Aerosols: The Last Frontier?
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.
21 February 2007 at 12:09
They’re still at it: environmental scholar Bjorn Lomborg, citing a 1997 atmosphere-ocean general circulation model, observes that “the increase in direct solar irradiation over the past 30 years is responsible for about 40 percent of the observed global warming.”
http://www.opinionjournal.com/columnists/pdupont/?id=110009693
Good ole Pete du Pont. All the deniers are on this sun kick, along with a revisionist history of the LIA, MWP, and out and out libel in my view over the hockey stick flaw. At some point can’t they be held accountable for lying? Is that an unalienable right?
[Response: If readers en masse refuse to purchase the paper until the editorial board stops using their perch to spread lies and propaganda, the WSJ might think twice next time. In the meantime, a google search on “Wall Street Journal editorial” might make you feel a bit better. –mike]
21 February 2007 at 12:16
re: the difference between 10 days (aerosols) and 100 years (CO2)
That’s a nasty conundrum: if we do something about active production of CO2, we accelerate the effects because of the lack of aerosols.
Does that sound like a recipe for fatalism? You betcha.
21 February 2007 at 12:35
I’m just terribly grateful to RealClimate for consistently posting this exceptional level of crisp exposition. You folks are really making a difference. Thank you for the generous contribution of your time toward providing this unparalleled forum.
21 February 2007 at 12:45
One of the big problems with aerosols is that they interact with clouds, that other “great unknown” in climate modelling. If clouds were well-represented by the models then untangling the influence of aersols would be hard enough, but… clouds are poorly represented in models at present [though much improved compared to ~10 years ago].
To be honest I don’t think this scientific problem is going to be “solved” soon enough to have an impact on the crucial policy decisions of the next 10 years.
21 February 2007 at 12:53
RE: Jeffrey (#2)
I prefer to see it as MORE inspiration to (1) figure out how to reduce CO2 emissions quickly - because the warming we’ve seen so far has been somewhat masked by aerosol, and (2) really work hard to understand all the aerosol climate effects more completely (look at those error bars!)
Remember also, the warming caused by increased CO2 has a time lag, so the sooner and more rapidly we reduce CO2 emissions, the better. We can do this, we just need to take the problem seriously, especially because of the (more uncertain) aerosol cooling.
21 February 2007 at 13:16
“Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, …”
That’s in decreasing order of abundance, I take it? That is, sea salt is the most abundant, followed by mineral dust, etc.? (The wording is a little ambiguous.)
21 February 2007 at 13:24
… 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.
What is a rough range of accuracy for the -1.2 W/m2?
My thought is that globally averaged net radiative forcing on a decade basis will change only slightly. It’s important not to loose focus on the need to act on drastically reducing our CO2 emissions.
21 February 2007 at 13:45
Very nice explanation, Juliane. Thank you for your insights. I actually just took part in a discussion at Open Mind, in which a skeptic raised this issue of the lack of understanding about the effects of aerosols on our climate, stating that the IPCC’s scientific understanding is “very low”. Like many other skeptics, he was first misrepresenting the actual level of understanding, and then used it as a reason for discounting any scientific statements about the effects of aerosols, because supposedly one must accept too many assumptions.
Anyways, I appreciate your information on the subject, which clearly demonstrates that, while there is room for improvement, much has already been discovered.
21 February 2007 at 14:34
Because we need to decrease GHG emissions dramatically in any scenario, it would seem wiser to reduce as quickly as possible despite additional warming caused by aerosol reductions. The longer we wait, the greater the likelihood that this ultimately unavoidable reduction in aerosols will have an even greater warming related effect.
21 February 2007 at 14:46
Hi Juliane
thanks for this interesting post
can we obtain the recent evolution, year by year, of aerosols forcing evolution (since 1990 for example)?
How can we understand “low level of understanding”?
Is this the understanding of accurate physical mechanism?
Does the given range of forcing depend on this understanding or on the measurements?
21 February 2007 at 14:49
What is Juliane’s URL?
Why do I never hear that the CO2 greenhouse effect is saturated ?
Why do I never see reference to the fact that CO2 absorption is concentrated in a wavelength band centered at 15 micro meters?
21 February 2007 at 15:21
Re #11: “Why do I never hear that the CO2 greenhouse effect is saturated ?”
Umm… How shall I say this? For much the same reason that you don’t hear a whole lot about the Earth being flat
21 February 2007 at 15:52
Juliane, a lecture I attended about a year ago pointed out that one of the problems with constraining aerosols is poor knowledge of their time history. It wasn’t till the middle to late twentieth century that direct measurements on large scales became practical. And because aerosols are often so short lived and so spatially inhomogenous, it is very difficult to reconstruct an adequate picture from proxies like ice core data. As a result, the speaker stated that a large chunk of the uncertainty comes from not having firm constraints on what the natural aerosol loading was circa 1750. I wonder if you could comment on this, and whether any progress is being made in paleo-reconstruction.
Secondly, could you comment on the degree to which the overall uncertainty is a matter of things that no one yet claims to understand versus areas where different researchers reach mutually incompatible conclusions. I recall that the indirect effect is known to give some examples of the second kind of uncertainty. If I recall correctly, the effect of high sulphate aerosol concentrations on cloud lifetime is one such area, where observational studies and cloud models have been disagreeing even on the sign of the effect (i.e. whether high sulphate leads to longer or shorter cloud lifetimes).
Thanks.
21 February 2007 at 16:04
Re: #6.
Yes, that order of aerosol components is in decreasing magnitude by MASS. Mineral dust and sea salt particles are primarily supermicron in diameter and thus constitute the bulk of aerosol mass. Depending on which estimate you read either mineral dust or sea salt it the largest aerosol component by mass.
However, by number (which matters more for aerosol-cloud interactions) the submicron particle types dominate. These include organic carbon, soot (also known as black carbon or elemental carbon), and secondary aerosol components such as ammonium sulphate and ammonium nitrate. Pure ammonium sulphate rarely exists in the atmosphere (though models often parameterise them this way!) and are typically internally mixed with submicron soot, organic carbon, etc. aerosols. The mixing state of aerosols is a very complicated but important topic that isn’t well represented in aerosol models (and even less so in climate models), partly because the models lack good empirical imputs for these parameters.
One interesting result of the mixing state concerns ammonium sulphate and soot. Soot is the most absorbing aerosol component and causes a positive radiative forcing. Ammonium sulphate is highly reflective, scatters solar radiation very well, and causes a negative forcing. However, when you have a particle that is internally mixed with soot and ammonium sulphate (say a core of soot with a coating of ammonium sulphate around it), the ammonium sulphate coating acts as a lens to focus light into the absorbing soot core. Thus by mixing the two aerosol components you end up with an overall more absorbing aerosol (higher positive forcing) than when the two aerosol types are externally mixed in their pure forms. This is one of the best examples of why aerosol mixing state is so important for modeling the effect of aerosols on climate.
21 February 2007 at 16:14
Re: #12 by James. L.D. Danny Harvey, “Global Warming - the Hard Science”, Prentice Hall, pp. 153-156 has forcing vs. CO2 concentration. The 15 micrometer band forcing falls in the logarithmic regime (bottom of p. 155). Fig. 7.11 shows some nonlinearity at twice the pre-industrial CO2 level.
21 February 2007 at 16:20
Believe it or not, skeptics have twisted this revision in aerosols to argue that climate change is not as bad we thought. I blogged about this new argument here.
21 February 2007 at 16:34
I guess “clean coal” (that reduces aerosol aspects, but not the CO2) is not a good solution to environmental problems, even though it would reduce acid rain; better to reduce fossil fuels altogether.
Re #1, I thought Lomborg was only a “money could best be spent elsewhere” denialist, who admitted that AGW was happening, but his heart went out to the masses suffering from other causes, who should be aided first. So now we see his true motive. And I wonder how much he really cares about the suffering masses.
And about the sun — without it the whole problem would be moot, including ourselves. So it is a major culprit, the way it just keeps shining and shining. I guess that small “solar irradiance” bar is for the increased shining, and does not represent the total shining?
Re # 5, I agree with you, Juliane, that we need to reduce GHGs, even though those same measures might reduce the cooling aerosol effect.
Somehow I wouldn’t worry about us reducing too fast, even if we put our whole heart, mind, soul, and effort into it (& all the contrarians converted to AGW believers & really started helping solve the problem, rather than dragging their feet kicking & screaming). Even in the best case reduction scenario the logistics are tremendous; it would take decades to get us down to the 70% money-saving cuts that are feasible with today’s technology, and a decade or so more to get the rich of the world (us) to actually sacrifice and reduce our living standards so as to get down to an 80%+ cut.
I know an architect who build a passive solar home that uses only a fraction of the gas & electricity of a comparable home (& that’s without PV panels). Considering all the homes & buildings now going up WITHOUT even the passive techniques that don’t have additional costs & that have been known about and practiced for over 2,500 years (see: http://www.amazon.com/golden-thread-years-architecture-technology/dp/0442240058 )….Today’s home are flagrantly inefficient….it seems like a big job to get architects to change habits. And then those wrongly-built houses & buidlings are here for decades to come. It really breaks my heart that at the first whiff of a problem (the 70s energy crunch) the field of architecture didn’t completely (or even somewhat) embrace passive solar techniques.
21 February 2007 at 16:49
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.
21 February 2007 at 17:01
Good article! But I do have to echo #10: what exactly does “low level of understanding” mean?
I take it to mean that we’re pretty sure about the direction of net forcing due to aerosols is, but we need more information to pin down the magnitude. Is this pretty accurate?
The reason I ask is because the length of the error bars in the “low understanding” categories vary a lot.
Re 11:
I’m confused about why you’d expect this article to address C02 as a greenhouse gas. There is plenty of information on this website about that topic. You clearly intended to say something with your questions, but I don’t get it…
21 February 2007 at 18:12
Scientists are acting as enablers, this is a social issue not a scientific one. I think scientists should show the world some tough love.
21 February 2007 at 18:37
Re: Post #20 by Paul M.
…..I think scientists should show the world some tough love…..
Paul, that’s a very astute comment. Thank you.
21 February 2007 at 19:53
I would like to know the calculation behind this part of the article:
First, I think it is more accurate to describe the lifetime of CO2 as half will be gone in 20 years, while 20% will remain in the air for centuries (at least according to this).
I would phrase the question as comparing the impact of running a coal plant for 100 years compared to replacing it with a clean energy source. The aerosol forcing of the coal plant will remain constant over the century, while the carbon dioxide impact will gradually accumulate. The key values, which I do not know, are the forcings from the amount of aerosol and CO2 emitted from the plant.
I find it surprising it takes a century of CO2 emissions to catch up to the aerosol forcing. It means that any action we take to reduce CO2 emissions will be effective on a longer time scale than I thought (which was already long), and the net effect will be less.
[Response: That’s exactly right: Juliane got that part wrong. We’ve been trying very hard to set the record straight. It is wrong to say that a CO2 molecule has a lifetime of “about 100 years” in the atmosphere. It takes more like 500-1000 years for the ocean to remove 80% of the Co2, and there’s a tail that extends for at least 10,000 years. Don’t know how we overlooked that, but c’est la vie. See David Archer’s article on CO2 lifetime in the archives. In any event, the main thrust of Julianne’s point, which is that CO2 stays around for a long time but aerosols disappear fast, is even more valid. Since sulfate pollution and other things that go with dirty coal have a lot of adverse environmental consequences one needs to get rid of, this just means one has to work even harder at reducing CO2. –raypierre]
21 February 2007 at 20:50
#18’s comment reminded me of another point that is often lost when discussing the climate effects of aerosols. The numbers given by the IPCC are GLOBAL averaged forcings. Aerosols, due to their short lifetimes, are not everly dispersed throughout the atmosphere the way greenhouse gases are (with the main exception being ozone). The aerosol burden in one region might be quite positive (due to high soot loadings as a simple example) while in another it might be strongly negative (due to high ammonium nitrate and organic carbon loadings from agricultural areas, for example). Thus, the REGIONAL climate impacts of aerosols really need to be considered, particularily their effects on precipitation. It is too simplistic to think that the negative (and quite uncertain) forcing of aerosols cancels out most of the positive forcings of GHG. Though it is easy to see how this misconception arises.
21 February 2007 at 21:20
Re #22: Somewhat closer to home, we have this article which gives a mean lifetime of about 30,000 years for a CO2 molecule, with the warning that this is a bit misleading due to the nonlinearity (or perhaps I should say the non-logarithmic linearity) of the decay process. It would be nice if we could agree on a metric for CO2 atmospheric lifespan to avoid confusion, but I’m too confused to propose a good one.
21 February 2007 at 21:30
Remember also, the warming caused by increased CO2 has a time lag
What is the lag time?
21 February 2007 at 21:52
Re #22 This question has been addressed in an earlier post,
Water vapor: feedback or forcing?
In the green comments to comment #85.
Here is an engineer’s view of the problem -
Residence time of atmospheric CO2
Bottom line - CO2 residence times are much longer than 20 years.
Hope this helps (and the links work)
21 February 2007 at 22:13
Re #26 and my own #24: Thanks for the link to Professor Lam’s paper. I am happy to accept his metric, which gives an answer for practical purposes of about 400 years, which is consistent with the rule of thumb given in the RC post I linked.
21 February 2007 at 22:25
So how fast is the science on this advancing? How important is this level of scientific understanding to the accuracy of the climate models. And how much does each of those LOSU values contribute to the uncertainties in the IPCC warming estimates?
21 February 2007 at 22:43
My question is this: Doesn’t the uncertainty in the aerosol forcing imply equivalent uncertainty in the other anthropogenic forcings? Since the overall increase in ghg related emissions is highly correlated if looked at averaged over the whole 200 years since industrialisation, error or uncertainty in one would imply more uncertainty in the balance between forcings, but less in the overall forcing than you’ve shown. I thought the overall forcing would have been calculated and cross-checked against the measured overall temperature differential. I imagined this to be quite accurate. On the other hand, when CO2 concentrations are increasing, and aerosols are decreasing (say due to the collapse of dirty eastern-bloc industries) it would be hard to tease out which was causing observed increases in temperature.
21 February 2007 at 23:29
In reply to Marcus L. #19, this article compares CO2 and aerosol forcing and examines the incremental change in forcing due to incremental changes in CO2 and aerosol concentrations. It is important to note that a certain percentage change in CO2 produces a smaller percentage change in forcing due to nonlinearity. I wonder if the climate forcing of a given aerosol (whatever this may mean, since aerosol concentrations are regional, as pointed out in #23 by Ryan Sullivan) is linearly related to its local concentration.
Also, CO2 absorption is due to molecular resonance and is narrow-band. Aerosol scattering is described (I think) by geometrical optics and is broad-band, compared to CO2. Radiation to space at the CO2 resonant frequency takes place from a much higher altitude than the reflection of short-wave sunlight or thermal equilibrium long-wave IR radiation from an aerosol.
While trying to write an intelligent comment I realize how complicated this stuff is. Enough already!
21 February 2007 at 23:34
The uncertainty in aerosol forcing looks unsettling, but this is a good example of the case where one needs to ask: What are the consequences of this uncertainty for our predictions of future climate? One way that the uncertainty affects the future is through the warming due to the decline in aerosols if we move to cleaner burning technologies. If the present forcing is large, the warming due to aerosol decline willl be larger, and conversely. The uncertainties in aerosol forcing also have some consequences for deciding which models have the “best” climate sensitivity, though the precise implications of that are a bit harder to see. One thing to keep in mind is that, if CO2 continues to increase, the CO2 increasingly overwhelms the aerosol forcing, so aerosol uncertainty may play less role in the future than in the past century. A further thing to notice is that, as one commenter mentioned, the aerosols are not uniformly distributed; there’s considerable regional variation. That means that a climate with a lot of CO2 warming partially offset in the global average by a lot of regional aerosol cooling is still a very different climate than one with no anthropogenic aerosols and less CO2. It’s still, as Hansen says, a different planet we’ll be living on.
I’d be interested in hearing more discussion from the aerosol experts here about what it would imply for the aerosol forcing to be on the low end of the range, vs. what it would imply for the aerosol forcing to be on the high end of the range.
Keep in mind also that, while the article chose to compare just CO2 radiative forcing with aerosols, there are other anthropogenic greenhouse gases. You could just as well take the sum of radiative forcing from the “lesser” greenhouse gases: CH4, N2O,halocarbons and ozone, which amount to about 1.2 W/m**2, and net them against roughly 1.2 W/m**2 of aerosol cooling. Put that way, you’d say the midrange estimate of aerosol forcing cancels out the radiative forcing from the lesser greenhouse gases, leaving us with the full effect of CO2 as if the “complications” weren’t there. Or, Jim Hansen, for some reason, likes (or liked) to say that the aerosols canceled out most of the CO2 radiative forcing, leaving us to see a little of CO2 plus a lot from the other greenhouse gases. That’s no more nor less valid, but it did open up his paper to a lot of misinterpretations by the media, which then got the message that Methane’s the problem, not CO2. The fact is that for the greenhouse gases, radiative forcing is radiative forcing, and it doesn’t make much sense to break it apart when comparing to aerosols. It makes sense to break it apart mainly because the different gases have different atmospheric lifetimes
21 February 2007 at 23:43
I have a question not directly related to the topic of aerosols but to a point that someone brought up higher in the discussion. Chevron and other big combustion players maintain blogs on which there are seemingly inexhaustible supplies of skeptics who can sound technical enough to be convincing for most of the general public. One of their favorite battle horses is the absorbtion spectrum of CO2, which they maintan is too narrow to account for the forcing attributed to the gas. Any help on that?
[Response: See the discussion of the spectrum of CO2 absorption in Dave Archer’s global warming text, and play with the online models. For a more technical discussion, take a look at my climate textbook (draft online, chapter 4, see geosci.uchicago.edu/~rtp1 ), or Goody and Yung. We don’t have an article on this because only the least informed skeptics are using the argument anymore, and nobody of any consequence believes it. Perhaps we ought to have something like that in the wiki, for general education. –raypierre]
22 February 2007 at 0:23
RE: 22
An estimate of anthropogenic CO2 inventory from decadal changes in oceanic carbon content
Proceedings of the National Academy of Sciences, (2007)
doi:10.1073/pnas.0606574104
http://dx.doi.org/10.1073/pnas.0606574104
…”Approximately half of the anthropogenic carbon released to the atmosphere from fossil fuel burning is stored in the ocean, although distribution and regional fluxes of the ocean sink are debated. Estimates of anthropogenic carbon (Cant) in the oceans remain prone to error arising from (i) a need to estimate preindustrial reference concentrations of carbon for different oceanic regions, and (ii) differing behavior of transient ocean tracers used to infer Cant.”…”In contrast to prior approaches, the results are independent of tracer data but are shown to be qualitatively and quantitatively consistent with tracer-derived estimates. The approach reveals more Cant in the deep ocean than prior studies; with possible implications for future carbon uptake and deep ocean carbonate dissolution. Our results suggest that this approachs applied on the unprecedented global data archive provides a means of estimating the Cant for large parts of the world’s ocean.”
Distribution and inventory of anthropogenic CO in the Southern Ocean: Comparison of three data-based methods
Journal of Geophysical Research 110 (c9), 09-2 (2005)
doi:10.1029/2004jc002571
http://dx.doi.org/10.1029/2004jc002571
…”North of 50°S, distribution and inventories of Cant are coherent with previous data-based and model estimates, but we found larger storage of Cant south of 50°S as compared to the midlatitude region. In that, our results disagree with most previous estimates and suggest that the global inventory of anthropogenic CO2 in the Southern Ocean could be much larger than what is currently believed.”
Response to Comment on “The Ocean Sink for Anthropogenic CO2″
Science 308 (5729), 1743d (2005)
doi:10.1126/science.1109949
http://dx.doi.org/10.1126/science.1109949
….”The correct determination of the magnitude and uncertainty of the oceanic uptake of anthropogenic CO2 and of the climate change feedbacks is of prime relevance to constrain the net balance of the terrestrial biosphere over the past 200 years”..
Climate-carbon cycle feedbacks under stabilization: uncertainty and observational constraints
Tellus B 58 (5), 603 (2006)
doi:10.1111/j.1600-0889.2006.00215.x
http://dx.doi.org/10.1111/j.1600-0889.2006.00215.x
…”However, the observational record proves to be insufficient to tightly constrain carbon cycle processes or future feedback strength with implications for climate-carbon cycle model evaluation.”
The Oceanic Sink for Anthropogenic CO2
Science 305 (5682), 367 (2004)
doi:10.1126/science.1097403
http://dx.doi.org/10.1126/science.1097403
…”The current fraction of total anthropogenic CO2 emissions stored in the ocean appears to be about one-third of the long-term potential.”
22 February 2007 at 0:57
As a counter-assertion to some denialists’ assertion of solar forcing being way bigger than the graph shows, I’d like to assert that the forcings from CH4, N20 and Halocarbons are double what the graphs indicate, and that the forcing from CO2 is less than half that. I have said before that this would imply a big policy shift. Neither the cold case evidence or the CSI evidence seems to be able to disprove this assertion. Am I missing some critical scientific experiment here that would change my skeptical mind? I’m not sure of the author of that graph, but what makes them so confident about their CO2 vs CH4 forcings being accurate to within 20% of their ranges or so?
[Response: The forcings by the greenhouse gases have little uncertainty because their concentrations are accurately measured in the atmosphere, and their infrared absorption properties are very accurately measured in the laboratory. The two are put together using highly accurate numerical methods that have little error. The small uncertainty is primarily because the radiative forcing depends on temperature profiles (well observed), water vapor profiles (somewhat less well observed) and cloud profiles (with additional uncertainties). The water vapor and clouds come in because you get little additional radiative forcing from greenhouse gas increases below a thick cloud. Similarly, water vapor competes somewhat with absorption due to the other greenhouse gases. There is no plausible ways these uncertainties could be stretched to accomodate the scenario you are proposing. –raypierre]
22 February 2007 at 0:57
Raypierre (#32),
If you want to see one aerosol expert’s view, look here.
I happen to think it’s greatly exaggerated (in fact my views have strengthened since writing that post), but then he’s the one writing in Nature and I’m the one merely blogging about it
(If you follow the links you will find RC has also talked about it before.)
[Response: All very good points to be reminded of, in the context of the present discussion. –raypierre]
22 February 2007 at 1:31
Thanks, everyone, for the interesting & informative discussion.
There were several comments and questions about uncertainty (error bars in the figure) and the assigned “level of scientific understanding” for each forcing. The full description of how these were derived in the new IPCC 4AR will be published with the full Technical Summary later this spring, but I assume they will be essentially the same as in the predecessor 2001 IPCC TAR. In that case, the error bars on the radiative forcing plots represented the spread in published estimates of the forcing for each source. The subjectively assigned “level of scientific understanding” (LOSU) was defined in chapter 6 of the TAR as representing “our subjective judgement about the reliability of the forcing estimate, involving factors such as the assumptions necessary to evaluate the forcing, the degree of our knowledge of the physical/chemical mechanisms determining the forcing, and the uncertainties surrounding the quantitative estimate of the forcing.”
So, to post #29: the LOSU don’t affect warming estimates at all.
Re #17: yes, the solar irradiance forcing represents the change since 1750, as are all the forcings on this plot.
Re #22 etc.: Thanks, Ray, for correcting my error on the CO2 lifetime. My apologies for perpetuating a misconception. I have, however, seen these “low” CO2 lifetimes thrown around a lot, presumably because as David Archer points out: “If one is forced to simplify reality into a single number for popular discussion, several hundred years is a sensible number to choose, because it tells three-quarters of the story, and the part of the story which applies to our own lifetimes.” (I put low in quotation marks, because I think 100 years would be plenty long enough to encourage some proactive public policy, if anything longer would) Some basics of why CO2 lifetime is effectively much longer than 100 years have been discussed in these comments by now, but for anyone interested in more detailed discussion, the full post from David Archer about CO2 lifetime, is here.
Re #30: Uncertainty in aerosol forcing doesn’t directly imply uncertainty in other forcings, because there are additional constraints on those other forcings beyond just the last 250 years. For a nice discussion of climate senstivity, check out this post.
Thanks, Ryan and others, for bringing up regional variability in aerosols. This is an incredibly important point, and one which I am certain will be addressed thoroughly in the technical summary portion of the IPCC report. So of course we shouldn’t make too much out of the globally averaged aerosol forcing.
22 February 2007 at 1:37
Let me amplify on #37: here’s the other RealClimate link (that James’ blog point to) I should have put in my comment about climate sensitivity and how uncertainty in aerosols relates to future climate projection:
http://www.realclimate.org/index.php?p=115.
thanks James.
22 February 2007 at 1:46
This was a great primer on aerosols. In my never-ending quest to understanding climate science the one area I was almost clueless on was aerosols
On raypierre’s response to Philippe Chantreau #33, sometimes people who are trying to learn about climate science and who hear alot of information in the popular media and the internet don’t realize how much of the science is not controversial within the scientific community. Some of the facts are so well settled that you will not see them a journal paper. To find out about it you have to consult a basic science textbook
22 February 2007 at 2:04
Sometime ago, I made a figure showing the removal of carbon dioxide from the atmosphere. A large chuck (70-85%) will be expected to be removed from the atmosphere after ~200 years, but in actuality it has just been transferred to the ocean and biosphere. Ultimately removing the excess carbon from the atmosphere-biosphere-ocean system occurs primarily through the production of carbonate rocks and may take 100s of thousands of years to fully occur.
Incidentally, as a historically anecdote, the TAR said that carbon dioxide had a residence time of 5 to 200 years. This memorable statement is based on the Wigley 1991 carbon cycle model where ocean uptake was modeled as a sum of five processes. Four of those processes had lifetimes ranging from 5 to 200 years. However, the fifth process had a lifetime of infinity! Wigley’s model, which in that iteration was only intended to predict a few thousand years, actually used the approximation that 15% of carbon never left the atmosphere. This got rather muddled when the IPCC used the 5-200 numbers but didn’t effectively convey that there was an additional portion that essentially never dissipates (from the point of view of human time scales).
22 February 2007 at 4:47
RE #18:
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.
22 February 2007 at 5:58
So the forcings of greenhouse gases would be greater if it was not for humans releasing counter chemicals which are often a product of burning coal in particular.
Therefore if we moved to clean coal technology we might not release much carbon into the atmosphere but also aerosols will lessen and hence warming would take some time to lessen in fact it might even rise tempoarily. I am presuming that aerosols are responsible for some of the so called global dimming effect?
[Response: For health reasons and acid rain reasons, you have to switch to clean(er) coal anyway. Even China is doing some of that already. 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 myself doubt that it’s a viable solution, but people do talk about it. –raypierre]
22 February 2007 at 8:53
One and all,–thank you, thank you, thank you for a sparkling discussion. Particularly, I thank Julianne Fry and Ray Pierre for bringing out the problem of conveying to lay folk like myself (who must take off their socks to count above ten) CO2’s life time. I say better a sudden and briefer rise in global heat with the shut-down of CO2 generators than with our Business As Usual. For psychologists it must be a puzzle that some of us understand compound interest in one context and not in another.
22 February 2007 at 9:00
RE: 43 In Pollutant Removal Race, CO2 Beaten by S
Although touched on in the introduction to this topic, it is important to clarify the difference between how easily and rapidly the sulfate aerosol precursors can be removed from fossil fuel sources vs. carbon from combustion sources or fuels.
Technologies already exist and are being employed to desulfurize coal and petroleum, either pre- or post combustion. The issue there is how rapidly these removal technologies can be applied to, e.g., a new coal-fired power plant in China or a new refinery.
There are no large-scale technologies for removing carbon or CO2 from these sources and probably won’t be for at least 20 years. Thus, it is more likely that the negative forcing from tropospheric aerosols will decline and become insignificant well before any significant removal of carbon or carbon dioxide takes place. Bottom line. Don’t count on that -1.2 or -1.5 W/m2 to help out after 2050 and expect to see it decline by several percent every year.
22 February 2007 at 9:16
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.
How does the expected life time of an aerosol being 10 days square up against a half life of a volcanic erruption of a 1-1.5 years
Hansen et al (1992).
Nick
[Response: The difference is whether the aerosols are in the troposphere (the lower part of the atmosphere where all the ‘weather’ is), or higher up in the stratosphere. The main removal process for aerosols is related to rain and clouds, and up in the stratosphere there isn’t any to speak of. Thus once things get up there (for instance after a large volcanic erruption), they stay around for a lot longer. - gavin]
22 February 2007 at 9:17
How is the LOSU determined? Is it based on the range of uncertainty re: forcing?
22 February 2007 at 10:00
Please can you give us some responses, for the third or fourth time, about recent evolution of aerosols forcings (with links if possible) and concerning the definition of LOSU and its application to the quantification of aerosols forcing?
If it is not possible, please, say it.
22 February 2007 at 10:04
Juliane-SO2 dissolves in water vapor to form H2SO3, sulfurous acid. Some of the sulfurous acid is subsequently oxidized to H2SO4, sulfuric acid. The sulfurous/sulfuric acid portion of the particles which we see as white haze is actually composed of tiny transparent spherical liquid droplets in the 0.01 to 10 micron diameter range.
Transparent spherical bodies, including water, are retroreflective i.e. they reflect light directly back to the source of illumination. A common example is the glass beads used in highway signage and road stripes to send light back toward the headlights of approaching vehicles. I am sure this effect diminishes or disappears at some low diameter around the wavelength of light or about 0.7 microns for visible light.
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?
22 February 2007 at 11:11
RE # 46 & 47: please see the 1st paragraph of my post #37 above. LOSU is a subjectively assigned label, which is NOT connected to uncertainty or quantitative estimates of forcing. Since the full 2007 IPCC 4AR isn’t out yet, you can look at the last report, (2001 IPCC TAR), chapter 6, for discussion of LOSU and uncertainty on the forcing plot. It can be found here.
22 February 2007 at 11:23
Digression: whenever Danny Bee or some other name posts another link and linked name to that page
climatechange3000 dot blogspotdot com
(that’s the all-hopeless “Climate Change and the End of Humankind, Circa Year 2500″ website) does that enhance their Google ranking credibility because it’s a link from
RealClimate?
Just asking.
[Response: No, because I deleted it. To anyone else reading, attempted google bombing or commerical postings in comments are simply going to be removed. -gavin]
22 February 2007 at 11:42
Oregon study showing increasing trend for direct normal solar irradiance. Decreasing cloud cover is blamed, but aerosols also mentioned.
http://solardat.uoregon.edu/download/Papers/DirectNormalTrends.pdf
22 February 2007 at 12:09
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]
22 February 2007 at 12:47
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.
22 February 2007 at 12:55
re#48
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?
22 February 2007 at 13:02
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…
22 February 2007 at 13:06
#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.
22 February 2007 at 13:06
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!
22 February 2007 at 13:34
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]
22 February 2007 at 15:04
I think aerosols play a minor role in climate change since they can be controlled.
22 February 2007 at 15:14
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).
22 February 2007 at 15:21
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: http://www.ehponline.org/docs/2002/110-5/forum.html
“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. “
22 February 2007 at 15:42
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 http://www.realclimate.org/images/ipcc2007_radforc.jpg 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?
22 February 2007 at 16:23
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 http://www.realclimate.org/index.php/archives/2006/04/global-dimming-and-climate-models/). 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]
22 February 2007 at 16:32
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…
22 February 2007 at 16:59
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.
22 February 2007 at 17:29
> 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.
22 February 2007 at 18:33
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.
22 February 2007 at 19:08
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.
22 February 2007 at 20:32
Gavin,
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.
Nick
22 February 2007 at 21:19
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.
22 February 2007 at 21:26
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.
22 February 2007 at 21:51
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 February 2007 at 21:52
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.
http://npat1.newsvine.com/_news/2007/02/21/580192-eminent-scientists-warn-of-disastrous-permanent-global-warming
22 February 2007 at 22:27
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!
22 February 2007 at 23:08
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
22 February 2007 at 23:17
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).
22 February 2007 at 23:36
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.
23 February 2007 at 1:57
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?
23 February 2007 at 6:05
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]
23 February 2007 at 8:05
[[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]
23 February 2007 at 8:08
[[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]
23 February 2007 at 8:20
“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.
or
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.
or
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.
or
4) Something else?
Whichever is being asserted, can the supporting calculations be summarised here, or accessed elsewhere?
23 February 2007 at 8:30
[[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.
23 February 2007 at 8:37
[[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.
23 February 2007 at 9:12
[[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….
23 February 2007 at 18:46
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.
23 February 2007 at 22:37
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?
23 February 2007 at 23:08
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.
24 February 2007 at 0:54
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.
24 February 2007 at 5:59
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?
24 February 2007 at 6:08
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…
24 February 2007 at 11:05
#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
http://www.agu.org/pubs/crossref/2007/2006GL028031.shtml
24 February 2007 at 15:11
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):
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…).
24 February 2007 at 16:16
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.
24 February 2007 at 21:20
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: http://bostonreview.net/BR32.1/emanuel.html.
[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]
24 February 2007 at 22:25
Article by Kerry Emanuel:
http://www.realclimate.org/index.php/archives/2007/01/the-human-hand-in-climate-change/
“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.
24 February 2007 at 22:56
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???
25 February 2007 at 1:25
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: http://eesc.columbia.edu/courses/ees/slides/climate/rt_calcs.gif 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 http://www.whrc.org/carbon/index.htm (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 http://www.bioedonline.org/news/news.cfm?art=1754 . 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.
25 February 2007 at 6:43
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
Website: www.ghgonline.org
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:
http://www.realclimate.org/index.php/archives/2005/01/global-dimming-ii/ . 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]
25 February 2007 at 7:48
#92
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 C