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  1. 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]

    Comment by ocean — 17 Apr 2006 @ 6:54 PM

  2. 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]

    Comment by Doug H — 17 Apr 2006 @ 7:06 PM

  3. 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]

    Comment by Blair Dowden — 17 Apr 2006 @ 7:28 PM

  4. 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?

    Comment by Paul Duignan — 17 Apr 2006 @ 8:48 PM

  5. 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?

    Comment by Ron R. — 17 Apr 2006 @ 9:19 PM

  6. 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?

    Comment by Roy D — 17 Apr 2006 @ 11:29 PM

  7. 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]

    Comment by muller.charles — 18 Apr 2006 @ 4:26 AM

  8. 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?


    Comment by Mark Lynas — 18 Apr 2006 @ 5:28 AM

  9. 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 ?

    Comment by pete best — 18 Apr 2006 @ 5:49 AM

  10. 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.

    Comment by Beate — 18 Apr 2006 @ 9:21 AM

  11. 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…)

    Comment by — 18 Apr 2006 @ 9:26 AM

  12. 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.

    Comment by Alan — 18 Apr 2006 @ 9:32 AM

  13. 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]

    Comment by Roger Hill — 18 Apr 2006 @ 9:34 AM

  14. 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*.”

    Comment by Richard Harvey — 18 Apr 2006 @ 11:04 AM

  15. #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. “

    Comment by Hank Roberts — 18 Apr 2006 @ 11:08 AM

  16. #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.

    Comment by Hank Roberts — 18 Apr 2006 @ 11:17 AM

  17. 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”.

    Comment by Beate — 18 Apr 2006 @ 11:45 AM

  18. 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?

    Comment by Sean Houlihane — 18 Apr 2006 @ 11:47 AM

  19. 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.

    Comment by Merry Maisel — 18 Apr 2006 @ 12:29 PM

  20. 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?

    Comment by Bryn Hughes — 18 Apr 2006 @ 12:52 PM

  21. 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.

    Comment by SqueakyRat — 18 Apr 2006 @ 2:13 PM

  22. 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.

    Comment by Dan — 18 Apr 2006 @ 2:40 PM

  23. #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]

    Comment by Don Baccus — 18 Apr 2006 @ 2:47 PM

  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]

    Comment by Barton Paul Levenson — 18 Apr 2006 @ 3:34 PM

  25. 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?

    Comment by Paul Duignan — 18 Apr 2006 @ 4:21 PM

  26. 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?

    Comment by Matt Middleton — 18 Apr 2006 @ 4:30 PM

  27. 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.

    Comment by Hank Roberts — 18 Apr 2006 @ 4:33 PM

  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?

    Comment by Barkley Rosser — 18 Apr 2006 @ 4:47 PM

  29. Beate,

    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]

    Comment by Ferdinand Engelbeen — 18 Apr 2006 @ 4:54 PM

  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.

    Comment by Ferdinand Engelbeen — 18 Apr 2006 @ 5:20 PM

  31. 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]

    Comment by ocean — 18 Apr 2006 @ 5:24 PM

  32. 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.

    Comment by Bryn Hughes — 18 Apr 2006 @ 6:32 PM

  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]

    Comment by Ferdinand Engelbeen — 18 Apr 2006 @ 6:55 PM

  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]

    Comment by Alastair McDonald — 18 Apr 2006 @ 7:37 PM

  35. re:32.

    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.

    Comment by Dan — 18 Apr 2006 @ 8:04 PM

  36. Post-script:
    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.

    Comment by Dan — 18 Apr 2006 @ 8:25 PM

  37. 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.””

    Comment by Hank Roberts — 18 Apr 2006 @ 8:54 PM

  38. 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. :)

    Comment by shargash — 18 Apr 2006 @ 9:08 PM

  39. 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:

    Comment by pat neuman — 18 Apr 2006 @ 9:18 PM

  40. 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:

    Comment by Hank Roberts — 18 Apr 2006 @ 9:20 PM

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

    Comment by Mark A. York — 18 Apr 2006 @ 11:54 PM

  42. 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.

    Comment by Bryn Hughes — 19 Apr 2006 @ 5:15 AM

  43. [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.

    Comment by jhm — 19 Apr 2006 @ 9:15 AM

  44. 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.

    Comment by tom brogle — 19 Apr 2006 @ 10:06 AM

  45. jhm
    Are you inferring that global dimming leads to global warming?

    Comment by Brian Forbes — 19 Apr 2006 @ 10:23 AM

  46. 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?

    Comment by Alisa Brooks — 19 Apr 2006 @ 10:38 AM

  47. 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.”

    Comment by Hank Roberts — 19 Apr 2006 @ 10:42 AM

  48. “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.

    Comment by Bryn Hughes — 19 Apr 2006 @ 11:01 AM

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

    Comment by Hank Roberts — 19 Apr 2006 @ 11:41 AM

  50. 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]

    Comment by Pete Conde — 19 Apr 2006 @ 12:43 PM

  51. In my opinion Jim Hansen and Peter Cox simplified that global dimming masks global warming to a 2-1=1 problem (light bulbs). This is surely the right approach for a five minute statement on TV but would not hold in the scientific debate.

    Jim Hansen’s light bulb illustration is correct at the top of the atmosphere. The aerosol dimming affects the surface of the Earth and is very important for the water cycle more than greenhouse gas warming. The surface energy budget is much more complicated than the top of the atmosphere energy budget. Our model results show that global dimming energy loss equals greenhouse gas forcing energy gain at the surface but this is not true for the top of the atmosphere.

    We think that the surface energy loss by global dimming is mainly compensated by reduced evaporation and reduced turbulence over oceans and land. These spatially varying changes lead to changes in winds and in moisture transport not only in regions where the aerosols are present (note the shown reduction in pan-evaporation in Australia).

    The interactions between global warming and dimming are very important and subject of ongoing research (we have an animation on the Lamont web site


    Comment by Beate — 19 Apr 2006 @ 12:46 PM

  52. RE:49
    Thanks Hank. My error…but please be patient withe me… I guess I should have asked:
    Is Dr. Hansen is trying to say that we need to look at the consequences of REDUCING Air Polution?

    onsequences of REDUCING GHG

    Comment by alisa brooks — 19 Apr 2006 @ 2:15 PM

  53. Did anyone had a Peak at this today?

    [Response: Peek? But anyway (just to quash the misinterpretations I’ve seen enthusiastically indulged in elsewhere…) this refers to an ongoing experiment, *not* to any published results. It means they have to start again, but doesn’t call into question anything you read elsewhere… – William]

    Comment by Robichaud — 19 Apr 2006 @ 2:38 PM

  54. Re #33 (comment):

    Raypierre, I hope (and expect) that water vapor hasn’t changed its absorption bands, that was why I asked for trends. If there is any influence of other absorbing (and partially reflecting) items in the atmosphere, this should be seen as a change of ratio between water absorption bands and total insolation. For SO2 there is a very strong decrease since 1975 in Europe. NOx also decreased, but far less than SO2.

    This is of course only for the primary effect (with open skies). But the huge change in SO2 emissions should be visible in the European stations which have long-term trends in both one or more specific absorption band(s) and in total insolation…

    Comment by Ferdinand Engelbeen — 19 Apr 2006 @ 3:48 PM

  55. 48
    see the actual paper, but to greatly oversimplify (I’m not a climate scientist)
    — clouds make for warmer nights (at night) by blocking some heat that gets radiated into space when the night sky is clear.
    — clouds make for cooler days (in the daytime) by blocking some heat that gets radiated from the sun to Earth when the daytime sky is clear.

    In the absence of contrails for that period of time, some heat was not blocked. They observed “changes in diurnal temperature range, mostly from increased daytime high temperatures” during that span of day/night time.

    Alisa, soot and dust and sulfates fall out of the air fairly quickly, so as soon as pollution control or dust storm plantings succeed, the particles decrease. So to whatever extent pollution is blocking some heat, that will change fast when pollution is better controlled.

    Google +diesel +coal +respiratory +cardiac for the reasons it’s still a good idea to reduce air pollution at the same time as reducing carbon dioxide production.

    Comment by Hank Roberts — 19 Apr 2006 @ 3:58 PM

  56. re 48.

    In my reply (39.) to your question about the cause of the (apparent) cold spell in the 1960s, I wrote that 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.

    Thus, I doubt aerosols were the reason for the dip in temperatures during the 60s. I also discussed this in my comments to RC in January, at:

    Also see average minimum temperatures at climate stations in the U.S., a few plots are at:

    Comment by pat neuman — 19 Apr 2006 @ 6:02 PM

  57. Further to 48
    “These results suggest that contrails can suppress both daytime highs (by reflecting sunlight back to space) and nighttime lows (by trapping radiated heat). That is, they can be both cooling and warming clouds. But what is the net effect? Do they cool more than they warm, or vice versa? “Well, the assumption is a net warming,” Travis says, “but there is a lot of argument still going on about how much of a warming effect they produce.”
    Is evidence that Global dimming causes Global warming

    Comment by Bryn Hughes — 19 Apr 2006 @ 7:53 PM

  58. Pat
    Theoretically, water vapour in the higher atmosphere is cooled (at night) forming cirrus clouds which prevent a higher proportion of the heat absorbed by the earth during the day from escaping.
    However theoretically in the 60s sulphate pollution formed more clouds during the the day making the earth colder.
    A cold cloudy day is generally followed by a cold night (clear or cloudy)

    Comment by Bryn Hughes — 20 Apr 2006 @ 4:10 AM

  59. Re #51:

    Beate, are there indications of changes in wind/moisture content for the SH? As my understanding of aerosol transport is that there is little exchange from the main sources in the NH to the SH, how do changes in NH aerosols and consequent NH changes in wind/moisture affect wind/moisture in the SH?

    Comment by Ferdinand Engelbeen — 20 Apr 2006 @ 4:42 AM

  60. Saw “Dimming the Sun” on Tuesday night. Have we passed the point of no return? If not, what actions could advert this catastrophe given less than a decade to act? Could the USA draft the necessary brainpower into a sort-of public works project? Does the USA have the will to act?

    Comment by amy quint — 20 Apr 2006 @ 1:11 PM

  61. Quick question: In the show ‘Global Dimming’ there were many shots of the pan evaporation apparatus. Also shown sometimes around those shots were shots of a clear glass globe on a stand of some sort. They one time showed a focused spot of sunlight burning a record of something into a thin metal strip. It seemed to me the glass globe was acting as a 360 degree magnifying glass, concentrating the suns rays on the thin metal, sheet or tape wasn’t clear in the shot I saw, to burn the record of something (declination of the sun perhaps) into the recording material.

    Does anyone know what sort of measuring device this was and what its function is?

    Best Wishes,

    Comment by BG — 20 Apr 2006 @ 8:58 PM

  62. #61: It sounds like a means of recording sunshine hours. Used to be a standard instrument (I remember my father changing the strips on a daily basis for his met reports), but I suspect it may have been superseded by solid state equivalents.

    Comment by Gareth — 20 Apr 2006 @ 11:43 PM

  63. Re 61: Sounds like Campbell Stokes sunshine recorder.

    Comment by Adam — 21 Apr 2006 @ 5:39 AM

  64. I saw the PBS show and have been very taken with the need for action, regarding the implications of global dimming on global warming. I am an artist and early childhood care provider. The one to three year olds that I work with deserve a life. Your discussion is very academic and I understand you are fulfilling your role as scientists. However I have been looking over a great deal of polling data recently and it is clear that Americans are very ill informed on science issues. I am sure this is not news to you. Here is an example:

    “Do you think most scientists agree with one another about whether or not global warming is happening, or do you think there is a lot of disagreement among scientists on this issue?” Most Agree 35% A Lot of Disagreement 64% Unsure 1%
    ABC News/Time/Stanford University Poll. March 9-14, 2006. N=1,002 adults nationwide
    I understand there is near unanimity among scientist on the broad aspects of global warming and its causes. This information is obviously not getting to the public. And as you must know certain industries are doing all they can to increase the perception of disagreement. I believe this problem is somewhat better in Europe.

    If I understand the issue correctly we have maybe twenty years to do an almost complete turn around on emission into our environment. (?) This would require a world wide and very intense campaign to get peoples attention so we can change individual, corporate, and governmental behavior. It is obvious that our present government and corporate structure is not going to collaborate much in this. We need to cross over from a scientific discussion to one that all people can understand. This PBS show was a good example, but I see that the BBC did a show on this topic a couple of year’s back that made a tiny stir and then faded away. The problem is that given bad news people want either to deny it or they become fatalistic, either of these will result in inaction.

    This is an issue that has the potential to connect a whole host of organizations and individuals, because of its implications to life here on earth. It requires all of us to put aside our individual pursuits and interests to join a greater cause. We have done this in the past to fight wars, and 30 million people worldwide came out to protest against invading Iraq. So there is some precedent of large-scale interaction in a single cause.

    What climate scientists need to do is write a clear statement about Global Warming, Global dimming and what can reasonable be predicted of there effects on life on earth, and then get every scientist in the field to sign it and then distribute it to every media outlet in the world. The misperceptions must be set aside, so every one of us can face this problem.
    My suggestion for a statement is as follows.

    We the undersigned are reasonably convinced that the problems presented by emissions from automobiles and industry will become obvious and acute within the next twenty years. We are also convinced that without a significant change in these human behaviors we will severely limit the possibilities of human survival beyond this century, and will certainly all life on earth. Of all the problematic effects presented by human behavior on the earth’s environments and climates, global warming is the most life threatening.

    Earths climate is very complex and many things could happen to steeply shift the gradual warming trend that we have been experiencing over the last fifty years, these include but are not limited to: the thawing of tundra world wide, the death of forests and their subsequent burning, the melting of ice sheets, the reduction of aerosols that cause the “global dimming” effect, and the release of the methane hydrates frozen in our oceans. Anyone of these could rapidly increase global climate change and sharply escalate its dangers to humanity, and a combination of them may prove disastrous.

    We also believe it is within humans capabilities to respond to the challenges presented by global warming by reducing the emissions going into the environment and implementing technologies that fill human energy needs without the heavy costs associated with fossil fuel use. However we will have to focus attention on these issues and encourage innovation, understanding, and cooperation across many disciplines and among all humanity in order to provide a reasonable chance that our children and grand children will have a healthy world to live in.

    Obviously any way you whish to change or use this suggestion is fine with me. We must all of us work to inform and inspire people to the greater possibilities open to us if we concentrate our actions on this issue. As scientist you have a unique role to play.

    I apologize for using so much of your time.

    Comment by david Iles — 21 Apr 2006 @ 12:01 PM

  65. Re #64 That sounds like a good idea to me. Perhaps the RC team could set up a web page where scientists could log in and sign up. I would keep the wording more or less the same as above, with spelling and punctuation corrected :-) If the text is changed then note that the more people involved in the changes then the less readable and understandable it will become :-(

    Comment by Alastair McDonald — 21 Apr 2006 @ 12:13 PM

  66. I’m still puzzled by the notion that there could be a decrease in surface evaporation (as highlighted in the program) and an increase in atmospheric water vapor content (required for operation of the water vapor feedback). This would seem to imply that there has been a decrease in global precipitation which is greater than the decrease in evaporation. Has such a decrease in global precipitation been observed?

    [Response:Not really. On any long time scales > a few weeks, precip is equal to total evaporation. The amount of water in the atmosphere is a statement about the residence time of the water vapour which can change independently of the evap/precip, and in fact goes like the temperature. – gavin]

    Comment by Jerry Steffens — 21 Apr 2006 @ 3:59 PM

  67. Follow-up to #66:

    But, isn’t it true that, absent an internal source, the rate of change of the mass of a given substance in a reservoir is equal to the net flux of that substance into the reservoir? So shouldn’t this equation be true: rate of increase of water vapor = evaporation – precipitation?

    [Response: The amount of water vapor in the air is very small, and the amount in the radiatively important mid troposphere is yet smaller. Though there is almost certainly a secular increase in water vapor in the air, the E-P budget is a difference of two large quantities and I am certain you would never be able to detect the net moisture source to the atmosphere by monitoring E-P. –raypierre]

    Comment by Jerry Steffens — 21 Apr 2006 @ 5:38 PM

  68. Re #65 & #66 Of course you are correct. This does imply that global rainfall should decrease, and that is what we are seeing locally in the south of England!

    However, the pan evaporation is not being measured globally. It is only being measured at sites on land. Most of the evaporation which feeds the rain is coming from the oceans. I am not sure how you measure ocean (as opposed to pan) evaporation :-) If the oceanic air blown onto the continents is more humid than before, then that would cause less pan evaporation. In other words, if the continental relative humidity is increasing then that would explain less pan evaporation, but AFAIK it is not happening.

    The original reports highlighted less sunshine as the cause. And their investigations pointed to thicker cloud, and cloud forming earlier in the day as the cause. This does not fit with the current climate models! Hence the attempt to grab at the straw of contrails in order to show that the models are correct.

    Comment by Alastair McDonald — 22 Apr 2006 @ 6:58 AM

  69. From what I read, something like 2/3 of CO2 in the air is converted to O2 by algae in the oceans. But ever since industrial revolution, most industrial waste has been dumped to rivers/lakes/oceans or buried in the ground but gradually released to the water after rainfalls. So eventually much of the solid and most of the liquid form waste ends up in the oceans. The environmental changes in the oceans by humans obviously have significant impact on global climate too.

    I’m a mathematician/digital signal processing engineer, so I feel excited reading all the statistical models. But the more I read, the more apparent it becomes that the changes in the oceans are at least as important as any other factor. The release of methane hydrates is also directly linked to ocean temperature.

    The first article I found about the changes in the oceans is the dead zones in
    the Gulf of Mexico were growing:

    I’ve also read quite a few news that fishermen all over the world witnessed dramatic decrease in their catches over the decades.

    It’ll be interesting to find any recent observations and prediction/errors from the statistical models.

    Comment by rinconj — 22 Apr 2006 @ 1:15 PM

  70. rinconji,
    you may want to lok at these articles;

    Comment by david Iles — 22 Apr 2006 @ 5:57 PM

  71. My high school environmental science class should see “Dimming the Sun”. They have been focusing on energy issues since September and global climate change specifically since January, and though they are all non-native speakers of English, I think they could understand it quite well. (They have also been doing a lot research on heating and energy conservation in our building.) They are ideal students to watch this film. But half the class is from Ethiopia. How will they respond to the part about the droughts in the Sahel and their likely cause? The whole film was jaw-dropping for me but that part was by far the most chilling. I don’t see discussion of it here. I’d like to get some sense of how strong the evidence is of a link beween those droughts and particulate pollution.

    Comment by Bronwen Lu — 23 Apr 2006 @ 1:21 AM

  72. we need a 60% – 80% reduction in CO2 in the next 10 yrs to impact global warming. i really would like to see a serious plan to achieve this goal. insulating your home, adjusting the thermostat, driving a smaller car and becoming a vegetarian will make a trivial impact. please list real actions and the corresponding percentage reduction that would result from those actions to achieve the stated goal.

    [Response: This overstates what is needed. What we really need to do is more like reduce the global emisssions by 10% in the next ten years, with steeper drops thereafter. Don’t discount what individual choices can do. Building energy use accounts for roughly a third of US CO2 emissions, and that’s a big chunk. Replacing your furnace and refrigerator with state of the art energy efficient models, adding home insulation and double-pane argon filled glass, driving a Prius instead of an SUV (or better biking, walking or taking public transit) all can make a huge difference. Support energy-efficient buiding codes in your town. Support natural gas cogeneration plants wherever feasible. There are lots of productive steps. –raypierre]

    Comment by jack fletcher — 24 Apr 2006 @ 10:46 PM

  73. Bronwen, the BBC transcript is online; it doesn’t have footnotes but points to CSIRO.

    BBC – Science & Nature – Horizon
    NARRATOR: Rotstayn has found a direct link between Global Dimming and the Sahel drought. If his model is correct, what came out of our exhaust pipes and … BBC –

    From memory, somewhere in the NASA spaceflight archives you can find astronauts who flew early and late saying that their 1960s flights saw the Earth’s atmosphere much clearer (before the second half of the carbon to date was burned); after those early years Earth’s air has appeared more and more translucent rather than transparent when viewed “sideways” from low Earth orbit — dirtier.

    Reminds me of this lyric:

    ‘Before Believing’ by Emmylou Harris (Danny Flowers)

    “How would you feel if the world was falling apart around you
    Pieces of the sky were falling in your neighbor’s yard
    But not on you.
    Wouldn’t you feel just a little bit funny
    Think maybe there’s something you oughta do.
    Solutions that never lay down before you
    The answers are all around….

    Comment by Hank Roberts — 25 Apr 2006 @ 7:41 AM

  74. big chunk? huge difference? what percentage reduction in total CO2 produced would each of those changes make? while each action suggested may make economic sense, i don’t see a significant impact on total CO2 reduction to change global warming. you are the first i’ve heard say only 10% reduction in 10 yrs. kyoto’s target is a 30% reduction in the U.S. how long until you think a larger (60%) reduction is needed? and then what other changes must be made to reach that target? we need a plan of action to tell our elected officials what to do.

    Comment by jack fletcher — 25 Apr 2006 @ 9:03 AM

  75. re: 72

    Excerpts from – Interview: Straight talk about climate change.
    Jerry Mahlman on dealing with your grandkids’ problem.
    Posted: April 2006

    Jerry Mahlman … Formerly the head of NOAA’s Geophysical Fluid Dynamics Laboratory, he’s now a senior researcher at the National Center for Atmospheric Research. …

    Mahlman: There’s a colossal misperception that if you bike to work
    once a week and recycle your garbage, then global warming will be
    fixed up. The problem is that, even if everyone did that, the attempt
    to stop global warming would fail by a factor of, oh, roughly of 100,
    from what we really need to be doing.

    For example, I was in Al Gore’s office when he was vice-president.
    And he asked me the question, “If we could hold the emissions of
    carbon dioxide into the atmosphere constant, would global warming go

    And I said, “If you were to hold the emissions constant, you would
    get up to eight times the carbon dioxide, or CO2, that there was
    before the Industrial Revolution. You would still be in a heck of a

    Comment by pat neuman — 25 Apr 2006 @ 9:41 AM

  76. I agree with Pat]’s post #75. I think it is bordering on the criminal for Ray to write that “What we really need to do is more like reduce the global emisssions by 10% in the next ten years, …” The US has increased its CO2 emissions by 20% since Al Gore’s day. A 10% reduction would not take them back to 1990s levels which Jerry Mahlman was saying were too high.

    Ice is melting in the Arctic, in Greenland, on tropical mountain tops, and in the Himalayas. That will change global albedo and provide a positive feedback on greenhouse gas warming. This is not alarmism, it is fact. We have already raised CO2 too high. The longer we delay doing anything about it, the worse the consequences will be.

    [Response: Note I said “global” emissions. It would be fair and feasible for the developed world to do more than 10% while the developing world did a bit less. If we can get a 10% reduction in GLOBAL emissions, including India and China, in the next 10 years that will be doing well. It’s a worthwhile target, and one that is probably achievable. How much CO2 emissions reduction we “need” in the next decade is dependant on your target CO2 level, and there is no one magic number on that. Take a look at the discussion following “Climate Sensitivity, Plus ca Change” and play around with Archer’s model. Look at the difference between doing a 10% reduction in ten years with steeper reductions afterword, vs. starting a crash-program of reduction earlier. In my view, it’s not sharp reductions we need IMMEDIATELY, but policies that prevent building of a lot of coal plants with 60 year capital life. If we build those in the next decade, we really will have a much harder time reducing emissions in the out-years. That’s where we most need IMMEDIATE action. –raypierre]

    Comment by Alastair McDonald — 25 Apr 2006 @ 11:06 AM

  77. thanks pat#75. i’m glad to see a leading expert expressing my concerns. i heard recently that if all U.S. households would switch to Fluorescent lights, it would be equivalent to removing 8 million cars from the road. that’s an impressive number and they match the best numbers i could find. i have made the switch in my home to save $84/yr on a $12 investment that lasts 5 yrs. but if all americans did the same, the total impact on CO2 production in the U.S. would be 2/3 of 1%. there are lots of quick fixes and scapegoats – none are significant. all of Ray’s suggestions in #72, would result in less than a 10% reduction in U.S. CO2 production. crush all the SUV’s and drive a hybrid. and yet it gets warmer. we have an interested and concerned group here willing to take action, but i cannot find a plan that will make any impact on global warming. i would like to see a plan by someone knowledgeable and experienced. can anyone point me to ;-)

    [Response: Nonsense. You’re being defeatist without cause. Wind only gives you 5% of your energy? Fine, get five percent of your energy from wind. Another 5% from solar. Figure in 20% reductions from energy efficiency. Cut transportation in half by driving Prius and replacing short-haul flights with trains. Add another 10% of capacity in the form of nuclear energy. Biodiesel (maybe from algae) to fuel those priuses. Maybe do a little carbon sequestration from new IGCC coal fired power plants. Natural gas cogeneration. Sheesh, we’re over 100% already. Mahlman is right that if you just hold emissions constant forever we’ll hit 8x Co2 or more, but that’s not what I was proposing. I said that if we make a start on getting the right market signals in, the really sharp reductions can be postponed for a decade or two. The saddest thing is that we’ve already wasted 20 years without doing anything. Defeatism will only result in wasting another 20 years. –raypierre]

    Comment by jack fletcher — 25 Apr 2006 @ 3:53 PM

  78. Re #77 There is no workable plan that would not hurt. For instance, we could ban all combustion engined vehicle globally and that would save 33% of the all CO2 produced. But even that would not meet the 60% reduction some people say is neccessary.

    Don’t forget that here in the UK we are using twice as many resources per head as is sutainable and in the US it is five times. If GWB manages to export the American way of life to China and India then we will soon all be starving.

    There is only one quick fix and that is to create a parasol by injecting water vapour into the stratosphere, ie more contrails. No, I don’t think that would work either :-(

    Comment by Alastair McDonald — 25 Apr 2006 @ 4:25 PM

  79. Some musings about the impacts of an increasing dustiness….

    There should be some impact on the cloud processes. It is not uncommon that the saturation vapor is exceeded inside clouds (i.e. 100 – 105 %RH is still a realistic measurement). This occurs when there are relatively few condensation nuclei around and the air is rising. More condensation nuclei should generate more and smaller liquid water droplets, at a lower altitude. This would mean that the latent energy is released at a lower altitude as well, and within a narrower altitude range.

    The same would happen at higher altitude when the droplets freeze and continue collecting vapor from their surroundings. Over a wide temperature range the clouds contain a mix of liquid droplets, ice crystals and vapor. Ice forms around icing nuclei, which are different from the condensation nuclei forming liquid droplets. (Liquid water droplets may exist down to -38 degC temperatures). As the saturation vapor pressure with respect to ice is lower than the saturation with respect to liquid water, there is also a process of transfer via vapor phase (evaporation from droplets, accredition on crystals). More nuclei makes for lower altitude latent energy release also in this case.

    Another aspect yet is that the evaporation rate is dependent on droplet diameter. For reasons I do not understand, in the micrometer world of cloud droplets the evaporation is less efficient than that from large water surfaces.

    More dust in the atmosphere probably changes the cloudiness, as well as the precipitation. This is the cornestone of the murky art of weather modification, but it is also behind the observation that hurricanes are seldom generated during the sandstorm outbursts from Sahara into the Atlantic.

    The impacts of these changes on the cloud formation proces in general and the radiation properties in particular are a question mark for me.

    Comment by Pekka Kostamo — 25 Apr 2006 @ 11:25 PM

  80. I find it difficult to stay on topic, especially when the topic is “Global dimming and climate models”, and I have been banned from discussing climate models :-(

    [Response: Come come, Alastair, don’t be glum. You haven’t been banned from discussing climate models, only from making certain claims about Kirchoff’s laws. I’m very glad you decided to stay with us anyway. –raypierre]

    Anyway, rather than my views, here are some new pages about climate models produced by NASA only two days ago.
    * Earth’s Big Heat Bucket
    The Earth now absors more energy than it emits back into space, and the excess heat is hiding in the ocean.

    They say that the oceans are warmming, and that they will continue warm even if CO2 is stabilised:

    Hansen remarks, â??Weâ??re putting in the pipeline additional change that will occur over the next several decades, and which will be difficult if not impossible to avoid.â?? By his estimates, the current energy imbalance is likely to produce an additional 0.5 to 0.6 degrees Celsius of warming in global average surface temperature on top of the 0.7 degrees that occurred between 1880 and 2000.

    What I argued in #76 is that with the Greenland ice already melting, then with another 0.5 C rise in global temperature (at least 1C in Greenland temperatures due to polar amplification) then it is now inevitable that the Greenland ice sheet will melt and that sea level will rise by 7 metres (over 20 feet.) A point which is often ignored is that as the Greenland ice melts the altitude of the ice surface will decrease. Since the temperature of the atmosphere falls with altitude, then the reduction of the altitude of the ice surface will mean that it will warm. In other words, there is a positive feedback which will mean that the melting of the Greenland ice will accelerate. Moreover, since the surface of melting ice is wet the enhanced greenhouse effect from water vapour will also accelerate the melting. Oh, sorry! The idea that surface effects could overide those at the tropopause was disposed of in “A busy week for water vapour”

    Moving swiftly on, in the NASA article, it mentions the danger of the West Antarctic ice shelves being melted by the warmer ocean. May I point out that as they thin the grounding line will retreat, and the mass of ice that is grounded will decrease. This will reduce the friction that is holding back the ice shelves, and they will accelerate towards the ocean also causing sea level rise.

    Presumably if I praise the models that will be acceptable, so I would just like to repeat what NASA have to say about the work of their man James Hansen:

    He developed three-dimensional models of global climate. In 1988 Hansen testified before Congress, describing how different levels of greenhouse gases might affect future temperatures. Over the next 17 years, observed temperatures closely agreed with Hansenâ??s 1988 predictions.

    They also show a diagram where the agreement is clear:

    I just wonder how clear it would be if the start date had been 1995 rather than 1993, or if the end date had been 2005 rather than 2003?

    Comment by Alastair McDonald — 26 Apr 2006 @ 6:55 AM

  81. re: 77 response: defeatist? never! what the cited article and i are saying is the solution is greater than the usual suspects. you are a numbers guy. run the numbers and see how much those horrible suv’s really contribute to the co2 footprint. i’ll counter your statement that defeatism is delaying action with naivety is delaying action. if people feel that the solution is just implementing the often stated easy fixes, there’s no need for them to act now. looking at co2 generation by sector, green electric power will have to be major component of the solution. that will need to start now to clear regulations and public objections to be available 10 yrs from now. are energy efficient cars, appliances, and home improvements part of the solution? do we want to wait until individuals can afford this or do we offer incentives? we need a coordinated plan to present to those in authority that this is what a bloc of voters want to do to achieve stated results.

    Comment by jack fletcher — 26 Apr 2006 @ 12:29 PM

  82. Hey Im very interested about the topic of Global Dimming. Are there books published about Global Dimming?Because where I live;Baldwin Park,CA in the library there dont have any books about the topic or anything related. I got to go to class.

    Comment by Javier — 27 Apr 2006 @ 1:39 PM

  83. Javier, you’re within the Los Angeles Public Library system. I just checked the Baldwin Park online library collection search via the Net and was able to find fifty-odd likely references fairly easily. Being local, you’ll want to narrow that down a bit.

    You will probably get best results quickest if you can actually physically go there, meet and talk to the person at the Reference Desk about how to find things there, and then how to branch out via Interlibrary Loan.

    Also use the Periodicals Index, which I know they have available — that will get you newspaper and magazine articles in addition to the books available.

    Comment by Hank Roberts — 27 Apr 2006 @ 2:23 PM

  84. If contrails are a significant contributor to the global dimming phenomenon, wouldn’t it make sense to mandate the use of cleaner-burning jet fuels for commercial aircraft? Or to require cleaner burning engines that more fully combust the fuel and reduce the particle emmissions?

    This seems like a very practical and attainable step in the right direction if such improvements are technological possible.

    Comment by Vince Ready — 28 Apr 2006 @ 4:53 PM

  85. I m very interested in the topic of Global dimming. THe global environment is changing constantly with the melting of ice in Himalayas,in Arctic and in the tropical greenlands. THis is changing the face of climate models. All this is due to the increase of CO2 in the air. Something has to be done for this as the consequences could be harmful.

    Comment by danny — 29 Apr 2006 @ 12:52 AM

  86. RE: #76 “The US has increased its CO2 emissions by 20% since Al Gore’s day.” Show me your data. That is a very wild claim especially considering the general movement offshore of heavy industry, the ongoging transition away from oil fired power plants and the trends in fuel costs.

    [Response: Not wild at all. CO2 emission up 19.6% since 1990, all GHGs up 21%. See the latest EPA report. – gavin]

    Comment by Steve Sadlov — 5 May 2006 @ 4:18 PM

  87. RE: #80. Based on considerations of the energy required to promote a solid – liquid or solid – gsas phase change, what would you think may be the least and greatest amoounts of time required to completely melt the mass of ice on Greenland? (others welcome to present their own data and views here …)

    Comment by Steve Sadlov — 5 May 2006 @ 4:21 PM

  88. RE: #80, 87. Thanks Steve for this question, for the estimates of sea level rise on which storm surges will ride are the most urgent question in my opinion for those of us with an urban planning-environmental policy bent.

    My understanding of working group conclusions preliminary to #4 has been that climate models have converged nicely or perversely, depending whether you come from a primary research or policy application standpoint, to approach a temperature increase curve unavoidable through about 2040 regardless of contemporary actions/BAU. Please correct me if this is outdated information.

    I wondered whether a corresponding sea level rise curve now exists, properly accounting for expansion and acceleration due to feedbacks from albedo and aerosols on down, and what that curve might imply for direct-hit major-hurricane surges in low-lying metropolitan coasts through the 2040s. I’d like some idea how much time we may have left to move Miami and major parts of Houston-Galveston out of harm’s way! (And are there other urban candidates over 5.5 million population at the seashore that I should guide these considerations toward?)

    Comment by Paul Martin Suckow — 6 May 2006 @ 1:56 AM

  89. Energy and global warming

    Renewable forms of energy such as wind, wave and solar energy are the only realistic ways to meet the worldâ??s long term energy requirements. Since these forms of energy already exist, and will continue to do so, they do not add to the worldâ??s total energy, and this is important because of considerations of global warming.

    The current climate change/global warming debate has become highly political, with some people maintaining that the warming over the last 150 years is due to the enhanced greenhouse effect caused by anthropogenic emission of carbon dioxide and other gases from the burning of fossil fuels. However, there is some scientific opinion that the claimed increase in the concentration of carbon dioxide in the atmosphere is based on flawed data obtained from ice core measurements in samples from the Antarctic, with some alleged preferential selection of values having occurred, although this has been strongly disputed.

    The principal argument for the enhanced greenhouse effect seems to be that climate models can be made to reproduce the observed warming only if the expected effects of the extra carbon dioxide are included in the models. If the carbon dioxide is removed, the models fail.

    However, amid these claims and counter-claims, one fundamental consideration seems to have been omitted, and this is the effect of the vast amount of energy currently being generated by mankind. I believe that IT IS THIS ENERGY ITSELF WHICH IS CAUSING THE GLOBAL WARMING. It is derived mainly from the chemical energy of fossil fuels, but whatever the source, the energy eventually ends up in the form of heat and as a change of state in melting ice into water.

    Greenhouse gases are emitted, but this is simply a side effect which correlates with the amount of energy produced.

    Energy consumption data was obtained from the website of the Energy Information Administration of the United States Department of Energy, which go back to 1970 and give figures for the whole world. From this data the quantity of ice which can currently be melted in one year is calculated to be 1200 Gigatons, where 1 Gigaton is one thousand million metric tons. This is about twice the amount estimated from practical observations for the whole world.

    On the basis of a simple â??total energyâ?? hypothesis, good agreement has been obtained with the reports of five sets of practical observations, namely the Arctic sea ice, the Greenland ice cap, the rise in temperature of the atmosphere in the Northern hemisphere, the melting of the Antarctic ice sheet, and the rise in temperature of the Antarctic troposphere. Further details can be provided if generally required.

    No consideration has been given in this hypothesis to anthropogenic greenhouse gases and so no reduction of these gas emissions will be able to solve the problem of global warming, which, indeed, must be occurring as evidenced by the melting of the worldâ??s ice.

    It also follows that no benefit can be gained by switching to nuclear or geothermal energy because the problem is simply one of the very energy being produced by mankind.

    Therefore, the only way to solve the global warming problem is by changing completely to the use of “renewables”, solar energy, wind energy and possibly energy from the waves. Since this energy is not “new energy”, its use does not add to the total world energy, and so has no net warming effect.

    Aubrey E Banner, Manchester, England

    [Response: Total energy use by humans is equivalent to 0.03 W/m2, much much smaller than the impact of GHG forcing and completely negligible except in localised environments like cities. – gavin]

    Comment by Aubrey E Banner — 6 May 2006 @ 10:55 AM

  90. Aubrey, you write
    >Further details can be provided if generally required.

    Please do provide details — if you have references/cites to published science with the numbers.

    Yes, there will be a visible similarity between graphs of fossil fuel use and warming — but such claims need numbers to deal with the science involved.

    As Gavin points out the total heat energy released by human use is tiny compared to the excess solar energy that is and will be captured by the increased atmospheric CO2.

    Comment by Hank Roberts — 6 May 2006 @ 11:47 AM

  91. We are causing global warming by burning coal because people are afraid of nuclear power. I have been surveying my neighbors. So far, only my family doctor had ever heard of background radiation. Nobody in my neighborhood had ever heard of any natural source of radiation. It is clearly the ignorance of the public that is the problem. A copy of the following paper possibly modified by a George W. Bush political appointee is available at:

    [full text of link deleted]

    Comment by Edward Greisch — 7 May 2006 @ 10:38 PM

  92. RE: #88. But you did not answer the actual question I posed. Many people make statements to the effect of “the Greenland Ice Cap is going to melt.” What does that mean? Does it mean that it will lose some of its mass? Melt completely away, exposing the land underneath? Or something in between? If it loses some of its mass, only, how long will that take? And is it possible that some larger effect, such as the end of the interglacial, will completely obviate that with a crisis of epic proportions in a quite different direction?

    Comment by Steve Sadlov — 8 May 2006 @ 7:43 PM

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