| Decadal trends in the aeolian dust record of the Sahara affect the global climate system and
the nutrient budget of the Atlantic Ocean. One proposed cause of these trends are changes in
the frequency and intensity of dust storms, which have hitherto been hard to quantify.
Because sand flux scales with the cube of wind speed, dune migration rates can be used as a
proxy for storminess. Relative changes in the storminess of the Sahara can thus be monitored
by tracking the migration rates of individual sand dunes over time. The Bodélé Depression of
northern Chad was selected as a target area for this method, because it is the most important
point-source of aeolian dust on the planet and features the largest and fastest dunes on
Earth.
A collection of co-registered Landsat, SPOT, and ASTER scenes, combined with
declassified American spy satellite images was used to construct a 45 year record of dune
migration in the Bodélé Depression. One unexpected outcome of the study was the
observation of binary dune interactions in the imagery sequence, which reveals
that when two barchan dunes collide, a transfer of mass occurs so that one dune
appears to travel through the other unscathed, like a solitary wave. This confirms a
controversial numerical model prediction and settles a decade-old debate in aeolian
geomorphology.
The COSI-Corr change detection method was used to measure the dune migration rates
from 1984 until 1987, 1990, 1996, 2000, 2003, 2005, 2007, 2008, 2009, and 2010. An
algorithm was developed to automatically warp the resulting displacement fields back to a
common point in time. Thus, individual image pixels of a dune field were tracked over time,
allowing the extraction of a time series from the co-registered satellite images without further
human intervention. The automated analysis was extended further back into the past by
comparison of the 1984 image with declassified American spy satellite (Corona) images from
1965 and 1970. Due to the presence of specks of dust as well as image distortions
caused by shrinking of the photographic film, it was not possible to automatically
measure the dune displacements of these scenes with COSI-Corr. Instead, the image
was georeferenced and coregistered to the 1984 Landsat imagery by third order
polynomial fits to 531 tie points, and the displacements of ten large barchan dunes
were measured by hand. Thanks to the 19-year time lapse between the two images
used for these ‘analog’ measurements, their precision is better than 5%, which is
comparable with that of the automated COSI-Corr analysis. The resulting dune
celerities are identical to the automated measurements, which themselves show little or
no temporal variability over the subsequent 26 years. The lack of any trend in the
time series of dune celerity paints a picture of remarkably stable dune mobility
over the past 45 years. None of the distributions fall outside the overall average of
25m/yr.
The constant dune migration rates resulting from our study indicate that there has been no
change in the storminess of the Sahara over the past 45 years. The observed dust trends are
therefore caused by changes in vegetation cover, which in turn reflect changes in precipitation
and land usage. This work highlights the importance of the hyper-arid Bodélé Depression,
which provides a steady but finite supply of aeolian dust to the atmosphere without
which nutrient fluxes and terrestrial albedo would be more variable than they are
today. |