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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. 1
    David B. Benson says:

    Clearly stated. Thank you.

  2. 2
    Chris Colose says:

    Thank you for the excellent post Dr. Shindell.

    Can you (or someone at RC) please provide the best type of sources used for assessing the time-evolution of aerosol emissions (or forcing) over the 20th century (particularly 1900 to roughly mid century-ish where records are likely poorly constrained). I’d imagine there’s good relaince on ground radiation measurements after subtracting the change in TSI. I know the ice core record shows up tracking the sulfate changes fairly well, but the paper by Stern and Kaufmman on “Estimates of Global Anthropogenic Sulfate Emissions 1860-1993” was the only paper I’ve really seen on the subject, which is fairly old.

  3. 3

    Thanks very much for taking the time to give this!

    I’ve been trying to talk sensibly about your paper at a large physics discussion forum, at “”. I write there under the name “Sylas”. The thread of discussion is at Only dirty coal can save the Earth. (I didn’t pick the title; I only joined in at message 11!)

    Physicsforum is not intended for fringe or crank science, but it is open to all comers. To try and manage the obvious difficulty, a special requirement has been laid down that only peer reviewed sources may be used in the Earth science section, where the physics of climate is often discussed.

    I hope I’ve been representing this work appropriately… I think so. (And if anyone can correct places I’ve misunderstood things, join in!) I’ll also put a link there for this article, as there are lots of folks who will, I think, be interested to see what Drew has to say on this directly. It’s certainly useful for me!

    Thank you! — Sylas

  4. 4
    EL says:

    Good Post

  5. 5
    Lawrence Brown says:

    This may be nitpicking and doesn’t affect the main thrust of the contribution,but, I believe, that the 0.6 and 2.4 W/m^2 figures in the IPCC AR4 are positive in sign and refer to the range in the total net anthropogenic forcing.

  6. 6

    Dr. Shindell,

    A fascinating post to compliment a fascinating paper. I’m curious to know if anyone has modeled the expected net forcing of projected global aerosol reductions over the next century (due to growing prosperity and our friend Kuznetz) taking into account changes in both sulfate aerosol and black carbon forcing. If I recall correctly, the AR4 WGIII projected a mean 50% decline in annual global aerosol emissions, though there was considerable uncertainty due to the wide range of socioeconomic scenarios examined.

    My back-of-the-proverbial-envelope calculations awhile back yielded 0.36 degrees C warming due to a 50% decline in sulfate aerosols (assuming a current anthropogenic SO2 combined direct and indirect forcing of -0.83 Wm-2 and a climate sensitivity of 0.87 K), though this left out any calculation of changes in black carbon forcings.

  7. 7
    Mike Hilson says:

    This confirms what I’ve always suspected. Air pollution camouflaged the
    effects of higher GHG until efforts were made to reduce it. Now imagine what
    will happen if/when China shuts down its coal fired generators.
    The net forcing will be higher for years as more rays reach the ground
    and are trapped by CO2. What is the break even point after such a scenario?
    Probably a decade at least.

  8. 8
    John Lang says:

    Shouldn’t the older models have captured these issues? It seems pretty clear to me that this should have been built in already.

  9. 9

    Lawrence Brown at #5: the aerosol forcings in IPCC 4AR are in the range -0.6 to -2.4

    Have a look at table 2.12, on page 204 of the WG1 report.

    It doesn’t actually quote the total aerosol there as a single range, but gives rather:
    -0.5 [-0.4, +0.4] (Total direct aerosol)
    -0.7 [-1.1, +0.4] (cloud albedo (indirect) effect, all aerosols)

    You add uncertainties as square root of the sum of squares. Hence the combined total aerosol there is

    -1.2 [ -1.17, +0.57 ]

    which to the single figure accuracy being used is -2.4 to -0.6.

    The total combined anthropogenic forcing is quoted at the head of this table, and it is 1.6 [-1.0,0.8]

    That gives 0.6 to 2.4

    I had never noticed this co-incidence before! But it is just a co-incidence. The graph in figure 2.20 on the previous page shows what is going on. As well as this rather poorly constrained aerosol forcing, there’s the much more accurately known greenhouse forcing, of +2.63 [-0.26,+0.26], which when added to the aerosol effect, just shifts it all across a bit.

    One of the great things about Shindell and Faluvegi is that it points a way to reduce the uncertainty of the aerosols. The best estimate global aerosol effect, however, remains about the same (-1.3 compared with -1.2)

  10. 10
    Eli Rabett says:

    Soot has a positive forcing

  11. 11
    Thomas says:

    I know it is off topic, but I think the recent paper by Romps and Kaung
    which purports to show that increases in tropical cyclone activity may be responsible for significant increases in stratospheric humidity. If this pans out, then it represents a previously unknown or underappreciated (at least to a layman like me) positive feedback mechanism. My gut feeling, is that stratospheric water vapor is so low that it wouldn’t have a strong warming effect, unless there was cloud formation -but I am not equiped to answer the question as to how strong such a feedback effect might be.

    Perhaps this could be the topic of a future discussion?

  12. 12
    Ray Menard says:

    Mike @7
    I tend to agree that China will have a big effect but rather than shutting down their coal, I think its far more likely they’ll place air pollution controls similar to what we did during the 70’s to improve local and regional air quality.

    The aerosol articles have been excellent.

  13. 13
    Mark says:

    John what makes you think the models don’t include these things?

  14. 14
    Ike Solem says:

    Very interesting indeed! What happens with the sea ice response as aerosols are added in?

    For more on why reducing black carbon aerosols is a good idea:

    Black Carbon Pollution Emerges As Major Player In Global Warming, San Diego CA (SPX) Mar 25, 2008
    Black carbon, a form of particulate air pollution most often produced from biomass burning, cooking with solid fuels and diesel exhaust, has a warming effect in the atmosphere three to four times greater than prevailing estimates, according to scientists in an upcoming review article in the journal Nature Geoscience.

    Scripps Institution of Oceanography at UC San Diego atmospheric scientist V. Ramanathan and University of Iowa chemical engineer Greg Carmichael, said that soot and other forms of black carbon could have as much as 60 percent of the current global warming effect of carbon dioxide, more than that of any greenhouse gas besides CO2. The researchers also noted, however, that mitigation would have immediate societal benefits in addition to the long term effect of reducing greenhouse gas emissions.

    There’s also the negative correlation between black carbon aerosols and precipitation:

    This has been widely noted, for details see:

    In this study, observed precipitation, MODIS data and meteorological sounding data over eastern central China were analyzed. The result shows that the precipitation in this region is significantly reduced during the last 40 years and this reduction of precipitation is strongly correlated to the high concentrations of aerosols.

  15. 15
    pete best says:

    So due to the seemingly very large error bars on these aerosols, thier forcing potential could still be casting a doubt over the forcing of GHG?

    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.

    This is slightly disoncerting for it does not and seemingly cannot let us know the true forcing we are experiencing for it could be low or high relative to GHG forcings and the uncertainty gives ammunition to the skeptics once again I suppose.

  16. 16
    Mark says:

    “So due to the seemingly very large error bars on these aerosols, thier forcing potential could still be casting a doubt over the forcing of GHG?”

    Uh, casting uncertainty.

    Is that uncertainty significant?

    Is the uncertainty reason to avoid change?

    Why did you not ask these questions? A skeptic would. Someone avoiding having to think about it will find anything or even make it up.

  17. 17

    Re #15 pete best: sigh, yes. But remember, uncertainty is just another word for risk. Contrary to what the “skeptics” seem to believe, it doesn’t respond to prayer :-(

  18. 18
    CM says:

    If black carbon has a significant positive forcing, is this good news or bad? Are there quick fixes for cutting soot while we try to figure out how to cut CO2? Or is it just bad news because some mitigation efforts (like promoting diesels over petrol cars) will turn out to be unhelpful?

  19. 19
    pete best says:

    Re #16 and #17, Its ok guys, I am just worried that the sensitivity of the climate is more so and hence more warming is going to happen than that predicted by the IPCC who is a very conservative body whose reports are not entirely scientific due to the political angle on the report.

    I fully accept that the error bars are within scientific tolerance, I just worry about the sensitivity a little bit. We don’t need even more warming now do we, we are going to get anough already and even more come another few decades of emissions.

  20. 20

    Hi Pete at #15: There is indeed a substantial uncertainty in the total forcing, and it is mostly from uncertainty on cloud and aerosol effects.

    No, this does not cast any doubt whatsoever on the greenhouse contribution, which remains one of the best defined and understood forcings involved. It can be calculated from first principles, using the same well established physics that applies for any calculation of electromagnetic transmission through a mix of gases. The methods work for the Sun’s photosphere, for a laboratory gas cell, or the Earth’s atmosphere.

    The uncertainty of the well mixed greenhouse forcing is about 10%, and that doesn’t depend in the slightest on the uncertainty with aerosols. They are different things.

    There’s no cause to be “disconcerted” by this, and neither is there any reason to obscure the real uncertainties and open questions that scientists are working on. We certainly don’t want to pretend that knowing the greenhouse forcing is the same as knowing all about climate!

    I agree with you that any uncertainty gives ammunition to so-called skeptics, but that’s only because they are not skeptics at all. Many of the these folks turn out to be some of the most credulous and naive simpletons on the planet, who’ll enthusiastically embrace any argument, however ridiculous, as long as it can be spun into denial of anthropogenic global warming.

    That shouldn’t make a difference for the rest of us, as we try to explain the various aspects of real climate science for an interested public.

  21. 21
    Eli Rabett says:

    Papers (about ten years ago? in PNAS?) by Hansen suggested that shutting down black carbon and methane emissions would buy time to start limiting CO2 emissions, which is much harder. We (eliminated water wastes) away the opportunity. It is a necessary step forward now but we will pay the opportunity costs as the effect will not be as large given the rise in greenhouse gas forcing.

  22. 22
    Lawrence Brown says:

    Re #9: I stand corrected. I was taking my numbers from figure SPM.2. Radiative Forcing Components, of the Summary for Policy Makers report of AR4. which gives numbers of 1.6[0.6 to 2.4] for total net anthropogenic forcing. Thank you for the clarification.

  23. 23
    RickA says:

    I don’t get it.
    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.

    Doesn’t this mean that there is a statistical chance that the entire total artic warming is due to BC?

    If that is the case – than doesn’t that mean that there is a statistical chance that zero % of the artic warming is due to C02?

  24. 24
    John H. says:


    There are plenty of reasons to be diconcerted. The totality of issues which indicate uncertainty should not be avoided while minimalizing a single apsect. Neither should you be more concerned about giving ammunition to skeptics than you are about the science.
    There is much to be skeptical about despite your opinions.


    Why don’t you ask questions? A skeptic would.
    Especially when it comes to the climate models.

    Someone avoiding having to think about it will find anything or even make it up.

    There are severe problems with AGW and the swarm of false claims being made by people at all levels.

    I am concerned. Why aren’t you?
    As an OSU professor and researcher Jane Lubchenco made false claims that ocean dead zones were linked to AGW.
    As the new head of NOAA Lubchenco said she wants to establish a climate information service modeled on the National Weather Service because Dr. Lubchenco “believes climate models are now sufficiently “robust” to help scientists start to do the same with climate, to help businesses, elected officials and regulators make good decisions on issues like where to put buildings or roads or wind farms.”

    “It is no longer enough to know what the wind patterns were for the last hundred years,” she said. “You want to know what they will be for the next hundred years — and they undoubtedly won’t be the same. So there are huge opportunities to provide services to the country.”

    Absolutely amazing. A new National Climate Service will soon be telling us where to put roads, buildings and wind farms 100 years out?

    Give me a break. There is no such ability in the climate models at all. None. Jane made that up.

    Will the models tell us it won’t be windy where windmills are now? Laughable.
    Then we have Thom Hartman saying on the air this week that if we don’t act fast the earth will become uninhabitable for humans and all other living creatures. Laughable.
    He said the MWP was local and limited to northern Europe and Greenland. Nonsense.
    Steven Chu is warning of increased hurricanes.
    Nancy Skinner says more category 5 hurricanes will be hitting the USA.

    On and on and on.

    And you’re claiming skeptics make up things?

    [Response: They do. – gavin]

  25. 25
    Ryan Sulivan says:

    Very nice post. The clear explanation regarding the masking effect of cooling aerosols was especially nice to see.

    I wonder how much is missed when models largely focus on TOA radiative forcing changes. Light-absorbing black carbon does not usually alter the TOA forcing much, but causes large changes in the distribution of radiation through the atmospheric column. It warms the lower atmosphere while cooling the surface (i.e. dimming). You can get at some of this by looking at changes in surface temperature, as you have done, but I’m wondering if you could comment more about what can be missed by just looking at TOA forcings.

    Also, I’m afraid I haven’t had time to read the original paper. In your model is black carbon treated as always externally mixed from sulphate?

  26. 26

    #24 John H.

    You’re funny, thanks for the laugh, …but it is way past April fools???

  27. 27
    SecularAnimist says:

    John H wrote: “There are severe problems with AGW …”

    I’m skeptical of that claim. What is your evidence? Evidence that hasn’t already been thoroughly debunked, that is.

  28. 28
    Walt Bennett says:

    Re: #24

    Gavin (inline),

    You seem to have ignored all but the last sentence of his post, or perhaps you failed to grasp his point: What isn’t being stapled onto AGW Theory and being treated as though it deserves the same high confidence?

    There is no significant difference between “making things up” and being deluded in your thinking. The first person knows they’re wrong and the second person doesn’t, but they’re both wrong.

    [Response: Does it really need to be explicitly stated that making things up is wrong? Please give me some credit. – gavin]

  29. 29
    Walt Bennett says:

    Re: #27


    He didn’t say there are severe problems with “AGW Theory”, he said there are severe problems with AGW as a propaganda tool, and he listed several of his concerns.

    The claims grow wilder and yet somehow more certain. If you don’t have a problem with that trend: Why not?

  30. 30
    Mark says:

    Walt, the reason why AGW is bad as a propoganda tool is that the facts are being used to promote it.

    And sometimes the facts don’t give a simple message.

    Anti-AGW doesn’t need to use facts, so they can ignore any that don’t stay “on-message”.

    Mind you, that’s not what the post said, is it. It said “problems with AGW theory”.

  31. 31
    SecularAnimist says:

    Walt Bennett wrote: “The claims grow wilder …”

    None of the claims that John H. referred to seem particularly “wild” to me.

    The claim that climate models could be useful in planning new infrastructure? What is “wild” about that? Surely it will be helpful to know where water supplies are likely to dwindle, or where coastlines are likely to be inundated, or where permafrost is likely to melt, and climate models can help to tell us those things.

    The claim that unmitigated global warming could render the Earth “uninhabitable”? We have evidence suggesting that CO2 increases from natural causes in the past, which were slower than today’s anthropogenic increase, caused massive global extinctions. And given what is known and observed about the ongoing effects of AGW, it is not difficult to envision mechanisms (e.g. large-scale die-off of forests, ocean acidification, methane releases from thawing permafrost and clathrates) by which that could happen again. Sorry to say, there’s nothing “laughable” about that.

    The claim that the so-called Medieval Warm Period was “local and limited to northern Europe and Greenland”? That’s not “nonsense”, as anyone capable of reading Wikipedia would know.

    The claim that global warming will lead to more powerful hurricanes? That’s mainstream climate science. Nothing “made up” about it.

    John H. obviously doesn’t like these “claims”. But he “rebuts” them with adjectives and attitude, not with substance.

  32. 32
    ccpo says:

    Re: John H. #24 “There are severe problems with AGW and the swarm of false claims being made by people at all levels.

    I am concerned. Why aren’t you?”

    Oh, I’m concerned. I’m concerned that “skeptics” lie. I’m concerned they focus on temps because that is the only area that has any wiggle room for them to make stuff up. I’m concerned that they never deal with Arctic sea ice (too obvious), but can’t say enough about Antarctic sea ice because it’s more stable, which allows them to lie and claim there’s nothing to worry about.

    I’m concerned that the polls and interviews of late indicate climate scientists are scared and are far more pessimistic than their public pronouncements indicate.


  33. 33
    Walt Bennett says:

    Re: #28 inline,


    I suspect that you are severely overworked, because you also misunderstood my point as well.

    Which was: the debunkerati do not own the copyright with regard to stretching information, to wit: a lot of potential climate change is being stapled onto AGW theory as though it carries the same weight.

    This was, I believe, the original commenter’s point, and it is one I have also been making for a long time, now.

    We must be careful to all remain skeptics, especially of new information. Yes, most of us accept AGW Theory as well developed and likely to be correct (within its error boundaries, of course); but we have no business saying with confidence what effect this will have on wind currents, ocean currents, storm activity or local changes.

    It’s clear that many people miss that point, thus it bears repeating.

  34. 34
    Walt Bennett says:

    Re: #31


    I believe my response to Gavin addresses your comment as well.

    I believe that we are observing a phenomenon here at RC and probably elsewhere:

    People have become convinced that AGW is real and has a great deal of momentum. Thus, the time for action is upon us, and yet the world seems slow to grasp this, making the situation even more desperate.

    The more information we get (“rivers are slowing”; “spring starts sooner”; “Arctic ice is at a historical low”) the more convinced we become that we must really act in a meaningful way as soon as possible.

    We start throwing this anecdote and that anecdote at the skeptics, as if to say: “Are you convinced now? (THROW.) How about now? (THROW.) Now?”

    And the more the skeptics reply: “I don’t see the hard line of evidence from AGW Theory to this factoid,” the more we look for things to throw at them, as if the sheer number of things we can find to throw at them ought to finally, at last, convince them.

    It won’t, and it shouldn’t.

    Much of what we are learning has no meaningful context. We don’t know precisely what causes it; we don’t know if it is short or long term; we don’t know if the eventual effect will be positive of negative with regard to global temps. What about any of that would make a skeptic stop being skeptical?

    It has always been and will always be problematical to point to individual observations as evidence of AGW. It remains enormously problematical to predict future effects of AGW.

    We aren’t going to win the argument by taking short cuts. We aren’t going to win it by convincing skeptics to be less skeptical.

    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.

    And so, you will never get your point across.

  35. 35
    Pete Wirfs says:


    Walt, it seems to me you wish to discuss public policy, and have chosen a scientific forum to do so. As a layman who comes here to read about the science, it seems that when you say;

    “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.”

    Perhaps it is you who has no interest in understanding the context of this forum?


  36. 36
    Hervé says:

    Nice paper and excellent post !

    Just surprised again (see my post #174 on the former item) that there is no comment about the following clarification:

    “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.”

    At a recent European Geophysical Union meeting (not this year), there were particular sessions about aerosols and Northern Hemisphere modes of variability. I attended both and was astonished to see that both scientist communities had the (only) explanation for the increasing surface warming over Europe since the 1970s: a change from regional (rather than global) dimming to brightening on the one hand, a trend in the North Atlantic Oscillation (a well-known regional mode of internal climate variability) on the other hand. While both communities were convinced of the AGW contribution to the observed temperature evolution, they were apparently ignoring each other !

    I am therefore very grateful to Drew Shindell for his important clarification and conscious that the study does not aim at providing very detailed regional estimates of aerosol forcings.
    Nevertheless, I’d like to know how confident Drew is on his estimate of unforced internal variability based on coupled GCMs ? Has he looked for example at the 20th century multi-decadal rainfall variability over tropical land and compared the GCM results with gridded observations ? This is not directly related to surface temperature but suggests (if my memory does not play a trick on me) that current GCMs could strongly underestimate the magnitude of multi-decadal climate variability.

    Am I right ? Even if I am, Drew’s paper is excellent and provides an original technique for estimating the aerosol forcing… but it might be interesting to pay a particular attention to the regions where internal climate variability is supposed to be low. Does the Arctic belong to this category ?

  37. 37
    sidd says:

    Mr Bennett writes:

    “We start throwing this anecdote and that anecdote …”

    Who is this ‘we’ ? I see data being discussed here. Not anecdotes.

    Mr. Bennett continues:

    “Much of what we are learning has no meaningful context.We don’t know precisely what causes it; we don’t know if it is short or long term; we don’t know if the eventual effect will be positive of negative with regard to global temps.”

    What is this ‘it’ ? Perhaps Mr. Bennett would care to give an example ?

    Perhaps he might comment on the putative lack of context in the paper we are discussing ?

    Or, perhaps, (be still, my beating heart) he might leave us alone to learn some science in peace?

    Now, as to the subject at hand: Would it be true to say that there is no impact from black carbon on Antarctic ice ?

  38. 38
    CM says:

    SecularAnimist (#31), I’ll trust current climate models to give us the general outlines you mention, like inundated coastlines and melting permafrost. (So I’ll pass on that beachfront property on the Maldives.) But should I trust them

    to help businesses, elected officials and regulators make good decisions on issues like where to put buildings or roads or wind farms

    as Lubchenco put it in the quote at #24? Are forecasts of regional and local impacts of climate change already so meaningful and reliable that climate models can serve as local area planning tools? If not, I’d worry about the impact of such claims on public confidence in the science.

    (recaptcha: “new drill” — how did the oil industry slip that one in?)

  39. 39

    Some folks are saying that the problem of uncritical acceptance of convenient data cuts both ways. And it’s true that you can find individuals on all sides making strong claims with weak foundations, and naive over-confidence.

    This not even close to being an equivalence of two sides on AGW.

    The place to go for forthright acknowledgement of genuine problems and uncertainties, wherever they lead, is the mainstream of science, and the IPCC reports which are based upon that mainstream. Drew’s paper is a case in point. Lots of people have fallen over themselves in haste to present Drew’s work as a useful correction to an improper scientific fixation on greenhouse warming. The truth is that Shindell and Faluvegi build on and extend a large body of work of aerosols which is straight out of the mainstream, developed by scientists who recognize the discovery of the anthropogenic greenhouse impact.

    The importance of greenhouse warming is basic physics, and the narrow confidence bounds for greenhouse forcing in IPCC 4AR are a true reflection of how solid this is as science. Using acknowledged areas of real uncertainty (aerosols, clouds, sensitivity, regional patterns of change, etc, etc) as a reason to be dubious of everything else about climate science is a fallacy. It arises because there is a strong desire to deny the anthropogenic emissions link; to the point where AGW-denial is riddled top to bottom with dubious hypotheticals, credulous acceptance of convenient speculations, and a strong thread of outright pseudoscience.

    This doesn’t mean everyone questioning AGW is a pseudoscientist. Many people are genuinely confused and simply don’t know who to trust.

    Walt (#34) is quite right that everyone involved needs to be careful about over reaching and stretching information. My point is that this caution is not needed for the great majority of scientists working away at the issues, but it is needed for nearly all of the “skeptics”.

    This is not a case of scientists needing to learn from the skeptics. It’s almost entirely the other way around.

  40. 40
    Chris Colose says:

    Walt, I do not feel the necessity to hijack this good thread with another “warmist vs. denialist” back and forth.

    AGW is not even a theory in the sense that you think it is. There’s also better things to talk about then if AGW is “right or wrong” which by itself is essentially meaningless. This isn’t the binary black and white world you think it is.

    Gavin, I do have another question concerning Koch et al (2009) on aerosols, of which you were a co-author. I was wondering if you can briefly elaborate on the physical mechanism whereby aerosols can still have a cooling effect during the cold season at high latitudes when there is little to no incident sunlight. The paper talks about a delayed effect, but how exactly does this work?

  41. 41
    John H. says:

    While I appreciate the opportunity to engage folks here on this AGW site, IMO some of you are misunderstanding some key points and making it difficult to have honest simple conversations.
    Let’s try and communicate.
    Making things up or wild embellishments without any basis are happening as I listed some above.

    Some of your responses were not right.

    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 reckless embellishment at best. Making things up IMO.

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

    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. This does cause severe problems with the hockey stick theory and by extension AGW.
    Especailly when considered along with many other problems.

    Steven Chu is making up his hurricane warning. Talk show host Nancy Skinner embellishes the falsehood with “more category 5 hurricanes will be hitting the USA.
    IPCC assessments in 1995 and 2001 concluded that there was no global warming signal found in the hurricane record.

    Yet it’s still a regular claim that Hurricane Katrina was caused by AGW.

    OSU professor Jane Lubchenco made up her claim that ocean dead zones were linked to AGW. That was accepted without ANY science and distributed around the globe. Google it and look at how far it flew.

    Even though Lubchenco’s her own research group “cautioned that it is unclear what — if any — link the dead zone has to climate change”.
    Yet Lubchenco made her report that made the link.

    Now that’s making things up. It’s wrong.

    This is no joke.
    The claims do grow wilder and yet somehow more certain.
    Mark views all of this as facts?

    SecularAnimist went wild right here claiming climate models locate future water shortages, permafrost melt and flooding etc.

    Now I am certain many of you know climate models do no such thing.

    They certainly DO NOT forecast wind patterns 100 years out as Lubchenco made up.
    Talk about a whopper. That’s as bad as her ocean dead zone tale.
    More “facts” Mark?
    Secular wandered with supposed massive anthropogenic global extinctions.
    He says it’s not hard to “envision”
    This is a science blog and he’s fantasizing. Making things up.
    And you wonder why skeptics are skeptics?
    But he’s also stuck on Wikipedia where the MWP is still misrepresented.
    And he has bought the hurricanes lie and calls it “mainstream climate science.”

    What a perfect demonstration of made up science being circulated morphed into “mainstream climate science”.
    Even when it contradicts the IPCC reports.
    IPCC assessments in 1995 and 2001 concluded that there was no global warming signal found in the hurricane record

    Are those “adjectives and attitude” and not “substance”?

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

    ccpo is concerned that skeptics lie. That they focus narrowly or something.
    Anyone following the work of skeptics scientists could hardly call it limited or narrowly focused.

    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?
    And that it’s about to reach the 1979-2002 average.
    Antarctic sea ice is more stable and as a continent is growing.

    Yet while all of these uncertainties, misunderstanding and fabrications flourish the alarms get louder by AGW proponents.
    scared and are far more pessimistic than their public pronouncements indicate.

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

  42. 42
    Doug Bostrom says:

    Regarding Walt’s #34 post, why not just stick with what apparently is done at Duae’s physicsforum by sticking with peer-reviewed material? Out go the anecdotes, right?

    But wait! Brief scrutiny of RC reveals that with the exception of book reviews and some general commentary it seems the RC principles already bases most topic threads on peer-reviewed material. Most if not all anecdotal input is dragged in from external sources, often of poor fabric and much the worse for wear.

    Walt’s suggestions that we find a common dialog and learn to understand differing points of view is admirable but at the same time some parameters on that idea need to be established.

    For instance, I can’t support a discussion on geochronology if I’m speaking with a person who believes the world is 6,000 years old; I simply don’t believe in magic, unsupported as magic is in the peer-reviewed literature. Just so, I can’t have a useful discussion with somebody who sincerely believes they’re struggling against a global conspiracy of scientists bent on unhinging civilization, a fantasy beyond magical thinking and into the realm of sad pathology.

    Equally, I can’t bring myself to have a serious discussion with somebody incapable of stringing enough thoughts together to understand the chain of causation that physics tells us will happen when we unleash a relatively sudden flood of a relatively large amount of CO2 into the atmosphere. Can I succeed in explaining long division to somebody unable to fathom multiplication?

    Somebody without a sufficient grasp of well-established science is not going to be able to have a rough understanding of why a competently assembled computational model of Earth’s atmosphere can deliver useful results. I personally think that’s why so many “skeptics” are so strangely suspicious of the concept and utility of simulations.

    So by all means, understanding where understanding is possible is a good thing. All the same, let’s also remember that contorted equivocation is pointless and counterproductive.

  43. 43
    MarkB says:

    Question for Dr. Shindell or those familiar with his work: How does your recent study differ significantly from recent work by Ramanathan & Carmichael? Does it mainly present details on regional effects?

  44. 44
    Lawrence Brown says:

    He’s Baa-aak. I hope this thread doesn’t degenerate into all kinds of OT arguments about the sincerity of one side or the other. The Shindell-Faluvegi paper is a lot more interesting on the effects of aerosols wrt AGW, the different impacts in different latitudes,the results of less sulfate emissions in western nations, etc,than whether a protagonist or antagonist is sincere or not. As Duae states in an earlier post, their work indicates a way to reduce uncertainty of the effect of aerosols, not the uncertainty of and any given individual’s position
    on climate change.

  45. 45
    Walt Bennett says:

    Re: #39


    AGW is in fact a theory, that human activity, specifically land use changes and greenhouse gas emissions, will warm the planet. It is an adjunct to GW theory itself, which states that greenhouse gases are what allow planet Earth to maintain a constant temperature which is 33K above what it would otherwise be.

    And to those who think this is a public policy nuance, you will need to reconsider that misguided notion.

    This is the heart of the matter.

    There is no question in most of our minds that AGW Theory is basically correct: short term climate sensitivity to a doubling of CO2 is 3*C+/-1.5*C. There is strong geological evidence that such an increase will cause significant sea level rise.

    Stop there.

    Every other “theory” (hypothesis, model result and so forth) is a guess. It is likely a highly educated guess, but who can say: in the year 2100 will there be more or less livable land mass than there is today? Will there be more or less food? Easier or more difficult shipping and transport? Will humans do poorly or maybe will they be better off?

    That’s where the doomsday scenarios come in. More desert, more storms, more severe storms, and on and on and on, mass extinction and on and on and on.

    That was the original commenter’s point, and I don’t know why it’s so hard to see, I honestly don’t.

    I have news for you: we’re all going to find out. In 2100 we will no longer be burning fossil fuels at anywhere near today’s levels, but by then atmospheric CO2 will be 500 or higher, because even after we stop contributing to the rise, the oceans will, the permafrost will, the lake beds will and so forth. It is as inevitable as it is inexorable.

    So we will see sea level rise, for sure, and some of today’s low lying lands will become bad places to live, and there will be mass migrations and certainly heavy loss of life. Again, I say: inevitable. What will emerge from that period?

    For all we know, a more vibrant planet and a more compatible and sustainable human footprint. Perhaps man and all other life will simply find a way to adapt to a planet where average seasonal temperatures are a few degrees higher than they are today.

    In any case, nobody knows, and the skeptics know who the fool is in that equation.

  46. 46

    #34 # Walt Bennett

    You continue to say things that are contextually irrelevant, even though you think they are relevant.

    We simply ‘know’ scientifically (with reasonable certainty based on relevant contexts), a lot more than you ‘believe’.

    And just because you think we don’t know those things reasonably well, or reasonably well enough to gauge their meaning in the context of future warming and climatic impacts on the human population and global economy, does not change the well reasoned science and the resultant well reasoned knowledge and understanding.

    You really don’t seem to have a good idea of the contexts involved from a scientific point of view.

    Yet you keep writing about how important your perspective is? Even though you have contradicted yourself many times as I, and others, have previously pointed out.

    This is kinda weird, don’t you think?

    Or do you think it is important to keep things as confused as possible to avoid needed action, due to your own perceptions? And if so, what is your motive, because it is obviously not based in the well understood science?

    You state:

    Much of what we are learning has no meaningful context. We don’t know precisely what causes it; we don’t know if it is short or long term; we don’t know if the eventual effect will be positive of negative with regard to global temps. What about any of that would make a skeptic stop being skeptical?

    1. Additional GHG’s impose additional forcing in the climate system
    2. The atmospheric lifetime of CO2 is hundreds of years.
    3. The paleo record combined with the quantitative analysis of addition GHG’s and reasonably expected feedbacks will produce positive forcing. The Magic cloud albedo of Richard Lindzen has no support in the paleo record (are you willing to state, and substantiate, it has not been warmer, or even much warmer than the current GMT?).

    How is it that you keep missing the science? Is your opinion getting in the way? It’s almost seems that your brain is stuck in the wrong gear?

    Lastly, IT’S NOT ABOUT EACH OTHERS POINT OF VIEW. It’s about the well understood science and the evidence modeled and measured.

    I also agree with Chris Colose, let’s not cloud this thread with such opinions, and he put it well, “warmist v. denialist” though I would add “v. science”.

  47. 47
    Doug Bostrom says:

    Walt #44: “Perhaps man and all other life will simply find a way to adapt to a planet where average seasonal temperatures are a few degrees higher than they are today.”

    Really, really quickly, right? And “all other life” lacking as it mostly does any notion of the future will have to adapt even faster.

    Right, sure, somehow all this adaptation will happen very, very quickly, with few or no casualties. There’s wild conjecture, all right.

    Here we are, once again, back at the pre-Enlightenment level and resorting to fantasizing a magical world where poor dumb beasts suddenly develop astounding powers of adaptation.

    As other posters have begged, can’t we move ahead from this incredibly infantile sort of conjecture? Why is that every single thread on this sight is quickly dragged down to the “EZ Science for Unicellular Organisms” level?

    I’m done with this sort of “discussion”. The science is in good hands, I’m sure of that, but the public understanding is stuck at about 6 years of age wth no improvement apparently possible.

  48. 48
    sidd says:

    Perhaps the moderators might consider a more stringent policy, based on relevance to the topic at hand.

    I come to this site to learn science. Yet I see the same old lies propagated in comments every day. I am sorry if I feed the trolls, but I sometimes feel compelled to respond. Hopefully, the more receptive will actually read these references.

    re: ocean dead zones:

    “Long-term ocean oxygen depletion in response to carbon dioxide emissions from fossil fuels’. Nature Geoscience, (in press)”

    re: wind patterns, Hadley cell expansion and increasing aridity:

    re: increased hurricane incidence and quotes from IPCC 2001 and previous: science has not stopped since 2001:

    re: MWP : dear god, not this again. MWP was not global, discussion on this site at

    Each of these references is easy to find, together with many more. Sometimes I feel that I am trapped doing homework for a horde of lazy children.

    Now a question on the lasting effects of black carbon. If we have a layer of black carbon on the surface of an ice sheet, how long would it last in the absence of further deposition. Let us say that we could stop depostion of black carbon on the Arctic ice sheets today. How long before the already deposited carbon is carried away by meltwater and albedo returns to normal ?

  49. 49
    Jim Eaton says:

    Re: #44 Walt Bennett

    Having just endured three days of summer temperatures in April, I’m not anxious to feel the effects of warming in coming years.

    On Monday, California broke temperatures records from the Oregon border to the Mexican border — sometimes by as much as 12 degrees F. Yesterday, more records were broken (but only by 11 degrees F). Today, it was only from Medford, Oregon to Fresno — only half the state.

    Yes, this is weather, not climate. But in my 60 years in California, I have never seen any spring event that shattered such high temperature records statewide. Maybe it’s time to start thinking that we really are seeing signs of warming now, not just debating a theoretical warming in the future.

  50. 50

    #40 John H

    While I don’t think that all creatures will die, likely a tremendous number of species are at risk and will die if we don’t act rather fast. Many species extinctions may now be inevitable based on current human effects, impacts, and that includes global warming.

    How does MWP hurt the hockey stick? Here is the hockey stick with the MWP which is included in IPCC reports toward the bottom of the page:

    It is reasonable that global warming will produce more cat 5 hurricanes. What is it about ‘warmer water gives more energy to hurricanes’ that you don’t understand.

    There is a lot going on in the oceans, maybe you should take a look at the ocean studies to understand the different human impacts on the earth oceans

    As to water shortages well, Hoover dam is expected to shut down in 2023 according to the folks at the dam due to all the water disappearing and there is a clear latitudinal shift pushing water north, which will cause in more southern regions… uh, what do you call it??? oh yeah, water shortages.

    As far as envisioning goes, it is not unreasonable to envision the changes and their effects (and the increasing costs). Same for you take on IPCC reports of hurricane signal. There is a visible trend already, so I guess you are one of those that believe we should wait until it’s worse, but you don’t understand the forcing and the inertia issues not to mention the feedbacks.

    Generally speaking you are a unreasonable on these issues.

    Arctic Sea ice mass is rapidly diminishing and the Antarctic WAIS is not stable, and while there is more snow falling down there increasing mass that actually confirms global warming and has been expected in the models.

    Generally, it’s a really good idea to study to understand the relevant contexts before you post such out of context stuff.

    As far as honest conversations, let’s be honest. Things are not exactly rosy in our environment, species reduction and extinction is a huge issue and it’s going to get worse… our best chance is to work hard and fast on solutions.