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  1. Isn’t higher air/water temperature a great factor in greater hurricane intensity? So does this mean with dust and increased heat, we might see fewer, but stronger hurricanes?

    Comment by Richard Pauli — 21 May 2009 @ 6:07 PM

  2. Where have you been since May 7? I missed you. Testifying to Congress, I hope.

    Comment by Edward Greisch — 21 May 2009 @ 6:11 PM

  3. The NAMMA field campaign in 2006 was designed to measure the effects of the SAL on hurricane development. THe NASA DC-8 was stationed in Cape Verde and flew missions into the SAL and into developing tropical cyclones. My research into the role the SAL plays in hurricane development is to first create an “accurate” representation of two developing tropical cyclones using GFS model initialization and WRF. Given this accurate representation use the LARGE and dropsonde data to improve the forecast via WRF-CHEM. Although not directly related to climate modeling much of the climate related work depends on an accurate picture of the role aerosols and dry air play in the development of deep concection

    Comment by yourmommycalled — 21 May 2009 @ 9:28 PM

  4. You forgot to post the important final lines of “Evan et al. (2009)”:

    “Our results imply that because dust plays a role in modulating tropical North Atlantic temperature, projections of these temperatures under various global warming scenarios by general circulation models should account for long-term changes in dust loadings. This is especially critical because studies have estimated a reduction in Atlantic dust cover of 40 to 60% under a doubled carbon dioxide climate, which, on the basis of model runs with an equivalent reduction of the mean dust forcing, could result in an additional 0.3° to 0.4°C warming of the northern tropical Atlantic.”

    Comment by Joe Romm (ClimateProgress.org) — 21 May 2009 @ 9:43 PM

  5. I had been under the (possibly wrong) impression that dust loading was undergoing a longterm increase due to anthropic ground disturbance -primarily grazing, and vehicular travel. Is this in fact the case, or do the climate-vegetation feedbacks overwhelm the land disturbance effects? A secondary, but I suspect important longterm issue could be the effect of dust deposition on ocean chemistry and biology, and probably upon the ability of the ocean to sequester CO2. Do you have any thoughts on these issues? Are we performing an inadvertant experiment in ocean fertilization?

    Comment by Thomas — 21 May 2009 @ 10:37 PM

  6. It seems counterintuitive to me that dust would decrease due to global warming. It seems if warming is dessicating the soil and plants, and perhaps causing more intense winds, there would be more dust, not less. Or maybe if there is less organic matter (due to GW killing off life), there would be less dust?? Somewhere I read dust is mostly composed of organic matter.

    On an unrelated topic, it seems MIT researchers have suggested that the more likely warming over the 21st century could be double what the latest IPCC report suggests (see http://www.climateark.org/shared/reader/welcome.aspx?linkid=128096 ).

    News reports are taking this and saying “GW could be twice as bad.” But I’m thinking that the bads — the harmful effects from GW — do not necessarily increase linearly with an increase in the warming, but could increase exponentially, or at least squared or cubed, or some nonlinear greater level…..that is, until life on earth more or less flatlines.

    Comment by Lynn Vincentnathan — 22 May 2009 @ 8:06 AM

  7. There seem to be some problems with this paper related to their aerosol optical depth trends, as well as with an oversimplified modeling effort. First, some background on work done in Barbados:

    Prospero 1980-81 (pdf)

    Thus, Saharan air outbreaks in general and dust in particular may have a pronounced effect on the atmospheric processes occurring over a large area. However, dust itself is a product of weather and climate. In this regard it is important to note that the dust concentrations over the tropical North Atlantic as implied by the Barbados measurements (Fig. 1) were much greater during the early 1970s than they were during the 1960s or in the later 1970s (Prospero and Nees 1977). The period of greatest dust concentrations coincided with the time of severe drought in the Sahel.

    See in particular Fig. 6 & 8.

    Now, let’s look at a 1997 paper (with Prospero as one of the authors) that uses the AVHRR data to generate some nice images of the above phenomena, pages 3-4. You can see that in the 1989-1991 winter period, the dust distribution is shifted southwards, as noted in the above paper, so winter dust measurements in the Barbados are lower than in Cayenne, French Guinea:

    Characterization of tropical aerosols over the oceans with the NOAA advanced very high resolution radiometer optical thickness operational product, Husar et al 1997

    Now, here is the complete paper that the Barbados graph (in original post) comes from , with an interesting quote:
    African droughts and dust transport to the Caribbean: Climate change implications, P&L 2003

    The Barbados annual dust cycle is linked to the cycle of dust activity in North Africa and to seasonal changes in large-scale atmospheric circulation patterns. During sum- mer, satellite images show dust outbreaks that emerge from the west coast of Africa, 4500 km east of Barbados, in pulses every 3 to 5 days, following behind easterly waves. About a week later, dust arrives in the Caribbean and the southeast United States

    Large changes are also evident from year to year and over the longer term. Concen- trations were low in the mid- to late 1960s but increased sharply in the early 1970s, and they have remained relatively high thereafter.

    Okay, now let’s look at Evan et al, who state in their introduction that:

    A smoothed time series of northern tropical Atlantic dust cover (Fig. 1) shows a maximum and minimum in dust activity that occurred in 1985 and 2005, respectively, and a downward trend in dust optical depth over the record.

    However, that figure doesn’t seem to show any such thing, and seems to contradict other work – so where does this aerosol trend record come from? Isn’t that a key part of the argument – that there has been a downward trend in dust optical depth, enough to account for the conclusion?

    I think I found the time series used for this paper, Figures 3 and 4 in the link below. It doesn’t seem to correlate very well with the Barbados record, however, and there is no other citation given:

    http://ams.confex.com/ams/pdfpapers/108119.pdf

    All in all, the paper is odd. Are the authors claiming, for example, that if aerosol trends had remained flat for the past 30 years, then there would have been 69% percent less change in Atlantic surface ocean temperatures? And why do the Barbados trends not match their aerosol record?

    P.S. Here’s a comparison of MODIS and AVHRR aerosol estimates:
    http://adsabs.harvard.edu/abs/2005JGRD..11010S09J

    For an explanation of the AOT and α terms:
    http://disc.sci.gsfc.nasa.gov/PIP/shtml/aerosol_angstrom_exponent.shtml

    All in all, their claim seems unsupported. This paper seems a bit more reliable:

    Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions, 2006

    “For the period 1906–2005, we find an 84% chance that external forcing explains at least 67% of observed SST increases in the two tropical cyclogenesis regions. Model “20th-century” simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases.”

    Comment by Ike Solem — 22 May 2009 @ 9:53 AM

  8. Ike,

    When you say “that figure doesn’t seem to show any such thing” as a downward trend in dust optical depth, that it contradicts other work and there is no other citation — are you sure those issues are real? Just eyeballing the graph it does seem to show a downward trend; and they do cite in their reference 1 a paper by Foltz and MacPhaden paper, which also finds a downward linear trend for optical depth (see Figure 2 of that paper).

    Comment by CM — 22 May 2009 @ 4:47 PM

  9. CH, I think this is another casualty of the “recent global cooling” meme. Even if it’s right, “eyeballing” doesn’t tell you whether something is really, statistically, going down.

    And this one at least from the information here has trend being stated but only track, not significance (which is required to see if the track is a trend) is shown to prove this.

    On another note, this is why spouting “correlation is not causation” is both required and ridiculous. There’s a correlation and that is making people look for a *mechanism*. The “looking for a mechanism” bit is why in so many cases, that meme is ridiculous. The causation is not “there’s a correlation” but “we noticed the correlation and think $THIS could be the cause”.

    For people like JonB, the causation is irrelevant. They want merely to see the correlation (and ignore any correlation significance tests). In those cases, the meme is required.

    Comment by Mark — 23 May 2009 @ 2:52 AM

  10. Seems to me that a dust infiltration would hamper convection over the Atlantic by raining out developing cells ( effectively seeding them ).

    This in addition to the ocean surface cooling due to dimming might really short circuit proto tropical storm/hurricane development.

    Comment by MacDoc — 23 May 2009 @ 4:06 AM

  11. Ike your post is quite thought provoking-thanks for taking the time and detailing the ambiguities in the dust record used by Evans.

    Comment by mauri pelto — 23 May 2009 @ 5:38 AM

  12. You have to wonder about the volcanic aerosols as well.

    See Santer et al. 2006 on Pinatubo:

    Variability on sub- to multidecadal timescales is superimposed on these overall increases in observed SSTs…part of this variability is in phase with fluctuations in the optical depth of stratospheric aerosols produced by massive volcanic eruptions…The relationship between SST variability and stratospheric aerosol optical depth is clearer in the PCR than in the ACR, particularly for the eruption of Mt. Pinatubo in June 1991 (Figs. 1 and 6). Regional differences in the observed SST changes after volcanic eruptions are expected, partly because of spatial differences in climate noise.

    Now, from the original post:
    Evan et al. calculate that between 1982 and 2007 the ocean surface warmed by 0.25°C/decade in the main region of Atlantic hurricane genesis (15-­65°W and 0­-30°N). For comparison, they calculate a warming trend of 0.18°C/decade due to a reduction of dust and volcanic aerosols.

    Are they mistaking the Pinatubo cooling and recovery (due to stratospheric volcanic aerosols) for part of the dust trend over Africa, and thus mis-attributing the warming trend to reduction in African dust aerosols? Pinatubo had a large but short-lived effect on global SSTs, after all.

    See Reynolds, June 1992, “Impact of Mt. Pinatubo aerosols on satellite-derived sea surface temperatures”.

    Satellite retrievals [of SST] in aerosol-contaminated regions are biased low because the infrared radiation from the surface is absorbed by the aerosol and the reemitted at the lower temperature of the aerosol

    And also: The atmospheric global dust cycle and iron inputs to the ocean, Mahowald et al. 2004

    Because of their large spatial coverage, satellite data have proven useful in evaluating dust sources, transport and deposition in global models. Both AVHRR [Husar et al., 1997] and TOMS AI [Herman et al., 1997] have yielded long term records of aerosol optical depth, but it is difficult to retrieve dust properties quantitatively from these records. Dust retrievals are often difficult because of the presence of aerosols other than dust and of clouds.

    New satellite observations from POLDER, MODIS and MISR will likely provide greater insights into dust distributions because of increased spectral, spatial and temporal resolution [Barnaba and Gobbi, 2004; Ginoux and Torres, 2003; Kaufman et al., 2004].

    The effect on hurricanes also seems uncertain. If dust arrives in pulse-like events following easterly waves, which initiate tropical storms in the central Atlantic, then there might be a temporal issue – the easterly wave initiates a tropical storm, but does the following dust affect it or not? I bet the dynamics matter a lot, in that case.

    Notice also that some people are trying to claim that Australia’s drought and record wildfires are due not to global warming, but rather to ‘plumes of dust’ from SE Asia. If you see such claims followed up by similar aerosol-based claims on hurricanes and SSTs in a science express paper, using apparently out-of-date methods and models and data, you have to wonder why – and why didn’t the reviewers raise these questions?

    For a good overview of recent aerosol research issues, see:
    http://www.phys.unsw.edu.au/RESEARCH/ATMOSPHERIC/atmospheric_research.html

    In particular, the section on “Satellite Observation of Aerosol Properties”.

    Comment by Ike Solem — 23 May 2009 @ 9:10 AM

  13. So everything is the cause of a positive feedback, and these feedbacks just add up like those systems I learned about in electrical engineering courses.

    With all these positive feedbacks existing in the climate, how did temperatures stay within a fairly stable range in response to changes, as we see from the hockey stick?

    [Response: LW_out = sigma * T4 - gavin]

    Comment by MikeN — 23 May 2009 @ 9:54 AM

  14. MacDoc that is exactly the goal of the NAMMA field campaign. How much dust and dry air does it take to hamper convection, is there a specific time in the development of the tropical cyclone where the ingest of the dust and dry air is most destructive?

    Comment by your mommy called — 23 May 2009 @ 2:12 PM

  15. MikeN wrote

    “With all these positive feedbacks existing in the climate, how did temperatures stay within a fairly stable range in response to changes,”

    Ocean and particularly cryosphere are huge dampers on atmospheric swings.
    Also atmospheric temperatures are a poor tracking for energy gain within the atmosphere as transforms occur continually to vertical or horizontal movement and convective columns with rain outs.
    Cyclonic activity is a big heat pump toward the poles where latent heat of melting ice shows as net glacial mass loss or loss of multi year ice.

    Focusing on temp graphs alone tells very little. The energy transforms in Tstorms alone are very complex starting with a kick off of a thermal from a “warmer than above” surface…see or land ….and can be right through sometimes to large hail which will have latent heat implications for tracking the energy path.

    Scaling that up to hurricane/cyclones which impact on a regional or global scale and even disturb ocean to significant depths and you can see why the interactions of dust/humidity/wind and even albedo from the dust is hard to puzzle out.

    I recall sitting on a beach in West Africa all day with no sunscreen and not really getting tanned due to the Sahara dust clouds over head….not really visible but certainly screening. One day the wind blew the other way….instant tan….big time fried skin.

    Aerosols seem to be a “here be dragons” terra incognita” just now.

    Comment by MacDoc — 23 May 2009 @ 7:08 PM

  16. MikeN, look up the terms in Gavin’s equation if you don’t recognize them. Short answer: the warmer the planet gets, the faster heat radiates away. If you’re asking why we’re not Venus, you can use the same methods to answer that question.

    Search with the search box at the top of the page for more:

    Try here: http://www.realclimate.org/index.php?p=212
    particularly the illustrations

    Also, put that equation into words and put the words into Google:
    http://www.google.com/search?q=longwave+radiation+budget+sigma+temperature+“fourth+power”

    http://cirrus.unbc.ca/201/manlec/node3.html
    The first specific hit returned with that search starts off:

    Stefan-Boltzmann’s Law
    States that the total energy emitted by a black body, integrated over all wavelengths, is proportional to the fourth power of its absolute temperature (Temperature in Kelvins).

    What’s sigma?
    sigma = Stefan-Boltzmann constant. The notes go on from there.

    Comment by Hank Roberts — 24 May 2009 @ 1:12 AM

  17. MacDoc or Ron, How is the dust in this case related to the Harmattan, the cold wind that blows the dust south during winter. I remember needing a jacket at this time of year despite being only 7 degrees north of the Equator. Cold and dry, and on some days it was even difficult to find the Sun in the sky. Damn, I probably still have some of that dust in my lungs!

    Comment by Ray Ladbury — 24 May 2009 @ 6:02 AM

  18. Mark #9, sure, eyeball impressions do not determine statistically significant trends — nor the absence of same. I was really just asking Ike why he said “that figure doesn’t seem to show any such thing” as Evans et al said it does, namely “a maximum and minimum in dust activity that occurred in 1985 and 2005, respectively, and a downward trend in dust optical depth over the record.” But this is probably a side issue to the other interesting stuff in that post.

    Comment by CM — 25 May 2009 @ 4:32 AM

  19. Given the eruption of dusty construction sites on Barbados , there is need establish a data collection site on a less industrial Windward Island , Grants in aid may be directed to the Mustique Academy Of Sciences, 1 Rue Royale.

    More seriously, it should be remembered that only micon-sized mineral aerosols and spores , not Saharan sand grains ,remain in suspension long enough for trans-Atlantic transport .

    Comment by Russell Seitz — 25 May 2009 @ 5:03 AM

  20. As a former scientist (physical oceanography) and as one whose first sailing experience was as a 1st year cadet aboard the USCG Eagle in 1954 when we encountered Hurricane Carol returning from Bermuda. We were fortunate to be in the safe semicircle of the storm. May I say that it was nonetheless the most thrilling experience of my life. In a way, it defined my future.
    Whether or not dust from the Sahara affects hurricanes, the corrrelation seems to justifiable reason to explore the possible cause and effect. Given the likely changes we can expect from global warming is even more compelling.
    May I suggest that an experiment of seeding potential hurricanes with dust particles might be worthwhile to prevent or reduce the damage that they cause on tropical islands and land masses.

    [Response: How would you know if it worked? - gavin]

    Comment by Richard Reinert — 25 May 2009 @ 9:39 AM

  21. Russell, I can speak from experience: The Sahara exports plenty of dust, not just sand. Harmattan winds blowing off the desert blanket much of West Africa in a choking haze in December through Feb. Blowing ones nose after a long motorcycle ride was, to say the least, memorable.

    Comment by Ray Ladbury — 25 May 2009 @ 10:36 AM

  22. “… Large particles fall down near dust sources, while a percentage of fine particles with size less than 1 micron increases with distance from dust sources. It is worth mentioning that a short-distance dust transport from the Eastern Sahara, through Egypt, into Israel brings both coarse and fine fractions. A long-distance dust transport from the Western Sahara, through Southern Europe, into the Eastern Mediterranean brings mainly fine fraction with size less than 1 micron. …”
    http://www.tau.ac.il/~harnik/lab/lab2-desert-dust-Pavel-Kishcha.doc

    Of course once they fall into the ocean they’re done; over land, the larger particles can be picked up repeatedly.

    I didn’t find mention of methods to discriminate the source, but I’d imagine there may be some distinguishable difference between construction dust from the islands (coral?) and Sahara dust. Anyone know for sure?

    Comment by Hank Roberts — 25 May 2009 @ 12:25 PM

  23. I recently finished my Masters Thesis on Saharan Dust transport to the Eastern United States. We examined dust levels over a 15 year period at various national parks along the east coast and the virgin islands. Quite accidentally we discovered that when dust concentrations were high hurricane activity was low and visa versa. There had been a couple of papers (Dunion and Velden (2004) comes to mind) that had mentioned the SAL effect noted above. We plotted max soil concentrations vs named hurricanes from a site at the Virgin Islands and found a good (although not great ) inverse correlation with an r-squared value of 0.7. We did not look at the data after 2004 which should provide 2 more good extreme points to look at.

    The more interesting thing to me would be that we can look at the Virgin Island max that occurs in June and get a good feel for what is to come the rest of the season. The thing I don’t know (and maybe someone else does?) is does the meteorology of the transport change or does the dust generation change? I think it is the former. I have always leaned towards the radiative effects being secondary to the synoptic meteorology that is more favorable to tropical storm development. The dust generation over the Sahara is likely to remain the same but the transport seems likely to change. And the conclusion we came to is that whatever transport meteorology is favorable to more dust coming to the US is also unfavorable to tropical storm development. Much more research needs to be done and this will likely not come from me or my advisor as this issue was secondary to our interest in positive identification of the saharan dust.

    Comment by Scott Robertson — 25 May 2009 @ 2:30 PM

  24. This is slightly off topic but in Ray’s post “Venus Unveiled” he made this statement…

    “These clouds account for the high reflectivity of Venus, but because they also reflect infrared back to the surface (unlike water clouds, which absorb and emit)”

    Can somebody explain to a novice the difference between an IR photon being reflected versus being absorbed and emitted. My understanding was that if a molecule had an energy state equivalent to the energy of the photon it would be absorbed by that molecule, if not, it would pass straight through. What am I missing?

    Comment by Daryl — 25 May 2009 @ 3:43 PM

  25. > can somebody explain
    Not me. But I can point to a bit that might help while we wait for an expert —

    – many gases including water vapor are transparent to visible light, but some like water vapor condense into particles/droplets. Those as they get larger, including water clouds in our atmosphere, scatter light in various ways. This site is consistently wonderful on the ways that happens: http://www.atoptics.co.uk/

    Venus has lots of sulfur, which makes reflective particles:

    http://dx.doi.org/10.1016/j.icarus.2008.05.020

    “… clouds with a single scattering albedo 1−a=0.01 …. value of 1−a agrees with the refractive index of H2SO4 (85%) and the particle radius of 1 μm. …

    Comment by Hank Roberts — 25 May 2009 @ 5:58 PM

  26. Is this Saharan dust partly a result of an intensification of the North Atlantic Oscillation (NAO) rather than exclusively relating to ENSO? I realize that ENSO may play a role in trade wind strengthening, but wouldn’t the phase and intensity of the NAO have a role in this as well?

    Comment by Stephen Berg — 25 May 2009 @ 7:36 PM

  27. Ray-

    You can check my 1988 -Naturwissenschaften- paper on dust transport for the particle size versus suspension lifetime stats, The bottom line is that as millimeter sized grains fall out in hours, and vertical convection systems seldom cohere for days , few particles larger than tens of microns make it across the pond.

    There is a whole literature on transatlantic transport of fungal spores, and bacteria some of which have been suggested as sources of Caribbean coral stress .

    Comment by Russell Seitz — 25 May 2009 @ 11:06 PM

  28. Daryl,

    Scattering discussions usually depend upon the size of the particle when compared to the wavelength of the light. Raypierre’s climate book in Chapter 5 gives a good overview of scattering, particularly Rayleigh and Mie scattering which help explain things why aerosols scatter light, why the sky appears blue, why CO2 clouds (say on ancient Mars) can scatter infrared radiation good but water clouds on Earth don’t have that effect, etc. On Earth, water clouds absorb IR so strongly before there is any opportunity for scattering.

    You’re correct about absorption: A molecule in the atmosphere can only absorb a photon only if the energy of the photon corresponds to the difference between the energy of two allowable states of the molecule (the transition from one energy state to another corresponds to the release or capture of an amount of energy h*v). The atmosphere is a great absorber at those discrete frequencies corresponding to an energy transition of an atmospheric gas.

    For practical purposes, in terms of a greenhouse effect, you can get a greenhouse effect from either reflection or absorption/emission. The latter usually dominates discussion because that’s what goes on here on Earth, but in many other planetary applications the scattering of infrared light plays a large role as well. A greenhouse effect governed by scattering of IR light would also not be sensitive to a cloud temperature (or the lapse rate in general) wheras the temperature change with altitude is the key behind the existence of the traditional absorption/emission GHG effect on Earth.

    Comment by chris colose — 26 May 2009 @ 1:00 AM

  29. Daryl #24, you could look up the terms refraction, reflection and scattering to see what you’re missing. Your description above does apply in practice to the special case of Earth’s water clouds, because they absorb thermal IR flux so strongly that you can ignore scattering. Other planets, different clouds. For a really thorough discussion, look up chapter five of Ray’s book online.

    Comment by CM — 26 May 2009 @ 4:43 AM

  30. One quick question.
    Is cause & effect being reversed here?
    Low number of hurricanes = less dust washed out of the air
    High number of hurricanes = more dust washed out.
    Can Saharan dust be identified in stratographic layers of sediment elsewhere, for instance during the African mega drought (Yep, looks like there is “Davis et al, Andean ice core records… Annals of Glaciology 43, 34-41″)
    An Africam mega-drought coincided with the LIA.

    Comment by Adam Gallon — 26 May 2009 @ 5:15 AM

  31. Daryl,
    Not sure of your physics background, but here’s an attempt to explain the difference. When light is incident on a material, one of 3 things can happen:
    1)It can pass through the material unattenuated. The degree to which it interacts with the material will determine how much the light is bent or refracted. Generally, different wavelengths of light interact to different degrees, so refraction tends to lead to dispersion–separation of colors of light as for a rainbow.
    2)It can be absorbed. This means that the light has the right energy such that it can raise the molecules in the material to a higher energy excited state. The excited state may subsequently decay by emitting light of the same wavelength in an arbitrary direction, or the excited molecule may relax some other way.
    3)It can be reflected. This means that the light is causing charges in the material to oscillate freely, like tiny antennae, resulting in the light being emitted at the same angle to the normal it went in at.

    Note that reflection isn’t changing the interneal energies of anything. Hopefully that helps.

    Comment by Ray Ladbury — 26 May 2009 @ 7:49 AM

  32. Daryl, further recommending the pointers to Raypierre’s book online, Ch.5, where he gives as clear an explanation as I’ve seen.
    Direct link to the book:
    http://geosci.uchicago.edu/~rtp1/ClimateBook/ClimateVol1.pdf

    Search therein for:

    “… a fundamentally different way, through reflection instead of absorption and emission. This will be discussed in Chapter 5….”
    and
    “… There is one last basic quantity we need to define, namely the index of refraction, which characterizes the effect of a medium on the propagation of electromagnetic radiation. It will turn out that the index of refraction amounts to an alternate way of representing the information already present in the scattering and absorption cross-sections. For a broad class of materials – including
    all that are of significance in planetary climate – the propagation of electromagnetic radiation in the material is described by equations that are identical to Maxwell’s electromagnetic equations, save for a change in the constant that determines the speed of propagation (the ”speed of light”)….”

    It’s still poetry, for those of us who don’t have the math to follow the equations, and the equations aren’t “what’s really happening” — they describe and predict what happens but not “what it is” — at the level of photons and molecules, we just can’t really say exactly what’s what.

    Unless I’ve missed a breakthrough.

    Comment by Hank Roberts — 26 May 2009 @ 9:30 AM

  33. Adam Gallon asks:”One quick question.
    Is cause & effect being reversed here?
    Low number of hurricanes = less dust washed out of the air
    High number of hurricanes = more dust washed out.”

    No, the dust originates from Africa, hurricanes do not go into Africa.

    Comment by t_p_hamilton — 26 May 2009 @ 11:51 AM

  34. I think t-p-hamilton’s got the wrong end of the stick here.
    I am aware that this dust does originate from Africa (It has been washed out over the UK before, covering vehicles, for instance, with a thin, dusty layer).
    The article is suggesting that there are fewer hurricanes in “dusty” years, I am suggesting that there is less dust, when there are more hurricanes.

    Comment by Adam Gallon — 26 May 2009 @ 1:00 PM

  35. Adam Gallon:”I think t-p-hamilton’s got the wrong end of the stick here.
    I am aware that this dust does originate from Africa (It has been washed out over the UK before, covering vehicles, for instance, with a thin, dusty layer).”

    In other words dust first. Now ask yourself, does effect precede cause? If not, then dust levels is not an effect of hurricane activity.

    Comment by t_p_hamilton — 26 May 2009 @ 2:56 PM

  36. Re #22 Hank,

    We used positive matrix factorization to identify sources in our study of saharan dust transportation and found up to 8 sources at any of the eastern US sites we looked at. The source we identified as road dust had most common soil elements as well as high amounts of ogarnic carbon, elemental carbon and sulfur. The saharan dust was identified as a calium-depleted soil signature. I’m sorry I don’t have a reference for you as we have yet to publish our results but hopefully will soon.

    Comment by Scott Robertson — 26 May 2009 @ 6:12 PM

  37. Thanks for the pointers and links guy’s, I’ll do some more reading.

    Comment by Daryl — 26 May 2009 @ 7:17 PM

  38. Thank you Scott Robertson! Looking forward to more when available.

    Comment by Hank Roberts — 26 May 2009 @ 7:30 PM

  39. t-p I think Adam is asuming the dust measurement is made far from the source. If more is washed out on the way over, that would be one reason to have less. I suspect the storms aren’t that widespread -even during a big hurricane year to wash out all that much. But, his question does have at least some logic behind it.

    Comment by Thomas — 26 May 2009 @ 10:00 PM

  40. The delightfully arcane unit of measure ‘furlong,’ most commonly used to describe the length of a horse race, is equivalent to one eighth of a mile. Thus a thousand furlongs is 126 miles. Probably not the distance you were thinking of.

    [Response: Yes it was (see footnote). - gavin]

    Comment by Larry Tolton — 26 May 2009 @ 10:45 PM

  41. Ron Miller wrote:

    The amount of dust crossing the Atlantic has been measured at Barbados since the mid 1960s (aptly by Prospero and colleagues).

    Aptly indeed: In a presentation given some 400 years ago, Prospero (1611) discussed the reduction of sunlight by airborne entities (parametrized as elves and spirits in the GSMs — general speculation models — available at the time). The 1611 paper appears, however, to suggest a positive correlation with tempest activity rather than an anti-correlation. It is worth citing the full reference:

    Prospero, D. of M. “Mutinous Winds Linked to Bedimming of Noontide Sun by Elves”, Tempest, vol. 5, no. 1 (1611).

    ABSTRACT:
    I have bedimm’d
    The noontide sun, call’d forth the mutinous winds,
    And ‘twixt the green sea and the azured vault
    Set roaring war: to the dread rattling thunder
    Have I given fire and rifted Jove’s stout oak
    With his own bolt; the strong-based promontory
    Have I made shake and by the spurs pluck’d up
    The pine and cedar

    (Full text: http://shakespeare.mit.edu/tempest/full.html)

    Comment by CM — 27 May 2009 @ 1:22 PM

  42. reply to #33 t_p_hamilton:

    Although hurricane do not occur IN Africa, their precursors African Easterly Waves do occur over Africa. Hurricane Helene was traced from central Africa to African coast as an easterly wave which then turned into a tropical depression, and finally a hurricane. The dry dusty air that was drawn into the easterly wave, tropical depression and Helene clearly came from Africa.

    Comment by your mommy called — 27 May 2009 @ 7:49 PM

  43. Gavin, what atmospheric circulation model do you use in your analysis?

    Comment by Antonio San — 27 May 2009 @ 10:19 PM

  44. CM Says (27 mai 2009 at 1:22 PM):

    “In a presentation given some 400 years ago, Prospero (1611) discussed the reduction of sunlight by airborne entities…”

    I believe you’ve misread that paper. It in fact describes an early attempt at geoengineering a solution to AGW (bedimming the noontide sun, and note that Prospero predates Arrhenius by nearly three centuries*), and some of the undesired consequences.

    *But note that he cites prior work by Sycorax et al.

    Comment by James — 27 May 2009 @ 11:27 PM

  45. It has just been announced (http://www.abc.net.au/news/stories/2009/05/28/2583676.htm?section=justin) that smoke from the February bushfires in Australia has reached Antarctica. This is unprecedented, and attests to the ferocity of the fires. The effects of these particles is uncertain – another potential devastating feedback mechanism?

    Comment by David Horton — 28 May 2009 @ 6:49 AM

  46. David Horton, how is this \unprecedented\? Perhaps taking the time to familiarize oneself with the atmospheric circulation patterns of the area will show that it is perfectly normal, natural and expected that those particles reach Antarctica through warm air advection pathways… Just look at a satellite animation.

    Comment by Antonio San — 28 May 2009 @ 11:23 AM

  47. Antonio San, “unprecedented” refers to evidence of this material having been deposited in Antarctica. That would show up in ice cores from Antarctica.

    Perhaps it exists. Have you any precedent for it?

    Paleo evidence of Australian fires:

    “USING PALAEO-SCIENCE TO UNDERSTAND ClIMATE CHANGE IN AUSTRALIA
    … As the geographic coverage, time resolution and detail of palaeo-climate records have improved, it has become possible to identify the effects of past climate variations on high impact events such as fire …. For example:
    • Evidence of past fire regimes has been predominantly derived from charcoal records extracted from terrestrial wetland sites and near-coastal marine cores….”

    http://www.climatechange.gov.au/science/publications/pubs/palaeo-evidence.pdf

    Comment by Hank Roberts — 28 May 2009 @ 1:10 PM

  48. Hmmm. Or you could read the article he points to, of course. I’ll save you the effort of a mouseclick:

    —excerpt—-
    The ferocity of the fires has seen smoke reach heights never seen before and it has now travelled south.

    Andrew Klekociuk from the Australian Antarctic Division says the smoke from the February fires arrived in the atmosphere in early March.

    “It’s an unprecedented layer of smoke that’s between about 14 kilometres and 20 kilometres above the surface,” he said.

    It was picked up by NASA scientists and researchers at Australia’s Davis Station using a light detection instrument called a lidar, which captures a vertical slice of the atmosphere.

    The intensity of the Victorian bushfires acted like a chimney, sucking smoke into the atmosphere and creating a fire-generated thunderstorm known as a pyrocumulonimbus.

    Washington-based scientist Michael Fromm agrees the phenomenon is unprecedented.

    “We have seen aerosols higher in the stratosphere by several kilometres than we have ever observed anywhere on the earth,” he said.

    “The evidence speaks to the effects of this storm being the most intense of any one we’ve observed throughout the years.
    —end excerpt—-

    Comment by Hank Roberts — 28 May 2009 @ 1:12 PM

  49. “The intensity of the Victorian bushfires acted like a chimney, sucking smoke into the atmosphere and creating a fire-generated thunderstorm known as a pyrocumulonimbus.

    Washington-based scientist Michael Fromm agrees the phenomenon is unprecedented.”

    Pardon me, but if this is KNOWN as a pyrocumulonimbus how can this be UNprecedented?

    Comment by Antonio San — 28 May 2009 @ 1:39 PM

  50. I was awaiting Antonio San’s next comment with baited breath, I wasn’t disappointed.

    Article: “Washington-based scientist Michael Fromm agrees the phenomenon is unprecedented. “We have seen aerosols higher in the stratosphere by several kilometres than we have ever observed anywhere on the earth,” he said.”

    Antonio San: “if this is KNOWN as a pyrocumulonimbus how can this be UNprecedented?”

    Priceless.

    Comment by Chris S. — 28 May 2009 @ 3:30 PM

  51. Chuckle. He’s found a cozy place over at WTF; not worth more time here.

    Comment by Hank Roberts — 29 May 2009 @ 11:24 AM

  52. Google pyrocumulonimbus and you will get many hits. See http://www.cosis.net/abstracts/EGU05/08569/EGU05-J-08569.pdf From the abstract, “Very recent investigations into pyroCb have revealed that this
    phenomenon has occurred with surprising regularity, and in both northern and southern
    hemispheres. Moreover, it has been learned that the historical record in the satellite
    era includes several cases of stratospheric aerosol layers, formerly attributed to volcanic
    eruptions, of pyroCb polluting the stratosphere”

    Comment by Jim Norvell — 29 May 2009 @ 3:18 PM

  53. Yes, it’s interesting to read these things:

    2009: “Washington-based scientist Michael Fromm agrees the phenomenon is unprecedented. ‘We have seen aerosols higher in the stratosphere by several kilometres than we have ever observed anywhere on the earth,’ he said.”

    2005: Polluting the stratosphere: an assessment of the impact
    of pyro-cumulonimbus
    M. Fromm (1), R. Servranckx (2), R. Bevilacqua (1), G. Nedoluha (1)
    1. Naval Research Laboratory, Washington DC, USA
    2. Canadian Meteorological Centre, Dorval, Quebec, Canada

    Review: “… higher in the stratosphere by several kilometres than we have ever observed anywhere on the earth,” he said.” (2009)

    Comment by Hank Roberts — 29 May 2009 @ 4:20 PM

  54. PS, to be fair, I only warn against mistakes I make myself (grin). I read “reached Antarctica” above and assumed “reached Antarctica” meant reached the continent, so the precedent would be the paleo record. Not so, obvious as soon as I read and quoted from the first link. Dr. Fromm is quoted, and wrote in 2005, on how high in the stratosphere smoke has been observed over the satellite record.

    Is smoke found in the paleo cores? Will this year show a layer? Dunno.

    Comment by Hank Roberts — 29 May 2009 @ 6:18 PM

  55. PPS, here’s how it works, though none of these pyrocumulus continue to rise — you can see the smoke rising and how when the smoke cools sufficiently the water in it condenses — it’s releasing heat of vaporization, and that heat energy lifts the water cloud much higher than the smoke is rising. A few times you can see, with the collapse of the vapor, what looks like rain or ice falling out. (This must have been a day with a significant inversion layer — warmer above — so the pyrocumulus did not keep rising into the higher level air)

    On a sufficiently big pyrocumulus in the right conditions, these would continue to climb and grow and carry smoke up with them instead of collapsing.

    Quite a video (time lapse, speeded up), from a California fire:
    http://www.youtube.com/watch?v=czYzu3OIjmY hat tip to ‘GoletaBrian’

    Comment by Hank Roberts — 29 May 2009 @ 6:38 PM

  56. Chris S. I guess Jim Norvell responded for me… and that too was priceless.

    Comment by Antonio San — 29 May 2009 @ 9:29 PM

  57. Um yes, the unprecedented part was reaching Antarctica. Even relatively small fires can, in the right conditions, create cumulus clouds above the smoke. In some circumstances (notably in central Australia) the clouds can even generate rain. The difference here was the immense height achieved because of the enormous temperatures and in turn getting into the Antarctic circulation. If massive bushfires are going to be increasingly common in Australia, does ash reaching Antarctic present a future problem either in terms of aerosols or blackening of snow?

    Comment by David Horton — 30 May 2009 @ 3:37 AM

  58. I don’t find anything about the smoke, or ash, reaching the surface.

    Here’s the report:
    http://www.aad.gov.au/default.asp?casid=36250

    Comment by Hank Roberts — 30 May 2009 @ 12:59 PM

  59. According to http://www.agu.org/pubs/crossref/2006/2006GL025827.shtml “The Bodélé Depression, Chad is the planet’s largest single source of dust.” Much of the dust is the silica shells of diatoms which were deposited when the area was a shallow lake. Because the shapes of the diatom particles are far from compact spheres, the ratio of aerodynamic drag to mass is higher, so I would expect this kind of particle to settle more slowly and become a larger fraction of the dust over time(or distance from the source).
    On the other hand, “Hygroscopicity and cloud condensation nucleus (CCN) activity were measured for three mineral dust samples: one from the Canary Islands, representing North African dust transported across the Atlantic…” and “Only the Canary Island sample generated from aqueous suspension showed appreciable hygroscopic growth at subsaturated conditions…” (http://www.agu.org/pubs/crossref/2009/2009GL037348.shtml), which may lead to higher rates of cloud formation and rainout of Bodélé sourced dust, and a different fractionation profile with time. I couldn’t find whether amorphous(diatom) silica is specifically more hygroscopic than crystalline silica and other minerals; question for Scott Robertson – did you look at the amorphous silica content of the dust, or other diatom identifiers? Could “hygroscopic growth at subsaturated conditions…” act to dry tropical waves and suppress hurricane formation?
    Other interesting abstracts i found (full articles paywalled) are:
    doi:10.1016/j.chemosphere.2006.02.052
    doi:10.1016/j.jaridenv.2007.12.007
    doi:10.1016/S0012-8252(01)00067-8
    doi:10.1016/j.gca.2008.05.037 “We thus define an iberulite as a coassociation with axial geometry, constituted by well-defined mineral grains together with non-crystalline compounds, structured on a coarse-grained core and a smectite rind, with only one vortex and pinkish color, formed in the troposphere by complex aerosol–water–gas interactions.”
    doi:10.1016/j.margeo.2007.09.003
    doi:10.1016/j.atmosenv.2006.08.024

    On a completely different off topic question, could the sinuous lead/open water in the arctic ice from ~120E to 165E and 70-75N represent a hydrate instability contour where methane fountaining is causing localized ice melting?
    http://www.iup.uni-bremen.de:8084/amsr/arctic_AMSRE_nic.png

    Comment by Brian Dodge — 30 May 2009 @ 4:27 PM

  60. Hank, I’m assuming the aerosols will settle out eventually?

    Comment by David Horton — 30 May 2009 @ 10:46 PM

  61. Re #59:

    On a completely different off topic question, could the sinuous lead/open water in the arctic ice from ~120E to 165E and 70-75N represent a hydrate instability contour where methane fountaining is causing localized ice melting?

    What you are seeing is the main pack ice beginning to draw away from the shorefast ice along the Russian Coast.

    Comment by Tom Woods — 31 May 2009 @ 1:57 AM

  62. Re #59

    Brian,

    We were simply analyzing the aerosol data generated by the IMPROVE network so we were only looking at elemental silicon, but the only silicon-containing source we identified was very characteristic of mineral dust (i.e. had common elements Ca, Al, Mg, Fe etc in ratios we expected for dust)I would expect any form a silica to be hydrophobic. My adviser published this paper (Perry, K. D., S. S. Cliff, and M. P. Jimenez-Cruz (2004), Evidence for hygroscopic mineral dust particles from the Intercontinental Transport and Chemical Transformation Experiment, J. Geophys. Res., 109, D23S28, doi:10.1029/2004JD004979.) The transport was from China and they think the hygroscopic nature of the dust is from pollution interaction (pretty likely in China) prior to transport.

    Comment by Scott Robertson — 1 Jun 2009 @ 7:41 PM

  63. David, re
    > assuming the aerosols will settle out eventually?

    I just looked briefly for mention of such layers in the ice cores from the past and didn’t find mention of smoke apart from volcanic ash in layers; I’d guess someone’s collecting what falls out each year and perhaps someone knows more. Believing it’s there is reasonable; being able to show it’s there is publishable (grin).

    Comment by Hank Roberts — 1 Jun 2009 @ 7:56 PM

  64. Hank – I wasn’t suggesting it is there, on the ice surface, already. And my impression is that there isn’t any evidence of such an event in the past, but I may be wrong (could volcanic ash be distinguished from bush fire ash, I guess so). But it just seems to me that this event and the arrival of aerosols in Antarctic air (at least), as well as the movement of desert dust into the Atlantic, may be other unexpected consequences of global warming, with potential feedback (positive or negative) effects. Dust from central Australia reaches well into the Tasman Sea (and has been found in sediments) and I think all the way to New Zealand. Pollution from China reaches America. Smoke from burning Malaysian rainforest covers huge areas. I hadn’t seen any discussion of such processes as part of the climate change modelling process, and I just wonder if this might begin now.

    Comment by David Horton — 1 Jun 2009 @ 8:57 PM

  65. There is a whole literature on transatlantic transport of fungal spores, and bacteria some of which have been suggested as sources of Caribbean coral stress . Çile, ben, aerosolların, sonunda dışarı yerleştirecek olduğunu farz ediyorum? Thank you

    Comment by Seviyeli Sohbet — 2 Jun 2009 @ 1:25 AM

  66. For the other (closed) post, see this for fun:

    http://www.physics.emory.edu/~weeks/research/tseries1.html

    It does apply to ocean cycles, ENSO influence, etc.

    David Horton:
    I hadn’t seen any discussion of such processes as part of the climate change modeling process, and I just wonder if this might begin now.

    See the most recent IPCC reports. Here’s the most recent one, the modeling section:

    http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter8.pdf

    Aerosols play an important role in the climate system. Interactive aerosol parametrizations are now used in some models (HADGEM1, MIROC-hi, MIROC-med). Both the ‘direct’ and ‘indirect’ aerosol effects have been incorporated in some cases (e.g., IPSL-CM4). In addition to sulphates, other types of aerosols such as black and organic carbon, sea salt and mineral dust are being introduced as prognostic variables (Takemura et al., 2005; see Chapter 2)

    Here’s the discussion of the data that goes into those assessments:

    http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf

    Also see Chapter Seven:
    7.5.2.4 Global Climate Model Estimates of the Total Anthropogenic Aerosol Effect

    Then, you had all the Pinatubo test case studies, which showed that for volcanic aerosols, the model predictions were pretty accurate.

    Finally, on black carbon reaching Antarctica:

    Continuous high-temporal resolution black carbon ice core records from Antarctica, Edwards et al. 2008

    Hope that helps. For more, see this recent realclimate post.

    http://www.realclimate.org/index.php/archives/2009/04/yet-more-aerosols-comment-on-shindell-and-faluvegi/langswitch_lang/fr

    Some of the most interesting conclusions of the study include those relating to the Arctic. For example, we estimate that black carbon contributed 0.9 +/- 0.5ºC to 1890-2007 Arctic warming (which has been 1.9ºC total), making BC potentially a very large fraction of the overall warming there. We also estimated that aerosols in total contributed 1.1 +/- 0.8ºC to the 1976-2007 Arctic warming.

    All told, there’s nothing involving aerosols that changes the basic relationships between fossil fuel combustion, deforestation, atmospheric CO2 content and global warming.

    Comment by Ike Solem — 2 Jun 2009 @ 1:38 PM

  67. Thanks for the references Ike, very useful. But “All told, there’s nothing involving aerosols that changes the basic relationships between fossil fuel combustion, deforestation, atmospheric CO2 content and global warming” – I never thought there was. Just wondered if the increase in bushfires and dust storms, with increasing severity spreading material far and wide, as the climate warms was yet another positive feedback mechanism.

    Comment by David Horton — 2 Jun 2009 @ 4:57 PM

  68. Thank you very much for the references and links in the article/comments.

    In regards to the Australian brush-fires, the droughts have become so severe that much of west Australia has lost all vegetation for miles…it’s a tragic thing to see first hand.
    ————
    http://www.tomorrowsworldcompetition.com/

    If you have strong feelings on water efficiency and/or flooding from global warming, then watch these and send them to people you know. Everyone can help turn climate change around.

    Comment by Mark — 5 Jun 2009 @ 5:05 AM

  69. A smoothed time series of northern tropical Atlantic dust cover (Fig. 1) shows a maximum and minimum in dust activity that occurred in 1985 and 2005, respectively, and a downward trend in dust optical depth over the record.ed hardy
    buy ed hardy
    Many gases including water vapor are transparent to visible light, but some like water vapor condense into particles/droplets. Those as they get larger, including water clouds in our atmosphere, scatter light in various ways.

    Comment by ed hardy — 22 Jun 2009 @ 2:38 AM

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