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RECONSTRUCTING CHANGING DUST FLUXES SINCE THE LAST GLACIAL: A NEW INFORMAL WORKING-GROUP
It is well established that dust fluxes from the land surface to the atmosphere were much greater during the last glacial phase, but no-one knows quite how much greater they would have been. It is important that we explore the possible role of dust in the atmosphere in reinforcing the cold and aridity of glacial climates, and in producing sudden global climate shifts (signs are from the latest ice core evidence that dust fluxes at the end of the Younger Dryas decreased very rapidly, even before the thermohaline circulation switched on). For understanding both past and future climates, this factor needs to be added to GCM palaeoclimate simulations.
The only realistic way to attempt this is to bring together regional specialists on arid region processes with climate modellers, and experts on regional palaeoclimates. For this reason I'm organizing an informal internet-based working group, open to anyone who can usefully contribute, to come up with a preliminary but realistically based set of comparable dust flux figures for 1) the Last Glacial Maximum/Global Aridity Maximum, 2) the Mid-Holocene 3) the late 20th century present-actual.
Open-ness is paramount; this is not intended to be a clique but rather a forum for individuals working in a certain direction with a common scientific goal in mind. In the general spirit of the QEN site, all results will be displayed publically at every stage, and open discussion and criticism are encouraged. Plagiarism by non-participants would of course be pointless, as the origin of the data will immediately be apparent to all who have previously visited this web site. Participants should expect occasional gentle prompting from me, to help ensure that the project moves at a reasonable pace (at least on the timescale of years)!
Jonathan Adams, Oak Ridge National Laboratory
The Plan (at least preliminarily) is this:1) Regional experts, bearing in mind a) paleovegetation maps (and their uncertainties), b) general geology and soils, and c) wind speeds (as output from previous GCM runs) suggest fluxes of particular particle grades to particular key heights in the atmosphere. The suggested fluxes will be for grid squares of 2 by 2 degrees.
Each expert or group begins by compiling a grid of dust flux for a particular region (e.g. the American south-west, the Sahal region), and the approach is to expand out from these regions, comparing and discussing all along with colleagues, 'fitting' the flux maps of different regions side-by-side until global coverage is reached. The flux maps will be published as our first joint paper.
2) The 'potential' dust fluxes for the global grid for each time slice are put into a GCM, the winds whip up dust into the atmosphere, and the effect on global climate is computed. This will be published as our second joint paper.
The Plan is always open to change. If it seems at all silly, please let me know!
List of Participants as of 19th March 1998.
Jonathan Adams (convener of the group)email@example.com
Quaternary biogeography, land surface interactions in climate changes, biogeochemistry.
Paleoclimatology; model-data comparisons for the Late Quaternary (specifically the Younger Dryas).
GCM modelling of contemporary climates.
Using GCM studies to understand rapid climate shifts, with particular reference to volcanic forcing.
Louis ScottScottL@plk.nw.uovs.ac.za General paleoclimatology and vegetation of Southern Africa during the Late Quaternary, deduced from offshore and lake cores.
Dust fluxes deduced from offshore cores, general paleoclimatology and arid-land environments of Southern Africa during the Late Quaternary.
Quaternary arid land environments and processes, paleoclimatology, biogeochemistry.
Paleoclimate simulations for the Quaternary and older periods, using GCM's.
Quaternary paleoenvironments and aeolian processes in Africa and the SW USA.
Contemporary modelling studies of local and regional dust flux from soil, vegetation and climate data.
Ocean core studies of Quaternary dust flux from the Australian landmass.
Broad-scale observations of dust-flux using satellite data.
Broad-scale observations of dust-flux using satellite data.
Long-distance transport of Saharan dust, Holocene periods of increased storminess, from studies of northern Scandinavean peat bogs.
Physics of wind erosion and aeolian dust fluxes.
History of dust transport recorded from lakes and loess, in arid regions of China.
Latest news on the project
Arid-land scientists towards estimating dust fluxes
At present the initial stage of the project is getting underway. I'm talking with Dale Gillette, Andy Kwarteng and Louis Scott about each of them making some initial dust flux estimates for the region they work in (the south-west USA, Kuwiat and South Africa, respectively). The approach is to start local and regional and then expand the coverage of the flux estimates out laterally until eventually the whole global land surface is dealt with. I'm hunting down other arid-land scientists to help us with other regions; if you know of anyone who might like to help, please let me know! On the basis of our discussions, I'm suggesting that the dust flux data be categorized into 1) <1 micro, 2) 1-2 micron, 3) 2-3 micron, 4) 3-4 micron. Flux values for the amount of dust leaving 1) the boundary layer and 2) getting up to ?'cloud' height, are also needed (the height in the atmosphere is important; if anyone has opinions on what height the upward flux should calculated for, let me know).
Vegetation maps available for estimating the dust flux
There are various versions of LGM, mid-Holocene and present-day vegetation available in gridded form on the web. For this study everything has to be on a 2 by 2 deg. grid. A useful source for the intial set of estimates is the 2-degree gridded Last Glacial Maximum land cover map compiled from the QEN review. A vegetation key, and other alternative distributions of Last Glacial Maximum (and Holocene) vegetation, are available at this page by Crowley & BaumIn our study we will also eventually include other hypothetical LGM land surface cover maps, to account for the range of uncertainty within the Quaternary community.
Note that some detailed (1 degree resolution) interdisciplinary reconstructions of land surface conditions for North Africa at 6,000 14C y.a. are downloadable from this link
LGM and Holocene wind data: gridded maps are now linked to this page
Since the entrainment of dust into the atmosphere depends crucially upon wind speed, as well as soil and vegetation type, it is necessary to have some rough figures for this, using GCM output (hardly perfect, but we really have nothing else to use for this!). Paul Valdes has pointed us towards the UGAMP model output (for LGM, mid-Holocene, and present-day), which has data on boundary layer wind fields on a 2 by 2 degree grid, same as for the paleovegetation maps above. I'm thinking that for our first study we should just calculate dust fluxes across the boundary layer and not bother with complexities of atmospheric transport once it gets aloft.
The grid of mean wind speed data for the LGM, mid-Holocene and present-day, based on GCM output, downloadable at this link. Note that there is a key to explain what the numbers mean, just above the table of links.
If you have opinions on what other detailed aspects of wind speed near the surface (e.g. maximum gust strength) are most useful for understanding dust flux up from the surface, let me know, and I will try to get hold of that sort of data.Jonathan Adams
(2nd April 1998)
"Dust aerosols, Loess Soils, and Global Change": A conference in Seattle, Washington set for this October.
Tom Gill's 'Aeolian Papers of the Month' page.
Wind erosion/aeolian processes discussion list.
If you feel that you can contribute to the Dust Flux Group, please e-mail Jonathan Adams, detailing your background of expertise: firstname.lastname@example.org