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Global Dimming and climate models

Filed under: — group @ 17 April 2006

Guest posting from Beate Liepert (LDEO)

On April 18th PBS will air the NOVA documentary “Dimming the Sun” which stirred up lively discussions among scientists and non-scientists when originally shown by BBC in the UK (under the name ‘Global Dimming’ – see our previous posts). [The NOVA version has been thoroughly re-edited and some of the more controversial claims have apparently been excised or better put into context [and we look forward to seeing it! - Ed.].

Global dimming is the phenomena of an observed reduction (about 1-2% per decade since ~1960) of sunlight reaching the surface of the Earth caused by air pollution (aerosols – small particles) and cloud changes. Some of this solar energy is reflected back out to space and this cooling effect is believed to have counteracted part of the greenhouse gas warming. The original version of the film focused mainly on the observational recognition of global dimming, but one aspect did not receive much attention in the film – namely the oft-claimed lack of global dimming in climate models. This led some to assume that climate modelers were ignoring air pollution other than greenhouse gases emissions from fossil fuel burning. Another implication was that climate models are not capable of adequately simulating the transfer of sunlight through the atmosphere and the role of clouds, sunlight extinction of aerosols and aerosol effects on clouds etc, and therefore model projections should not be trusted. The NOVA version will address this issue more prominently by adding an interview with Jim Hansen from NASA Goddard Institute for Space Studies. Along this line, I’d like to elaborate on aerosols in climate models in more detail.

It is indeed true that the first climate change simulations were predominantly concerned with greenhouse gas forced climate change. Albeit very early papers argue that man-made aerosol changes might cause a cooling (Rasool and Schneider, Science 1971 and Bryson, Science 1974). And already in 1990, J. Hansen and A. Lacis (Science 1990) published a paper where they explicitly discuss the importance of anthropogenic aerosol forcing: “Sun and Dust versus Greenhouse Gas Forcing”. The authors list direct effects of increasing concentrations of light scattering sulfate aerosols, light absorbing carbonaceous aerosols like soot and even aerosol effects on cloud properties (indirect aerosol effects). Finally they conclude that “… solar variability will not counteract greenhouse warming and that future observations will need to be made to quantify the role of tropospheric aerosols … ”. The surface dimming effect was not yet considered an important climate factor. Back then, state-of-the-art climate models changed the reflectivity at the top of the atmosphere to account for the climate effect of increases in man-made aerosol emissions. Using cloud properties from independent climate simulations and weather forecast models to provide monthly mean water vapor and temperature fields, Kiehl and Briegleb (Science 1993) estimated a top of the atmosphere global mean human-related sulfate aerosol forcing of -0.3 W/m2 in contrast to the a +2.1 W/m2 greenhouse gas forcing. A year later, Jones, Roberts and Slingo (Nature 1994) added the indirect aerosol effect – the impact of increasing sulfate aerosol concentrations on cloud droplet sizes – which make look cloud darker. They used empirical relations to link the number of aerosol particles and number of cloud droplets to cloud droplet radii for their estimates. Other groups as well, started testing new prognostic cloud schemes for general circulation models that were able to capture the microphysical processes of cloud formation (e.g. MPI in Hamburg – Lohmann and Roeckner, Climate Dynamics 1996). These climate-modeling developments were compared (Wild et al., JC 1995) with the then available observational data of the surface solar radiation (incidentally the same data sets were used by Russak (Tellus 1990), Stanhill and Moreshet (Climatic Change 1992) and myself (Liepert et al., Contr. Atm. Physics 1994) to reconstruct the history of global dimming). One conclusion was that then-current models did not include enough aerosol absorbtion in the atmosphere (and Wild and I (GRL, 1998) wrote a paper on the “Excessive Transmission of Solar Radiation Through the Cloud-free Atmosphere in GCMs”). Note that at the end of 1990s these more complex climate models with a more physically based prognostic cloud scheme were run as equilibrium experiments hence transient 20th Century changes could not be used directly for comparison. But is has always been clear that anthropogenic aerosols are so temporally and spatially variable that long-term means are not adequate in assessing the actual aerosol forcing.

Several publications on model validations and improvements based on surface solar radiation records followed and I was involved in two of these studies. We analyzed the simulated multi-decadal changes in the direct tropospheric aerosol forcing in the NASA GISS GCM and utilized global dimming time series of the United States and Germany to assess the temporal change prognosed in the model (Liepert and Tegen, JGR 2002). Ina Tegen’s aerosol model was one of the first that added time variations in carbonaceous aerosol components (including black carbon – an absorbing aerosol). We concluded that increasing absorption might actually play a stronger role than expected.

Climate simulations are the primary tools for explaining and understanding observations that might otherwise seem counterintuitive. For instance, how global dimming can go hand in hand with global warming. In 2004, at the MPI in Hamburg my coauthors and I (Liepert et al. GRL 2004) analyzed output data from a brand new version of the ECHAM general circulation model (GCM) that incorporated a fully interactive aerosol module and aerosol-cloud-scheme. This model interactively calculates aerosol chemical transformation, aerosol transport, rainout and fallout processes and even aerosol formation for some species (e.g. sulfate). We showed that, in the model, global warming caused changing rainfall patterns that fed back on aerosol distribution and composition supressing the water cycle (i.e. evaporation) as had been observed. The key to explaining the apparent contradiction was that the surface forcing changes can be very large without affecting the top-of-the-atmosphere radiation as much.

All major climate models now have some representation of aerosol physics though they range in their complexity – e.g. from top of the atmosphere aerosol forcing to highly interactive aerosol-cloud modules. The role that aerosols play in issues like the Sahel drought (Rotstayn and Lohmann, JC, 2002) or the Asian Brown Clouds (Ramanathan et al., PNAS 2005) is starting to be understood (and both these examples are featured in the documentary), but we do not as yet have a clear picture of exactly how aerosols and the other human-related forcings have affected climate.

More recently, many modeling groups ran 20th Century climate simulations in support of the Intergovernmental Panel on Climatic Change 4th Assessment Report (IPCC AR4) that include representations of the aerosol direct, indirect and semi-direct aerosol effects. The models show a global dimming effect of between 1 to 4 W/m2 over the 100 years with simultaneous global warming between 0.4 and 0.7°C (Romanou et al, under revision) which match the observational dimming quite well.

Overall, in the fifteen years from the 1990 Hansen and Lacis paper to the IPCC AR4, major steps forward have been made in implementing aerosols in climate models and hence matching observations of global dimming. However, it would be misleading to claim that the new appreciation for the surface energy balance changes implied that modelers a few years back were ignorant about the role of aerosols in other aspects of climate change. It is indeed a very complex problem.

92 Responses to “Global Dimming and climate models”

  1. 1
    ocean says:

    I am sure this is a stupid question for the scientists who post here, but I am new to this: do aerosols, especially the dark ones, have an insulating effect for the lower atmosphere therefore increasing the amount of heat trapped there while reflecting sunlight from the upper atmosphere?

    [Response: There is nothing obvious about aerosol effects, so no questions are stupid! Reflective aerosols (sulfates, nitrates) cause light to scatter back out to space and so have a cooling effect, but they also absorb a little and casue a local warming (this happens with volcanic aerosols as well). The dark ones (basically soot) absorb more and darken the earth relative to what it was and so end up warming the planet. Aerosols also have effects in the long-wave and interact with clouds and chemistry so the net effects are pretty complicated - hence the uncertainty in assessing their forcing. - gavin]

  2. 2
    Doug H says:

    A little more explanation of these different effects of aerosols would be helpful: “light scattering sulfate aerosols, light absorbing carbonaceous aerosols like soot and even aerosol effects on cloud properties (indirect aerosol effects).”

    Good post. Thanks.

    [Response: Sorry, one of our previous posts went into this in some detail - I should have linked it: - gavin]

  3. 3
    Blair Dowden says:

    Why does the NOVA documentary (and this review) focus on “Dimming the Sun”, when according to your Global Dimming may have a brighter future radiation at the surface has been brightening since 1990? And given that the rise in global temperature has been fairly steady for the past few decades, what (if anything) does this dimming / brightening cycle tell us about climate sensitivity?

    [Response: The post 1990 'brightening' is less certain than the longer term dimming and is more fraught with data quality and length of time series issues. It may also be restricted to European and US stations, rather than Asian ones. We'll see though once the data series get longer. Does this mean anything for 'climate sensitivity'? In the specific sense of the climate response to 2xCO2, no it doesn't. In the more general sense of how is the planet reacting to all the changes we've made, then yes, the surface energy budget is a big (but complicated) part of that. - gavin]

  4. 4
    Paul Duignan says:

    If, one were to imagine for a moment, a radical conservation campaign which reduced global carbon dioxide emissions by say 30%. Does anyone have any idea of what that might do to aerosols? Would it significantly reverse global dimming and would that translate into short-term temperature increases?

  5. 5
    Ron R. says:

    I am suspecting that there is a direct link between tropical deforestation and the increasing strength of El Nino’s. If so, how might this tie in with climate change? Can someone speak to this?

  6. 6
    Roy D says:

    There was a Science paper in 2003 by Anderson et al. that claimed that forward calculating aerosol models (models directly based on aerosol physics) predicted about 50% more forcing than indirect models (models that use temperature change to infer expected aerosol forcing). Has modeling work improved upon the disagreement since then?

  7. 7
    muller.charles says:

    Hello. I’m a beginner in climate topics (and moreover, a French beginner), so please be tolerant with my naive questions.
    The first question : it was said last year (Wild 2005, Pinker 2005) that Earth encounters a straight brightening since 1980′s And now we’re back to global dimming. So, what is the real current trend (past two decades) ?
    The second question : global dimming / brightening affect solar radiation budget on atmosphere and surface. Could we expect a response of Earth temperature to that specific phenomena, relatively independant of ghg forcing ?

    [Response: The effects of aerosols approximately (though not exactly) can be just linearly superposed on the effects of GHG's. However, in looking at the effects of aerosols, it is important to distinguish between the effect on the top of atmosphere radiation budget and the effect on the surface radiation budget. Some aerosols (like soot, which absorb solar radiation) reduce the surface radiation but have little effect on the top of atmosphere -- they can even increase the solar energy absorbed. The effect of such aerosols is tricky, though. If the absorption is in the lower troposphere and doesn't shut off convection, the effect is to change the precipitation (less solar energy reaching the surface to drive evaporation), not temperature. Absorbing aerosols in the lower stratosphere or upper troposphere can actually cool the surface even if they increase solar absorption. That's because when you absorb solar energy in a soot layer high up, only half the energy gets radiated back downward to the surface as IR. The rest gets re-radiated back upward as IR. The extreme form of this is Nuclear Winter, where a thick soot layer shuts off tropospheric convection entirely and reduces the surface temperature to something approximating what the Earth would have without a greenhouse effect. Add to that the fact that many aerosols are also active in the infrared, and add to the greenhouse effect. As the article says, it gets complicated. Aerosols which dominantly reflect sunlight, and indirect aerosol effects which reduce the particle size in clouds, have a pretty unambiguous cooling effect. --raypierre]

  8. 8
    Mark Lynas says:

    What I don’t get about the programme is that it cites the 9/11 studies which apparently show a cooling in the atmosphere when the US air fleet was grounded and there weren’t any vapour trails in the sky. But I thought that cirrus contrails had a warming effect? Any pointers?


  9. 9
    pete best says:

    I presume that what is being said here is that “global dimming” kept global warming from reaching projected climate model estimates but now that humankind has cleaned up some of the pollution then more light will reach the earth and hence more heat will be released from the earth to be trapped by CO2 and the like. Could the effect be dramatic and bring forward 2C of warming by some decades or are we talking about a lot of scientific effort here to answer the detractors and skeptics ?

  10. 10
    Beate says:

    The term “brightening” seems misleading in my opinion. “Dimming” (caused by air pollution) didn’t go away in the last years. There are still lots of pollutants in the atmosphere (in the US and Europe as well, not just in Asia) but less that ten years ago in many regions of the world. And these pollutants still dim the sunlight and change clouds.

  11. 11 says:

    I apologize because this post is not directly related to the topic being discussed. But I wanted to suggest a possible article topic to someone associated with this site.

    A lot of mileage has been gained from the ice core samples in Greenland and elsewhere. An expensive proposition by most scientific standards but small compared to say a military project.

    What other information gathering initiatives could be funded right now to give us information? How much would they cost and what would they be designed to determine? Is there any discussion about implementing any of them?

    This is a pretty large question, and not designed to address any one person’s area of expertise. And I was most curious about projects that might take a large amount of funding, say a billion or so US dollars (please don’t laugh, I think it might be unrealistic but considering the possible ramifications…)

  12. 12
    Alan says:

    RE #9. I understand what you mean, you get so used to the shills you start to habitually ignore some things. When I first heard the “dimming” claim a while back I also thought it was an industry beat-up, this is not to say the “skeptics” haven’t tried to twist it for their own benifit.

  13. 13
    Roger Hill says:

    Watts in watts out, I can only hear the Enterprise institutes of the world saying “see pollution is just great” more pollution will solve all of our problems.

    Scotty beam me up!

    [Response: I haven't heard any group proposing more pollution to offset CO2. It would be pretty dicey because: (1) the geographical distribution of aerosol cooling is very different from GHG warming and, (2) Aerosols have a lifetime of only a few weeks in the atmosphere, so if there were a world depression that shut down polluting industry, you'd see the full GHG radiative forcing hit you within a few weeks. Talk about abrupt change! There have been other geoengineering proposals, though. Teller's group was proposing putting a ring of fine copper wire fragments in orbit to reflect some sunlight, and there has also been talk of shading by mirrors in space. As geoengineering goes, these strike me as pretty fanciful compared to proposals to remove CO2 from the atmosphere by chemical processes, and sequester the resulting gas. That's pretty fanciful, too, but at least it can be tested on a pilot basis with current technology. --raypierre]

  14. 14
    Richard Harvey says:

    Just a remark on a small typo. The phrase :

    “[...] the indirect aerosol effect – the impact of increasing sulfate aerosol concentrations on cloud droplet sizes – which make look cloud darker.”

    should read:

    “[...] the indirect aerosol effect – the impact of increasing sulfate aerosol concentrations on cloud droplet sizes – which make look cloud *brighter*.”

  15. 15
    Hank Roberts says:

    #8, Mark, how contrails work — sum up both day and night effects.

    Quoting below from this (first quick Google result) — you can do better but this article’s footnotes/cites will lead you to more info:

    “… These thin clouds slightly cool Earth’s surface by blocking some incoming sunlight, but they moderately warm the lower atmosphere by trapping a portion of Earth’s outbound infrared radiation….
    “… (DTR) – the difference between the day’s high and low temperatures… During the 3-day hiatus of air traffic…, the average DTR was a little over 1°C wider than normal, even though the average DTRs computed for the 3-day periods immediately before and after that period were below normal.
    “…. areas of the country typically blanketed with aircraft contrails in mid-September…showed the largest changes in diurnal temperature range, mostly from increased daytime high temperatures. “

  16. 16
    Hank Roberts says:

    #14 — there is a typo, but I believe clouds with larger droplet size do look darker, “threatening rain” clouds typically do. Can’t cite that at the moment though.

  17. 17
    Beate says:

    Just a brief clarification to 14. “[...] the indirect aerosol effect – the impact of increasing sulfate aerosol concentrations on cloud droplet sizes – which make look cloud *brighter* / *darker*” This actually is not a typo. From the satellites perspective clouds look “brighter” (famous ship track image) because they reflect more sunlight back to space. But from the surface point of view these clouds look “darker”. And since we talk about dimming at the surface I chose the word “darker”.

  18. 18
    Sean Houlihane says:

    This is remarcably well timed, given the accuracy of the BBC’s climate prediction experiment – which seems to have actually given some insight into the effectiveness of attempting to model a complex system. Question… If the program had not actually crashed in 2013, would the results have been believed?

  19. 19
    Merry Maisel says:

    wrt #14: “which make look cloud” is simply not English. Although all posters have ignored it, could the original writer re-render this sentence in standard English? Is it
    “makes clouds look/appear” brighter or dimmer? If English is the language of science, let’s use English. English has standards also, easy to look up and verify, including idiomatic word order and agreement in number, tense, and case. Thanks, Merry.

  20. 20
    Bryn Hughes says:

    Actually the BBC programme stated that Global Dimmming was caused by Jet Aircraft Con Trails.
    The number of these con trails has increased steadily since 1960.
    There was an increase in diurnal temperature range by 1 degree centigrade over the USA immediately after 9/11 when all aircraft were grounded for 3 days with the days being warmer and the nights being cooler (Dr David Travis Wisconsin U.)
    Haven’t sulfate aerosols reduced in recent years? Weren’t they the cause of the cold spell in the 1960s?

  21. 21
    SqueakyRat says:

    Congrats to RC and Gavin Schmidt on the attention in Nick Kristof’s slightly silly though well-intended column. The piece is behind the NYTimes subscription wall but includes a link to RC. You guys are starting to make a difference.

  22. 22
    Dan says:

    re: 20.
    A change of 1 degree C in the diurnal range in the immediate days after 9/11 may have also been simply due to the synoptic pattern. A relatively dry Canadian high pressure system was over a large portion of the country which could have easily caused a larger diurnal range.

  23. 23
    Don Baccus says:

    #21: I fear that Kristof’s column, though well-intentioned, may backfire. He quotes some of the wilder speculation about methane clathrates that were in the piece that appears at Real Climate, with just a hint that these outcomes are thought to be extremely unlikely.

    Of coures he states that he’s enganging in a bit of hyperbole (actually he says “fearmongering”) but he’s given the other side a lot of opportunity for quote mining.

    [Response: On the contrary -- Kristof did an excellent job of pointing out methane clathrates as an attention-getting possible catastrophe which is possible though highly uncertain. He states very explicitly that the more certain consequences of global warming ought to be sufficient to get action, and it seems pretty clear that what he is really lamenting is the necessity to claim the end of the world before you can get any attention from the government. Heck, even when you do claim the end of the world and make a good case for it, you don't always get action. Kristof's campaign to get the world to do something about Darfur (where the world has indeed ended for millions of people) is a case in point. As for quote mining, the dark side of the Force is so good at quote mining that if people worried about that they'd never say anything. --raypierre]

  24. 24

    with regard to #19 and “wrt #14: “which make look cloud” is simply not English. Although all posters have ignored it, could the original writer re-render this sentence in standard English? Is it
    “makes clouds look/appear” brighter or dimmer? If English is the language of science, let’s use English. English has standards also, easy to look up and verify, including idiomatic word order and agreement in number, tense, and case. Thanks, Merry. ”

    Why you which make look cloud not equals like? For have perfectly good can understand meaning of sentence is the same! Also.

    [Response: Ah wise-guy Master Levenson is. -- yoda]

  25. 25
    Paul Duignan says:

    Regarding the response to #13 “As geoengineering goes, these strike me as pretty fanciful compared to proposals to remove CO2 from the atmosphere by chemical processes, and sequester the resulting gas. That’s pretty fanciful, too, but at least it can be tested on a pilot basis with current technology.” Any comment on the amount of energy this would take. What if a large part of the remaining oil reserves were used to do this. Would that have any chance of making a difference?

  26. 26
    Matt Middleton says:

    So then do you still believe that the claim in the program of a global temperature increase of 10C is still outlandish, or do you believe that this is now in the range of possibilities?

  27. 27
    Hank Roberts says:

    Using an irreversible fix for a problem that’s due to variables seems ill-advised.

    (If a wild card comes up — the sun gets a bit cooler, or the solar system moves into some interstellar dust cloud, or one of the dust rings orbiting along with Earth has a thick spot develop affecting us — it’d be a lot easier to uncork sequestered CO2 or burn coal than to fine tune a belt of orbiting shade-dust or adjust a sunshade at L1, I bet.)

    Lagrange 1 point is unstable; a sunshade there would need adjustment every 23 days or so:

    Dust in space:

    Of course I’d love to see NASA have the capability — but let’s fiddle with Venus first, okay? Once it’s on the way to being successfully terraformed, that’s soon enough to make astronomical-sized changes to Earth.

  28. 28

    One aspect of this involves longer term trends. The environmental Kuznets curve phenomenon is relevant here in that higher income countries tend to reduce their emissions of both aerosols and suflates, although these are still rising in the very important countries of China and India, sources of so much controversy in the Kyoto negotiations. But in the US and Western Europe and Japan, they are definitely declining. OTOH, CO2 is known not to decrease as income rises and continues to increase in emissions (and certainly in ambient quantities). I have even seen speculation that part of what turned around the rough cooling between the 1930s and 1970s and turned it into the more recent warming trend was this switch as the richer countries started to cut back the cooling aerosol sulfates but continued to increase CO2, although if the numbers reported in above are correct, it would seem that the cooling was not strong enough to be responsible for that.

    BTW, on an earlier post I noted the issue of the nature of the nonlinearity of GHG amounts to warming, which is logarithmic. Raypierre reported it is in the models, which I knew. The issue was the values of the parameters and how they interact with other effects going on. One of these indeed is this sulfate aerosol cooling, which is well covered here. However, that raises the issue that Raypierre reported it to be linear on top of the presumably logarithmic CO2 effect. Is it linear? How well understood is this?

  29. 29


    Although there may be more understanding of what aerosols are doing, I still have a lot of unanswered questions (see here), which make me rather skeptic about the overall cooling effect of aerosols, as currently implemented in climate models.

    - If tropospheric human-made sulfate aerosols have the same physical properties as stratospheric volcanic ones, the overall primary cooling effect would be around 0.025 K, which is not very impressive. I didn’t find an explanation for any difference in properties in the literature.
    - I didn’t find a measurable effect on temperature trends between less and more polluted areas in Europe, as result of the large reduction in sulfate emissions (Raypierre did give a reference to a study, but that is behind a -rather expensive- issue fee wall).
    - Aerosol emissions are largely in the NH, but warming of the oceans is far higher in the NH (if corrected for area). There is very little exchange of aerosols between the NH and SH.
    - Aerosol emissions are largely in the NH, but global dimming was also measured in Australia and down to the South Pole…

    My impression is that much of the global dimming is water vapor related, not aerosol related…

    [Response: If you think that, perhaps you can tell us what water vapor shortwave absorption bands have been missed in the past century during which the spectroscopy of water vapor has been extensively studied. Or did you perhaps mean "clouds" rather than water vapor directly? I wonder how you are distinguishing between aerosol effect and cloud effects due to the indirect aerosol effect. --raypierre]

  30. 30

    Re #3 (comment)

    Gavin, according to the graph in Wild ea. in Science, in Asia, most India stations see an increased dimming, while most China stations show decreased dimming. While this may be attributable to increased aerosols in India, for China this is hardly the case.
    And in Climate sensitivity and aerosol forcings the range of aerosol forcings is directly connected to the doubling CO2 sensitivity of 1.5-4.5 K. Of course, that are the ranges for different GCM’s, but I suppose that if one changes the aerosol forcing in one GCM, the result would be a change in 2xCO2 sensitivity too.

  31. 31
    ocean says:

    Ok, what is the difference between the “cloud” effect and the aerosol effect?

    [Response: The (direct) aerosol effect arises from the radiative influence of the tiny aerosol particles themselves -- sulfate droplets, soot particles, mineral dust, etc. The indirect aerosol cloud effect arises from the role of aerosols as nucleation sites. Certain aerosols can make the typical drop size in clouds get smaller, resulting in a more reflective cloud. In addition, climate change -- whether due to CO2 increases, aerosol forcing, or anything else -- will generally cause the cloud properties to change regardless of what aerosols do to cloud particle size. This would also cause a change in the amount of solar radiation reaching the surface. --raypierrre]

  32. 32
    Bryn Hughes says:

    Re 22
    But the skies were clear of vapour trails surely that was the main cause of the change throughout the USA rather than your theoretical synoptic pattern.
    Dr Travis is a good enough climatologist to recognise if such a pattern had caused the increased range and would have come to a different conclusion than the one he arrived at.

  33. 33

    Re #29 (comment)

    Indeed Raypierre, I meant water vapor and clouds, but I am not sure what is more important. As far as I have read, most insolation measurements are done over a broad range of short waves (as energy input), without looking at specific absorption bands (but I may be wrong!). If water vapor increases, absorption in the near IR will increase and thus affect the overall incoming energy. Do you have information about stations which have done long-term spectroscopy measurements of incoming sunlight?

    If clouds are involved, that can be from aerosols, or from (natural) internal or forced variability. The secondary effect of less aerosols on clouds would be in the higher latitudes for Europe and North America, where the radiation balance doesn’t change much with more or less clouds (according to Philipona). The increasing SE Asian aerosol emissions are at low latitudes, which should increase tropical cloud albedo and lifetime, but the recent trend in the tropics is that there are significantly less clouds…

    Together with the global dimming (and reverse) in Australia and the South Pole, this points to a non-aerosol (probably natural) origin…

    [Response: So far as I am aware there are no long term high spectral resolution time series of the solar radiation, but last time I checked the water vapor molecule had not changed much over time. Based on that, I'm pretty willing to trust laboratory spectroscopy. It would take a darn huge overlooked band to give you the kind of signal the global dimming folks are seeing. NO2, on the other hand, has important solar absorption bands and in very polluted air could conceivably be present in sufficient concentrations to make a difference -- particularly if the absorption is amplified by multiple scattering in clouds. --raypierre]

  34. 34

    Venus has lots of clouds, and it does not have man made aerosols. Why are aerosols neccessary to cause global dimming? Could it no just be that as global temperature rises so does absolute humidity? That would cause larger water droplets in the clouds for the same number of condensation nuclei.

    Larger droplets means more blackbody radiation back to earth from the clouds so global warming, but also more reflection to space so global dimming.

    [Response: The clouds on Venus are mainly composed of sulfur dioxide and sulfuric acid; you could almost say they have more in common with sulfate aerosols on Earth than they do with water clouds. (almost, but not quite). Regarding your global dimming question, it is entirely possible, in principle, that water cloud changes could cause global dimming without aerosols. It's a matter of doing the observations and modelling right to try to separate changes in cloud patterns from aerosol effects. Not easy. However, there is no simple or direct connection between temperature and cloudiness, not even via absolute humidity. There are well-known cloud mechanisms that either increase or decrease cloudiness with temperature, increase or decrease droplet size, or increase or decrease cloud height. The simplistic reasoning for water vapor (warmer equals more, even aloft) seems to work, for rather subtle reasons. There is no such simplistic reasoning that works for clouds. That's why they're such a headache. Note also that larger cloud drops (with fixed total water mass in the cloud) reduce the cloud albedo. They don't increase it. Reflection sort of goes with net cross section area. You get a lot more of that with a hundred trillion small drops than you get with one drop the size of a beachball. --raypierre]

  35. 35
    Dan says:


    With all due respect, I was not referring to a “theoretical” synoptic pattern at all. I was referring to the *actual* surface synoptic patterns on 9/11 and on the following days at, specifically 01091112.gif et seq. Yes, lack of vapor trails may also be a factor but the synoptic pattern could just as easily be an equal or stronger factor. It appears that a somewhat-modified Canadian high pressure was over the substantial portion of the US. Which could have easily caused a larger diurnal range in that period. Without a detailed study and analysis, they is no certainty at all that the lack of vapor trails was the cause of the change.

  36. 36
    Dan says:

    According to Dr. Travis’ article in Nature magazine at, “we attribute at least a portion of (the daily temperature range) anomaly to the absence of contrails”, which is not to say that they were the “main cause” of the anomaly.

  37. 37
    Hank Roberts says:

    They talked to interviewers about that system you mention, I think — see the 2nd quote below.

    Did you check Google Scholar or PubMed for other studies?

    Canada and Northern Mexico did not halt aircraft flights. Temps from those areas would be interesting, especially right along the borders; probably someone’s looked at them (I imagine they had fewer flights since nothing entered or left the USA during those days). A lot of data mining becomes possible when something like this happens.

    More quotes found among other places here:

    “Other studies looked at cloud cover before the advent of heavy jet traffic in the 1960s and afterwards, but these studies really provide circumstantial evidence.”


    ” “Satellite images showed that cloud cover on Sept. 11 was light, but that cloud cover and humidity increased on the 12th, 13th and 14th,” says Carleton. “These clouds and greater humidity should have suppressed the range, but the temperature range was still the largest in 30 years.”"

  38. 38
    shargash says:

    I just watched the show on PBS. There was lots of stuff with Hansen, which I gather was new relative to the British version. It was very interesting, but I haven’t digested it all yet. I’m anxiously awaiting followups from the scientists. :)

  39. 39
    pat neuman says:

    re 20. Comment by Bryn Hughes … cause of the cold spell in the 1960s?

    I doubt aerosols were the reason. At most U.S. climate stations, the average overnight minimum temperatures (even in winter) followed the same warm 30s-40s/cool 50s-60s pattern as the average daily means and global temperatures. It is unlikely that aerosols cooling would have had a large effect overnight.

    The late 30s and 40s had frequent warm El Ninos while the 50s-60s had cool La Ninas, causing the apparent cooler 60s.

    What caused the warm late 20s-mid 30s? Large areas of the Midwest and West had unusually dry conditions 20s-mid30s, creating the dust bowl years. The 20s-mid30s may have been a period of unusually high solar radiation due to a period of globally low cloud cover, which allowed unusually large amount of solar radiation to reach oceans and land surface, leading to subsequent frequent El Ninos late 30s/40s.

    Summer average dewpoints at Minneapolis, 1903-2004 at:

    Average minimum temperatures at some climate stations in the U.S. at:

  40. 40
    Hank Roberts says:

    More here, including much more in the way of comparisons.

    “A logical extension to this research is to determine where contrail coverage would have occurred had commercial aircraft not been grounded during Sept. 11-13, 2001. The analysis of AVHRR imagery available for this period indicates numerous occurrences of single contrails produced by military aircraft. Moreover, the analysis of imagery available for the grounding period shows contrails occurring just over the border in Canada on multiple occasions. Both observations suggest that contrail outbreaks would have occurred in portions of the U.S. had commercial aircraft not been grounded between September 11-13, 2001.”

    Google’s HTML version without images here:

  41. 41
    Mark A. York says:

    It was a great show. Very clearly presented. Hansen, in my view is a true American hero.

  42. 42
    Bryn Hughes says:

    Re 35
    Surely Dr Travis made such a study before he published.
    Re 39
    ” is unlikely that aerosols cooling would have had a large effect overnight”
    If the days were colder then would not the nights be colder too?
    Everything Ive read to date tells me that the cold 1960s were caused by sulfate aerosols.

  43. 43
    jhm says:

    [Response: I haven't heard any group proposing more pollution to offset CO2.....]
    I note that President Bush, after the release of yet another attempt to find some science to support our ignoring GW, spoke of the of the reflective aspects of soot being one of the positive factors, or confusing factors in the debate.

    Having watched the show last night, my question concerns radiational cooling. The 9/11 data used were temperature spreads. How much of this spread is a result of lower night time temps? On the same idea, would not thicker, more reflective cloud cover increase the warmth? In other words, does radiational heating out trump radiational cooling? I know this is phrased badly, but I feel that the idea is clear.

    Also, I was wondering about the importance of the location of the pollution. It seems that the effects of the same level of pollution over different areas (with different albedo, over ocean, near large populations, et cetera) will vary widely. Perhaps we could find safe places to pollute.

    I’m sorry if this has been asked above.

  44. 44
    tom brogle says:

    Re 43
    Is it suggested then: that although less radiation arrives at the ground a higher proportion of it is absorbed by the earth because it it prevented from radiating out by the contrails.
    That means that Global Dimming could lead to Global Warming.

  45. 45
    Brian Forbes says:

    Are you inferring that global dimming leads to global warming?

  46. 46
    Alisa Brooks says:

    OK, I’m really confused now…I got this from the PBS website that discussed the program: In the early 21st century, it’s become clear that air pollution can significantly reduce the amount of sunlight reaching Earth, lower temperatures, and mask the warming effects of greenhouse gases. Climate researcher James Hansen estimates that “global dimming” is cooling our planet by more than a degree Celsius (1.8°F) and fears that as we cut back on pollution, global warming may escalate to a point of no return.

    Do you think that Dr. Hansen is trying to say that we need to look at the consequences of REDUCING GHG?

  47. 47
    Hank Roberts says:

    jhm — see the full paper, link is in response #40.

    You asked if the increased spread was due to nighttime cooling; see the last line quoted in #15, “changes in diurnal temperature range, mostly from increased daytime high temperatures.”

  48. 48
    Bryn Hughes says:

    “changes in diurnal temperature range, mostly from increased daytime high temperatures”
    But if the night temperatures were lowered when there were no contrails,then it would mean that the contrails are warming the earth.

  49. 49
    Hank Roberts says:

    Alisa — ‘air pollution’ is soot, sulfates, dust particles — solids. Greenhouse gases are methane, CO2, water vapor, etc. — gases.

  50. 50
    Pete Conde says:

    What can we do to help with global warming? what is expected from the people of the world?

    [Response: Better energy efficiency in all sectors -- homes and commercial buildings are an especially big target. Use trains and public transit. Drive a small car, preferably a hybrid. Stop building coal fired power plants, and make sure that the coal plants that are built use IGCC technology with good plans for ultimate carbon sequestration. Vote for legislators (whatever their party) who understand the problem and the need for action. If you can manage to be a vegetarian, and to eat more locally grown food, that also helps. --raypierre]

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