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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:

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!

Jonathan Adams (convener of the group)

Quaternary biogeography, land surface interactions in climate changes, biogeochemistry.

Hans Renssen

Paleoclimatology; model-data comparisons for the Late Quaternary (specifically the Younger Dryas).

Jan Polcher

GCM modelling of contemporary climates.

Casper Ammann

Using GCM studies to understand rapid climate shifts, with particular reference to volcanic forcing.

Louis Scott

Tim Partridge

Dust fluxes deduced from offshore cores, general paleoclimatology and arid-land environments of Southern Africa during the Late Quaternary.

Hugues Faure

Quaternary arid land environments and processes, paleoclimatology, biogeochemistry.

Paul Valdes

Paleoclimate simulations for the Quaternary and older periods, using GCM's.

Nick Lancaster

Quaternary paleoenvironments and aeolian processes in Africa and the SW USA.

Dale Gillette

Contemporary modelling studies of local and regional dust flux from soil, vegetation and climate data.

Bill Nickling

Paul Hesse

phesse@laurel.ocs.mq.edu.au

Ocean core studies of Quaternary dust flux from the Australian landmass.

Charlie Zender

zender@ncar.ucar.edu

Broad-scale observations of dust-flux using satellite data.

Andy Kwarteng

Broad-scale observations of dust-flux using satellite data.

Lars Franzen

Long-distance transport of Saharan dust, Holocene periods of increased storminess, from studies of northern Scandinavean peat bogs.

John Gillies

Physics of wind erosion and aeolian dust fluxes.

Fahu Chen

History of dust transport recorded from lakes and loess, in arid regions of China.

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 & Baum

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.

(2nd April 1998)

Useful links

"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: jadams@arts.adelaide.edu.au