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Of tempests, barren ground and a thousand furlongs of sea

Filed under: — group @ 21 May 2009

Guest commentary by Ron Miller, NASA GISS

Several studies have shown that hurricane activity is generally reduced during years when there is a thick aerosol haze over the subtropical Atlantic. The haze is comprised mainly of soil particles, stripped by wind erosion from the barren ground over the Sahara and Sahel. These particles are lifted into the atmosphere and carried by the Trade winds as far as the Caribbean and Amazon basin. Plumes of dust streaming off the African coast are easily recognized in satellite imagery, and were even described by Charles Darwin during his voyage on the Beagle.

The amount of dust crossing the Atlantic has been measured at Barbados since the mid 1960s (aptly by Prospero and colleagues). These measurements show a threefold increase in dust between the original part of the record and the mid 1980s at the peak of the Sahel drought, when the region was unusually vulnerable to wind erosion. African dust crosses the tropical Atlantic within the Saharan Air Layer (SAL), an elevated duct of air between about 2 and 5 km in altitude. Because of its continental origin, this air is not only dusty but extremely dry.


Figure 2: Monthly mean dust concentration measured at Barbados. Arrows mark years with large El Niño events, which are irrelevant here (Prospero and Lamb, 2003).

There is an observed anti-correlation between dustiness and tropical cyclone days in the Atlantic (Evan et al, 2006). This anti-correlation might indicate the a direct influence of dust on hurricanes, or a connection between the dry air the dust resides in and hurricanes, or might even be related to a much larger scale pattern which controls both hurricanes and dustiness. If there is a connection, one hypothesis is that entrainment of dry SAL air rapidly strangles a developing cyclone because of the low humidity that accompanies the dusty air, while the dust itself has no direct effect. An alternative hypothesis is that the reduction in sunlight beneath the dust layer cools the ocean surface, whose temperature is a well-known predictor of hurricane activity (at least at the basin scale). Thus it is plausible that decadal variations in dustiness could contribute to decadal variations in hurricane activity, but how big might such an effect be?

A recent article in Science by Evan et al. (2009) is one of the few attempts to quantify the contribution of both dust and volcanic aerosols to the observed warming within the tropical Atlantic. The authors infer the amount of total aerosol using the Advanced Very High-Resolution Radiometer (AVHRR) satellite instrument and screen for locations where dust is present (they note that other aerosols might be mixed with the dust, but neglect this overlap). They also assume that dust has no effect where there are clouds. However, where the SAL extends over low marine clouds, the dust (since it is darker than cloud) might have an opposing effect to that seen in clear sky regions, although this is hard to quantify. They then calculate the contribution by dust and volcanic aerosols to observed changes in sea surface temperature (SST) during the satellite record between 1982 and 2007. During this period, the aerosol amount varied with dust export from Africa, but also from major eruptions by two volcanoes (El Chichon in 1982 and Pinatubo in 1991), each of which left a reflective layer of sulfate droplets in the lower stratosphere for a couple of years.

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. That decreasing aerosols account for two-thirds of the observed warming might suggest that other factors like the increase in greenhouse gas concentrations (combined with anthropogenic aerosol changes) made a relatively modest net contribution to the warming (and by implication to observed trends in hurricane activity). For the natural aerosols, they calculate that stratospheric aerosols made roughly twice the contribution of dust over this period.

So how did they do this calculation? Firstly, they use a relatively simple model to relate SST to the reduction in net radiation into the ocean surface, prior to any climatic response. This forcing is calculated using the total aerosol amount inferred from the AVHRR data. Variations in SST due to variations in heat transport by ocean currents or diffusion into the thermocline are neglected while contributions by changes in evaporation, turbulent transfer, and surface radiation are estimated as being proportional to the anomalous air-sea temperature difference. Cooling of the ocean by aerosols must therefore be offset by a reduction in heat lost from the ocean to the atmosphere.

They note a key simplification is their neglect of any change to the surface air temperature when calculating anomalous air-sea temperature difference. This would require an atmospheric model along with a consideration of aerosol forcing at the top of the atmosphere (TOA). There is a strong relationship between surface air temperature and TOA forcing (at least at large spatial scales). As a consequence, the ocean-atmosphere flux depends upon not only forcing at the surface but the forcing at the TOA. By neglecting the effect of the changes in surface air temperature upon SST, Evan et al. may be underestimating the impact of the aerosols on their calculated trend. This is especially important for volcanic aerosols, whose TOA forcing is large and comparable to the surface forcing, as opposed to absorbing aerosols like dust where the surface forcing is larger than at TOA. However, balancing this effect is the neglect of heat diffusion into the thermocline which would reduce the ocean cooling. It is not a priori obvious which effect is more important, especially since the atmosphere can balance the forcing by adjusting lateral heat transport, which would also influence the anomalous surface air temperature.

Another way to test the importance of atmospheric changes would be to calculate both the TOA and surface forcing using the satellite measurements, and then impose this transient forcing in a general circulation model that calculates both the atmosphere and ocean response. That too would have problems, given that the models are not perfect, but it would be a useful check on the order of magnitude of the inferred effects. Indeed, assessments of the causes of tropical Atlantic trends using the IPCC AR4 models (Santer et al, 2006) come up with a much larger component due to anthropogenic effects, though those models did not include dust forcing changes.

Using their methodology, Evan et al. find that a decline in total aerosols contributed around two-thirds of the observed warming in NH tropical Atlantic SST between 1982 and 2007. Most of this is due to the two major volcanic eruptions (El Chichon and Pinatubo) that cooled the ocean early on in this period (and so lead to a warming once they were no longer present). However, the attributed aerosol trend would have been smaller had the satellite record extended a decade earlier. The estimated contribution of dust changes to the observed trend is small, roughly one-quarter of the total trend.

Whatever its impact upon SST, dust might impact other factors contributing to cyclone intensity (Emanuel, 1995), in particular, the reduction of the air-sea heat flux and temperatures in the upper troposphere. Unfortunately, global models don’t quite have the resolution to explicitly calculate all these effects.

Ultimately, the effect of dust upon hurricanes is important because, like ocean temperatures, African dust export is expected to change during the 21st century in response to global warming and changes in African rainfall. One study shows that dust production is expected to decrease (Mahowald and Luo, 2003), though given the diversity of Sahel rainfall projections and the preliminary state of vegetation models, this is not necessarily going to be a universal response.

The calculation by Evan et al. is an interesting first step to quantifying the effect of dust changes on SST, but there plenty of issues left to investigate.

Footnote: For some presumably poetic reason, the Bard neglected to note that the Main Development Region is more like 25,000 furlongs across and the Sahara is about 2 billion acres.


69 Responses to “Of tempests, barren ground and a thousand furlongs of sea”

  1. 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?

  2. 2

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

  3. 3
    yourmommycalled says:

    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

  4. 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.”

  5. 5
    Thomas says:

    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?

  6. 6
    Lynn Vincentnathan says:

    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.

  7. 7
    Ike Solem says:

    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.”

  8. 8
    CM says:

    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).

  9. 9
    Mark says:

    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.

  10. 10
    MacDoc says:

    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.

  11. 11
    mauri pelto says:

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

  12. 12
    Ike Solem says:

    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”.

  13. 13
    MikeN says:

    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]

  14. 14
    your mommy called says:

    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?

  15. 15
    MacDoc says:

    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.

  16. 16
    Hank Roberts says:

    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.

  17. 17
    Ray Ladbury says:

    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!

  18. 18
    CM says:

    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.

  19. 19
    Russell Seitz says:

    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 .

  20. 20
    Richard Reinert says:

    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]

  21. 21
    Ray Ladbury says:

    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.

  22. 22
    Hank Roberts says:

    “… 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?

  23. 23
    Scott Robertson says:

    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.

  24. 24
    Daryl says:

    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?

  25. 25
    Hank Roberts says:

    > 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. …

  26. 26
    Stephen Berg says:

    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?

  27. 27
    Russell Seitz says:

    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 .

  28. 28
    chris colose says:

    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.

  29. 29
    CM says:

    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.

  30. 30
    Adam Gallon says:

    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.

  31. 31
    Ray Ladbury says:

    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.

  32. 32
    Hank Roberts says:

    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.

  33. 33
    t_p_hamilton says:

    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.

  34. 34
    Adam Gallon says:

    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.

  35. 35
    t_p_hamilton says:

    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.

  36. 36
    Scott Robertson says:

    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.

  37. 37
    Daryl says:

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

  38. 38
    Hank Roberts says:

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

  39. 39
    Thomas says:

    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.

  40. 40
    Larry Tolton says:

    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]

  41. 41
    CM says:

    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)

  42. 42
    your mommy called says:

    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.

  43. 43
    Antonio San says:

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

  44. 44
    James says:

    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.

  45. 45
    David Horton says:

    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?

  46. 46
    Antonio San says:

    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.

  47. 47
    Hank Roberts says:

    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

  48. 48
    Hank Roberts says:

    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—-

  49. 49
    Antonio San says:

    “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?

  50. 50
    Chris S. says:

    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.


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