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Yet more aerosols: Comment on Shindell and Faluvegi

Guest post from Drew Shindell, NASA GISS

Our recent paper “Climate response to regional radiative forcing during the twentieth century”, has generated some interesting discussion (some of it very ‘interesting’ indeed). So this post is an attempt to give a better context to the methods and implications of the study.

First, some history. Global model responses to aerosols have been looked at since the early 1990s (Taylor and Penner, 1994; Mitchell et al, 1995, Santer et al, 1995). These studies and subsequent ones have shown that when a forcing is spatially concentrated, the regional climate response does not closely follow the spatial pattern of the forcing. These two figures show an example of that from two recent models (GISS ModelE and GFDL). Despite extremely large localized forcings over the industrialized areas, the climate response is spread out much more broadly in the zonal direction. Similarly, although forcing is extremely large over India and Southeast Asia, those areas show only very weak warming. In particular, the Arctic climate response can be quite different from what the local forcing would imply.

Figure 1. Ensemble mean annual average 1880–2003 radiative forcing (Fs, the top-of-the-atmosphere forcing with fixed SSTs and sea-ice, left column) and the surface air temperature (SAT) response (ºC, local linear trends, right column) from 5-member ensemble simulations driven by tropospheric aerosols including their direct radiative effect only (top row) and both their direct and indirect (via cloud cover) effects (bottom row). [Shindell et al., 2007].

Figure 2. Annual mean-adjusted radiative forcing (W/m2) between years 2100 and 2000 from tropospheric aerosols and ozone changes simulated under an A1B scenario (top) and annual surface air temperature change (°K) from the 2000s (years 2001–2010) to the 2090s (years 2091–2100) due to those same short-lived species in the GFDL model [Levy et al., 2008].

In our paper, we wanted to characterize the geographic forcing/response relationship more clearly. Prior studies had looked at particular scenarios or time periods when forcings were typically changing over much of the world (albeit most strongly in certain regions). So we put idealized forcings from GHGs, aerosols, and ozone in the tropics, mid-latitudes and polar regions to see what would happen. The results showed that the temperature response in the tropics, like the global mean, is only mildly sensitive to the location of forcing. That is, you get an enhanced tropical response to forcing in the Northern Hemisphere extratropics (where you can activate strong positive feedbacks like snow/ice albedo), but the enhancement is only 40-50% over that found with forcings applied elsewhere. In contrast, the extratropical zones are much, much more sensitive to local radiative forcing than to tropical forcing or to forcing in the opposite hemisphere. So to quote from the paper

“global and tropical mean temperature trends during the twentieth century would have been quite similar if short-lived-species radiative forcing had been distributed homogeneously rather than being concentrated in the northern extratropics. Regional concentration of forcing contributed to the departures of Northern Hemisphere mid-latitude and Arctic temperature trends from the global or Southern Hemisphere extratopical means, however.”

We then used the regional forcing/response relationships to derive the aerosol forcing needed to explain the observed global and regional temperature trends. Our results have a substantial uncertainty range which arises primarily from the influence of unforced, internal variability. The global mean preindustrial to present-day aerosol forcing we calculate is -1.31 +- 0.52 W/m2, consistent with the IPCC AR4 range of -0.6 to -2.4 W/m2.

We also estimated aerosol forcing for the tropics and Northern Hemisphere mid-latitudes for several time periods, and compared with historical emissions estimates to tie the forcings to sulfate or black carbon (BC) aerosols when possible. The results show, for example, that nearly all CMIP3 models require strong aerosol cooling at Northern Hemisphere mid-latitudes during the 1931-1975 period to capture both the global mean trends and the NH mid-latitude versus Southern Hemisphere extratropics temperature trends (many CMIP3 models had both sulfate and BC, but not necessarily the correct amounts as modeling their forcing directly is quite uncertain, hence we compared the CMIP3 models’ responses to non-aerosol forcings with observations to see how well they could do without aerosols). During the last 3 decades (1976-2007), the best fit to the temperature responses in the models require negative forcing from tropical aerosols but positive forcing from Northern Hemisphere mid-latitude aerosols. It’s the latter, the positive Northern Hemisphere mid-latitude aerosol forcing that leads to the strong warming impact on the Arctic as well, as the Arctic responds to mid-latitude and local forcing, but the local forcing is primarily driven by mid-latitude emissions that are transported to the Arctic, so the overall climate response ends up being closely tied to Northern Hemisphere mid-latitude emissions. Given the strong sensitivity of the Northern Hemisphere extratropical zones to aerosol forcing, it’s then understandable that those areas could have cooled during the mid 20th century when the aerosol forcing we calculate was substantially larger than greenhouse gas forcing (in absolute magnitude).

A big uncertainty is still the influence of unforced internal variability, which we estimated from coupled ocean-atmosphere climate runs. Though that contribution is large, it was still not large enough to account for many of the mid-latitude and Arctic temperature trends without including aerosol forcing. For many cases, the influence of aerosols and internal variability were comparable in size. Though the influence of internal variability leads to a substantial uncertainty range in our results, they are nonetheless useful as other techniques of estimating aerosol forcing of climate have comparably large or larger uncertainties. These include ‘forward’ modeling from emissions to concentration to optical properties (e.g. see [Schulz et al., 2006]), and various estimates based at least in part on satellite observations (see this previous post).

Some of the most interesting conclusions of the study include those relating to the Arctic. For example, we estimate that black carbon contributed 0.9 +/- 0.5ºC to 1890-2007 Arctic warming (which has been 1.9ºC total), making BC potentially a very large fraction of the overall warming there. We also estimated that aerosols in total contributed 1.1 +/- 0.8ºC to the 1976-2007 Arctic warming. This latter aerosol contribution to Arctic warming results from both increasing BC and decreasing sulfate, and as both were happening at once their contributions cannot be easily separated (unlike several earlier time periods we analyzed, when one increased while the other remained fairly constant). Though the uncertainty ranges are quite large, it can be useful to remember that the 95% confidence level conventionally used by scientists is not the only criteria that may be of interest. As the total observed Arctic warming during 1976-2007 was 1.5 +/- 0.3ºC, our results can be portrayed in many ways: there is about a 95% chance that aerosols contributed at least 15% to net Arctic warming over the past 3 decades, there is a 50% chance that they contributed about 70% or more, etc.

It’s also worth considering how to interpret the effects of decreasing sulfate during the past 3 decades. To try to make sure that the complex role of aerosols wouldn’t be misunderstood, when referring to the recent warming due to aerosols at Northern Hemisphere mid-latitudes and in the Arctic, we stated in the conclusions of the paper:

“much of this warming may stem from the unintended consequences of clean-air policies that have greatly decreased sulfate precursor emissions from North America and Europe (reducing the sulfate masking of greenhouse warming) and from large increases in Asian black carbon emissions.”

So it is incorrect, or at least quite incomplete, to say that that controls on air pollution such as those created under the Clean Air Act in the US have caused the recent warming. In the absence of increasing greenhouse gases, our large historical emissions of sulfate precursors would have led to substantial cooling from sulfate, and the subsequent reduction in emissions would have brought temperatures back towards their previous level. So reduced sulfate does not cause warming in an absolute sense, only relative warming compared to a time when emissions were larger. Over the mid-20th century, sulfate precursor emissions appear to have been so large that they more then compensated for greenhouse gases, leading to a slight cooling in the Northern Hemisphere. During the last 3 decades, the reduction in sulfate has reversed that cooling, and allowed the effects of greenhouse gases to clearly show. In addition, black carbon aerosols lead to warming, and these have increased during the last 3 decades.

For an analogy, picture a reservoir. Say that around the 1930s, rainfall into the watershed supplying the reservoir began to increase. However, around the same time, a leak developed in the dam. The lake level stayed fairly constant as the rainfall increased at about the same rate the leak grew over the next few decades. Finally, the leak was patched (in the early 70s). Over the next few decades, the lake level increased rapidly. Now, what’s the cause of that increase? Is it fair to say that lake level went up because the leak was fixed? Remember that if the rainfall hadn’t been steadily increasing, then the leak would have led to a drop in lake levels whereas fixing it would have brought the levels back to normal. However, it’s also incomplete to ignore the leak, because then it seems puzzling that the lake levels were flat despite the increased rain during the first few decades and that, were you to compare the increased rain with the lake level rise, you’d find the rise was more rapid during the past three decades than you could explain by the rain changes during that period. You need both factors to understand what happened, as you need both greenhouse gases and aerosols to explain the surface temperature observations (and the situation is more complex than this simple analogy due to the presence of both cooling and warming types of aerosols).

Hence the implication should not be that cleaning up the air causes warming, but that air pollution plays a substantial role in climate, and we can better understand regional climate changes during the past by taking this into account. Economists have argued that inclusion of a broader array of climate forcing agents leads to more cost-effective strategies to mitigate climate change (e.g. [O’Neill, 2003]), so that taking into account the large impact of air pollution and its ancillary effects on human and ecosystem health may also lead to better solutions for climate change.

114 Responses to “Yet more aerosols: Comment on Shindell and Faluvegi”

  1. 51
    Mark says:

    Walt, there’ll be less land. There’ll be less food (unless we go back to eating tree roots). We’ll be worse off.

    Otherwise, prove there’ll be more land. Prove that there’ll be more food.

    The Oracle Says: naughty cap

    I suggest you wear it.

  2. 52
    Mark says:

    “Many people are genuinely confused and simply don’t know who to trust.”

    Yet somehow they trust the people who make trillions off the status quo when they say that the status quo is fine as much as the people who make a wage off their research when they say their research is correct.

    This is willful confusion, if it is confusion at all.

  3. 53
    Petro says:

    Is it possible to just throw away John H and Walt Bennett from this thread? There are ample places their can present their BS. We had a nice scientific discussion about to start until John H came with his hackneyed agenda. You do not need to kill them, just move these comments to the end of some other discussion, which have already degenerated.

  4. 54

    Walt Bennett writes:

    We have to find a common dialog and learn to understand each others’ point of view. Most of you in here have NO INTEREST AT ALL in that.

    That’s because we don’t think w DO have to “find a common dialog and learn to understand each others’ point of view.” I understand the denier point of view perfectly well, but I have no desire or interest in trying to dialog with them. Why should I? Their minds are made up. I’m interested in talking to people who are on the fence, or who are trying to learn, but trying to find a “common dialog” with pseudoscience crackpots is a waste of time. What “common dialog” can there be with someone who thinks the greenhouse effect violates the second law of thermodynamics, or that global warming is caused by the sun? Should I look for common dialog and understanding with people who reject relativity or think Apollo 11 never landed on the Moon? Why?

  5. 55

    John H. writes:

    There is nothing in the IPCC reports, or other science that established a threat that Hartman claimed, “if we don’t act fast on AGW the earth will become uninhabitable for humans and all other living creatures”.

    I guess it depends on how you define “acting fast.” If we don’t act fast, AGW will get much worse, and if it gets bad enough, it could trip geophysical feedbacks which would amplify the problem to the point where we couldn’t solve it. If that happens, and the Earth’s mean global annual temperature is doomed to rise 6 K or more, then it may indeed become uninhabitable for humans. At +6 K ocean chemical changes may lead it to bleed out hydrogen sulfide, and if that happens we’re all dead.

    That is reckless embellishment at best. Making things up IMO.

    See above.

    None of the various sea rise-100 million climate refugee etc. scenarios spell out what Hartman made up.

    If half of Florida and all of Bangladesh are under water a hundred years from now, then there will be AT LEAST 100 million climate refugees. That’s not made up either.

    His MWP message is stale and wrong too.
    The MWP has been firmly established as a global warming, according to data published by 695 individual scientists from 405 separate research institutions in 40 different countries.

    Care to cite a source, preferable from a peer-reviewed science journal? Your list is impressive but lacks any specifics. As I understand the MWP was NOT global and was certainly not warmer than today.

    This does cause severe problems with the hockey stick theory

    It might if it were true, but it isn’t.

    and by extension AGW.

    Except that AGW doesn’t depend on the “hockey stick,” whatever you’ve read in climate denier blogs.

  6. 56
    ccpo says:

    ***# John H. Says:
    22 April 2009 at 5:30 PM

    In the mean time why can’t we have honest conversations?***

    To be honest, I don’t think there are any honest skeptics, professional or not. I must explain, however, my concept of honesty. I’m not a fan of the PC speak, nor of extreme complexity. Simpler is often better. So, wrt climate denialists I don’t care about the **degree** of the lie, only that it exists. Why? Because we know too much about the sources of the lies, disinformation, etc. We know it was largely the result of the Marshall Institute and Exxon. These are facts. Virtually all of the “skepticism” exists because of these two organizations, whose sole goal, also fully documented, has been to not even prove the science wrong, but to simply sow doubt to slow down and/or prevent action to protect political and/or financial interests.

    Thus, all this “doubt” is manufactured. There is no room for consequential doubt in the science. To wit:

    Virtually all of the anti-AGW “science” is traceable to the two organizations and their many proxies. Given this, pretty much all “skeptic”-ism is a result of ideology or lies. (Self-delusion, btw, is just another kind of lie.) It truly is simple: all of you are either too tied to your ideology, too tied to your delusions, too tied to your fossil fuel-abetted incomes (oil touches everything, no?) or are plain old brainwashed.

    The proof is simple: How many studies are there supporting AGW? How many thousands? How many supporting anti-AGW? A handful? A couple handfuls?

    How many climate scientists support AGW? 97% according to the most recent poll.


    Simple fact. According to the link above wrt meeting the 2C target: “86% of the experts told the survey they did not think it would be achieved.”

    So, not only do they support the basic science, they are expecting disaster at a clip of 86% of those polled!

    But let’s let the science talk: Can you name me even five scientifically sound, peer reviewed papers that are unambiguously supportive of an anti-AGW stance?

    The only survey of science papers I know of done back in ’03 or ’04 found the ration of pro/anti at 1,000:1. I see zero evidence this has changed.

    There was another review done last year that found 90% of all anti-AGW books were linked to the network referenced above of political activism.

    Then, there are the global changes being seen. The trends, the changes, the extinctions. And, this is where the science starts, not what it falls back on when challenged. Or are you not aware of scientific method? Observe > hypothesize > test > record > analyze > conclude. It doesn’t start with “assume” nor does it start with “model.” It starts with observe. I literally have seen no comments, no papers, nothing, that started with a simple observation of the natural world from skeptics. They are all starting from the point of attempting to support the presupposition that AGW science is wrong.

    If there are any out there, do share.

    And what of the Arctic? If not AGW, then what? What of methane concentrations rising as we start finding evidence of increasing clathrate releases, permafrost thawing and thermokarst lakes?

    What of the global temps? What of the habitat changes? What of changes in rainfall amounts and patterns? What of the strength of cyclones/hurricanes? What of earlier springs? What of extreme weather events? What of models understating the speed, depth and breadth of changes? What of the scientists scared by what they see?

    The evidence is so vastly overwhelming in its totality that to claim it is weak and uncertain is ridiculous. You are conflating uncertainty in specifics with uncertainty in the overall weight of the evidence. In other words, you are lying. (You are free to choose the form of lie you wish to be labeled with, but know this: it is a lie of some sort.)

    ***There is nothing in the IPCC reports, or other science that established a threat that Hartman claimed, “if we don’t act fast on AGW the earth will become uninhabitable for humans and all other living creatures”.***

    That is a lie. The possibility of rapid climate change of 7+C in as little as a decade is known. The possibility of a world 6+ degrees warmer is well known. Both of these could so disrupt carrying capacity so as to knock humanity off the the face of the planet, and definitely would render our current civilization a ruin.

    If you find some use in trying to say the person should have made a distinction between all dying and virtually all dying and civilization being rent asunder, go ahead, but you’re playing games. (Another form of lie.)

    Re: hurricanes. You’re using findings based on science and measurements that are at best more than ten years old? This is just another form of lie.

    Dead zones: you have proof they aren’t?

    Do you really think there won’t be extinctions when there already are?

    I tire of your childish rhetoric. You are taking a simple fact of science and warping it into a huge lie: Uncertainty in facts and the inability to establish 100% causation = not possible, not happening.

    You are also holding all AGW-concerned people to the standard of science. This is a science blog, but not all posters are scientists. It’s the same stupidity visited on AGW activists about Gore. Gore said! 1. Who cares what Gore said about climate science? He’s not a scientist. 2. He can say anything he wants as a private citizen.

    I, as a non-scientist, am free to suppose, to guess, to assume, to extrapolate without having to prove anything at all. In fact, if I fully believe a danger exists, I am ethically and morally bound to speak out. The caveat being I must be truthful.

    E.g., after IPCC 4 came out, I, non-scientist, immediately concluded A. ice melt was much higher than stated, B. Arctic melt and Antarctic melt was higher than stated, C. that SLR was accelerating and that AGW in general was moving much, much faster than stated.

    How was I able to do this? Simple: real world observations and IPCC4 didn’t match up.

    But, you see, I CAN do this because I am not bound by the same constraints as scientists. For you to pretend, in order to support your agenda of do-nothing, that I and all other activists are constrained by scientific rules and 100% certainty is a gross distortion of reality. It is nothing more than a tactic aimed at slowing reaction to AGW. That is, it’s a lie.

    ***There’s nothing wrong with my skepticism, attitude or approach.***

    Oh, there very much is. See above.

    ***Anyone following the work of skeptics scientists could hardly call it limited or narrowly focused.***

    Because you assert it? First, they produce practically no science of their own. Why? (Rhetorical.) There is nothing for them to produce. They prefer to spend their time, like W. Soon, pretending to debunk the actual science being done.

    Show us the vast work of anti-AGW science. Please, set up a web page that lists the vast array of this science, then another with the AGW-supporting science. I dare you.

    ***ccpo says they “never deal with Arctic sea ice”?

    Huh? Well that’s ccpo making things up.
    How can it not be known that Arctic sea ice is a regular, well monitored topic of research and discussion?***

    By skeptics? Show me their research.

    And that it’s about to reach the 1979-2002 average.

    Now that’s just another huge lie. It’s not terribly important for record keeping what the ice does on the up or down slope. You are pulling the same slight of hand G. Will pulled a few months back. Did it get anywhere near the average at its highest extent? No. The peak of extent was one of the lowest in the record.

    Why lie by omission?

    Antarctic sea ice is more stable and as a continent is growing.

    And, if true, is one of the least relevant metrics you could consider. Why do you emphasize that, emphasize the extent of Arctic ice at a non-peak period, but not mention at all that Arctic ice **mass** is down 80%? Or that the lowest 5 extents have all been in the last five years? Or that old ice is down to around 30%? You know… issues that actually MATTER.

    I don’t find you an honest opponent. This is not a surprise, obviously.


    reCAPTCH: its epcot
    I.e., fantasy. How very true, eh, Mr. H.?

  7. 57
    Walt Bennett says:

    Re: #53

    Bart (and by proxy all others who commented on my comment):

    I am not suggesting some sort of equalized discussion with deniers; I am saying that skepticism is real, necessary and more reality based than the views of many “warmers”.

    I am saying that the fool is the person who thinks he knows all he needs to know.

    Mark: there will likely be less total land in 2100; I asked, will there be more or less LIVABLE land? You don’t know and you can’t know.

    That’s the sort of skepticism I am referring to. Nothing “infantile” about it. I fully accept that we are warming and will continue to warm and that there will be significant change resulting from that.

    And I hardly think a comment here and a comment there warrant any sort of “take away his keyboard!” response. What are you so afraid of?

    Anyway, we can have a nice robust discussion about this at

    I welcome all views.

  8. 58
    steve says:

    ref #55 “Dead zones: you have proof they aren’t?”

    The dead zones, at least in the Gulf of Mexico, are not a particularily recent problem. I recall reading about these many years, possibly decades, ago as a result of the run off of fertilization occuring in the Mississippi basin. I see no reason to believe the fertilization has decreased and therefor no reason to believe the cause as understood then is not the cause now. Perhaps it has been disproven that this was actually the cause and I am unaware of it?

  9. 59
    Rando says:

    Lots of bovine affluvium in this thread, which is typical. I’d bet more than half (three quarters?) of the comments and responses to comments are irrelevant to the topic introduced in the lead-in discussion.

    [Response: Indeed. We’re going to try and get back on topic and try and control the noise. So people who want to argue about policy can do it somewhere else. Comments are either on topic or deleted. – gavin]

  10. 60
    dhogaza says:

    Perhaps it has been disproven that this was actually the cause and I am unaware of it?

    Or perhaps the scientist in question, an oceanographer at Oregon State University before joining the Obama administration, was talking of the intermittent dead zones being studied off the *Oregon* coast, not the gulf.

    Changes in the timing of upwelling off the Oregon coast has been linked to global warming.

  11. 61

    I would say they produce high ejecta mass eruptions of bovine effluvium. I would not say it is an aerosol, but it sure does have a sort of stench in the cloud of obfuscation they seem to be producing that could fog the unsuspecting readers vision.

    I am generally against censorship, but the relevance level needs to be higher. John H., Walt Bennett and Todd Bandrowsky seem to mostly distract and eat up valuable time and space with obtuse arguments (if they write with relevance it would be a different story). It’s Red Herring day every day with these guys. They, like others, may also be trying to get the banner of RC banned me so they can whine in the denialosphere? They distractions sort of remind me of Lomborg:

    and the ‘do you beat your wife (yes/no)’ poll

    Back to aerosols, I started an item on aerosols, relevant comments and criticisms welcome.

  12. 62
    RickA says:

    Could someone comment on my earlier comment #23. It seems to me that this study holds out the possiblity that all or most of the total artic warming is due to black carbon – and not to CO2. I believe this is directly on topic and would appreciate some thoughts on my observation.

  13. 63
    MS says:

    I do have a question, which is hopefully on topic.
    Discussing the effects of aerosols on warming and the decrease in temperature in the 1940’s.
    I have been wondering if the World War had any (maybee very small) part in this. Could the dust and chemicals have any measurable effect on regional or global climate for probably just a short period?

  14. 64
    Walt Bennett says:

    My link has been wrong; it was missing the “s” in “blogspot” and directed to a religious site.

    The link should work now:

  15. 65
    Lawrence Brown says:

    This is sad. The contribution by Drew is very interesting. Check out the subtle interplay between the large increases in black carbon from Asia and the reduction in sulfate emissions and the effects in mid latitudes of the northern hemisphere and the arctic, yet we’ve been bombarded with a lot of OT from the ususal suspects. WB hasn’t once mentioned aerosols in his numerous posts on this thread. Not one time!Is it possible to use his words, that he has NO INTEREST AT ALL in this topic. This post deserves better.

  16. 66
    Mark says:

    “It seems to me that this study holds out the possiblity that all or most of the total artic warming is due to black carbon – and not to CO2.”

    So just because *some* of the warming *can* be attributed to black carbon, you decide that there’s a possibility that ALL or most of it is.


  17. 67
    Adam Gallon says:

    Like all of the climate models, there is a huge temperature range here, that may, or may not, be attributed to the elements being modelled.
    Once again, the debate is based around the models and the question, again, is “How reliable are the models in predicting the future”?
    Taking the GISS Model E predictions as of 2003, current global mean temperatures are .15C below those predicted by the model.
    The models are not proving to be very good at prediction, they may be “tweakable” to give a reasonable representation of the past, but that isn’t of much use to us if they can’t predict the future with a reasonable degree of accuracy.
    This quote gives me problems.
    ” During the last 3 decades (1976-2007), the best fit to the temperature responses in the models require negative forcing from tropical aerosols but positive forcing from Northern Hemisphere mid-latitude aerosols”
    So we have both +ve & -ve forcings needed to get the models to fit the measured temperatures and the same aerosols are supposed to do this?

  18. 68

    Response to RickA at #62. Yes, the paper is saying that the immediate cause of most of the Arctic warming is the impact of aerosols and black carbon.

    From the paper: (page 298) ” During 1976-2007, we estimate that aerosols contributed 1:09 +/- 0:81 C to the observed Arctic surface temperature increase of 1:48 +/- 0:28 C.”

    The paper looks at regional response to greenhouse, and CO2 in particular, on page 296; and concludes that the regional response to CO2 is similar to the response for other forcings. That’s not a comment on total magnitude, but on whether there is any localized amplification of the global CO2 effect. There isn’t… and that implies the CO2 impact should be about the same in the Arctic as anywhere else.

    The Arctic is warming much more rapidly than the global mean, according to the paper, because of a local aerosol and black carbon effect that is locally larger than the underlying global greenhouse driven trend.

    Disclaimer. I’m not a climatologist… I’m just reading the paper as best I can.

  19. 69
    David B. Benson says:

    RickA (62) — I think
    Accuweather Climate Blog
    or else maybe
    has a recent thread about an attribution study regarding black carbon in the Arctic. Both sites are linked on the sidebar.

  20. 70
    steve says:

    ref #60 dhogaza thank you, apparently I was off topic from even the off topic topic

  21. 71
    RichardC says:

    The temporary dip in temperatures around the middle of the 20th century was substantially(?) caused by aerosols. What is the current aerosol release compared to back then? Is this period a second aerosol dip – though a degree F warmer? If one counts the late 19th century, this could even be the third dip. If so, interestingly enough they are all approximately 50 years apart and 1F higher each time.

  22. 72
    Ike Solem says:

    From the IPCC FAR, Chpt 2:

    Aerosol effects on radiative forcing are divided into four categories:

    1) direct effect – “the mechanism by which aerosols scatter and absorb shortwave and longwave radiation, thereby altering the radiative balance of the Earth-atmosphere system.”

    2) cloud albedo effect – “the microphysically induced effect on the cloud droplet number concentration and hence the cloud droplet size, with the liquid water content held fixed.”

    3) cloud lifetime effect – “the microphysically induced effect on the liquid water content, cloud height, and lifetime of clouds.”

    4) semi-direct effect – “the mechanism by which absorption of shortwave radiation by tropospheric aerosols leads to heating of the troposphere that in turn changes the relative humidity and the stability of the troposphere and thereby influences cloud formation and lifetime.“

    Two other definitions to keep in mind:

    Radiative forcing: “the change in net (down minus up) irradiance (solar plus longwave; in W m–2) at the tropopause after allowing for stratospheric temperatures to adjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values.”

    Surface forcing: “the instantaneous perturbation of the surface radiative balance by a forcing agent. Surface forcing has quite different properties than RF and should not be used to compare forcing agents (see Section 2.8.1). Nevertheless, it is a useful diagnostic, particularly for aerosols.”

    Here is the relevant section 2.8.1 quote:

    For example, for absorbing aerosol, the surface forcings are arguably a more useful measure of the climate response (particularly for the hydrological cycle) than the RF (Ramanathan et al., 2001a; Menon et al., 2002b). It should be noted that a perturbation to the surface energy budget involves sensible and latent heat fluxes besides solar and longwave irradiance; therefore, it can quantitatively be very different from the RF, which is calculated at the tropopause, and thus is not representative of the energy balance perturbation to the surface-troposphere (climate) system.

    Incidentally, here is a quote from the FAR on the split between fossil fuel and biomass as sources of black carbon aerosols:

    “Carbonaceous aerosol emission inventories suggest that approximately 34 to 38% of emissions come from biomass burning sources and the remainder from fossil fuel burning sources.”

    Finally, this was the conclusion of the Fourth IPCC report on the level of knowledge as of five years ago or so:

    “The increase in the knowledge of the aerosol-cloud interactions and the reduction in the spread of the cloud albedo RF since the TAR result in an elevation of the level of scientific understanding to low.”

    Considering that much of the influence seems to come from the indirect and semi-direct aerosol effects, does this study mean that “low” gets switched to “medium”, possibly even to “medium-high”? :)

    P.S. This comment immediately brings to mind the “cool phase of the PDO”

    “nearly all CMIP3 models require strong aerosol cooling at Northern Hemisphere mid-latitudes during the 1931-1975 period to capture both the global mean trends and the NH mid-latitude versus Southern Hemisphere extratropics temperature trends”

    The “cool phase of the PDO” is from 1945-1980, roughly… it’s looking more and more like the existence of predictable multidecade oscillations in the oceans is a myth – there are certainly fluctuations, but there is no reason to assume that future ‘cycle’ strength and timing would be any more predictable than El Nino is – assuming that the ocean-based ‘multidecadal cycle’ notion has any validity at all.

    Interestingly enough, the NAO index also seems to have that curious superimposed cool period on top of the short-term fluctuation, 1935-1980 or so… but that’s the AMO… which also has a similar “cool period”. According to the API/George C. Marshall crowd, we are about to plunge into another ‘cooling ocean cycle’ – and the last time they said that was in the 1990s and it was a ‘cooling solar cycle’ that we were about to enter. Curiouser and curiouser…

  23. 73
    Bill Hunter says:

    Aerosols are an interesting topic but I am unclear on what value is derived from trying to model aerosols to explain decadal oscillations in global temperatures.

    I say that because oscillations likely predate much of this. Below is a link to fish scale deposits. These fishes are very abundant in the eastern pacific and one tends to dominate in rough relationship to ocean SSTs and have done for some time.

    There may well be some anthropogenic signals in the climate record but to ignore natural ones risks far overstating the anthropogenic effects.

    Interestingly, on the flip side of this anthropogenic activities (fishing) has long been fingered for declines in sardine abundances.

    Somebody somewhere has to be laughing at us!

    Seems to be some kind of an infectious disease. Perhaps somebody can work on a cure for that problem.

  24. 74

    MS — yes, World War II had an effect in that the global economy went into high gear coming out of the Depression. Industrial production picked up massively in order to turn out weapons, and there was very little in the way of pollution control. One result was regional pollution disasters such as Donora, PA (1948) and London (1952).

  25. 75
    CM says:

    RickA (#62): it was noted in the post that

    As the total observed Arctic warming during 1976-2007 was 1.5 +/- 0.3ºC, our results can be portrayed in many ways: there is about a 95% chance that aerosols contributed at least 15% to net Arctic warming over the past 3 decades, there is a 50% chance that they contributed about 70% or more, etc.

    Perhaps someone numerate following this thread will work out the corresponding probabilities of 80%, 90%, etc. of the net Arctic warming being due to aerosols.

    But be careful drawing conclusions about the role of CO2.

    The paper infers the aerosol forcing from the difference between observed warming and the warming that can be accounted for by other forcings modeled, with uncertainties that are dominated by natural variability. The positive CO2 forcing is already in there before they work out the aerosol influence.

    So if you want the whole observed Arctic warming since 1976 to be due to black carbon, I guess you have to posit natural variations cooling the Arctic enough to cancel out the warming due to CO2 and other GHG, natural, and ozone forcings.

    It would still be the case that CO2 is warming the Arctic. That is, given the same natural cooling and the same aerosol warming, the Arctic would not warm as much in the absence of increased CO2. See the dam analogy in the post.

    Correct me if I’m missing something, folks, I’m here to learn…

  26. 76
    Matt says:

    Thank you for the clear post- some much needed context. The dam analogy was particularly helpful.

    Re #59 and Gavin’s response:
    I’d like to propose a reason for the high levels of ‘bovine effluvium’ and constant seepage of policy discussion into these threads. Until it is addressed the problem is likely to continue, regardless of how much RC authors insist this is a ‘science only’ discussion blog. Namely that ‘we’ (ie nonscientists/lay people) desperately want to know what climate scientists’ opinions are on proposed climate change policies, and it’s very hard to find these opinions elsewhere on the web. So we trawl the blogs where climate scientists occasionally break cover, trying to glean some idea of their thinking.

    I can understand why Gavin and others are keen to maintain the distinction between their work as scientists and their private views on policy, but as the science becomes more ‘settled’ these policy discussions are going to get more and more important. Most people accept the reality of AGW now, and the focus is shifting to how much, over how long, what we should be doing about it and how urgently. On the last two points particularly, there is a huge amount of uninformed opinion cluttering up the internet. It would be nice to see more informed opinion on the issue- if not on this blog than elsewhere. (And whose opinions could be more informed than those of working climate scientists?)

  27. 77
    pete best says:

    Re#76 If AGW was incorrect science would have demonstrated it for Science is set up to shoot itself down. Thats how as I understand science works. Peer review and scrutiny thereafter before anything is written into the annals of knowledge. Thats the issue with the skeptics, they attack everything in desperation but submit nothing for the scientific process to do its job.

    450 ppmv of CO2 is almost certain. Antarctica started to form then so WAIS is a definite issues of being severely eroded along with Greenland in the Arctic as it formed when Co2 was less than todays level hence it unease. The Arctic sea ice will continue on its increased summer melt (area and thickness) and will recover in the winter to but maybe even that is under threat.

    Lets hope that aerosoles have a small sensitivity whcih means that GHGs will have less effect as warming already expereinced will be due to more GHG required for its temperature rise. If Aerosoles have a higher -ve sensitivity effect then if we eliminate them then GHG will mean a stronger warming.

  28. 78
    Douglas Wise says:

    re #76.

    I agree. I am currently finding what I’m looking for at

  29. 79
    SecularAnimist says:

    I regret that I have been “suckered” into responding to off-topic comments by denialists with replies that were also off-topic.

    I appreciate with much gratitude that this site is about climate science, not climate/energy policy, renewable energy, nuclear energy, etc.

    Many of us who understand and accept what science has learned about AGW, while we continue to be interested in the ongoing development of scientific knowledge and understanding (the focus of this site), realize that science has already learned enough to know that the problem is real, the danger is grave, and the need for a policy response is urgent. And we naturally want to discuss those topics, particularly when an article on this site brings into sharp focus just how serious the problem of AGW is. But there are indeed other sites where discussion of political, economic and technological responses to the reality of anthropogenic global warming are more appropriate (e.g., and I will try to keep that in mind.

    Having said that, I think that the denialists are less likely than readers like myself to respect the moderators’ requests that comments stay on-topic. And if the denialists are permitted to post extended off-topic regurgitations of bogus, scripted, ExxonMobil-funded talking points, often with a tone of belligerent arrogance and derision towards those who accept the science, should those comments go unanswered?

  30. 80
    RickA says:

    #66 Mark
    As I said in my post #23:

    1.1 + or – .8 gives a range of .3 – 1.9 C for BC contribution
    1.5 + or – .3 gives a range of 1.2 – 1.8 C for total artic warming.

    What struck me about these two ranges is that the top end of the black carbon warming exceeds the top range of the total artic warming (i.e. 1.1 + .8 = 1.9, is larger than 1.5 + .3 = 1.8), so I infer from this that it is possible (maybe 5% possible – I don’t know how to calculate the exact probability) that all of the artic warming from 1976 to 2006 was due entirely to black carbon – and therefore none was due to C02.

    [Response: The two ranges are not independent. – gavin]

    I don’t know how much of the artic warming is due to black carbon – it is just interesting to me that it is statistically possible that all of the artic warming could be due to something entirely different than CO2.


  31. 81
    RickA says:

    #75 CM
    It is not that I want the entire artic warming to be due to black carbon. Rather, it is that if there is a possibility that the entire 1.5 +- .3 of artic warming is entirely due to black carbon; and given that the artic is supposed to be the most sensitive to global warming, then maybe we don’t really understand what is going on. Maybe we don’t really understand how CO2 impacts climate.

  32. 82
    RickA says:

    #80 – Gavin in line comment

    The two ranges are not independent.

    I assumed that the total artic warming was a measurement which measured total artic warming, from whatever source – whether it was CO2, black carbon, other cause, or some combination.

    Therefore, if the top end of warming attributable to just black carbon (1.9 C) exceeds the total warming observed in the artic (or more accurately there is some small probability that this is the case) – then all of the observed warming could be due to black carbon.

    Therefore, I do not assume they are independent, but in fact that the black carbon warming is a subset of total artic warming.

    [Response: You misunderstand. The conditional probability of BC provided 1.9C of warming given a total warming of 1.8C is zero. Generally, the joint pdf of the attribution and total warming will likely show a strong correlation between the two. – gavin]

  33. 83
    David B. Benson says:

    Adam Gallon (87) — My understanding is that the ABC aerosols have both global cooling and regional warming effects; ain’t simple.

    RickA (81) — We certainly understand how CO2 impacts climate on centennial scales.

  34. 84
    Marcus says:

    RickA: I’d also encourage you to read the post more closely:

    “black carbon contributed 0.9 +/- 0.5ºC to 1890-2007 Arctic warming (which has been 1.9ºC total)” pretty clearly states BC’s contribution to Arctic warming in comparison to the 1.9 number, and it is clearly less than that number.

    Now, in the next sentence, “We also estimated that aerosols in total contributed 1.1 +/- 0.8ºC to the 1976-2007 Arctic warming. This latter aerosol contribution to Arctic warming results from both increasing BC and decreasing sulfate”: therefore, part of this is the warming from black carbon, but part of this comes from a _decreased cooling_ from sulfates.

    Note, also, that in a 3 decade period for a regional signal, there are a lot of influences. So, one might write an equality like:

    CO2 + BC + SO2 + internal variability + solar changes + … = observed change (+1.5 (+-0.3)). So if there are other influences in the Arctic that might have led to cooling in the past 3 decades, then there can still be warming from CO2 _even if_ aerosols explain “all the warming”.

  35. 85
    Mark says:

    “Rather, it is that if there is a possibility that the entire 1.5 +- .3 of artic warming is entirely due to black carbon; ”

    Rick, how can it all be from black carbon?

    If there’s aerosols, CO2, H2O, clouds, ice albedo, Ucle Tom Cobblers and all, how can all the heating be from black carbon? Did the rest give up because black carbon was there??? Did he throw them out of the equation because he’s Big Black Carbon and he ain’t gonna share the artic with ANYTHING that will affect the climate…?

    If you boil a pan and pour in cold water so that the temperature doesn’t rise, then you rub the pan which friction causes the temperature to rise, is all the heating of the water in the pan on the stove because of you rubbing it???

  36. 86
    llewelly says:

    Matt 24 April 2009 at 8:03 AM :

    Namely that ‘we’ (ie nonscientists/lay people) desperately want to know what climate scientists’ opinions are on proposed climate change policies, and it’s very hard to find these opinions elsewhere on the web. So we trawl the blogs where climate scientists occasionally break cover, trying to glean some idea of their thinking.

    But there are other global warming blogs which are much better for policy. For example,

  37. 87

    John H #41: what’s your source for your claim that

    The MWP has been firmly established as a global warming, according to data published by 695 individual scientists from 405 separate research institutions in 40 different countries.


    I found a web site making similar claims. I had a closer look at the data they were presenting and the dates don’t line up. Some spots were warmer 500 years ago, others 1,000 years ago, others 1,500 years ago.

    You can look at the temperature records at any randomly selected list of locations and you can be guaranteed that you will find some with unusually warm or cool periods that do not correspond to any worldwide trend.

    Take a look again at whatever source you thought was so credible and report back if you find something different. Better still, write a paper and get it published.

  38. 88
    Hank Roberts says:

    co2science is the source of that assertion. Caveat emptor, or, sourcewatch.

  39. 89
    James C. Wilson says:

    I did not see a reference to CCSP 2009. Maybe I missed it.

    CCSP 2009: Atmospheric Aerosol Properties and Climate Impacts, A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. [Mian Chin, Ralph A. Kahn, and Stephen E. Schwartz (eds.)]. National Aeronautics and Space Administration, Washington, D.C., USA, 128 pp. (Synthesis and Assessment Product 2.3 or SAP2.3)

    It has been noted in this thread (and by IPCC) that uncertainties in the direct and indirect effects of aerosol and differences among models concerning the treatment of clouds account for most of the uncertainty in climate sensitivity. The current range is so wide as to be of limited use. We need better estimates of climate sensitivity to make better policy.

    The exec. summary of SAP2.3 asserts that three things can help reduce uncertainties due to limited knowledge about particle properties and impacts:
    “(1) improving quality and coverage of aerosol measurements, (2) achieving more
    effective use of these measurements to constrain model simulation/assimilation and to test model parameterizations, and (3) producing more accurate representation of aerosols and clouds in models.”

    The magnitude of these needs and undertakings does not seem to be widely understood. Again the exec summary reminds us that the differences in climate sensitivities among models is large and necessarily balanced by large differences in aerosol effects (after all the models do largely agree in producing our modern climate). Direct cooling by sulfate varies by a factor of 6 among models. More variation was seen in the treatment of black and organic carbon. Some models ignore indirect effects, others have indirect effects varying by a factor of 9.

    These uncertainties result in estimates of the impact of doubling CO2 that range from 2 to 4.5 degrees. Quoting the Exec Sum of SAP 2.3 again: “Development of new spacebased, field and laboratory instruments will be needed, and in parallel, more realistic simulations of aerosol, cloud and atmospheric processes must be incorporated into models.” Our lack of understanding casts doubt on predictions of future climates. Estimates of anthropogenic contributions to the direct effect vary from -0.7 to -1.5 w m-2. This is not nothing when viewed in the context of net anthropogenic forcing of +1.6 w m-2.

    Note that development of spacebased and field instruments is needed. We still do not have all the measurement tools that are needed!!!!

    The spatial and optical variability of aerosol is huge. The ability to predict abundance and impact is limited. The ability to measure from space is limited. Ground stations providing the physical, chemical and optical characterizations are limited. The RC posts have been good, but limited. But the GCR goofyness is the least of the worries about aerosol. The fundamentals deserve more attention.

    Chuck Wilson

  40. 90
    John Finn says:

    Dear RC

    I’ve just had chance to have a proper read through the Drew Shindell post. I have a few initial questions

    1. In the Abstract to their paper, Shindell & Faluvegi refer to “historical emission estimates”. Is there a source for these estimates.
    2. Due to the complex nature of it’s absorption properties, increases in CO2 results in a logarithmic response. What about aerosols? Can we assume a linear response. For example, if a given level of emissions results in a net forcing of -1 w/m2, can we take it that doubling that level of emissions will result in a net forcing of -2 w/m2 [assume the proportions of BC, SO2 etc remain constant].
    3. Does Drew consider seasonal effects – and what are they? For example, the reflective properties of aerosols in the arctic winter would, I assume, be less relevant than during the summer, while the cloud-forming properties might actually result in warming.

    Final question

    4. Would Drew Shindell have any objection to being contacted directly.


  41. 91
    RickA says:

    #84 Marcus
    #85 Mark

    Thank you for your posts. Yes – I was not considering these aerosols (other than black carbon), H2O, clouds, or ice albedo.

    I can certainly see how those effects could mask or negate the warming due to black carbon.

    However, those numbers were not presented in the post – so I don’t know what they are.

    Also, since we are speculating – couldn’t all the other effects just cancel out – leaving the entire artic warming due to just black carbon? It doesn’t seem like the data totally exclude that possibility.

  42. 92
    Mark says:

    re: 90

    1. The paper should say. Read it.
    2. Due to the complex nature, a simple answer is not possible.
    3. Ask Drew.
    4. Ask Drew.

  43. 93
    Hank Roberts says:

    Click the supplementary info link for the entire list of references. Those should answer the question about sources.

    For the full paper, an academic library will have full access; any public library can borrow the journal for those who want to read it.

  44. 94
    Mark says:

    “Also, since we are speculating – couldn’t all the other effects just cancel out – leaving the entire artic warming due to just black carbon? It doesn’t seem like the data totally exclude that possibility.”


    Because without the increased CO2, there would have been cooling.

    Whatever that cooling would have been is the magnitude of heating done by CO2.

    The physics may cancel out, but that doesn’t mean that the warming was due to just one. You may as well say “leaving the entire artic warming due to just CO2”. Would you accept that?

    The data excludes the possiblity because the data is the reflection of reality. Reality doesn’t say “Well, we have CO2 and H2O and ice albedo but they all cancel out, so anything else that comes in now is all that’s going to happen”. It lets it ALL happen. It doesn’t cancel out thinks like a student cancelling out elements in two simultaneous equations so that they can work out the value of X and Y.

  45. 95
    Matt says:

    re 86:
    climateprogress ok I guess, but lots of political (as opposed to policy) discussion- and very US-centric.

    re 78
    Thank you for the pointer. Had a good look at the site and it certainly ticks a lot of boxes- I think this is the closest yet.

    Otherwise, the best I’ve found is David Mackay’s:
    But it’s a poor (so far) relation to his book. There’s not much discussion/other views, and definitely UK-centric.

    I guess what we’re looking for is a ‘policy’ blog which is international in outlook, apolitical, and regularly frequented and commented on by working climate scientists. Will keep looking.

    Sorry to continue off topic. But I think there is a real need and finding the right home for this kind of discussion will free up RC for what it does best.

  46. 96
    John Finn says:

    Mark Says:
    26 April 2009 at 11:39 AM

    re: 90

    1. The paper should say. Read it.

    I would but the link in the post only links to the abstract which tells me that the “complete document is not available”.

    2. Due to the complex nature, a simple answer is not possible.

    The “complex nature” refers to CO2 not to aerosols. Why should aerosol forcing be “complex”? That’s really what I’m asking.

    3. Ask Drew.

    I would, but I do know that Gavin, for example, has worked with Drew and if I remember correctly has co-authored papers with him. I’m not suggesting that Gavin necessarily speaks for him, but he may be able to suggest the best approach in order to get questions answered.

    4. Ask Drew.

    You are suggesting that I ask Drew if he minds being contacted directly, but to do so I woould have to contact him directly. Drew Shindell posted an article (which is of interest to me) on RC. I would like to know more so I’m just clarifying the position. Is Drew fielding queries via the RC blog or is it best to contact him directly. I fully understand that the regular RC contributors may not necessarily have the answers to my questions.

    ….and why are you responding on their behalf?

  47. 97
    RickA says:

    #94 Mark

    The physics may cancel out, but that doesn’t mean that the warming was due to just one. You may as well say “leaving the entire artic warming due to just CO2″. Would you accept that?

    If I was told that total artic warming was 1.5C plus or minus .3, and

    CO2 warming for the artic was 1.1C plus or minus .8, then

    yes – I would allow for the possiblity that all of the warming in the artic was due to CO2.

    Because then there would be a small possibility that total CO2 warming was 1.8C (it is within the range), which equals the top end of the total artic warming (1.5 + .3 = 1.8C).

    Of course, that would mean that all the other cooling forcings would have to negate the other warming forcings (black carbon warms and volcanoes cool – maybe all these cancel out).

  48. 98
    Mark says:

    RickA, the physics doesn’t cancel out. They all add up constructively and destructively.

    And you don’t do your addition like that.

    Here’s an example.

    The spread of the hand from fingertip to thumbtip is about 8-10 inches.

    Measuring your height from that “ruler” and going to the nearest “hand” means you’re 8 hands tall.

    The error from that is 0.5 hands from “nearest”. 20% error from the variation of hand sizes and because I didn’t inchworm my way up, another 5% error there from start of one hand to the next.

    So the error is .5+ 8*.25. 2.5hands.

    Your hip is less than 1.5 hands high.

    Does this mean you don’t have a hip???


    So why do you insist that the warming has no CO2 component just because it has error bars.

    Your attempt to use maths this way is in considerable error.

    THEY DO NOT CANCEL OUT. They all sum together. Without one of them, they would not total the same result.

    If you don’t understand you’re willfully ignorant and unworthy of response since you will only hear what you want to hear and trying to educate you is a waste of everyone’s time.

  49. 99
    Mark says:

    NOTE: the ***results*** may cancel out.

    But they don’t stop each element having its effect.

    If you push with a force of 10N on one side of a plate and I push with 10N on the other side of the plate, the ***result*** cancels out.

    This doesn’t mean neither of us are pushing.

    If I push 12N, the result cancels out YOUR push. But it doesn’t mean your push didn’t have an effect and definitely doesn’t mean you didn’t push at all. If you hadn’t been pushing, the resultant force would have been 6x larger.

  50. 100
    Ike Solem says:

    Bill Hunter says:

    “I say that because oscillations likely predate much of this. Below is a link to fish scale deposits. These fishes are very abundant in the eastern pacific and one tends to dominate in rough relationship to ocean SSTs and have done for some time.”

    April Fool’s Day was some time ago, wasn’t it?