks in ice, some water percolates inside the glacier. In the early phases of the process, the water re-freezes liberating the latent heat and the glacier internal temperature increases. Uneven temperature fields result in uneven thermal expansion and further breakage.

    When the interior temperature has warmed to 0 degC, water remains in liquid form, eventually lubricating the glacier base. It also lubricates the interfaces between ice blocks. Wet ice against wet ice is very slippery indeed. A two mile high pile of wetted ice blocks is definitely unstable. Surface melting occurs now up to 1,5 km above sea level, which is a major concern for Greenland and the Western Antarktis.

    Surface melting in Eastern Antarktis obviously starts when temperatures rise above 0 degrees at the altitude of 10 000 ft (3 km) above sea level. Assuming the average lower atmosphere lapse rate of 6,5 degC/km, this roughly means +20 degC temperatures at sea level. When the Antarctic ice is gone, re-glaciation must start at a substantially lower altitude. The sea level temperatures must then be much lower, based on the lapse rate cited.

    How this translates into CO2 ppms, I am not able to say, considering all the short-term and long-term feedbacks. My quite un-educated initial guess is that the hysteresis might be as large as 100 ppm.

    Antarktis is a huge area with a climate of its own, and may have some quite special features. For instance, does it have a troposphere? Tropopause altitude on the Equator ranges from 12 to 15 km, on high latitudes just between 3 to 7 km. At 3 km altitude (ASL) and near the pole there is at least at times just a boundary layer interfacing directly with the stratosphere. No evaporation, no convection, no fronts, just some jet streams experienced as surface winds. Externally forced intrusions of humid air masses provide occasional snowfalls.

    The Eastern Antarktis will be stable for a long time ahead – which can not be said of Greenland and West Antarktis where the destabilization processes are underway already.

  6. 106
    Pinguiinimies says:

    Pekka, I think there are a few misconceptions in what you wrote.
    1) under pressure (inside the glacier) ice melts already at a lower temp than 0C.
    2) also the EAIS will start melting at the edges, not
    the top. But the whole ice sheet is like a plastic blob
    of putty, slowly deforming under pressure. Snow gets
    added at the top, gets compacted on the way down, and
    then gets squeezed out sideways towards the coasts.
    This in spite of ice being a solid. As a result, the
    annual layers in the ice get paperthin towards the
    Ice cores are always drilled from the summit, where there
    is no sideways movement even at depth, and you retrieve the
    longest history down to the bedrock.
    In this picture of a slowly deforming pancake, the flow
    is stationary. As much ice leaves at the edges into sea,
    as is added at the top by precipitation. But if you
    remove ice (shelf or coastal glaciers) from the
    edges (processes you sketched), pressure equilibrium is
    disturbed and deformation (ice flow) speeds up also further inland.
    These phenomena are poorly understood currently.
    3) about hysteresis, I believe that on a time scale
    long enough to accomodate isostatic re-adjustment,
    there will be no hysteresis for ordinary glaciations/deglaciations.
    It is different for the “snowball Earth” episodes,
    where the ice reaches 45 deg latitude resulting in a
    runaway glaciation. And later, a runaway deglaciation. Hysteresis
    and runaway feedback go together: they mean that there
    are two stable ice geometries for the same
    CO2 forcing.

  7. 107
    pete best says:

    How well does science understand non linear systems, or how well do climate scientists understand non linear systems. The climate systems is a complex one we are told consisting of many interacting elements, outputs of some inputs to others, the systems making up the climate or earth system are coupled and hence complex. But how much of this is linear and how much no linear and if non linear how long before we see dare I say it, strong behaviour away from the norm?

    I learned that in a complex dynamical system have several inputs to a system (simple predator and prey scenarios seem to work) whereby when the input is within a certain range all is well, predator and prey are nicely balanced. However perturb one or more of the inputs and the system can start deviate from the norm in terms of its behaviour. Large scale change is observed when prey die out (virus or too many predators) or vice versa. Birth range of animals change etc and fall out of balance whihc causes some very interesting dynamics to occur. Normally the systems find its way to chaos under certain paramater conditions.

    So is climate modelled this way? Is James Hansen non linear fears for albedo feedback and others leading us to more unpredictable patterns of weather the same as Lovelocks GAIA in which the balance is being perturbed. The Sunshine parameter remains the same, but GHG and land use changes have changed, so have dimming particles for that matters. Is it too difficult to model? Does we need to rely on rea world data more then what the models show us.

    Are these nonlinearities understoodm is 6C from 3C something than scientists missed until now because we are too focused on the linear in science?

  8. 108
    Jim Bullis says:

    Re 56 re 29

    Chuck Booth, Thanks for the solubility data.

    Sorry for not making a better question. I was meaning to get at something about temperature and depth effects in combination.

    If you think about my premise that the ocean surface will warm only slightly but the heat energy will be shifted to ever greater ocean depths, then the needed information would be a function of pressure as well as temperature.

    The importance of solubility at the ocean surface as a function of temperature still exists, but the impact would be much moderated if that temperature increase was only slight.

  9. 109
    Rod B says:

    Jim (94), interesting comparison, but it’s kinda apples v oranges; concentration significance ala physiology and human health has no bearing or relation to concentration significance ala radiation absorption.

  10. 110
    Chuck Booth says:

    Re # 108 Jim Bullis

    If I understand your point correctly (CO2 solubility at depth will be a function of both temperature and pressure), the following may have the information you are wondering about:

    Zeebe, R. E., and D. A. Wolf-Gladrow, Carbon dioxide, dissolved (ocean). Encyclopedia of Paleoclimatology and Ancient Environments, Ed. V. Gornitz, Kluwer Academic Publishers, Earth Science Series, in press 2008.
    (Dr. Richard E. Zeebe, Department of Oceanography, University of Hawaii School of Ocean and Earth Science Technology;

    See also:
    Science 16 July 2004:
    Vol. 305. no. 5682, pp. 362 – 366

    Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans

    Richard A. Feely, Christopher L. Sabine, Kitack Lee, Will Berelson, Joanie Kleypas, Victoria J. Fabry, Frank J. Millero

    Science 10 January 2003:
    Vol. 299. no. 5604, pp. 235 – 239

    Anthropogenic CO2 Uptake by the Ocean Based on the Global Chlorofluorocarbon Data Set

    Ben I. McNeil, Richard J. Matear, Robert M. Key, John L. Bullister, Jorge L. Sarmiento1

  11. 111
    Martin Vermeer says:

    Re #109 Rod B: You’re right, but yet… the general argument that small numbers of molecules can have a big effect, stands. Whether the interaction is with photons or with other molecules, is more like a technical detail.

    Another spectacular example is uranium 235, only occurring at 0.7% in natural uranium. Still thermal neutrons can maintain a chain reaction in it due to its huge fission cross-section.

    The permanganate example in #104 is also a good one (although I think the amount should be 2.7 g per 10 litres). The colour will clearly show… that’s interaction with photons :-)

  12. 112
    Lawrence McLean says:

    Re: #111 Martin,

    The way I figured it is:

    H2O: 18 grams/mole
    KMnO4: 158 grams/mole
    Which is 8.8 times the mass/mole of water

    1960’s CO2 ~ 320 ppm

    For something with the same molar mass as water that would equate to:
    (in 10 litres of water = 10,000 grams)
    (10,000 ÷ 1,000,000) * 320) grams = 3.2 grams

    Potassium permanganate is 8.8 times the molar mass of water that gives:
    3.2 * 8.8 = 28 grams

    How am I wrong?

  13. 113
    Martin Vermeer says:

    Re #112 Lawrence: I see you did it by molar ratios. I did it in my head by mass ratio (and miscalcuated).
    Hmmm. Yes, by molecule (actually, anion) count your approach is the right one.

  14. 114
    Rod B says:

    Martin (111) said, “…You’re right, but yet… the general argument that small numbers of molecules can have a big effect, stands. ..”

    Reasonable point.

  15. 115
    sidd says:

    Has anyone got a preprint of the paper referred to in the following link ?

    Svetlana Jevrejeva et al predicting sea level rise range of 0.8-1.5 m by century end.

  16. 116
    Jim Galasyn says:

    This is comforting:

    Bush plans White House speech Wednesday on climate change

    WASHINGTON (AP) — In a Rose Garden speech Wednesday, President Bush will outline the way he things the United States can reduce greenhouse gas emissions, and challenge lawmakers on climate change legislation up for debate in June.

    White House press secretary Dana Perino said Tuesday that Bush will not outline a specific proposal, but instead will lay out a strategy for “long-term” and “realistic” goals for curbing emissions.

    Bush wants every major economy, including fast-growing nations like China and India, to establish a national goal for cutting the emissions believed responsible for global warming.

    “This will ensure that all major economies, like France, Germany, China and India play a role in any international agreement so as to avoid a future Kyoto-like mistake,” Perino said.

    A new global warming pact is being crafted to succeed the first phase of the 1997 Kyoto Protocol. It requires 37 industrialized nations to reduce greenhouse gas emissions an average of 5 percent below 1990 levels by 2012. The United States is the only industrialized nation not to have ratified Kyoto, but it agreed with nearly 200 other nations at a conference in Bali in December to negotiate a new agreement by the end of 2009.

    Bush also is to talk about his concerns with legislative proposals likely to be considered during Senate floor debate in June, as well as frame a discussion about pending action in the regulatory arena, she said.

    “We believe there a right way and a wrong way to address this problem,” Perino said, adding that there is no legislative proposal on the hill that the administration supports.

  17. 117
    Jim Galasyn says:

    And re the Bush speech mentioned in 116, who could be surprised at the outcome:

    Bush Announces Greenhouse Gas Strategy — Surprise! It’s Bad
    By Alexis Madrigal

    President Bush delivered a speech today in which he outlined a new strategy to “effectively combat climate change.” Unfortunately, on my initial read of excerpts from his speech, it seems deeply, deeply flawed.

    Bush is finally publicly admitting that emissions should be curbed because they affect the climate. That’s fine, but his proposed response doesn’t logically follow from that statement of fact. If you think the greenhouse gas emissions are a problem, then you should craft a solution that reduces them. But Bush, instead, is suggesting that no mandatory caps be put in place, that no specific targets be fixed in the near term, and that no moratorium be called on coal plant production.

    Essentially, he just says, let’s wait ten years and then do something. The initial IPCC draft roadmap called for rich nations like the US to make reductions of “25-40 percent below 1990 levels by 2020.” Bush’s plan would not only result in no reductions, but would actually allow for continued growth in emissions until 2025!

    This speech strikes me as a desperate, half-baked ruse to stave off the real climate change legislation that is almost sure to come down the pike during the next Presidential Administration. On this issue, a new President cannot take the Oval Office fast enough, be it Obama, McCain, or Clinton.

    Bush also consistently conflates energy security goals–mythical independence from foreign oil, say–with climate change goals–primarily reducing carbon dioxide emissions from burning hydrocarbons. Don’t get it twisted: ethanol is about growing our own, not kicking our habit.

  18. 118
    Dan says:

    re: 116 and 117. It is of course no small coincidence that Bush made the speech now because Earth Day is Saturday. Each April is when politicians such as Bush throw the environment a supposed bone.

  19. 119
    pete best says:

    Would it be fair to say that this increased Earth Sensitivity of 6C is of concern or is too speculative to usurp the standard measure as yet? Is the jury still out or has Hansen improved the standard measure of CO2 sensitivity. I mean that a doubling of preindustrial CO2(e?) to 560 ppmv is going to result in a more likely 6C rise!

    400 ppmv of CO2 then means in increased sensitivity from 0.2C per decade or nothing much for a much longer period, ie; this increased sensitivity is not here yet but will make itsels know as we climb towards some currently unknown (2 to 3C) tippng point?

  20. 120
    Ian Greenwood says:

    Hi folks

    Thanks for the comments, site recommended by Tyndall Centre UK as worth a look.

    Can anyone ponder a guess? Even if stability is achieved at +6 degree Celsius, surely there will be quite fast melting of land-supported ice once the Arctic floating ice has melted i.e less than 50% of ice surface remaining in the Northern hemisphere. During the summer months the sun’s energy is relatively trapped in the North due to climate cells, so will not the mountain ice be melting at double the rate it is today? So will not that result in less melt-water and less irrigation possible/stability in the river systems fed by that mountain melt-water? Has this been included for in the modelling so far? Look out USA/EU/Asia for further food price rises as this ice rapidly reduces in the Rockies/Alps/Himalayas?

    I’d appreciate any comments, Thanks. (Global citizen)

  21. 121
    Mark A. York says:

    Moved from the current thread.

    There’s some pretty libelous things said in this Esterbrook interview by that opponent of mine. Doesn’t take long to find them either. Downright shameful. Somebody should hand them their hat.

  22. 122
    R. Gates says:

    What about methane?

    While it is certainly a much smaller factor in the GCM (at least currently), why have no targets been set for it? Besides being so much more potent a GHG, though with a much shorter lifetime in the atmosphere, it also has the added danger of leading to a positive feedback loop, and hence, wouldn’t a so called “tipping point” for methane in the atmosphere be much more of a concern? Locking in a target for CO2 at whatever level, seems like only half a victory if methane continues to rise.

  23. 123
    Vincent van der Goes says:

    From some of the responses I understand that if we would use up all oil and gas reserves, but leave coals in the ground, the result would be a CO2 level of 450 ppm. Is that correct? What if we would burn all fossile fuels, including coal? How high could the CO2 level get, in a worst case scenario?

  24. 124
    pete best says:

    Re #123, if we convert coal to liquids, gas to liquids, mine heavy tar and oil sands then we might do even more damage CO2 wise. Coal is not going to stay in the ground of any kind as it is a local resource in around 70 countries and means energy security for them.

    Scary eh.

  25. 125
    Hank Roberts says:

    R. Gates, what we can target is what humans produce that gets into the atmosphere. Methane as natural gas? Fix the leaky pipes. Doable, being done.

  26. 126
    Ross says:

    Does CO2 amplify temperature and if so can you briefly explain how?

  27. 127
    Hank Roberts says:

    Sure. Click the “Start Here’ link at the top of each page, and click the first link under Science at the right side.

    Use the Search box, also at the top of the page, for any terms you want explained.

    How much math and science have you had in school so far?

  28. 128
  29. 129
    Ray Ladbury says:

    Ross, Ever hold your hand under a heat lamp at a burger joint, etc.? The heat you are feeling is infrared radiation being absorbed by your skin. CO2 absorbs infrared radiation that would otherwise radiate away from Earth’s surface and atmosphere and back into space. Since more energy is coming in (via sunlight) than is leaving (via outgoing IR radiation–the only energy leaves the climate) the planet must heat up. Does this help?

  30. 130
    Scott Holladay says:

    There are claims, eg in ,
    that the inferred low correlation between temperature and C02 concentrations (based on the article by Paul N. Pearson and Martin R. Palmer, “Atmospheric Carbon
    Dioxide Concentrations over the Past 60 Million Years,” /Nature/, *206*
    (17 August 2000), 695-99) over the last 60 million years somehow invalidates all the work of the IPCC and the rest of the climate science fraternity. I am a scientist, but my PhD was in exploration geophysics–I would be grateful if you could explain where this line of reasoning goes off the rails.

    [Response: Look at figure 6.1 in IPCC AR4 – that has an updated set of all the CO2 estimates pre-Quaternary (including Pearson and Palmer’s work). First thing you notice is that there is a lot of variability among the different methods so P&P aren’t likely to be the last word on this. Secondly, if you try and do a detailed comparison where the data are good enough (see Royer et al, 2006), then as best we can judge, there is a correlation between temperature and CO2 over long time scales where the CO2 is changing because of tectonic or weathering effects etc. – gavin]

  31. 131
    Scott Holladay says:

    Thanks, Gavin–your comment and reference were very helpful. As a follow-up, I understand that ice core data and other sources clearly indicates a phase lag of C02 relative to temperature during pre-industrial times. I have read that it is well-known that this lag has reversed during the industrial period due to anthropogenic GHG emissions, but I have not been able to find a graph that clearly shows this. Can you point me to a suitable reference (preferably one that I don’t need a journal subscription to read–IPCC would be great!)

  32. 132
    Chris Colose says:

    # 131 Scott

    Might I suggest that you’re thinking about this relationship wrong. There has been a lot of emphasis on the blogosphere about “what comes first in the Vostok record” but the two are tied into an intrinsic relationship. CO2 causes temperature rise through well documented radiative principles, specifically inhibiting the efficiency at which the planet loses heat. CO2 will also respond to temperature because of ocean chemistry principles (i.e., gas solubility is lower in warmer water, and that goes in the atmosphere) and also vegetation and other responses. Once you get ocean outgassing though, you’ll also get a positive feedback whereby CO2 amplifies whatever the initial warming effect was (maybe milankovitch orbital variations). But thinking about the two dichotomies too hard is like trying to decide if the chicken or egg comes first.

    Throughout most of the ice core record, you’re going to expect CO2 to lag temperatures because there is not much reason to expect a massive external release of carbon to happen by itself. It should be rare in the geologic record, and random, not cyclic. If you look hard enough in the ice core record though you’ll find instances where CO2 comes first (Marine Isotope Stage 14.2) or precedes a deglaciation, etc. If you look back farther, to say the Paleocene-Eocene boundary there appears to be a massive release of CO2 which caused a large spike in temperatures.

    Today, CO2 is clearly causing (so preceding) a large chunk of the temperature rise. If you want a supporting reference see this paper. What’s more, the magnitude and rate of CO2 rise is far beyond what you’d get with just a CO2 outgassing feedback, and we also have isotopic evidence which unequivocally establishes the source of CO2 release, and it’s fossil fuels, not the oceans (which are a net sink now) or terrestrial biomass. The fact that levels are highest in at least 800,000 years (and likely longer!) is not a coincidence.

  33. 133
    Scott Holladay says:

    Thanks, Chris. I had a look at the paper that you referenced (Caillon et al, 2003). While it does discuss paleoclimatic temperatures and CO2 levels in the context of ice core results, I could not find within it direct support for your statement that “CO2 is clearly causing (so preceding) a large chunk of the temperature rise.” Perhaps this was not the paper that you intended to reference.

    The isotopic evidence that you cite for the fossil-fuel origin of high and increasing CO2 concentrations in the atmosphere seems unequivocal. Similarly, it seems well established we are well beyond the highest CO2 concentrations seen in the last 800k years. Thank you for pointing me to that material.

    Regarding the rest of your comments–yes, it is obvious that there is a feedback process at work and that it must factored in–we’ll take that as read. What I’m getting at is that, while it has been repeatedly stated (in many comments at RealClimate, and in the IPCC report, and in media reports) that there exists an unequivocal lead in CO2 concentrations over temperature increase (which we can contrast with the well-established paleoclimate temperature lead over CO2 levels), a graphical or tabular representation of this linkage would be much more helpful from an educational standpoint than verbal statements.

    So this is what I’m looking for: a plot (or a pair of stacked plots with a common time scale) that shows both a well-established temperature versus time series (or a collection of such series) with a corresponding set of CO2 concentration vs time estimates running for the last (say) 500 years. The icing on the cake would be a similar plot with a longer time scale that runs back to the end of the last ice age.

    While such figures would not tell the whole story of radiative forcing and feedbacks, they would be immensely valuable tools for non-experts like me to use in explaining the difference between the pre-industrial and post-industrial GHG-temperature regimes to others.

  34. 134
    Chris Colose says:

    #133, Scott

    The conclusion in Caillon et al is pretty clear:

    from the recent anthropogenic CO2 increase.
    As recently noted by Kump (38), we
    should distinguish between internal influences
    (such as the deglacial CO2 increase) and external
    influences (such as the anthropogenic CO2
    increase) on the climate system. Although the
    recent CO2 increase has clearly been imposed
    first, as a result of anthropogenic activities, it
    naturally takes, at Termination III, some time
    for CO2 to outgas from the ocean once it starts
    to react to a climate change that is first felt in the
    atmosphere. The sequence of events during this
    Termination is fully consistent with CO2 participating
    in the latter 4200 years of the warming.
    The radiative forcing due to CO2 may serve
    as an amplifier of initial orbital forcing, which is
    then further amplified by fast atmospheric feedbacks
    (39) that are also at work for the presentday
    and future climate.

    The Kump paper they mention is also a good read that goes over this. is a good site with the graphs you may be looking for.

  35. 135
    David B. Benson says:

    Scott Holladay (133) — While I ccan’t show you strictly pre-industrial, the following three plots ought to do.

    Decadal average temperatures since 1850 CE:

    Keeling curve of atmospheric CO2 concentrations:

    Human-caused emissions of CO2:

    A serious attempt to put these together:

    and for dessert, a light-hearted attempt to put these together:

  36. 136
    Chris Colose says:

    After David posts these graphs I probably want to make a further point (I still don’t think the right question/request is being made). I think you’re trying to eyeball a graph and see if CO2 started rising around 1850 or something, and make sure that temperature started rising later (maybe 1900). The better question is probably, from a radiative forcing perspective, what’s the magnitude of the anthropogenic forcing compared to natural forcings? Or, what is the lag time in the climate system between forcing and a full realization of the temperature response? Or, what is the rate and magntitue of CO2 outgassing for a given temperature rise over a specified time period, and the possibility of massive releases from the ocean/biosphere in a warmer world?

    A few things though– a lot of warming before 1950 had to do with natural variation, such as increases in solar, lack of volcanoes, and other things. A clear anthropogenic signal that diverges from the noise of natural variation is not clear until the ’70s or ’80s. Secondly, the climate system takes decades to come to equilibrium and see the full response (or most of it) realized. What’s more, the “warming in the pipeline” (which is what we are commited to even if atmospheric concentrations stabilize) is proportional to the climate sensitivity, so if sensitivity were a lot higher than we think (say the high end at 4.5 C), a stabilization at 385 ppmv would not result in a temperature stabilization for a while, and we’d still be in store for another degree or so. So, a side-by-side comparison of CO2 and temperature as function of time over the industrial era will not give a complete picture to answer your question.

  37. 137
    Scott Holladay says:

    Re 134: Chris, I had seen and understood the passage that you quote here. I guess that I should have used the words “graphical” or “numerical” support in 133 rather than “direct” support–sorry for my lack of clarity.

    Re 135: David, thank you very much–the fourth plot is just what I was looking for.

  38. 138
    Scott Holladay says:

    Re 136: I was convinced long before we had this discussion, Chris, but you’ve broadened my understanding considerably. Thanks for taking the time to help me out.

  39. 139
  40. 140
    Robert Stenson says:

    In view of the present discussion of the role of carbon dioxide in effecting global temperature I would like to know of any laboratory or bench experiments that show a temperature- CO2 concentration curve within the range of currently measured atmospheric CO2 levels.
    You would think it fairly easy to set up multiple containers with different gas concentrations and similar incident radiation.
    Some crude runs have been reported with 100% CO2 or with increased but unreported or unmeasured CO2 concentrations but I know of none with a precise CO2-temperature relationship in the climactically important range.

    I realize that the laboratory scenario is simple in the extreme but it would prove useful in thinking about atmospheric CO2 effects, or for the effects of any other GHG for that matter. I would appreciate any comments, references, or help.

  41. 141
    Mark Singer says:

    I came late to this thread, finding it due to an interest in climate policy recommendations. Simply put, to me the urgent question currently is, “What should be humanity’s atmospheric CO2 target?” And that is quickly followed by the question, “How do we get there from here?”

    There is no discussion in this thread (nor elsewhere on, apparently) of Cox and Jones, Illuminating the Modern Dance of Climate Change, Science, 19 September 2008 pp. 1642-44. Using additional data, Cox and Jones constrain earth’s climate sensitivity to what previously had been the high end of the range.

    The Cox and Jones article requires that the subject matter of this thread be re-opened and reconsidered.

    IPCC4 concluded the debate about, “Is human-caused global warming real?” to most reasonable people (even though open to revision based on later evidence). So realistically and pragmatically, climate science now must provide policy advice on what to do about it (even though that answer, too, is tentative and subject to revision).

    So I would like to re-open or re-invigorate this discussion in view of recent developments.

    I apologize if I should be asking this instead in another discussion thread. If so, you can simply “tell me where to go.” ;-)


  42. 142
    David B. Benson says:

    Mark Singer (141) — My amateur opinion is that CO2e needs to be reduced to about 300 ppm. One way to get there, the least expensive known to me, is via enhanced mineral weathering (enhnced carbonate formation). Here are some links.

    Olivine weathering:

    Peridotite weathering:
    “Rocks Could Be Harnessed To Sponge Vast Amounts Of Carbon Dioxide From Air”:

  43. 143
    Hank Roberts says:

    Mark Singer, try:
    — the “Start Here” link at the top of each page
    — the first link under Science in the right hand sidebar
    — the Search box at the top of the page (try “target” as the search)

    Target Atmospheric CO : Where Should Humanity Aim?

  44. 144
    Mark Singer says:

    Thanks, Dave (No. 142). I also saw that article in Nov. 6 Science. Maybe when the price of carbon gets high enough someone will commercially exploit that. But that leaves the question of what price carbon, which makes target CO2 very relevant.

    Thanks, Hank (No. 143), but I’m aware of Hansen et al., and there are numerous references to Hansen earlier in the thread. I assumed it was no coincidence that this thread started 7 April 2008, the very day Hansen et al. (v1) was submitted. (It was last revised 15 Oct 2008, version v3.)

    So, not being a climate expert, I’d like the experts to weigh in on:
    (1) Does the consensus of climate experts agree with Hansen et al.?
    (2) How does the consensus factor in the new results of Cox and Jones?
    (3) And ultimately, what should be the target atmospheric CO2?

    . . .

  45. 145
    Chris Colose says:

    Mark, I think most would agree that we cannot systematically continue to emit greenhouse gases and not expect changes in climate…and the faster the CO2 rises the more worries because rates of change matter as much as, if not more than, overall magnitudes.

    “450 ppm” seems to be a popular threshold number but you won’t get a large “consensus” on a single number like you would on other things which are well established, in part because “how much CO2 = dangerous” is in some sense a judgment call and subjective in nature. Also, given different responses from ecosystems, sea ice, mountain glaciers, etc you might get different responses from various fields of expertise. Dr. Ove Hoegh-Guldberg agrees with a 450 ppm limit for reasons of ocean acidification rather than just the warming.

    Hansen does not appear to be too off the mark with much, so if we hit 450 ppmv it’s probably not going to be a walk in the park in terms of impacts.

  46. 146
    Hank Roberts says:

    Mark, if you’re the person who cited that study to the EPA, good work. It popped up with Google.

    It’s only a month-old paper

    so won’t have made it into the consensus, but the IPCC grinds slowly as you know. It will be interesting to hear from the climate scientists once they’re thought about it.

    This came out about simultaneously:

    Amateur readers like me here of course can opine without reading, but “some guy on a blog says” isn’t useful. Interesting paper tho’. I’ll note what I found where for others who may want to read what they can.

    Science is paywalled of course, but the supplementary data page is downloadable.

    This (see para. 2) might be the abstract of it, or a summary:

    Climate change: Illuminating the modern dance of climate and CO2

    Climate and atmospheric CO2 concentrations are closely linked: the climate affects the stores of carbon on the land and in the oceans, and CO2 influences climate through the greenhouse effect. This coupled relationship between climate and CO2 will thus have a big impact on how the climate changes through the 21st Century. There is already evidence that the strength of ocean, and especially some land carbon sinks are weakening and becoming less efficient, and coupled climate-carbon models suggest that this relationship will persist into the future. The implication is that there will be more CO2 in the Earth’s atmosphere, leading to more global warming. Here, Cox and Jones ask the question “by how much, and how quickly will there be more CO2 in the atmosphere, as a result of increasing temperatures.” Using estimates from first-generation climate-carbon cycle models (C-CC models), and palaeo-climatic evidence from the Little Ice Age (the period 1500 to 1750), Cox and Jones show that the predictions from C-CC models may be too conservative and that CO2 in the atmosphere will probably increase more rapidly than the models suggest. This has implications for the development of policies that seek to stabilise atmospheric CO2 at a given level, and for the level of change which the world might experience and have to adapt to.

    Source: Cox, P. & Jones, C. (2008) Climate change: Illuminating the Modern Dance of Climate and CO2. Science, 321, 1642-1644.

    This looks like it may contain the same information– can you tell from what you know already?

  47. 147
  48. 148
    Hank Roberts says:

    PS, looking at related papers by the same authors is often useful; this one paper you mention is more interesting in the context of the previous work. It’s always useful to see if, and how often, a paper gets cited over time, and by whom.

    Clearly these authors are taken seriously, and their earlyer work has been much cited and quickly after publication. Because of that, I would expect given their earlier work that this is already being factored into the developing consensus. It’s not a sudden surprise from outside, it’s a contribution.

    So — one question for the modelers.

    What does it suggest that they are “Using estimates from first-generation climate-carbon cycle models (C-CC models)” — are these the ones policymakers have been hearing about, or is this somewhat correcting the historical record?

    That is — are there later generations of models of that sort already being considered, and are current estimates already incorporating this thinking?

    (That would be an answer to policy-wannabe folks who have contended publicly that the models from decades ago were wrong so today’s can’t be better. That’s alternate history — they were likely bidding for a policy job in an administration that didn’t happen — but still, it’ll be asked again.)

    I’ll shut up now, having figured out that the real scientists here already know about Cox and Jones’s work, and await further education.

  49. 149
    Mark Singer says:

    Yes, Hank (No. 146), I cited Cox and Jones to the EPA in the Clean Air Act rule making proceeding. I also cited it to the EPA at a public hearing in the CCS rule making proceeding. I hope this did some good.

    Thanks for the cites. No surprise that it was mentioned by the UK Climate Impacts Program, since they’re both at Exeter in the UK. I think the last cite was from work by Cox and others in 2000.

    Concerning Chris’s statement (No. 145) that,
    “. . . you won’t get a large “consensus” on a single number like you would on other things which are well established, in part because “how much CO2 = dangerous” is in some sense a judgment call and subjective in nature. ”

    Your observations about “judgment call” and “subjective in nature” are at once so true, and yet so very discouraging. But to leave this statement alone, just like that, the world could be seriously screwed. (Please pardon my blunt language.)

    With deference and all due respect, it is not that simple. The context of a discussion is critically important. Unfortunately, statements that are perfectly appropriate in one context can be quite misleading when taken out of context. Scientific discussions occur within a cultural context. Most scientists eschew value judgments and are loath to venture into the policymaking arena. But what might be impermissibly subjective in the context of a discussion of scientific research can, and often should, be considered as highly pertinent expert opinion in a policymaking discussion.

    It is really not an overstatement to say that some of the most urgent and critically important discussions in the history of humanity are now taking place concerning humanity’s response to global climate change.

    These discussions are taking place at various levels: among climate change experts (think of specialized websites, online journals, listservs and the like); among scientists who are not climate change experts but who are nevertheless involved in scientific discussions and influential publications (think of Science magazine, for example); among policy makers (they have journals and blogs; not sure what publication example to give); among ‘popular’ news media (pick your own example: Economist, NY Times, Time, etc.); and so on. The level of technical detail and scientific depth is usually inversely proportional to the breadth of the audience. Yet scientists must not ignore the importance of asserting leadership and influencing public opinion. Meanwhile, policymakers are like Harry Truman who said, “I’m tired of economists who say, ‘On the one hand … and then on the other hand.’ Send me a one-armed economist.” This is totally reasonable from the policymaker’s perspective. Policymakers can’t use and don’t want scientific exegesis. They need the expert’s best judgment. Period.

    When a message is communicated to a broader audience, the context changes. The rules of discourse change. What once was (appropriately) subjective in one context becomes (appropriately) expert opinion in another context. But, importantly, the urgent need for informed, unbiased and good faith scientific judgment now is even more critical.

    Because now you can add to this mixture the venal sowers of confusion and doubt at the behest of vested interests. Stir into the mixture politicians and lobbyists appointed to inappropriate governmental positions, who deny, deceive, delay and obstruct, gag scientists and rewrite reports to downplay risks and accentuate the limits of knowledge rather than what is known albeit inconvenient to their vested interests.

    How in the world can a system like this produce enlightened leadership and policymaking in the public interest? Well, for eight years it hasn’t, by any reasonable standard of judgment. And this is true despite the courageous example of dedicated scientists and public servants, including but not limited to James Hansen.

    Chris Colose probably didn’t expect or deserve this response and I suppose I owe him an apology. But I am earnestly pleading that climate scientists must do what they are by professional orientation loath to do.

    The President, members of Congress, policymakers within the EPA, opinion leaders in the Fourth Estate, business leaders, teachers at all levels, and, yes, the American public, urgently need to know climate scientists’ and other scientific experts’ considered best judgment about:

    — What should be humanity’s target atmospheric CO2?
    — What emissions restrictions are required, and in what timeframe, to achieve that target?
    — What are the consequences and costs of failing to take the steps required to achieve these emissions restrictions?

    These answers must be given in as clear, simple and understandable terms as the context and subject matter may allow. Be blunt.

    . . . .

    Providing current best scientific judgment is better than policy paralysis. Granted, it’s all tentative and subject to revision based on later evidence. The accumulation of evidence and scientific knowledge hopefully converges on answers with ever increasing accuracy. (Pace Thomas Kuhn: A paradigm shift is essentially a mental event that seldom adduces reduced numerical accuracy of previous scientific predictions.) This is likely to be true even for nonlinear dynamic systems because answers in a policy context are usually first order approximations that are not extrapolated so far into the future that the nonlinearities dominate the outcome. Besides, as Hansen et al point out, model uncertainties can cut both ways. But again, caveats and quibbles aside, current best scientific judgment is better than policy paralysis.

    I hope you will all heed this call to action.

    . . .

  50. 150
    makale says:

    think the the precautionary level is 280 ppm. What this preprint (and it is a preprint, not a paper) is arguing is that the sensitivity is larger than has been assumed up until now and so a “safe” temperature (a la Exeter) corresponds to a lower concentration (350 rather than 450). It does so so much that a scenario is developed to show how 350 ppm might be achieved (fig. 6). So, the preprint takes the risk quite seriously. We don’t know what it will be once it is a paper. Hansen has a habit of being right, but there may be some flaw in the analysis that a referee catches.