RealClimate logo

Note 3/23/2021: we had a few hiccups with comments after moving the site to https/SSL. Hopefully they're fixed now. Please let us know if there are remaining issues.

Aerosols: The Last Frontier?

Filed under: — group @ 21 February 2007 - (Português)

Guest commentary from Juliane Fry, UC Berkeley

The recently released IPCC 2007 Fourth Assessment Report Summary for Policymakers reminds us that aerosols remain the least understood component of the climate system. Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, inorganic salts such as ammonium sulfate (which has natural as well as anthropogenic sources from e.g. coal burning), and carbonaceous aerosol such as soot, plant emissions, and incompletely combusted fossil fuel. As should be apparent from this list, there are many natural sources of aerosol, but changes have been observed in particular, in the atmospheric loading of carbonaceous aerosol and sulphates, which originate in part from fossil fuel burning. While a relatively minor part of the overall aerosol mass, changes in the anthropogenic portion of aerosols since 1750 have resulted in a globally averaged net radiative forcing of roughly -1.2 W/m2, in comparison to the overall average CO2 forcing of +1.66 W/m2.

Figure SPM-2, shown here, compares the radiative forcing for greenhouse gases and other climate forcing agents, along with an assessment of the level of scientific understanding (“LOSU”) for each component. In this figure, it is clear that while aerosols contribute the largest negative (cooling) radiative forcing, the level of scientific understanding of their climate influence is “low” to “medium-low”. The aerosol effects are split into two categories: (1) direct effects, meaning the scattering or absorption of radiation by aerosols influencing the net amount of energy reaching the Earth’s surface, and (2) indirect effects, such as the cloud albedo effect, referring to how the presence of aerosol increases cloud reflectivity by providing a larger number of nuclei for cloud droplets, reducing the amount of energy reaching the surface. This is a step up from the last report, where the LOSU for aerosols was very low to low, and no most likely value was assigned at all for the ‘indirect’ part.

This figure also visually hints at why improving our understanding of aerosol’s role in climate is so important: while overall net radiative forcing is positive (warming), aerosols provide the dominant negative (cooling) forcings. Hence, the aerosol currently in our atmosphere is acting to mask some of the greenhouse gas-induced warming. This means that as we get our act together to reduce fossil fuel use to improve air quality and address global warming, we need to be mindful of how changes in emissions will impact aerosol concentrations and composition.

In addition, our deficient understanding of aerosol forcing also hinders our ability to use the modern temperature record to constrain the “climate sensitivity” – the operative parameter in determining exactly how much warming will result from a given increase in CO2 concentration. The determination of climate sensitivity has been discussed in this forum previously here. The sensitivity parameter can be derived by examining historical records of the correlation of CO2 concentration and temperature taking into account other contemporary changes. Aerosols contribute significantly to the uncertainty in climate sensitivity because we cannot model their historical impact on the temperature record with sufficient accuracy, though additional constraints on climate sensitivity such as the last ice age do exist. A better understanding of aerosols then may well facilitate more accurate predictions of future climate responses to changing CO2.

The relative lifetimes of CO2 and aerosol in the atmosphere result in the expectation that reducing fossil fuel use will accelerate warming. A CO2 molecule has a lifetime of about 100 years in the atmosphere, while an aerosol particle has an average life expectancy of only about 10 days. Therefore, if we instantaneously ceased using combustion engines, the (cooling) fossil fuel-related aerosols would be cleaned out of the atmosphere within weeks, while the (warming) CO2 would remain much longer, leaving a net positive forcing from the reduction in emissions for a century or more.

So, what do we need to learn about aerosol to narrow those error bars in Figure SPM-2? To accurately model aerosols’ climate impact, we need to know about the whole lifespan of the aerosols: their diverse sources, aging processes (and how those affect radiative properties), how they mix together and the mechanisms and timescales for its removal from the atmosphere. As the IPCC 2007 4AR will make clear, we’ve come a long way in our understanding of atmospheric aerosol, but there is still plenty of room for improvement.

184 Responses to “Aerosols: The Last Frontier?”

  1. 51
    Mark A. York says:

    Dr. Mann, that does have a comforting effect. Thanks.

    [Response: Mark, google searches on “Inhofe” or”Crichton” are similarly satisfying. Combinations are often even better, e.g. “Crichton” and “climate” or “Crichton” and “Inhofe” ;) -mike]

  2. 52
    Hank Roberts says:

    Henry asked: “My question is: does the retroreflectivity of the larger droplets, i.e. back towards the light source, play into the sulfur aerosol/water droplet issue or is it simply averaged out by the bulk effect of all the aerosol particles present in the apparently white haze?”

    Not a climatologist speaking, but anecdotally — I suspect it mostly averages out and the light spreads in all directions, with some amount of reflection directly back toward the source. That’s from looking at the ‘Glory’ (circular rainbow around one’s shadow) and also from camping on mountains in thin cloud at night under a full moon and seeing absolutely no shadows at all, even under trees and inside tents; the moonlight spreads everywhere, scattered by the mist. It happens in daylight as well, though perhaps less magically. This effect was appreciated by the early photographers, working in Paris especially, as a way of getting even lighting for very long exposures required by early emulsions, I recall.

  3. 53
    Pascal says:


    thanks juliane

    Ok for this important (for me) confirmation of LOSU’s no-influence on uncertainty.
    I apologize for my insistance but for my question about the recent trend of aerosols forcing, can I hope a response?

  4. 54
    James says:

    Re #41 response: ” If one insists on keeping aerosols in the atmosphere as part of the solution, one is going to need to put them there deliberately, and in a way that is less environmentally destructive than burning coal dirty.”

    I noticed an article in a recent Science News concerning the effects of a regional-scale nuclear war (using about 100 Hiroshima-size bombs). Seems as though the aerosols from that would nicely counteract AGW :-) Of course the problem is that you have to repeat every couple of decades…

  5. 55
    Daniel Goodwin says:

    #39 and others here have referred to the very long-term reduction of atmospheric CO2 (and de-acidification of sea water) via the production of carbonate rock. I haven’t seen anything detailing these processes and how they work. Where, for instance, do we find the carbonate-rock deposits which sequestered the CO2 which spiked during the PETM (55 mya)? Thanks for your help.

  6. 56
    Steve Sadlov says:

    I have grave concerns regarding the likely introduction of massive amounts of SO2 and H2S over the next 50 – 100 years, as the huge native coal deposits in China are exploited. Take a boat ride on one of the many channels in the Pearl River Delta, and already, you will see coal piled up on the banks and being shuttled around on barges. It is burned both for direct heat for industrial processes as well as for power. If you want to see an amazing (and frightening) sight, along one of the southwestern channels, as you look out toward the west, you will see “the plain of stacks” – the most smokestacks I have ever seen in one view, anywhere on the planet.

    Add this on top of the growing dust quantity also coming from China, Mongolia, Eastern Russia and the Central Asian countries. All of it is stretching out across the North Pacific – and beyond!

  7. 57
    Tom Huntington says:

    Great Post, THANK YOU

    I am concerned about one aspect of the figure reproduced from the IPCC AR4 SPM. It seems to be missing radiative forcing from black carbon, or the science has moved beyond these forcings. In recent analyses of climate forcings from Hansen and Sato as well as the IPCC TAR figures there are positive forcings assigned to black carbon (+0.39 W/sq m) from fossil fuel burning – this is in addition to the change in snow albedo on account of soot deposition of +0.17W/sq m. In addition the forcings show +0.11 and +0.08 W/sq m for the effect of soot from biomass burning on black carbon aerosols and deposition on snow respectively. Unfortunately, I don’t have the citation for the figure I am looking at [Figure 28. (a) a specific estimate fo climate forcings for 1750-2000, (b) same as (a) but the effective forcings partially sorted by sources.]

    Can someone tell me why they are not included in the AR4 SPM?

    [Response: They are. They are simply grouped in with the ‘Total Aerosol’ bars, except for the specific effect of black carbon on snow affecting the albedo (which is separated out). -gavin]

  8. 58

    I think aerosols play a minor role in climate change since they can be controlled.

  9. 59

    Re #52 and #30.

    What type of optics regime you are in depends on the ratio of the radiation’s wavelength to the object’s diamter. The interaction of solar wavelengths and atmospheric particles are primarily dictated by Mie Scattering, the most complicated type of light scattering (as opposed to geometric or Rayleigh). Mie scattering is highly directional, meaning that the scattering is not uniform in all directions from the particle. There is a strong angular dependence.
    For atmospheric radiation the Single Scattering Albedo (SSA) and Scattering Asymmettry Parameters are two important factors. SSA is the ratio of scattered light/light extinction (extinction = scattering + absorption). So a very refelective aerosol type (eg. ammonium sulphate) has an SSA ~ 1 while an absorbing aerosol (eg. soot) has an SSA ~0.3-0.4.
    The Scattering Asymmetry Parameter describes the angular dependence of the scattered light. In general, particles of a similar or larger diameter to the incident wavelength of light tend to mostly forward scatter light. Smaler particles tend to forward and backward scatter in roughly equal amounts.

    If you then consider that real particles in the atmosphere are not just complex mixtures but are also often not spherical (which is what Mie theory is for), then it gets really complicated.

    Note that I’m not really an expert of atmospheric radiation though. A good resource is “A First Course in Atmospheric Radiation” by Grant W. Petty.

    As for the sensitivites of light scattering for CO2 versus typical aerosol components, I’m not sure but this is a very good question. Usually “mass extinction/absorption coefficients” are reported for different types of aerosols to describe this. While aerosol composition is usually measured and reported on a mass basis (with some particle size segregation as well), the optical properties of an aerosol really depend on it size distribution (i.e. number concentration as a fucntion of particle size).

  10. 60

    Re #56: I recently read a very interesting article about the huge amounts of GHGs released from coal mine fires (which I guess are pretty common). This would also release a lot of particulate matter, though I don’t think anyone has studied or directly measured those, yet.

    The article:

    “The CO2 production of all of these fires in China is more than the total CO2 production in The Netherlands,” Rosema says. This amounts to 2-3% of the annual worldwide production of CO 2 from fossil fuels, or as much as emitted from all of the cars and light trucks in the United States. “

  11. 61
    Mark Hadfield says:

    RE 18: “Aerosols have a net radiative forcing of -1.2 W/m2 against a CO2 forcing of 1.66 W/m2. If I were an AGW convictee, the closeness of these figures along with the uncertainty regarding aerosols would concern me.” and 40: “It is staggering to consider that the error bar for the forcing caused by aerosols includes a negative number that is larger than even the highest number in the error bar for the positive forcing of GHGs. If I were a skeptic, this is clearly the fact I would harp on.”

    Enough with the hypotheticals, people, what is your point? I don’t care what you would harp on if you were (heaven forbid) a sceptic. What do *you* conclude from the large uncertainty in aerosol forcing?

    It is my understanding of that if the total aerosol forcing (direct plus cloud albedo) were really at the far negative end of their plausible range, i.e. -2.7 rather than -1.2 W/m2, then the total net anthropogenic forcing would have to be near at the bottom end of its plausible range, i.e. 0.6 rather than 1.6 W/m2. To reconcile this with the observed anthropogenic warming would require large climate sensitivity. But we know that the climate sensitivity is round about 3 degC (expressed as equilibrium sensitivity to a doubling of CO2) and James Annan has argued convincingly (to me at any rate) that it can’t be much higher. So large negative aeorosol forcings are excluded by other evidence.

    N’est-ce pas?

  12. 62
    Steve Latham says:

    Sorry, but I have a kooky question: Would there be any benefit to understanding aerosol forcings, etc, by having one day per year in which commercial flights aren’t scheduled in the US or world for 12 or 24 hours? I was originally thinking about something similar to “don’t drive your car to work day” for airplanes (because they burn a lot of carbon and because they’re easier to regulate than cars), but then I thought back to global dimming and a documentary in which a guy found the sky very clear after 9/11 (I think this RC posting talks about it Anyway, I wonder if such an annual experiment (say on 9/11 or Earth Day) would help climatologists.

    [Response: Well, the contrail effect is so small it’s hard to be conclusive on the basis of that. What would be a really lovely experiment would be to shut down all coal burning worldwide for about 10 days. If this were done at a time when there was good satellite coverage from sats that can measure aerosol properties, the results would be extremely valuable. I read that China will shut down a lot of industry near Beijjing for the Olympics. Maybe something could be done with that, looking on a regional scale. Good idea! –raypierre]

  13. 63
    Roger William Chamberlin says:

    I think all this discussion is fine longterm , but overloooks that the world is already in BIG trouble in many places and few people seem to be aware that the CO2 already there is gonna have consequences , let alone what we add to it because one cannot stop the way men do things very quickly [if at all until the fail!]…

    Thus we NEED to buy some time even for this elegant discussion else it becomes moot … the machines are already in the design phase to put [mostly salt] sea-water crystals into the clouds in the Southern Ocean to increase reflectance in a manner similar to that used by nature in aerosols generated from the peaks of waves in storms…

    The point is that it is really already too late and we need to use our existing knowledge , however slight, in controlled manner [with monitoring and review of results] to DO something NOW , immediately, as a matter of life-threatening urgency .

    IF everyone was simply informed and went out at weekends to plant a tree [willows are easy to grow] or some food plants, or anything green , one every scrap of soil or in pots or on roofs [sedum is good on roofs]then we should at least be STARTING the enormous task instead of making it worse by continuing OUR way of life …

    Equally we could bombard our sluuggish governments with petitions proving that the people DO want this problem solved and NEED leadership now, not the dithering inaction we see … only governments stand in the way of industry getting going on CO2 injection into porous ocean rocks [where under pressure the CO2 is dense enough not to ever escape]

    There are colossal tasks of changing feedstocks for industry that should be tackled before the oil gets more expensive and could in principle provide the review necessary of efficiency and recycling in all industrial processes.

    Even new houses are not being built to take account of what we know about the future which is absurd waste of resources adding to future burdens in changing infrastructure

    Our seas lack but relatively tiny ammounts of micronutriments [notably iron for phytoplankton enzyme systems] to becaome able to absorb massive ammounts of CO2 quite naturally and so boost fisheries that we choose mostly to meaninglessly deplete by over-fishing] and turn the excess CO2 into FOOD for humans … nature just needs a tiny ammount of help to let the ‘sea deserts’ bloom into a partial solution to world food shortages in many places, which simply must now increase unless we act now…

    The point is not that we don’t need to know about better aerosol reflectance, but that we need to start NOW with something ocs’ in many ways it is already to late to be starting and we need an all-out emergency response by everyone, today , not tomorrow…

    We have to use what we have and develop it as we go because elsewise there is no means to do someting better tomorrow [our civilisations could so easily fail as things go wrong in agriculture , the seas are already dying from CO2 acidification and it will kill the whole food chains with us at the end]

    Hoping that the greater storms due to climate chaos will put more salt aerosol up in the clouds and solve the problem just ain’t enough to justify not sitting around any more, but doing something positive to reverse the trends IN ONE’S OWN LIFE before sitting down again to see if one can find an even better way of helping the earth for everyone…

    people really are dying in increasing numbers already, the problem is here and now and bigger than any crisis man has ever faced before, bigger than world war… if it were war then people would take it seriously and mobilise with what they have , what they know, but this crisis is one we are buying for our future, it is more insidious than war… but it really needed action yesterday, so it is time to act to do something to remedy the situation, the time for talking comes after one has put in place all the actions one can in one’s own life… in every aspect of it … then begins the task of educating everyone else in the world … and only when one has finished that is it time to devise a better solution because one has bought the time in which to do so…

  14. 64
    Lynn Vincentnathan says:

    It sort of occurred to me that our emissions (GHGs + aerosol = long-term warming, off-set a bit by some short-term cooling) are sort of like a “balloon” morrtgage in which a family pays well below even the interest due for a number of years, then the monthly payment greatly jumps up to pay off that unpaid innterest (plus principle), which is well beyond what a family would have had to pay had they gotten a fixed morrtage. This may not a good idea if you plan to retire by that time, keep that house, & live on less income. And a very bad idea if your house is in hurricane alley.

    The problem with this analogy is that there are also harms from the aerosols, such as acid rain (and to some extent sulfur dioxide) corroding forests, lakes, soil, property, and lungs. It would be interesting to figure if the aerosols lead to net good from cooling WHILE in the atmosphere or net harm, considering these other harms — which would be of a longer time duration.

    I’d think the fact that CO2 emissions by their extreme longevity in the atmosphere, are even more harmful than many are estimating (since most reports only go up to 2100, and it’s hard to estimate harms 10,000 years from now). But from a false-negative avoiding perspective, the situation looks very very bad. We should be thinking how much harm today’s emissions will cause over their lifetime in the atmosphere (or in carbolic acid in the ocean), plus the aerosol harms (& slight benefits). The whole picture over the life-time of harms & benefits. Even though it would be near-impossible to quantify.

  15. 65
    Hank Roberts says:

    > China will shut down a lot of industry near
    > Beijjing for the Olympics. Maybe something
    > could be done with that, looking on a regional
    > scale. Good idea! –raypierre]

    Great idea. If it’s possible to plan ahead by lining up satellite time and also to enlist local monitors around the various pollution sources that are in theory being shut down, to detect cheating site by site (like nighttime smoke), that’d give a very good source data set. I smell PhD material, if there are students there now who’d be able to start collecting baseline data in advance and continue afterward.

  16. 66
    Steve Latham says:

    Re # 65: Of course, the same experiment will basically be done in the opposite direction as China builds numerous new coal plants in the next while.

  17. 67
    Terry says:

    Where I live large amounts of wood are used for warming houses. The wood burning stoves are very efficient and burn hot giving off little smoke once they get going.The wood is sustainable being off cuts from timber yards and plantations and from fallen trees. It has been found to be the most economical as well. Where are the problems with this compared with burning oil or gas for heating or generating electricity from coal.

  18. 68
    Nick says:


    So aerosols from a volcano go high into the atmosphere, and so take a long time to settle out.

    Ditto for aircraft.

    The graph and the article talk about aerosols, and not specifically anthropogenic aerosols.

    What’s the break down, because that needs to be clear.

    What is the expected life span of the different components, because it looks like the 10 days is selective picking just aerosols that are just low level.


  19. 69
    David B. Benson says:

    Re #68: Nick — The latest super-eruption of Mt. Toba, about 74,000 years ago, resulted in sulfates(?) in the stratosphere which persisted for 3–6 years.

  20. 70
    Steve Bloom says:

    Completely off topic:

    This press release shows U.S. internet searches for “global warming” hit .01% of the total for the four week period ending February 18. This is actually a very high number for a specific issue (noting that it doesn’t include terms like “climate change,” “climate warming” or “climate science”) and reflects a huge increase over a year ago. RC is not in the list of top sites, but I suspect benefited since it’s prominently linked by many of the ones that are (many of which didn’t even exist until recently). What’s most gratifying is that there isn’t a single denialist site on that list (which is confirmed by a current google search, in which the first denialist site doesn’t show up until the middle of page four). RC co-author William Connolley, who has taken special charge of making sure the Wikipedia climate pages are accurate, will be particularly pleased since WP got the most hits of all (14.5% of the total).

    The press release includes an interestingly-shaped graph that may arouse a feeling of irony in some.

  21. 71
    Marco Parigi says:

    Enough with the hypotheticals, people, what is your point? I don’t care what you would harp on if you were (heaven forbid) a sceptic. What do *you* conclude from the large uncertainty in aerosol forcing?

    I still conclude that scientists are being unskeptically selective in how they are calculating uncertainties. To me the observable quantities are the measured temperature increase in 200 years correlated with increases in all these “suspects”. I believe this correlation gives us the confidence to say how much the overall effect is because it is based on actual temperature measurements. In a sense, the total forcing is calibrated to actual data for this planet. Meanwhile, the calculations based on the physical properties and measured concentrations do not have this same directly observable calibration. There may be other effects associated with exactly the same compounds (or different compounds) that are just not observable directly because of the cross-correlation, and the models have only coincidentally matched due to manmade things generally rising in unison.

  22. 72
    pat neuman says:

    re: 63 … to DO something NOW , immediately, as a matter of life-threatening urgency. …

    There are many eminent scientists saying about the same thing as you did.

    “The pace of change and the evidence of harm have increased markedly over the last five years. The time to control greenhouse gas emissions is now.”

    “These events are early warning signs of even more devastating damage to come, some of which will be irreversible,” warned the board.

  23. 73
    Philippe Chantreau says:

    Thanks for the tip on CO2 absorbtion’s spectrum. I have basic and patchy science background so this is challenging for me but this site is definitely outstanding!

  24. 74
    Richard LaRosa says:

    Since we are facing the problem of regional drought, the effect of aerosols on regional precipitation should be considered. A recent AGU paper found that aerosols decreased precipitation over east Asia. “Direct and indirect effects of anthropogenic aerosols on regional precipitation over east Asia” Jour. of Geophysical Research, v.112, D03212, doi:10.1029/2006JD007114,2007

  25. 75
    Daniel Morris says:

    I have heard a claim that the planet would be 60-70 degrees F cooler if green house gases were removed (leaving oxygen and nitrogen). As CO2 is the dominant greenhouse gas, most of the greenhouse effect appears to be related to this one gas, directly or indirectly (H20 into air). I bring this up because establishing CO2 as essential in making the Earth habitable implicitly gets one half way to addressing most skeptic arguments (with the public at least).

  26. 76
    Steve Bloom says:

    Re #71: Marco, that reasoning is the purest of handwaving. Recall that the behavior of the various atmospheric constituents has been of great interest for many years prior to concern being expressed about global warming, and nothing like what you speculate about has been found. Your idea that the observed behavior of GHGs would just disappear in the atmosphere seems especially fanciful.

  27. 77
    Marco Parigi says:

    Re: reply to #34 There is no plausible ways these uncertainties could be stretched to accomodate the scenario you are proposing. –raypierre]

    Firstly, I’d like to thank raypierre for taking the time to respond personally to my assertion. It seems that you are talking about uncertainties in the numerical models putting everything that is known to be uncertain. One more recent uncertainty of CH4, ie the stratospheric water vapour has been listed separately there and I thought some scientists had asserted it to be far higher than your graph shows. Some of these substances can also be catalysts, set off positive or negative feedbacks, that could be quite significant but as yet unobserved or attributed incorrectly. Has some form of regression analysis been performed regionally to tease out these separate forcings from actual data rather than forcings calculated from first principles?

  28. 78
    Caz says:

    Re: #13

    ” And because aerosols are often so short lived and so spatially
    inhomogenous,” ….

    …. wouldn’t we expect the indusrialised nations in the Northern
    Hemisphere to be cooling (or warming at a lower rate) than those in
    the SH. i.e. the exact opposite to what is actually happening

    [Response: … if every thing else was equal, sure. But it’s not. Principally the amount of land in the Northern Hemisphere is much greater than in the South, and since land has a much quicker response (less heat capacity than deep oceans), it warms faster. That turns out to be a significantly stronger effect. -gavin]

  29. 79

    [[Why do I never hear that the CO2 greenhouse effect is saturated ?]]

    Because it isn’t. When the line center is saturated, there is still absorption in the wings. You might want to look up how line shapes are calculated and observed. Goody and Yung’s “Atmospheric Radiation” (1989) is a good source, but some of it can also be found in Houghton’s “The Physics of Atmospheres” (2002).

    [Response: What’s more, even for a grey gas where absorption is independent of wavelength, the greenhouse effect would never saturate no matter how much you put in. The reason is that the greenhouse effect arises from the fact that adding a greenhouse gas moves the altitude from which radiation escapes to higher levels, where temperature is lower. Since radiation goes down like the fourth power of temperature, that makes the radiation to space lower (for fixed surface temperature). As long as there is some colder air up there, adding more greenhouse gas makes more greenhouse effect. It doesn’t even need unsaturated bands (though for CO2 these do indeed help). One thing that makes cold air aloft is convection, which lifts and cools air. However, as you add more greenhouse gas, that cools the stratosphere, making even more cold air available. This is all explained in Chapter 4 of my ClimateBook. The lack of saturation of greenhouse effect is how you can make Venus have a surface temperature of 740K. You’d never get that hot if you ran out of greenhouse effect at the piddling few hundred parts per million we have on Earth. –raypierre]

  30. 80

    [[Wow. “aerosols remain the least understood component of the climate system.”
    Aerosols have a net radiative forcing of -1.2 W/m2 against a CO2 forcing of 1.66 W/m2. If I were an AGW convictee, the closeness of these figures along with the uncertainty regarding aerosols would concern me. ]]

    The feedback from water vapor and ice/albedo, and the warming from other greenhouse gases such as methane, nitrous oxide, and ozone, makes the warming component a lot larger than 1.66 W/m2. There’s no question that the warming dominates over the cooling.

    [Response: Actually, the feedbacks aren’t relevant for this comparison, but all the other forcings obviously are. As you say, there is very little possibilty that the net forcing is negative. -gavin]

  31. 81
    Nick Gotts says:

    “A CO2 molecule has a lifetime of about 100 years in the atmosphere, while an aerosol particle has an average life expectancy of only about 10 days. Therefore, if we instantaneously ceased using combustion engines, the (cooling) fossil fuel-related aerosols would be cleaned out of the atmosphere within weeks, while the (warming) CO2 would remain much longer, leaving a net positive forcing from the reduction in emissions for a century or more.”

    Despite all the comments, I’m still having trouble understanding exactly what Juliane is asserting here, and on what basis. Is it:
    1) If we stopped all fossil fuel use now [and everything else continued unchanged], it will be more than a century before the reduction in enhanced greenhouse effect due to the CO2 from post-1750 fossil fuel use leaving the atmosphere outweighs the reduction in reflection of solar irradiation due to the aerosols from fossil fuel use leaving the atmosphere.
    2) If we stopped all fossil fuel use now [and everything else continued unchanged], it will be more than a century before net positive forcing falls below the level that would result if we continued using the same amount of fossil fuel in the same way as now.
    3) If we stopped all fossil fuel use now, it will be more than a century before net positive forcing falls below the level that would result if we continued using fossil fuel on a “business as usual” path.
    4) Something else?
    Whichever is being asserted, can the supporting calculations be summarised here, or accessed elsewhere?

  32. 82

    [[I believe this correlation gives us the confidence to say how much the overall effect is because it is based on actual temperature measurements. In a sense, the total forcing is calibrated to actual data for this planet. ]]

    CO2 radiative forcing is NOT based on empirical measurements. It is calculated from the known properties of CO2 and the amount of CO2 in the atmosphere. It won’t change with new data. We’ve known about thousands of CO2 lines for decades and the calculations, while intricate, are straightforward.

  33. 83

    [[I have heard a claim that the planet would be 60-70 degrees F cooler if green house gases were removed (leaving oxygen and nitrogen). As CO2 is the dominant greenhouse gas, most of the greenhouse effect appears to be related to this one gas, directly or indirectly (H20 into air). I bring this up because establishing CO2 as essential in making the Earth habitable implicitly gets one half way to addressing most skeptic arguments (with the public at least). ]]

    Most of the 33 K greenhouse warming on Earth is caused by water vapor. It’s not really possible to break it down by individual agents, but without being too far off you might make it 21 K from H2O and clouds, 7 K from CO2 and 5 K from other greenhouse gases.

  34. 84
    Lynn Vincentnathan says:

    [[Why do I never hear that the CO2 greenhouse effect is saturated ?]]

    It seems to me that the denials want to have it both ways. Sometimes they say it’s prepposterous to think that the tiny ppm of CO2 could have any effect at all, and the tiny additions to this from humans, even less possibility of an effect. Then you hear there’s too much CO2 up there to have any effect, just too crowded.

    Seems to me that the 1st case is correct in pointing out that these molecules are really spread thin — with the fantastic, counter-intuitive fact that they do help warm the earth. AND seems there’s plenty of more room up there for more of them….

  35. 85
    PHE says:

    Re 80 (BPL) and other comments.
    The IPCC Summary for Policymakers – that which is regularly claimed to represent the ‘scientific concensus’ demonstrates (on the basis of current knowledge) that it is perfectly feasible that negative forcing components outweigh the positive forcing component of CO2. CO2 could be 1.49, and aerosols could be -2.7.
    We so often hear that ‘the science is settled’, that ‘there is no room for doubt’, etc. You can add up all the postivie forcings, but what these statements assume is that manmade CO2 emissions are causing AGW. The IPCC Summary for Policymakers itself demonstrates this is far from being a certainty.
    It is clear I am a ‘skeptic’ (‘denialist’ is a naive and immature term for those with their head firmly in the sand – or is it the clouds?). Most with a view on this subject already have relatively firm views, so I don’t expect to change these. My principle concern is that it must be accepted that the debate continues.
    Personally, I strongly believe in protecting the environment and maintaining a sustainable world. To always accuse ‘skeptics’ as being ‘in the pay of the fossil fuel industry’ is a nonscense (some are, but many are not). What I can never accept is the distortion of the scientific process. By this, I do not mean the belief in AGW – which is an acceptable point of view (if based on judgement and not faith) – but what I include is the ridiculous claim that ‘the science is settled’ and that it is impossible to believe otherwise unless you have some ulterior motive.
    My reassurance is that this site only exists because the debate exists.

  36. 86
    Rod B. says:

    a clarification (maybe I’ll find it later, but don’t want to forget):

    [[While a relatively minor part of the overall aerosol mass, changes in the anthropogenic portion of aerosols since 1750 have resulted in a globally averaged net radiative forcing of roughly -1.2 W/m2,…]]

    Is -1.2W/m2 currently? what does “global average since 1750” mean?

    Is the -1.2 W/m2 only from anthropgenic aerosols? What do the (relatively much greater??) natural (?) aerosols do? The graph shows
    -1.2 W/m2 total aerosol forcing. Can someone straighten me out?

  37. 87
    Blair Dowden says:

    Re #81: Let me attempt a crude calculation of how long it takes before a reduction in fossil fuel use lead to cooling. I will start by assuming all aerosols and carbon dioxide come from fossil fuel burning (in reality it is only part of both, so maybe the errors roughly cancel). According to figure SPM-2 from the recent IPCC Summary for Policy Makers, stopping fossil fuel use would result in an immediate increase in forcing of 1.2 watts per square meter (W/m2), the sum of the two aerosol forcings. The present forcing from carbon dioxide is 1.7 W/m2. Assuming the relationship between CO2 concentration and forcing is linear (not true: it is logarithmic, each unit of decrease will have more effect, but that is good enough for this crude calculation) we need to get rid of 70% of the CO2 to remove 1.2 W/m2 of forcing.

    Reading from the CO2 decay rate from this page, it will indeed take one hundred years to reduce CO2 by 70%. However the result is very sensitive – if we assume we only need a 50% CO2 reduction (because of the large uncertainty in aerosol forcing or other problems with the figures I use), the time is reduced to 20 years.

    Even the more optimistic result indicates we get net warming for 20 years after reducing fossil fuel use. I wonder how many people are really aware of the long time scales involved in this issue. There are no quick fixes. This is not a justification for doing nothing, but we must be realistic.

    I would love to see a better calculation than this one.

  38. 88
    yartrebo says:

    My understanding is that aerosol emissions from fossil fuel burning are likely to be heavily reduced regardless of what happens to CO2 emissions. Smog is very noxious and countries like China are likely to seriously reign it in soon (my guess is within a few years – they probably would have tackled it earlier if it weren’t for the chronic electricity shortages). The developed world has been quite successfully tackling smog for decades and gross emissions have been dropping.

  39. 89
    Charles Muller says:

    In order to estimate the forcing of aerosols, we need a good microphysics of each kind of particles, but also, more basically, a quantification of these particles (like we’ve with GHGs atmospheric concentrations) and a quantification of natural particles (to assess the relative role of anthropic ones in their indirect effects). Do we have such estimates at a global scale? Does uncertainty deal with a low physical understanding or just with a difficulty to count up?

  40. 90

    Some time ago that I have commented here. As a lukewarm skeptic (I am sure that more CO2 causes some warming, but I am far from convinced that we are heading to a catastrophic warming), I have reacted several times on this forum about aerosol forcing and its effects.

    To begin with, there is an offset in response between aerosol cooling and GHG warming. If (anthro) aerosols have a huge influence, then the sensitivity for GHGs is high, if aerosols have a low influence, then GHGs have a low influence too. This is reflected in the first graph of the RC discussion about climate sensitivity and aerosols, and has huge consequences for projecting the future.

    There are several indications that anthropogenic aerosols have a small influence on climate.

    – While SO2 emissions (which give the strongest cooling) are nearly constant since about 1975, there is a huge shift in regional sources. Europe (and North America) have made huge reductions, while SE Asia firmly increased. One should see a huge change in temperature trends within Europe between less polluted and more polluted areas in the main wind direction, but these are not measurable.

    – Aerosol models underestimate natural aerosols. One measurement off the coast of West Africa found about 90% salt spray, while the model calculated 50% anthro (sorry, lost the reference). Another investigation found that the load of natural (SOA – VOC induced secondary oxydised) aerosols in the free troposphere was underestimated with a factor 2-100 in chemical transport models (see Heald ea.).
    The mass ratio SOA/SOx (SO2+sulfate) aerosol is app. 2:1 to >10:1, between 0.5 and 5.5 km altitude. These are natural fine aerosols with similar properties as sulfate aerosols.

    – About 90% of anthro SO2 is emitted in the NH. Very little of this passes the equator, thus the influence must be maximal in the NH. This is not reflected in the temperature trends, as the NH warms faster than the SH. Not only because there is more land in the NH (which indeed warms faster), but also the heat content of the NH oceans increases substantially faster than the SH oceans, if corrected for area. This points to a more positive W/m2 radiation balance for the NH than for the SH.
    If we accept the forcings as reflected in the SPM, then GHGs forcing is evenly distributed over the NH and SH (overall about 3 W/m2). But as 90% of the anthro aerosols are emitted in the NH, this would give some -2.2 W/m2 cooling from aerosols in the NH, against -0.2 W/m2 in the SH with a global average of -1.2 W/m2. That means that the NH should have an overall radiation balance of +0.8 W/m2 (all forcings included) since 1850, and should be warming far less rapidely than the SH with about +2.8 W/m2.
    As the NH is warming more rapidely than the SH, this -again- points to a low influence of aerosols and consequently a low climate sensitivity for GHGs…

  41. 91
    Charles Muller says:

    #90 Please to read you, Ferdinand.

    I’m still skeptic about conclusions concerning aerosols forcing evaluation (either high or low). Some works on recent trends (better known that 1750-2000 means) suggest their effects could be important, after all.

    If we accept the global dimming / brightening transition measured by GEBA / BSRN stations as a physical reality rather than a statistical artifact (Pallé 2005 ; Wild 2005, 2007), and if we consider these trends over lands are mainly driven by anthropic aerosol emissions (Streets 2006), you have a slight warming (for Tmin, cooling for Tmax) between 1960-85, and a strong warming (Tmin and Tmax) between 1985-2002. Doesn’t it plead in favour of an important negative aerosol forcing ?

    One can look at the interesting paper of Wild et al. 2007 (they conclude that the recent increase of T cannot be attributed to surface insolation trends 1960-2000, at least on GEBA network, because surface insolation 2000 is still under the 1960 level, in spite of downward aerosol emissions over industrialized countries) :

    GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L04702, doi:10.1029/2006GL028031, 2007
    Impact of global dimming and brightening on global warming
    Martin Wild, Atsumu Ohmura, Knut Makowski

  42. 92

    Re #91,

    Charles, I beg to differ about the role that anthro aerosols play in global dimming. I have not (yet) read the 2007 Wild ea. paper, but had some discussion on global dimming in the past here (comments #23 and #25):

    About the suggestion that aerosols may be involved: Although the emissions in Western Europe have sharply declined (over 50% since 1975), the timing doesn’t correspond to the decrease in insolation until 1990 and the recovery thereafter. Neither does that correspond to the increase in solar radiation in Australia and Antarctica since 1990, where human made aerosols have no measurable impact. Moreover, the global emission of sulfate aerosols is near steady since 1975, but with a large shift from Europe (and North America) toward SE Asia. This may be seen in the data of India, but China with an explosively increasing industry shows increasing insolation…

    I don’t think that aerosol emissions in China currently are lower than in the 1990s.

    Thus the main problem with the aerosol-dimming connection is that the timing of regional/global emission trends and regional/global dimming trends don’t match.

    As said in comment #25 of the above RC link, it seems that clouds and water vapor (especially in Europe) are mainly responsible for global dimming and brightening…
    Btw, the first link in comment #25 doesn’t work anymore. It was referring to the (free copy) work of Philipona and Durr, “Greenhouse forcing outweighs decreasing solar radiation driving rapid temperature rise over land”, GRL, VOL. 31, L22208, 2004, now only available for a fee. The “greenhouse forcing” in that article seems to be mainly from increased water vapor (probably NAO induced…).

  43. 93
    Hank Roberts says:

    When someone who can follow the science gets the full text of Wild and has time to read it, please comment. It’s an interesting abstract.

  44. 94
    Daniel Morris says:

    RE: #75 and 83

    Dear Mr. Levenson,

    I am fully aware that water functions as the dominant greenhouse gas. I am not trying to break out the individual contributions. The question is whether the absence of CO2 and methane would also reduce the concentration of water to the extent that surface temperature would drop about 60 degrees F.

    Perhaps I should indicate the number came from an article by Immanuel Kerry:

    [Response: That’s Kerry Emanuel. If the Earth had a pure Nitrogen or a Nitrogen/Oxygen atmosphere, having no greenhouse effect, the temperature would be about 255 Kelvin — if the Earth had no oceans to freeze over. The actual mean temperature is more like 285K, so the net greenhouse effect (neglecting ice-albedo feedback) is more like 30K (which is also 30C). Now, since we have oceans that would freeze, the fact is that without the greenhouse effect the oceans would freeze over, increasing the reflectivity of the planet. That drops the temperature to 230K or less. That would say the greenhouse warming is something over 60C. For more details see Chapter 3 of my ClimateBook –raypierre]

  45. 95
    Hank Roberts says:

    Article by Kerry Emanuel:

    “Air is …composed almost entirely of oxygen and nitrogen, …. Such molecules barely interact with radiation….. If that is all there were to the atmosphere, … the average temperature of the earth’s surface …. Accounting for the amount of sunlight reflected back to space by the planet ….. works out to be about 0°F, far cooler than the observed mean surface temperature of about 60°F.”

    Raypierre has written about science fiction atmospheres; he probably knows if it’d be possible to change only to zero CO2 and keep the planet warm enough not to freeze out the water, but that’d be a slippery slope to a snowball planet, I imagine.

  46. 96
    Lynn Vincentnathan says:

    It sort of occurred to me that with more water vapour in the air under GW conditions, there is a greater chance for SO2 & NOx to become sulfuric acid and nitric acid (I don’t know if my chemicals are correct, but you know what I mean), at least in contexts where areas of increased WV & SO2 & NOx converge. Could this be another negative effect of GW???

  47. 97
    Ike Solem says:

    Regarding the issue of residence times, it seems a bit hard to apply to CO2 in the current environment. See the description of residence time calculation at: The steady-state assumption is off, since CO2 in the atmosphere is currently increasing at something like 2.0 ppm/year (and accelerating). We also know that only half the CO2 emitted by human activity stays in the atmosphere, and then you have the biosphere’s photosynthesis and respiration. The biosphere flux to the atmosphere is >100 in, 100 out, while the human flux is 6 in, 3 out. (GtC) The biosphere appears to operate in steady state, unless there is organic carbon burial going on (fossil fuel formation) or land-use changes (deforestation). Trying to figure out the residence time of methane is even worse, since that’s largely determined by atmospheric chemistry (which ends up converting CH4 to CO2 – another flux term). The oceans have 50x as much dissolved inorganic carbon as the atmosphere, but the mixing time is slow (slowing?)…there’s a good site on all this at (Maybe the pre-industrial residence time of CO2 is well-defined, however)

    This all relates to the potential carbon cycle-feedback responses, which were briefly mentioned in the fourth IPCC summary. There aren’t any climate models that I’ve heard of that include CO2 cycle effects, but it’d be ridiculously difficult because you’d have to include photosynthesis and respiration (and human behavior!). However, perhaps the IPCC should include emissions scenarios for both strong and weak CO2 feedback effects.

    Regarding aerosol measurements, there are airborne mass spectrometers that can get this data; for example see J. Schneider et al 2003 Aerosol Sampling (pdf) which mostly shows that tropospheric aerosols are highly variable between regions and even in the morning vs. the evening. Trying to directly measure the indirect aerosol cloud effect? That seems difficult.

    Regarding the Wild paper on aerosols and global dimming, a brief synopsis from Science is available at . It seems clear that aerosols from volcanoes can have a significant effect; see “Significant decadal-scale impact of volcanic eruptions on sea level and ocean heat content” Church 2005 (abstract). (Incidentally, inducing short-term cooling by setting off volcanoes with nuclear weapons is a bad idea, for many reasons, but is on par with the other geoengineering proposals).

    Thanks for the clear discussion of this complex topic.

  48. 98
    Dave Reay says:

    At a meeting last Friday our group at the University of Edinburgh discussed a recent EOS article by Gerald Stanhill on Global Dimming and Brightening (see this link).

    The reduction in solar radiation at the Earth’s surface he cites for the period 1958 to 1992 is huge: 20 W/m2. We just could not reconcile this with measured changes in global surface temperature during this period, or with the ‘global brightening’ reported since 1992. Can anyone explain why such an apparently huge change in solar radiation has not dominated the surface temperature signal? Compared to anthropogenic forcing over this period (around 2.5 W/m2) the global dimming/brightening story would seem to be very important. Stanhill seems to believe that AR4 has overlooked this issue, but I’m not familiar enough with the relevant literature on this to know if it’s really as important as it appears to be.

    Thanks in advance.

    Dr Dave Reay
    University of Edinburgh
    Recent books: Climate Change Begins at Home (Macmillan) and Greenhouse Gas Sinks (CABI)

    [Response: There are (at least) two problems with Stanhill’s analysis. First, he confuses the surface forcing with the radiative forcing from CO2 (defined at the tropopause). The former, while interesting, does not imply very much about the latter (which is what the global temperature generally responds to). We have a paper in press that shows clearly that ‘global dimming’ and ‘global warming’ are not mutually exclusive concepts. Secondly, his estimate of 20 W/m2 for the surface forcing in no way can be described as a ‘global’ average. This has been discussed here previously: . There will undoubtedly be some responses to the Stanhill piece at some point, and we’ll go into some more detail at that point…. – gavin]

  49. 99
    Charles Muller says:


    Ferdinand, the general problem with timing / spatial convergence is that we need a good evaluation of both phenomena (dimming/brightening and anthropic aerosol emissions) and that’s hard to get. Comparisons between measurements from surface pyranometers (like GEBA and BSRN networks) and air-quality assessment would probably be the more precise for local analysis of land-station evolution (but may be too scarce for a continental / global assessment, notably the much more problematic nebulosity).

    As I mention in my previous post, Streets at al. 2006 (thereafter) fond a rather good correlation between trends in surface insolation and in SO2/black carbon emissions.

    GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L15806, doi:10.1029/2006GL026471, 2006

    Two-decadal aerosol trends as a likely explanation of the global dimming/brightening transition

    David G. Streets, Ye Wu, Mian Chin
    Abstract- Global average trends in solar radiation reaching the Earth’s surface show a transition from dimming to brightening that occurred in about 1990. We show that the inter-annual trend in solar radiation between 1980 and 2000 mirrors the trend in primary emissions of SO2 and black carbon, which together contribute about one-third of global average aerosol optical depth. Combined global emissions of these two species peaked in 1988â��1989. The two-decadal rate of decline in aerosol loading resulting from these emission changes, 0.13% yrâ��1, can be compared with the reported increase in solar radiation of 0.10% yrâ��1 in 1983â��2001. Regional patterns of aerosol and radiation changes are also qualitatively consistent. We conclude that changes in the aerosol burden due to changing patterns of anthropogenic emissions are likely contributing to the trends in surface solar radiation.


    For China precisely, the draft of another work from Streets 2006 can be read here (link > pdf). He observes a peak around 1994-1996 for carbonaceous aerosols and SO2 over East Asia:


    Anyway, I agree with you, anthropic aerosols probably don’t tell us the whole story. For example, Wong et al. 2006 find the same trend over Tropics (20 N – 20 S TOA SW upward from ERBE, compared to other sources) and they acknowledge nebulosity rather than aerosols is a better candidate for this Tropical brightening from 1980s to 1990s.
    Wong, T., B. A. Wielicki, R. B. Lee, III, G. L. Smith, K. A. Bush, and J. K. Willis, 2006: Re-examination of the Observed Decadal Variability of Earth Radiation Budget using Altitude-corrected ERBE/ERBS Nonscanner WFOV data. J. Climate, 19, 4028-4040

  50. 100
    James says:

    Re #97: You say “Incidentally, inducing short-term cooling by setting off volcanoes with nuclear weapons is a bad idea, for many reasons, but is on par with the other geoengineering proposals.”

    Is it such a bad idea, though? Certainly it’s not something to do lightly: I’d consider it the terrestrial analogue of coronary bypass surgury. Far better to convince your patient to exercise & eat reasonably, and so make the risk & expense unnecessary.

    Unfortunately, we seem to be faced with a patient who isn’t willing or able to make such lifestyle changes. Even supposing the world could be convinced to take a cold-turkey cure for its addiction to fossil fuel burning, it seems reasonable to suppose that just the ongoing effects of having 6+ billion people living on the planet – no matter how gently each one treads – might push the climate to the point where feedbacks become self-sustaining.

    In such a case, I for one would really prefer to have other options available. Setting off a few nuclear explosions in unpopulated areas (perhaps in mid-ocean) certainly seems preferable to the predicted consequences of severe global warming. Not to mention that, unlike all the other geoengineering schemes I’ve read of, it could be done on short notice, and at little expense.