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Global dimming and global warming

Filed under: — gavin @ 14 November 2007

Readers might remember a minor kerfuffle in EOS (the AGU house journal) in February this year in which Gerald Stanhill claimed to find a paradox in the contemporaneous effects of global warming and global dimming (a long term reduction of surface solar radiation, mainly due to aerosols and clouds). The article attracted attention mainly because the paradox was claimed to “pose [a challenge] to the consensus explanation of climate change”.

Rather than point out the subtle confusions (between surface and tropospheric forcing, and local and global signals) here, I and two co-authors wrote a comment to the journal. After a number of avoidable and unavoidable delays, this comment (along with another one and a reply) have all now appeared in EOS (Nov 6 edition). By now of course, the original piece has been long forgotten and so the point in having the correspondence printed is unclear, but still…

For those that care, I’ll link our comment once it’s been posted on the GISS website (now available here), but the bottom line is clearly seen in the following figure:

That is, if you take all of the IPCC AR4 models (now called the CMIP3 ensemble), then over the twentieth century all of them show varying degrees of global warming, while at the same time they show significant global dimming. An earlier paper of ours had pointed to the aerosols (unsurprisingly) being the dominant cause for long term changes in dimming, but that changes in clouds on a decadal basis were responsible for much of the shorter term variability. Thus there doesn’t appear to be much ‘paradox’ left to worry about – both dimming and warming are seen in models and in observations.

Apologies for appearing to push my own papers here (not something we like to do particularly), but the published comment would have been better done as a blog post in February. There may be a lesson there….

73 Responses to “Global dimming and global warming”

  1. 51
    Aaron Lewis says:

    RE 46
    The burning oil wells in the Desert Storm Theater did put a lot of particulate matter in the air. As we were preparing to put the fires out, it was noticeably dark. Some towns left their street lamps on all day, and I have photographs where the street lamps are brighter than the sun. However, the particulate matter was rather large, formed at near ground level, and at low velocity. Therefore, it tended to fall out of the atmosphere rather rapidly. Near the fires, big black flakes of soot would fall like snow. Finer soot was quite easy to see with a 10X hand lens on glaciers of Nepal, but finding oil fire soot on the glaciers of California required sampling and analysis. There is consensus that most of the material washed out of the atmosphere within a year of the last of the fires being extinguished.

    Also, the area had been overgrazed for an extended period of time, and large numbers of trucks had been used off road after 1972, so that the war caused little additional disruption of the desert pavement and additional dust production. Oil spills during and after the war were less than the long term operational average. (E.g., Prior to the war crude oil was commonly sprayed on broad expanses of sand to stabilize it against wind drift.) Over all, the 1991 Gulf War had minor long term impact on the global environment.

  2. 52
    Richard Ordway says:

    PaukM wrote: “Bangledesh Cyclone 1600 plus dead….probably closer to 2600. It’s difficult to comprehend this if one is not there. Welcome to the new age of climate change.”

    Yes Paul, not pretty. I’d like to add that it’s important to remember that usually no single incident can be attributed directly to climate change…it’s averages, averages, averages that you have to look at.

    Did rising sea levels due to global warming contribute to it…probably (storm surge)…but it is hard to measure.

  3. 53
    Lynn Vincentnathan says:

    RE #52, OTOH, even if GW contributed only a tiny amount to the Bangladeshi cyclone, it would be that last “umph” of wind intensity or sea surge that did the most damage.

    I guess we cannot attribute such events (micro-level) to GW (macrolevel stat-based phenomena)–I think that’s called the “ecological fallacy.” However, by standards that do not require such scientific rigor, perhaps we can–at least that tiny increment that did the most damage.

    And I suppose we can say with more (scientific) confidence that increased weather intensity events such as the Bangladeshi cyclone (along with sea rise complicating them) are expected or are in line with our increasing GHG emissions.

    We may not be hauled into court and found guilty over the Bangladeshi cyclone, but we could reduce our GHGs now with the hope of reducing the likelihood of increasing intensity of such events in the future. That’s what I’ve been doing since 1990, and when Hurricane Rita struck in 2005 and damaged a friend’s house, I told him I’d been reducing my GHGs in hopes that events such as that would be reduced or minimized. He said that gave him some consolation, that someone cared. Victims tend to be attentive to the motives of their perpetrators.

  4. 54
    weather tis better... says:

    #52 comment – I actually thought the death toll was pretty small for the area. I suspect we’ll hear that advanced warning and effective evacuation minimized deaths. In addition to the 1991 cyclone, a 1986 cyclone took 139,000 and the reigning king of cyclones took 300-500,000 Bangladeshis in 1970. I read somewhere that over the past 130+ years 200 million Bangladeshis have died in about 80 cyclones. It’s a densely populated, very poor, low lying country uniquely subject to these disasters.

  5. 55
    PaulM says:

    As I am not a scientist I really cannot contribute to the scientific discussion and I probably am more like the guys on the street corner with the “world is going to end” sign. But I also know the world is heating up and this planet will be drastically different in 200 years even if every single person on the planet as well as the seven billion to come learns what global warming is and actively takes steps to mitigate it. When you say I trivialize death or am attributing one small event to global warming your very human propensity to obsfucate the reality of what is occurring shows through. The earth is effing heating up eventually to a point where it will be unsustainable for life, that is my stand. Bangledesh is first in line and it will continue from there. I know people don’t want to hear this, and so be it.

  6. 56
    Lawrence Coleman says:

    You just cant say that this last cyclone to hit bangladesh was due to GW or was partly causded by it. All you can do is map trends over the past years as to the path of the cyclones, their intensity, the barometric measurments etc and plot a graph and see whether the rate is getting more frequent or the wind speeds in the vortex are getting higher. Recent istory shows that in the last 20 years there is a clear trend to many more cyclones in the cat 4-5 range globally and correspondingly greater destruction they exact.

  7. 57
    Lawrence Coleman says:

    Re: Paul M…thats exactly the problem..people dont want to hear it..the ostrich mentality is alive and well. How are we all going to take an extive stance to fight this calamity when soo many people are in denial? Al Gore is losing many hairs over that one to. re..his new book ‘An assault on reason’.

  8. 58
    Lawrence Coleman says:

    I’m buddhist by training so I tend to see things as they are..and that also means seeing the wood for the trees. Putting it bluntly we will not have a habitable world in 200 years if a nigh miracle doesn’t occur right now!! That’s why all these contrarians make me furious..what they are doing is muddying the pond so noone knows which way to turn or act. So what if 99.99 of all the climate scientists are wrong..I wont be embarrased to go out on the street..I would rather shout..I WAS WRONG at the top of my voice with a grin from ear to ear. But what…just what if ALL those scientists are right. If the contrarians ecourage us to do nothing and they are wrong which they most likely are..then the world dies..forever!!!! So Contrarians.. you better be 100% sure that you are right..or stop muddying the picture for everybody case you’re wrong. If the world gives 150% on mitigating GW and then some completelely obvious finding shows that humans are not to blame..then no harm done. Me… I’d tend to believe and back all those thousands of scientists who have studied many many years on understanding what makes climate tick..any rational human being would!!

  9. 59

    PaulM #55: I honestly think you are misreading the response to your posting on the Bangladesh hurricane. This is a scientific blog, and as frustrated as many climatologists are about the world “not wanting to hear” their message, they will continue to object to sending out messages that state more than the facts will bear. You can lose your scientific reputation only once.

    Yes, anthropogenic climate change is expected to make hurricanes more destructive, as does (especially for Bangladesh) the projected faster sea level rise. This is a statistical projection, i.e., climate.

    As for any concrete hurricane, saying that it is due to climate change is not even untrue, rather it is a completely meaningless statement. A butterfly wing flaps in Tombstone Gulch, and you won’t even see the same hurricanes next year as without it: names, times, intensities, landfall; all different. There just isn’t a way to ‘label’ a hurricane, like ‘this one is due to GW’, but ‘this one would have happened anyway’. This is weather.

    So it’s about the science, not about facing reality.

  10. 60
    b says:

    A few things we/I can do now:
    I will turn off a few extra lights.
    I will not water my lawn as much and never in the winter.
    I will drive a slower until I buy an electric car.
    I will plant two trees this year.
    I will flush my toilet less. If it is yellow, let it mellow, if it’s brown send it down.
    I will act, but not spread panic.

    I will re-post this comment 3 times.

  11. 61
    Richard Ordway says:

    In our division meeting this Friday at a national research laboratory that deals with climate change, the question came up: Will future generations condemn us for not speaking up more about climate change?

    The question was not answered. This issue is not taken lightly by many in the scientific community.

  12. 62
    Lynn Vincentnathan says:

    RE #59, & “A butterfly wing flaps in Tombstone Gulch, and you won’t even see the same hurricanes next year as without it”

    But then we could blame it on that danged butterfly :)

    Non-scientists could take a different tact from the usual science scientists pursue, and that is assume the cyclone is impacted to some extent by AGW, since we do know GW is going on and heating the ocean, etc. Make GW and its effects the null hypothesis. Then let the naysayers try and prove at .05 significance (95% confidence) that GW did not in any way increase the intensity of the cyclone. (I’m not sure, but I do believe they wouldn’t be able to do so.)

    We don’t have scientific reputations to worry about, and I think this tact works well for me. For instance, when Andrew struck Florida in 1992 ?, I thought, “Yep, global warming. Hope others take note and start reducing their GHGs.”

  13. 63
    mg says:

    61. Richard, my personal view is that 10 years from now the public will condemn climate scientists for their reticence about sea level rise.

  14. 64


    It’s “take a different tack,” referring to a sailboat’s tacking into the wind. Taking a different tact would be presenting your story more or less politely.

    -The Grammar Police (Dragnet theme up)

  15. 65
    Ray Ladbury says:

    mg, re:63; while you may be correct, this is not really fair. Climate scientists have been playing this issue exactly correctly–publishing consensus while emphasizing that there may be substantial error on the high side of the estimate. Science is inherently a conservative enterprise, and so has to emphasize consensus. This is precisely why the accusations of alarmism are so unfair. Perhaps what is needed is a climate mitigation engineering discipline, where we plan for the worst and try to bound the problem–but that is not what climate scientists do.

  16. 66

    #62 Lynn: sure you could so that. You could also adopt the habit, everytime the weather turns moderately unpleasant, of exclaiming “just prove that it’s not due to global warming!”. Every thunderstorm, snow shower, dense fog, early autumn night frost, heat wave,… See how long it takes for people not only to stop listening [edit] :-)

  17. 67
    lgl says:

    “The satellite data have not yet been processed (AFAIK) for the 2000+ period” ???
    Good some have computers:

  18. 68
    Hank Roberts says:

    And from the grammar police review commission, a reminder that often someone writing “tact” means “tactic” (grin).

    The difference between a tack and a tactic is that you can sit on a tactic to keep anything from happening, but if you sit on a tack ….

  19. 69
    henry says:

    [Response: The averaging period for the dimming is so that it is lined up with the satellite obs, and for the SAT it was for graphical convenience. In each case the different trends are apparent in the early part of the record. The models only go up to 2000 with observed forcings and that is why they are truncated then. The satellite data have not yet been processed (AFAIK) for the 2000+ period. – gavin]

    Maybe I’m just not understanding the phrase “graphical convenience”. What I mean is: If the swl data only goes back to 84, why can’t the surface temp average cover the same period (84-99)?

    [Response: Sure it could. What is your point? – gavin]

  20. 70
    henning says:

    I don’t believe blaming the recent cyclones on GW is reasonable. Wouldn’t you have to praise GW for the relatively mild cyclone season we’ve had this year as a consequence? I’ve listened to a public lecture given by Hans von Storch on the subject last year. According to him, the cyclone activity during the last couple of decades wasn’t special in any way.

    [Response: This issue has been much discussed here before (see here, and also all of these. Increased activity by various metrics for the Atlantic basin almost certainly show a significant increase that is likely tied to anthropogenic climate change. Things are less clear for the other ocean basins. But this is getting way off topic. -mike]

  21. 71
    Jeffrey Davis says:

    relatively mild cyclone season we’ve had this year

    2007. First year, 2 CAT5 hurricanes made landfall. The US isn’t the only country hurt by hurricanes.

  22. 72
    Aaron Lewis says:

    RE 63,65, & 68
    Time to recruite some “Civil Engineers” to our cause!
    That would save our reputation, and change the tactics with tact.

    There are two ways to lose our reputations. One is to “over predict” and be alarmist; and the other is to under predict and have people unprepared and perish as a result. I would rather be considered alarmist, than to betray the trust of people that are depending on me to keep lookout. Indeed, society supports the science of climatology so that society does have lookouts and watchmen for impending climate hazards.

    I was considering the state of polar ice sheets in geologic times past when CO2 levels were at 380 ppm when, the other night, a Civil Engineering Manager asked, ”What do you think of the new IPCC Summary for Policy Makers?” I told him (in 5 pages) that for his purposes, it was understated because scientists and engineers use a different definition of conservative. I told him, if he was using it as a basis of design for public safety engineering, then he needed to apply safety factors of 100 or a 1,000. Since some of his structures are required by law to have a 95 % confidence of withstanding, and protecting the public against a 200-year return event storm, is there anyone here that feels my response was “Alarmist”?

    We need to get standards agencies involved with developing realistic basis of engineering design. Some are lagging behind see for example, and even C&EN printed, (OK, most C&EN readers have worked in the Petrochemical industry at some time.)

    (Civil Engineering Manager works for a large government organization that “does not have a view on global warming.”)

  23. 73
    Jack Roesler says:

    I just watched “Dimming the Sun” for a second time.

    That is one of the most powerful presentations on AGW available. I donated a copy to our local high school. They should have it about now, so I’m anxious to hear their response.

    I can’t imagine anyone arguing with the problem we have after watching that movie.