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Losing time, not buying time

Filed under: — raypierre @ 6 December 2010

Control of methane, soot, and other short-lived climate-forcing agents has often been described as a cheap way to "buy time" to get carbon dioxide emissions under control. But is it really?

Expectations for the outcome of the Cancun climate talks seem to be running low, and the suggestion has emerged that maybe we should forget about controlling CO2 emissions for now, and instead do something with short lived climate forcing agents like methane or soot. This is often described as "buying time" to put CO2 emissions controls into place. For example, in a recent New York Times Op-Ed, Ramanathan and Victor write:

"Reducing soot and the other short-lived pollutants would not stop global warming, but it would buy time, perhaps a few decades, for the world to put in place more costly efforts to regulate carbon dioxide." — Ramanathan and Victor

The idea that aggressive early action to control short-lived climate forcing "buys time" to do something about CO2 has often been pushed in the past, e.g. in various newsletters and press releases associated with the UNEP Atmospheric Brown Cloud program, for example

"The BC reduction proposal is not proposed as an alternative to CO2 reduction. At best, it is a short term measure to buy a decade or two of time for implementing CO2 emission reduction strategies." — Ramanathan, writing in the UNEP Black Carbon Newsletter.

To be fair, it should be acknowledged that such pleas for more attention to short-lived climate forcing are almost invariably accompanied by a salutary reminder that it is really CO2 that needs to be gotten under control, as in the quote above. Achim Steiner, writing in the same issue of the Black Carbon Newsletter writes "Paying attention to black carbon should not distract people from the real issue at hand, carbon dioxide." A similar sentiment is expressed in the Ramanathan and Victor op-ed. While emphasizing the central importance of CO2, Penner et al. argue that "…to provide short-term relief from climate warming, the short-lived compounds that induce warming need to be brought under control within a timescale of a few decades." (They also make the intriguing suggestion that doing so might provide a global experiment that could help constrain climate sensitivity.) Writing in Science, Stacy Jackson concludes that "… a focus on CO2 may prove ineffective in the near term without comparable attention to pollutants with shorter lifetimes"

All of this is well-intentioned stuff, none of it denies the central importance of CO2, and I’m sure there are many benefits to be had from reducing soot emissions sooner rather than later. Given the large agricultural component of methane emissions, keeping these emissions from growing in the face of a the need to feed a growing number of people is a serious challenge that must ultimately be met. But still, these proposals tend to convey the impression that dealing with the short-lived forcings now will in some way make it easier to deal with CO2 later, and that’s wrong. In this post, I will explain why.

To get a feel for the issues in play, we’ll first take a look at methane vs CO2. This provides a clean example, because methane has a straightforward, well-characterized warming effect which is easy to compare with that of CO2. If you’re just looking at the concentration of methane and CO2 at a given time, the methane/ CO2 equivalence is pretty easy to figure, since you can turn them both into the common currency of top-of-atmosphere radiative forcing. For example, doubling CO2 from 300 ppm to 600 ppm yields a clear-sky radiative forcing of 4.5 W/m2. Doubling methane from 1ppm to 2 ppm yields a radiative forcing of 0.8 W/m2, but since we started from such a low concentration of methane, it takes many fewer molecules of methane to double methane than to double CO2. Per molecule added, methane yields about 54 times as much radiative forcing as CO2. Note that most of this effect has nothing much to do with any special property of methane, but arises simply because the radiative forcing for most greenhouse gases is logarithmic in concentration, so you sort of get the same radiative forcing for everybody upon doubling their concentration — but if you start with somebody whose concentration is low, it takes many fewer molecules to double. That means that the CO2 equivalent of methane depends on what concentration you are starting with. If you started from a concentration of 10ppm, then the equivalence factor drops to 10. If you start out with equal amounts of methane and CO2 (300 ppm), then the equivalence factor drops further to 0.5. In that sense, methane is, intrinsically speaking, a worse greenhouse gas than CO2, though the crossover is at values that are so high they are only relevant (at most) to the Early Earth. ( I ran these calculations with the Python interface to the NCAR radiation model, provided in the Chapter 4 scripts of my book, Principles of Planetary Climate. They are done using an idealized clear-sky atmospheric profile, so the numbers are a bit different from what you’ll find in the IPCC reports, but it’s nice to have a calculation simple enough you can re-do it yourself.)

Things get a lot trickier when you try to bring time into the problem, because methane and CO2 have vastly different atmospheric lifetimes. Methane oxidizes to CO2 in about 10 years, and since we are dealing with so little methane, that extra ppm of CO2 you get after it oxidizes adds little ongoing warming. That means that the methane concentration in the atmosphere is determined by the methane emission rate averaged over the previous ten years, and the methane component of warming disappears quickly after emissions cease. In contrast, about half of CO2 emitted disappears into the ocean fairly quickly, while the other half stays in the atmosphere for thousands of years. Therefore, the atmospheric burden of CO2 in any given year is determined by the cumulative emissions going back to the beginning of the Industrial Revolution, and the warming persists for thousands of years after emissions cease. Over the long term, CO2 accumulates in the atmosphere, like mercury in the body of a fish, whereas methane does not. For this reason, it is the CO2 emissions, and the CO2 emissions alone, that determine the climate that humanity will need to live with for a time that stretches into the future at least as long as the time since the founding of the first Sumerian cities stretches into the past. The usual wimpy statement that CO2 stays in the air for "centuries" doesn’t begin to convey the far-reaching consequences of the amount of CO2 we decide to pump out in the coming several decades.

As a reminder of that, here’s a graph from the NRC Climate Stabilization Targets report (of which I was an author) summarizing how cumulative carbon emissions set the climate thermostat for the next 8000 years and more.
NRC Cumulative Carbon effect on Climate
The numbers on each curve gives the total cumulative carbon emissions (in gigatonnes) during the time when human activities continue to emit carbon. These results are based on calculations by Eby et al using the UVIC coupled carbon/climate model, and they are really just a reprise of what Dave Archer has been telling all of us for years (e.g here, here and here). It turns out that it matters little to temperature whether all the CO2 is emitted in a carbon orgy near the beginning of the fossil fuel era, or spread out over a few hundred years. It’s cumulative carbon that counts, and pretty much it is the only thing that counts. A cumulative emission of a trillion tonnes of carbon just might keep the Earth below a warming of 2ºC, in line with earlier estimates equating the European Union target warming threshold with cumulative carbon (see our Trillion Tonne post). The peak warming scales approximately linearly with cumulative emissions, and the warming you get at the peak is pretty nearly the warming you are stuck with for the next millennium, with only slight declines beyond that. We are currently about halfway to our first trillion tonnes, but given the miracles of exponential growth, we are going to get there pretty quickly if nothing changes. If you go beyond, and dump 2355 gigatonnes into the atmosphere before kicking the fossil fuel habit, then the global mean temperature will still be 3ºC warmer than pre-industrial in the year 8000. That gives plenty of time for bad stuff to happen, including deglaciation of Greenland, loss of the West Antarctic Ice Sheet, or a destabilizing PETM-type soil carbon release. Note further that these calculations were done with a model designed to have a climate sensitivity similar to the IPCC median. Therefore, even if you hold the line at a trillion tonnes, there is still about a 50% chance that warming will exceed 2ºC.

Let’s suppose, however, that we decide to go all-out on methane, and not do anything serious about CO2 for another 30 years. To keep the example simple, we’ll think of a world in which methane and CO2 are the only anthropogenic climate forcing agents. Suppose we are outrageously successful, and knock down anthropogenic methane emissions to zero, which would knock back atmospheric methane to a pre-industrial concentration of around 0.8 ppm. This yields a one-time reduction of radiative forcing of about 0.9W/m2. Because we’re dealing with fairly short-term influences which haven’t had time to involve the deep ocean, we translate this into a cooling using the median transient climate sensitivity from Table 3.1 in the NRC Climate Stabilization Targets report, rather than the higher equilibrium sensitivity. This gives us a one-time cooling of 0.4ºC. The notion of "buying time" comes from the idea that by taking out this increment of warming, you can go on emitting CO2 for longer before hitting a 2 degree danger threshold. The problem is that, once you hit that threshold with CO2, you are stuck there essentially forever, since you can’t "unemit" the CO2 with any known scalable economically feasible technology.

While we are "buying" (or frittering away) time dealing with methane, fossil-fuel CO2 emission rate, and hence cumulative emissions, continue rising at the rate of 3% per year, as they have done since 1900. By 2040, we have put another 573 gigatonnes of carbon into the atmosphere, bringing the cumulative fossil fuel total up to 965 gigatonnes. By controlling methane you have indeed kept the warming in 2040 from broaching the 2C limit, but what happens then? In order to keep the cumulative emissions below the 1 trillion tonne limit, you are faced with the daunting task of bringing the emissions rate (which by 2040 has grown to 22 gigatonnes per year) all the way to zero almost immediately. That wasn’t very helpful, was it? At that point, you’d probably like to return the time you bought and get a refund (but sorry, no refunds on sale items). More realistically, by the time you managed to halt emissions growth and bring it down to nearly zero, another half trillion tonnes or so would have accumulated in the atmosphere, committing the Earth to a yet higher level of long-term warming.

Suppose instead that you had focused all efforts on reducing the growth rate of CO2 emissions from 3% to 2%, averaged over 2010-2040, forgetting about methane until the end of that period. In this scenario, the cumulative carbon emitted up to 2040 is only 713 gigatonnes, giving more time to avoid hitting the trillion-tonne threshold. The warming from CO2 in 2040 is about 1.2C, but we have to add in another 0.4ºC because we haven’t done anything to bring down methane emissions. That brings the warming to 1.6C, which will increase further beyond 2040 as the cumulative carbon emissions approach a trillion tonnes. However, since methane responds within a decade to emissions reductions, we still get the full climate benefit of reducing methane even if the actions are deferred to 2040. The same cannot be said for deferral of action on CO2 emissions.

The following cartoon, loosely based on Eby’s calculations shows two illustrative scenarios: one in which early action is taken on methane, at the expense of allowing cumulative CO2-carbon emissions to rise to around 1.7 trillion tonnes, and another in which action on methane is delayed until 2040, allowing cumulative emissions to be held to a trillion tonnes. The curves can be diddled a bit depending on how much short term warming you get from controlling additional short-lived gases, and how much extra cumulative carbon emissions you assume goes along, but it is really hard to come up with any scenario where you come out ahead from acting early on the short-lived forcings instead of going all-out to reduce the rate of CO2 emissions.

There are a few greenhouse gases other than CO2 that have lifetimes sufficiently long to lend some urgency to their control. That would include HFC23 with a lifetime of 260 years, CFC13 with a lifetime of 640 years and SF6 with a practically unlimited lifetime. Most of the rest are more like methane than they are like CO2 (e.g HFC31 at 5 years)

Absorbing aerosols — soot, loosely speaking — have a number of complex regional effects that make it difficult to treat their climate impact on an equal footing with that of well-mixed greenhouse gases. Soot falling on snow or ice has an unambiguous warming effect, manifest particularly strongly at high latitudes and high altitudes. For airborne absorbing aerosols, though, it is hard to even know whether they have a warming or cooling effect on surface temperature, or leave it more or less unchanged. Except over high albedo surfaces, airborne aerosols mainly heat the atmosphere by direct solar absorption, at the expense of reduced solar absorption at the surface. When the shading is not too strong, the main consequence is a reduction of the convection that would ordinarily carry solar energy from the ground to the atmosphere. This profoundly influences precipitation, and the atmospheric circulation, especially in the tropics. In extreme cases, the atmospheric absorption can even shut down convection completely, leading to stabilization of the tropospheric lapse rate and a severe surface cooling, as in the Nuclear Winter limit (see also the more elementary discussion of this limit in Chapter 4 of Principles of Planetary Climate).

A further consideration is that most activities that emit soot also emit precursors to reflecting aerosols which cool the planet. It is unlikely (and probably undesirable) that one would be able to limit one without also limiting the other. Hence, the net implication of the black carbon component is probably that it will help offset some of the warming caused by eliminating sulfate aerosols. That’s good, but it’s not what you bargained for if you were expecting a cooling for your money. The main thing about soot and the stew of toxic emissions going into the Atmospheric Brown Cloud , though, is that there are compelling human health, agricultural, and regional climate reasons to eliminate them, regardless of the side effect on global temperature. These are things that need to be done regardless of the climate implications (positive or negative), just as there is a need to supply the developing world with reliable clean water. It is pointless to make an already complicated climate negotiation yet more complicated by wrapping such things into the mix. It is nonetheless worth noting that many of the things one would do to reduce soot emissions, such as substituting natural gas for coal, or burning coal in cleaner, more efficient power plants, also would tend to reduce CO2 emissions, and such double-wins are of course to be sought and pursued ardently (note Gavin’s op-ed on co-benefits of CO2 reduction).

IPCC-style Global Warming Potentials attempt to trade off radiative forcing against lifetime in a Procrustean attempt to boil all climate forcings down to a single handy-dandy number that can be used in climate treaties and national legislation. In reality, aerosol-forming emissions, short-lived greenhouse gas emissions, and CO2 emissions are separate dials, controlling very different aspects of the Earth’s climate future. CO2 emissions play a distinguished role, because they ratchet up the Earth’s thermostat. It’s a dial you can turn up, but you can’t turn it back down. CO2 is a genie you can’t put back in the bottle. Climate forcings should not be aggregated. Each category should be treated in its own right. Otherwise, there are perverse incentives to do too much too soon on short-lived forcings and too little too late on CO2.

379 Responses to “Losing time, not buying time”

  1. 1
    Russell Seitz says:

    ” as in the Nuclear Winter limit”

    Really? How realistic were the TTAPS parameter assumptions ?
    It’s hazardous to predicate natural analogs on phenomena that may not exist- like the instant appearance of homogenous stratospheric aerosols of optical depth 20.

    [Response: See my comment to Bart further down. The climate physics that leads to surface cooling is correct, even if TTAPS estimates of how much soot a nuclear war lofts are wrong. That’s why I also referenced the analytical calculation in Chapter 4 of my book, which brings out the physics with less baggage. –raypierre]

  2. 2
    ScepticMatthew says:

    Suppose instead that you had focused all efforts on reducing the growth rate of CO2 emissons from 3% to 2%,

    You make a persuasive case, but is that a policy that you advocate?

    [Response: I’m not advocating any policy. I’m just pointing out the consequences of various actions. Knowing consequences is essential to any democratic decision-making on policy. –raypierre]

  3. 3
    Andy says:

    Perhaps the folks who recommend taking baby steps before tackling carbon emissions think technology will be able to cheaply pull out carbon from the air within a few decades. This is so like a bad drug experience where the junkie thinks it will be easier to quit tomorrow than today so why not shoot up now.

  4. 4
    Chris Ho-Stuart says:

    You say: For example, doubling CO2 from 300 ppm to 600 ppm yields a clear-sky radiative forcing of 4.5 W/m2.

    The number I have usually heard is 5.35 W/m^2 per natural log; which works out to 3.7 W/m^2 per doubling. Is this a typo, or an old value from the second AR? Or is a difference between clear sky forcing and a more general forcing with cloud considered?

    [Response: That’s what you get with a clear-sky calculation with 50% relative humidity using the moist adiabat patched to an isothermal stratosphere. IPCC numbers (some of them) are based on a more complex calculation taking into account distributions of clouds and water vapor and the global variations of temperature. Here, I wanted to stick to the simplest kind of calculation, that is easiest to explain and easiest for somebody else to reproduce. I don’t think the simplifications very much affect the point on relative strength of radiative forcing for methane vs. CO2. –raypierre]

  5. 5
    Esko Pettay says:

    One huge area where sulphate aerosols don’t affect human health is open ocean. Ocean going vessels emit sulphate aerosols and produce a cooling effect. This cooling is reduced once the sulfur limits become effective in 2020-2025. At the same time CO2 emissions from shipping will keep increasing. That is an example of a well intended but potentially dangerous decision. Obviously it is necessary to reduce aerosol emissions in coastal areas to protect human health.

    How about buying time by continuing sulphate aerosol emissions in areas where they have minimal negative effects on human health or the environment. The resources thus saved should be used to reduce CO2 emissions instead.

  6. 6
    John Mashey says:

    Nice post, thanks! I think the first graph is very useful all by itself.

    Two questions:
    1) Are there plausible estimates for date ranges on each of the 5 lines w2hen one would expect Greenland to have melted? etc? That would be nice to see plotted on the same graph.

    [Response: Depends on which estimate you use. The IPCC range of committment to melting Greenland ranges from about 2°C of global mean warming to about 7°C. So the timeline is anywhere between tomorrow and a very very long time from now.–eric]

    2) It’s not instantly obvious why the bottom two lines have sharp jiggles.
    Can you say a few words about that, or point me where it explains this?
    (I’ll go read the links later, can’t right now.

    [Response: I defer to Ray on this.]

    [Response: Nobody likes the sharp jiggles, but they are there in the UVIC model, which is the only one we had access to that did a series of runs suitable for making a graph like the one I show in the post. The jiggles come from ocean circulation changes that affect the amount of cold upwelling, I think primarily in the Southern Ocean. There is some CO2 signature associated with the jiggles, but the main effect on temperature is coming from fluctuations in ocean heat storage/release. The UVIC model has a very primitive atmosphere, but a decent 3D ocean, so I wouldn’t completely discount the reality of this phenomenon. It needs to be examined in a more comprehensive ocean-atmosphere model. –raypierre]

  7. 7
    Dave Werth says:

    Since methane oxidizes to CO2 (& 2H2O) in a relatively short time then wouldn’t it be reasonable to just consider it as added atmospheric CO2? Seems like that would at least be a good first order approximation for methane’s long term effect.

    [Response: Yes, but the amount of extra CO2 from methane is very small — remember, we’re adding ppb of CH4 each year, but ppm of CO2.–eric]

  8. 8
    Jacob Mack says:

    In other words the IPCC range is far too broad for Greenland.

    Focusing on methane buys more time for more research to be funded.

  9. 9

    Jeekers, “this perfect storm is just getting more and more perfect”

  10. 10
    Hank Roberts says:

    Esko, as to delaying sulfate controls on shipping, the impact is around the port cities now but if you look at this, it’s likely to be more widespread with industrialization. Those of us in countries that industrialized slowly over a couple of centuries find it hard to imagine just how astonishingly fast industrialization is happening in Asia: Interaction of Ozone and Sulfate in Air Pollution and Climate Change By Nadine Unger — March 2006

    “… change in annual average sulfate aerosol and ozone air pollution at the Earth’s surface by 2030. There are large increases in pollution in subtropical regions, especially Asia. Over the Indian subcontinent the surface sulfate aerosol amount changes from around 400 pptv in the present day to around 2000 pptv at 2030 and the surface level ozone increases from around 35 ppbv to 60 ppbv. …”

  11. 11
    Edward Greisch says:

    A clue:

    Ignore the non-article by Ramanathan and Victor. Change the system. First do the
    Root Cause Analysis.

  12. 12
    Paul Kelly says:

    Can you do a similar analysis of carbon soot? Isn’t it a more important forcing than methane?

  13. 13
    Dave Werth says:

    Eric, yes of course, I understand that but every little bit adds up. But methane’s not worth worrying about much until we get a handle on CO2 which is the thrust of the story.

  14. 14
    GlenFergus says:

    this perfect storm is just getting more and more perfect

    Seem so RP. If there is some (slight) hope, perhaps it lies in what Aleklett’s group and others are saying about resource limits. E.g., in the first graph above, is there 4000 Gt of realistically extractable carbon to emit? They appear to think not. As I’ve said before, time for RC to address this one. It’s not going away and it does seriously affect the answer, even if it’s far from your specialty. AGW is a multidisciplinary problem.


  15. 15

    I think there a few reasons usually put forward for *also* focussing on shortlived forcings (not to the detriment of the focus on CO2):

    – anciliary health benefits (soot, ozone)
    – quicker effect on global temperature (a direct result of their shorter lifetime. This is the other side of the same coin as to why we shouldn’t forego the focus on CO2)
    – gridlock in global climate negotiations (that’s more of a pragmatic-political reasoning, assuming/hoping that the shortlived forcings will be easier to tackle and be less severely opposed, about which people disagree)
    – it will pave the way for CO2 reductions later. I’m very doubtful of that though, as I don’t see how or why. It could also backfire, as in lowering the sense of urgency to tackle CO2, esp if the temperature rise is reduced, which will naturally lower the political will to do “even more”. I.e. does short term success (in reducing global avg temp) help or hamper long term success (for which strong CO2 reductions are mandatory)? In the current political climate it may very well hamper rather than help

    Reducing shortlived warming agents (with a limited atmospheric lifetime) doesn’t reduce the slope (i.e. the long term rate of warming) very much, but rather shifts the line of temperature vs time to the right (or effectively downwards) by a little. Its benefit is constant in time.

    Reducing longlived emissions (e.g. CO2, which therefore accumulates in the atmosphere) reduces the slope (i.e. the long term rate of warming). Its benefit grows over time.
    So what’s at issue here is also whether one is more concerned about the short term or the long term effects of climate change and air pollution (health). To me, the long term climate effects that we’re at risk of committing ourselves to, are of most concern. That’s a personal judgement of course, based not only on science but also on my values and worldview.

    These issues and the op-ed by Ramanathan was also discussed at Kloor’s ( ) and at my blog ( )

  16. 16
    Calamity Jean says:

    Good point GlenFergus, “[I]s there 4000 Gt of realistically extractable carbon to emit?” Coal is already rising in price and being shipped internationally. If the cost of coal is pushed up more with restrictions on black carbon, renewables may become more attractive because of lower price. One large reason why China is working so hard to install wind and solar power is the terrible smog problem they have caused themselves by burning coal.

    It’s still going to be ugly, worldwide. Peak Coal won’t do much to save us from climate change.

  17. 17
    bratisla says:

    I would like to understand one point : you say that CH4 is much more short-lived in the atmosphere than CO2 ; that means that the CH4 \threat\ due to permafrost disappearance is mainly due to the fact that huge amounts of CH4 could be released, adding therefore a significant contribution of CO2 in the atmosphere ? Or is this \threat\ a non-significant one ?

  18. 18
    The Ville says:

    “How about buying time by continuing sulphate aerosol emissions in areas where they have minimal negative effects on human health or the environment. The resources thus saved should be used to reduce CO2 emissions instead.”

    So you want a technology on ships that allows engines to pump out sulphates, particulates etc at sea, but when in port switch on a cleaning system???

    But in any case sulphates are short lived in the atmosphere and CO2 is long lived. Which I equate to passing on the buck to future generations.

    Plus they only partially counter radiative forcing, so all you would be doing is slowing down the inevitable warming.

  19. 19
    Geoff Sherrington says:

    The historical development of global warming could easily have taken a path that there was an unexplained gap between observations and models and that the gap was due to imprecision in measurement of albedo as epitomised by soot.

    Soot would then become the demon. The logical path might well have led to the implication of GHGs, but this is a toy story and we shall never know. (For example, soot on the ocean does not seem to count for much).

    The case for CO2 as the demon is still weak. You mention above if “all the CO2 is emitted in a carbon orgy near the beginning of the fossil fuel era”. What is your take on an orgy where all the known fossil fuel reserves are burned in a year? What temperature effect would you expect then? Does this not set an upper limit to deltaT?

    I think not, because although it is thrown about that the T response to CO2 is exponential, it is less clear where it sits on the growing curve. Maybe it has already maxed out and adding more does not matter. If you are going to invoke an exponential, you have to add its properties or shape and which part of the shape is relevant.

    Also, because I mention T response to CO2, I do not concede that there is not a mix-up between the dependent variables.

    It’s all so far from settled.

    [Response: Geoff, you are confusing the past with the future. The question of what extent non-CO2 forcing matters — in explaining the 20th century temperature variations for example — becomes less and less uncertain as CO2 concentrations increase. Regarding ‘dependent variables’, please again recognize that the idea that recent CO2 increases is driven by temperature is wrong, plain and simple. If you don’t understand why, please try reading this article.

  20. 20
    tc says:

    Couldn’t the same basic argument be made in relation to REDD programs? It seems that in focusing on carbon in terrestrial ecosystems, which has long been part of the carbon cycle, REDD is diverting attention from the real long term issue of industrial emissions of carbon from fossil fuels, which have been out of the cycle for millions of years.

  21. 21

    GS 19: The case for CO2 as the demon is still weak.

    BPL: Not if you understand the physics involved, it isn’t.

  22. 22

    RS 1: ” as in the Nuclear Winter limit”
    Really? How realistic were the TTAPS parameter assumptions?

    BPL: Schneider and Thompson’s 1984 “Nuclear Autumn” paper had the plume heights too small by a factor of three. Thus less sulfate in the stratosphere and a deceptively warm Nuclear Winter. See TTAPS 1991.

    RS: It’s hazardous to predicate natural analogs on phenomena that may not exist- like the instant appearance of homogenous stratospheric aerosols of optical depth 20.

    BPL: What do you think the effect of incinerating several hundred cities would be?

    [Response: Note that my links were to the newer Robock work, but it’s irrelevant to my point whether or not any of that work is correct about the real impact of a nuclear war. Regardless of whether a real nuclear war could loft that much soot, the simulations show the basic climate physics of what happens when you put extremely large amounts of absorbing aerosol into the system. –raypierre]

  23. 23
    Keith Kloor says:

    “Let’s suppose, however, that we decide to go all-out on methane, and not do anything serious about CO2 for another 30 years.”

    Somehow, I doubt that Ramanathan and Victor are suggesting that carbon reduction efforts be put aside for three decades.

    [Response: You can put your own interpretation on what “a few decades” means. In the Black Carbon newsletter it says “one or two decades.” Other writings on this issue leave the time frame for “buying time” unspecified. Even two decades without action would be bad enough, and it’s not hard to imagine it stretching to 3 if you go down that path. In any event, my goal here is to make sure people understand what happens if pushing early on control of short lived forcings results in continued unabated emissions of CO2. I don’t think that has been made clear by advocates of early, aggressive control of the short-lived forcings. –raypierre]

    At my site, over at another thread devoted to this issue, a pretty smart commenter made this observation:

    “The fact is that political capital does not exist to implement carbon reduction policies. That simple reality can’t be wished away. The goal should therefore be to build capital which, IMO, requires time and continuous effort. Incremental success on secondary and tertiary issues will help. Success in those areas will not only build political capital but will also improve the chances for some kind of carbon reduction scheme. The reason is that if you can demonstrate, for instance, that methane reduction or whatever policy is workable, then carbon reduction doesn’t look so scary to people which lowers the political capital necessary to bring that about.”

    It’s all well and good to remind people that carbon is public enemy number one, but I think this post by raypierre (while understandably science-based) ignores one of the main rationales for focusing–temporarily, not 30 years–on those secondary climate forcings:

    It’s to “buy time” while building momentum towards the necessary political conditions to tackle carbon.

    [Response: I am not going to comment on strategies for successful international negotiations, since anything that I say would be rank amateur speculation. It is fair to point out that negotiations are going nowhere, and to look for ways to revive them. It is fair to say that getting action on carbon is hard. You could argue that any action of any type that shows international collaboration on anything related to climate will hasten the day that carbon emissions are brought under control. But none of that “buys time.” Every day that goes past without reducing the rate of CO2 emissions lost is a day irretrievably lost. But beyond that, I’m not sure why you think that there is less low-hanging fruit on CO2 than there is on methane. There is a lot of low-hanging fruit in power plant energy efficiency, and in end-user energy efficiency. And as I said at the end of the post, for soot there is a lot of room for co-benefits on reducing both soot and CO2. Much the same could be said for mercury and CO2. –raypierre]

  24. 24
    Esko Pettay says:

    The Ville@18
    “So you want a technology on ships that allows engines to pump out sulphates, particulates etc at sea, but when in port switch on a cleaning system???
    But in any case sulphates are short lived in the atmosphere and CO2 is long lived. Which I equate to passing on the buck to future generations.
    Plus they only partially counter radiative forcing, so all you would be doing is slowing down the inevitable warming.”

    Reducing CO2 emissions is the priority. But it is worrying to reduce cooling emissions while the warming emissions keep increasing. It is possible to use sulfur removing scrubbers in ships and use them only near coast. That way we could protect human lungs and still keep the cooling effect in open ocean. IMO (International Maritime Organization) treaty might reduce the ships’ cooling impact from 0.58 to 0.27 watts per square metre. The optimal solution of course is to minimize both GHG and aerosol emission. But since that is not going to happen anytime soon we may have to keep emitting these climate cooling things to avoid or delay dangerous tipping-points.
    But once again; the most important thing is to reduce CO2 emissions. Need to go now….

    Lauer, Axel et al: Assessment of Near-Future Policy Instruments for Oceangoing Shipping: Impact on Atmospheric Aerosol Burdens and the Earth’s Radiation Budget, Environmental Science and Technology, 43, 5592-5598, 2009.
    Fuglestvedt, Jan et al: Climate forcing from the transport sectors – Proceedings of the National Academy of Sciences, approved October 5, 2007,
    Fuglestvedt, Jan, et al: Shipping Emissions: From Cooling to Warming the Climate – And Reducing Impacts on Health, Environmental Science and Technology, 43: 9057-9062, 2009.

  25. 25
    Jenik Hollan says:

    John@6 2):
    “Bumps in the warming curves in the left panel are because of adjustments in ocean circulation in response to warming in this particular climate model and
    should be thought of as illustrative only”
    (from Prepublication copy, Figure S3 caption)

  26. 26
    Paul says:

    Quick ‘back of the envelope’ calculation.

    Area of global arable land 14,633,840 km2
    Mass of soil (to 1 m) approximately 13,000t/ha (1.3 million t/km2)
    Increase soil organic matter 1% (by wt) 13,000t/km2 – not too difficult
    1.9 trillion t SOM or 1.14 Trillion t C (soil organic matter is about 60% C)
    I know this is not new, but are we looking in the wrong place?

  27. 27

    #19–“Maybe [temperature response to CO2] has already maxed out and adding more does not matter.”

    And maybe you haven’t been paying attention. This has been studied rather a lot. I’m not going to argue or cite; just start with the “start here” tab at the top of the page, and keep reading.

  28. 28
    harrywr2 says:

    The Chinese have $500 billion in their nuclear power build budget.

    Staff training issues(How long does it take to create an ‘experienced’ nuclear plant operator) and various industrial capacity limitations makes it pretty close to impossible to spend that entire budget in less then 20 years.

    Finland has a similar problem. They only have 2 citizens with PHD’s in nuclear physics, both past retirement age, who is supposed to oversee the nuclear operator treaing courses?

    In most of the world, with the exception of the US nuclear,hydro and wind are already ‘cheaper’ options.

    It’s not a matter of ‘buying time’ for more expensive options. It’s buying time to build the human and industrial infrastructure to adopt ‘cheaper’ options.

    Even in the US, the NRC doesn’t have adequate staff to review nuclear license applications in a timely manner.

  29. 29
    DrCloud says:

    It seems to me that there are really two issues conflated in this interesting analysis.

    There should be no surprise that Ram is advocating controls on BC emissions, given his long focus on atmospheric aerosols.

    The question of methane control — analyzed quite nicely here — seems a bit different, however.

    There are links, certainly — have a look at forward scattering across a feed lot sometime — but the issues of mitigation as a way to buy time for CO2 emissions controls seem different. As noted, if we get our attention diverted form CO2 emissions by focusing instead on methane emissions, we’re just playing ostrich.

    On the other hand, many sources of BC aerosols are more amenable to fixing — diesel emissions, cooking fires, ships at sea seem like low-hanging fruit. Perhaps those can be tackled without diverting too much attention from the real problem.

    As noted, getting the global community to work together on something (hell, anything) that would address these problems would be a step in the right direction. So far, we’ve got mostly gridlock.

  30. 30
    harrywr2 says:

    It’s not about buying time for more ‘expensive’ options.

    It’s about buying time to build the human and industrial infrastucture to adopt cheaper options.

    The Chinese have $500 billion in their nuclear build budget. Unfortunately they only have experienced staff to run 10GW of nuclear power plant, never mind 200GW. It’s going to take 20 years for the Chinese to create enough ‘trained and experienced’ staff to run 200GW worth of nuclear reactors.

    Even in the US, the NRC has insufficient staff to process nuclear license applications in a ‘timely’ fashion.

    In Finland they only have 2 nuclear physicists, both past retirement age. Who is supposed to teach the training courses?

    The price of steam coal is over $100/tonne in Europe and Asia. Burning coal is no longer cheap.

    Coal is only cheap in the US Midwest and Rocky mountain states. All the electric utilities in the US Southeast are patiently awaiting NRC approvals.

    There isn’t a way to train a senior nuclear plant manager in less then 20 years without risking a Chernobyl.

  31. 31
    Earl Killian says:

    It looks like the numbers in this post all assume that reflective aerosols, e.g. SOx, stay at current levels. However, on the timescales used in this post, we stop burning coal and oil, one way or another (we will either stop burning them to limit CO2 or we will reach both peak coal and peak oil this century). Once the these sources of reflective aerosols are gone (and their lifetimes are short), the temperature increase should exceed the estimates presented in this post.

    [Response: You are absolutely right that the loss of reflective aerosols would add another increment to the warming, and make it harder to stay under 2C. I somewhat skirted the issue of reflective aerosols since I didn’t want to make an already complex subject yet more complex. However, I do think that one should think of absorbing aerosols and reflective aerosols as a package, all of which needs to go away for reasons independent of climate. Figuring the net side-effect of that on climate is an important task. Estimates vary widely about what that net effect will be, in part because of poor constraints on the aerosol effect on clouds. That deserves a separate post of its own. One of the more intriguing remarks in the Penner essay I cited in the post is that a major effort to control aerosols, if monitored closely, would give us a better idea of how big the aerosol effect was on 20th century climate, and therefore a better handle on climate sensitivity. –raypierre]

  32. 32

    One question, one observation.

    Question: What is a realistic estimate for fossil fuel reserves? Can total fossil fuel consumption really emit as much as 4275 GtC? Atmospheric CO2 is up from 2,175 gigatonnes to 3,000+ (i.e. up from 280 ppm to 386+ ppm). That implies emissions of 1,625+ gigatonnes of CO2, or 225 GtC. Only a twentieth, so far, of the 4275 GtC shown on the graph. Given that we are close to peak oil, are the high numbers realistic?

    [Response: In the NRC Climate Stabilization Targets report, we deliberately steered clear of taking a position on how much fossil fuel carbon remains. My own personal assessment formed from reading background material on that is that beyond the roughly trillion tonnes of carbon remaining in more or less known economically recoverable coal reserves, we don’t have much idea of how much coal is really out there. The 5000 gigatonne ballpark figure often quoted from Rogner’s review article is based on very sketch survey responses, not real geological estimates. Rutledge thinks even a trillion tonnes would be pushing it, but when I ran this by Klaus Lackner recently, Klaus thought that hydrocarbon extraction would get so good that we’d probably start running out of oxygen before we ran out of coal. One thing on the side of Rutledge is that it’s much harder to hide coal deposits, which need to be near the surface and are always on land, than it is to hide oil deposits. Hence, the chances of major undiscovered coal deposits are (so the story goes) slim. All we can really say is what the world would be like if we do burn 4000 gigatonnes of carbon (which at present rates we get to in about 2100). It could be that we just run out of coal before we get there, but banking on that is not a policy. –raypierre]

    Observation: Scientists who have used “emissions” as their independent input variable, and “temperature increase” as their dependent output variable, have conditioned us to think of “emissions” as the cause of “global warming.” But emissions of CO2 are driven by the energy technologies we choose. And global warming isn’t really the end result – the end result is climate change and marine species killed off by ocean acidification. I believe we would be better off if we set aside the “emissions –> global warming” framing, and replaced it with “Dirty energy technologies –> severe climate damage, severe marine life damage.” Then the goal becomes Protect the Climate, Protect the Oceans. And the action step becomes Shift Our Capital Budgets to Clean Energy Technologies. I think the public will respond more positively to an appeal that links technology choice to climate consequences, and asks everyone to participate in protecting the climate by shifting our energy technology base to one that is climate-safe and ocean-safe.

  33. 33


    Thanks, harry. (#29)

    I have to deal sometimes with this guy who is a real nuclear enthusiast: well-informed in certain respects, yet as totally dismissive of all other energy sources as being “too expensive” in any but a few marginal situations, as he is absolutely rah-rah about any nuclear option–no matter how “blue sky” it currently remains. (Yes, that includes fusion, and exotica yet farther out than that.)

    Water isn’t a problem for him, waste isn’t a problem for him (since it’s all going to be recycled for fuel, supposedly)–who knows, maybe a lack of human capital will be a problem he might deign to recognize. Or more likely, that the casual reader lurking might recognize.

    BTW, do you have a source I can point to on this?

  34. 34
    Aaron Lewis says:

    I have 3 questions:

    1) How will CH4 trapped in permafrost, and released as the permafrost melts affect the projected CH4 and CO2 concentrations?

    2) How will methane clathrate decompostion as the the sea bed warms affect projected CH4 and CO2 concentrations?

    3) How will biogenic CO2 and CH4 produced as the Arctic warms affect projected CH4 and CO2 concentrations?

    [Response: There are some answers to these things in the Earth System Sensitivity section of the NRC report. Basically, they are all in the “known unknowns” category. There is enough near-surface carbon to add another trillion tonnes or so, maybe more, to what humans add by fossil fuel burning. The circumstances under which it would be released are unknown, but the PETM says that such things can happen. A related question is how much would be released as methane and how much by CO2. By some estimates there is enough carbon in clathrate methane that you get significant warming even after it oxidizes to CO2. If the methane is released suddenly, you get a big warming spike, followed by a reduction after it oxidizes. If it is released slowly, you get a longer period of elevated methane concentration, accompanied by a steady buildup of CO2 as the outgassed methane is oxidized to CO2. None of this is in any climate projection yet, and the “trillion tonne” target doesn’t take into account the risks of such a release. –raypierre]

  35. 35
    Keith Kloor says:


    Thank you for the thoughtful response. I can understand the concerns about a potential loss of urgency on the CO2 problem that might result from turning the focus to short-term climate pollutants. That would be a risk. But the thrust of the NYT op-ed is probably best captured in these closing words:

    “For too long, overly ambitious global climate talks have focused on the aspects of global warming that are hardest to solve. A few more modest steps, with quick and measurable effects, are a better way to proceed.”

    Again, I will offer a link to another observation made by a commmenter at my site:

    “We are, in a way, trapped between the immovable object of political reality and the unstoppable force of nature’s timetable. I guess if I had one overarching point in all this it’s this question: What strategy is likely to bring about co2 reductions the soonest? My basic premise is that continuing to spend political capital pushing for co2 reductions now is only going to result in more failure and an even longer delay than what we’re already facing. If my premise is correct, then it makes sense to alter the strategy (not the goal) and take a more indirect approach.”

    [Response: I have no quarrel with that implication, if that is what you think the op ed is actually saying. But the implication about “buying time” is just false advertising. It also invites the sort of reaction you see in those people who go to a lot of trouble to recycle their plastic (a certainly worthy thing) but then hop in their SUV because they feel they’ve already done their bit. I don’t know why you continue to defend an aspect of the op-ed that is just demonstrably wrong on physical grounds. –raypierre]

    Lastly, let me point out that one of the best rationales (IMO) for the deeply flawed Waxman/Markey cap & trade bill was that it was a start: something to build on and to generate momentum towards stronger climate policies down the road.

    [Response: It’s important to make a start, but it’s also important that that “start” be in the right direction, and in a direction that can be scaled up later to more effective CO2 emissions reductions. Putting a price on carbon — any sort of price — is a start down that path, but it is less clear how how you get that direction out of “other things first.” The sort of thing that does set you on the right path are global power plant efficiency targets, which directly get at the CO2 problem, but also can be justified for human health co-benefits, and possibly for climate side-benefits through absorbing aerosols as well (depending on how much of that is offset by loss of reflecting aerosols). –raypierre]

  36. 36

    @ 29, Steve Johnson.

    Estimated cumulative emissions from fossil fuel use, cement production and land-use change since industrialization began are ~ 543 GT of carbon. (and counting).

    The 225 GTC you reference relates to the “airborne fraction”, with the rest being absorbed by the ocean and terrestrial systems (as you allude to in any event…). Just a nit… cheers…

  37. 37
    ScepticMatthew says:

    2, raypierre: I’m not advocating any policy.

    I forgot to add, Thank you for an informative and nicely written post.

    That looked to me like a worthy and achievable goal. Other people here do advocate or repudiate policies, and I think that what you wrote should get some discussion.
    29, harryrw: It’s about buying time to build the human and industrial infrastucture to adopt cheaper options.

    I am glad you wrote that.

  38. 38
    Ray Ladbury says:

    Keith Kloor’s commenter says: “The fact is that political capital does not exist to implement carbon reduction policies. That simple reality can’t be wished away.”

    When political reality collides with physical reality, which one do you think will win?

  39. 39
    Ray Ladbury says:

    Geoff Sherrington@19, Sorry, but WTF are you talking about? Who is bloody suggesting that temperature response to CO2 is exponential? If you can’t even be bothered to learn the science (e.g that temperature scales logarithmically for the current range of concentrations), then why should we waste our time with your opinions?

  40. 40
    Aaron Lewis says:

    Isn’t doing the anthropogenic carbon budget without including natural sources a bit like doing our household budget and leaving out the rent, car payment, tuition, food, and utilities so we can spend the full $2,000 in our bank account for beer?

    We are not students anymore. Do we really think Mother Nature is not going to collect her due? A carbon budget that does not include at least a place holder/estimate for natural sources does not inform the policy. Instead it gives an excuse for delay.

  41. 41
    Hank Roberts says:

    Ray, are you sure you _want_ to encourage GS to elaborate? He will, you know.

    SM: harryrw wrote “It’s about buying time” — Ray followed up inline above: “the implication about ‘buying time’ is just false advertising.” Still happy with harryrw writing that, or will you help explain to him why Ray’s right?

    My guess: those “buying time” are buying extended time for fossil fuel emission, and are borrowing from the future to do it. It’s a bad choice.
    Always ask them who they’re really buying time for. I’d call it weasel wording but I have more respect for mustelids. It’s lawyer wording.

  42. 42
    Keith Kloor says:

    “I don’t know why you continue to defend an aspect of the op-ed that is just demonstrably wrong on physical grounds.”

    I’m actually not comfortable at all defending that aspect of the op-ed, as it was the one thing that raised a red flag for me:

    I also think the op-ed authors erred in not using more precise language and for leaving the impression that the CO2-first approach could be downgraded for 20 or 30 years.

    But I don’t think that’s what they meant to convey (it would be nice if at least one of them participated in this discussion), because they’re also saying, in my reading, that the world could “put in place more costly efforts to regulate carbon dioxide” during the “few decades” it takes to build the necessary political space and momentum– via the “more modest steps” they suggest.

    [Response: Keith, I’ll leave it to you to be the psychic and tell me what they meant to say. I’m only interested in what they wrote, and even more than that, what conclusions people may draw from what they wrote. –raypierre]

    Thus, I think a more relevant question to ask this: would it be acceptable to “buy time” with a decade (as opposed to 20 or 30 years) focused on the more modest steps outlined in their op-ed? Again, the premise being that this helps establish the political framework and momentum to then segue into the harder, carbon mitigation actions.

    [Response: And the right answer would still be that “other things first” even for just a decade, would not buy time in any sense. The longer you wait before starting in on reducing the growth rate of CO2 emissions, the hard it will be to keep the Earth’s climate change within reasonable bounds. If you want to make the politically based argument that any international agreement bearing on climate breaks the ice and gets things moving (a dubious argument, in my view, but that’s just an amateur opinion) you could make that argument just as well for things that have co-benefits in reducing the growth rate of CO2 emissions — as some soot proposals do. –raypierre]

    What about that timeline?

  43. 43
    Andy says:

    Re: Kloor’s posts. I would guess that controlling black carbon would be easier because people can see it and it makes it hard to breathe. I’ve read a number of interviews where Republican legislators (i.e. global warming deniers) state that reducing harmful air pollutants is where we should focus our attentions as a means of dodging the global warming thing.

    The U.S. EPA’s best tool for reducing carbon dioxide emissions maybe to further restrict emissions of black carbon, sulfur dioxide, mercury, ozone generating chemicals such as nitrogen oxides and other pollutants. Reductions in these emissions means that power generation from natural gas, solar and wind becomes more competitive with coal, thus a reduction in carbon dioxide is acheived.

    I see this as kind of a “don’t throw me into the briar patch” sort of situation for the U.S. Say the Democrats give in on carbon dioxide regulation in return for more stringent controls of “traditional” pollutants such as the above list. The end result may well be strong reductions in carbon dioxide through less use of coal and higher vehicle mileage requirements. For a while this could work. Though a downside would be that tackling the next fossil fuel addiction, natural gas (methane) maybe all the harder.

    [Response: This sort of thing would at least get things moving in the right direction, namely a reduction in the growth rate of CO2 emissions. Putting on the hat of a cynic, I see your argument as reason to keep soot out of the climate negotiations, because the more certain congresspeople realize that controlling soot might also help control CO2, the less they are likely to want to do anything about soot — even if they would otherwise think it is a good thing to control soot. –raypierre]

  44. 44
    Karen Street says:

    Tangential question: according to EPA, Greenhouse Gas Emissions Reporting from the Petroleum and Natural Gas Industry (pdf), methane release associated with petroleum and natural gas are higher than previously reported.

    Where do I go to see an official summary in English, including significance?

  45. 45
    Ray Ladbury says:

    About the only advantage I can see to this proposal is that it would give legislative bodies and governments some experience in ACTUALLY DOING SOMETHING. It might have been a great idea 30 years ago when we actually had time to play with. However, as we frittered away that time by debating about science established over a century ago, we do not now have such luxuries. It is time to act or devil take the hindmost generation–our grandchildren.

  46. 46
    Hank Roberts says:

    > soot

    Soot from wood and dung fires near the Himalayas: “08 Feb 2010: Black Soot is Main Cause Of Himalayan Glacier Melt …. Aerosols and black carbon from air pollution may be responsible for as much as 90 percent of the melting taking place in Himalayan glaciers ….”

    Black carbon is from fossil fuel (diesel engines); wood/dung cooking fires.
    So it’s counterproductive to do anything that continues use of fossil fuel.

    Fortunately there’s an option to increase local biomass use to burn cleaner, already being funded:

    Reducing soot will “buy time” for the Himalaya glaciers, unless it doesn’t — it’s not clear overall what soot does to global climate

    Clouds: “Global model studies of soot effects on clouds …. Most … indicate that the net cloud response to absorbing particles is cooling. This suggests the need for caution when pursuing mitigation of soot in order to cool climate.”
    “New cookstoves, which while still burning biomass (wood, crop waste, dried animal dung) reduce indoor air pollution by 80%, reduce fuel usage by 50% and decrease cooking times by 40%.” — and don’t need to be trucked in.

  47. 47
    Martin Vermeer says:

    When political reality collides with physical reality, which one do you think will win?

    That’s a trick question, right?

  48. 48
    Hank Roberts says:

    What to control first? high-sulfur fossil fuel; I know high sulfur diesel is still burned in heating oil furnaces in the USA. Most is used in transportation.

    “… solar-absorption efficiency was positively correlated with the ratio of black carbon to sulphate. Furthermore, we show that fossil-fuel-dominated black-carbon plumes were approximately 100% more efficient warming agents than biomass-burning-dominated plumes. We suggest that climate-change-mitigation policies should aim at reducing fossil-fuel black-carbon emissions, together with the atmospheric ratio of black carbon to sulphate.”
    Letter abstract

    Nature Geoscience 3, 542 – 545 (2010)
    Published online: 25 July 2010 | doi:10.1038/ngeo918
    Warming influenced by the ratio of black carbon to sulphate and the black-carbon source

  49. 49

    Two decades is the amount of time we need to buy to break even, as a rational carbon policy should have set in two decades ago. The way I see it is that the time we “buy” just compensates for our past foolishness, not for our future foolishness.

    If I may be allowed a moment of armchair economics…

    The foot-draggers say we should delay policy change for as long as possible because we will be wealthier in the future and better able to afford to act. The problem is twofold: 1) as long “as possible” may already have expired and 2) even in the absence of climate impacts, conditions have changed enough that future growth in per capita wealth along the model of the last 200 years is in no way guaranteed.

    The reason to delay impacts for as long as possible (even at the expense of the long term outcome) is the flip side of this argument. At some point climate change may well become so severe that per capita wealth will begin a long term, accelerating downturn. At that point, no mitigation at all will be affordable. This argues for mitigation as early as possible because we can’t afford it once it’s too late; but it also argues for mitigation whose effects are as early as possible.

  50. 50
    Edward Greisch says:

    29 harrywr2: See and

    The new factory built reactors are so much simpler to operate that the trained operators and physicists are not needed. Also look at the Navy’s nuclear powered submarine fleet. Their operator requirements are much lower than what you quote. Hyperion is planning to offer reactors to power commercial ships.

    The NRC will only have to process 1 application for 4000 reactors to be built by Hyperion.

    Chernobyl is not possible in the US because:
    1. We do not have any reactors that are that primitive.
    2. American reactors have containment buildings that are pressure vessels.
    3. Coal fired power plants spew as much radiation in only a few years as Chernobyl did. Coal contains uranium, thorium and arsenic. The “disaster” was mostly hype.