Geoscience Reference
In-Depth Information
(Cakmur
et al.
, 2006). Models tend to be calibrated to
data remote from source regions using Aeronet data or
the Miami Aerosol Group measurements (see Chapter
20) (Cakmur
et al.
, 2006).
tunnel experiments rather than more realistic assessments
in the field. Solutions to some of these difficulties lie
in implementing targeted research efforts such as instru-
mentation of key dust source regions at a scale that is
sympathetic to the resolution of the climate models.
In the meantime, some research on projected change
in dust emissions has resorted to less-sophisticated meth-
ods, which correspond more with the intermediate cate-
gory of approaches to projecting climate change impacts.
Washington
et al.
(2009), for example, analysed climate
projections from the IPCC AR4 data set for the Bodele
in Chad. Of the 10 models studied, 8 show an increase
in mean annual surface wind speed by the last decade of
the twenty-first century compared with 1971-2000 and
all 10 show an increase during January to March (max-
imum
The surface winds in most dust models are known to
seriously underestimate winds over the key source re-
gions such as the Bodele (Koren and Kaufman, 2004;
Washington
et al.
, 2006), many by 50 % or more.
The climate models have a coarse resolution, particu-
larly global climate models, making it difficult to repre-
sent the processes of deflation realistically. As a result,
the model components that specify emission tend to be
tuned to background dust loadings remote from source
rather than made to satisfy observed physical processes
crucial in deflation. Even when emission data from key
source regions are available to constrain models and res-
olution is improved by running regional climate models
at horizontal resolutions of 26 km or better, such as the
study of five regional models for a single deflation event
covering several days in the Bodele Depression, model
emission still differed by an order of magnitude (Todd
et al.
, 2008).
0.2 m/s). Choosing only
those models with a realistic simulation of current cli-
mate limits the selection to three models, two of which
(MRI and GFDL) show dry conditions over the Sahel
during the twenty-first century. Near-surface zonal winds
over the Bodele Depression in MRI enhance considerably
during the course of the twenty-first century based on
decadal means. In the decade 1991-2000, easterly winds
exhibit an average peak speed of 9 m/s within the core of
the low-level jet, extending from 15-17
◦
N, 17-20
◦
E and
fragmenting over Lake Chad. By 2091-2100 the spatial
coverage of the jet has increased, extending from 3-21
◦
E
across the region with a clear focus of expansion over
the Bodele.
Since dust mobilisation arises from synoptic-scale
events in the Bodele, the extremes of daily wind speed
distribution and their frequency of occurrence in each
month are crucial components in modelling dust output.
The frequency of winds stronger than 10 m/s is markedly
greater in the latter half of the twenty-first century (Figure
24.5). The percentage of January to March (JFM) days
with winds exceeding 11 m/s (a rough threshold for de-
flation in the basin) increases from 45 % (1991-2000) to
49 % (mid-twenty-first century), exceeding 56 % by the
end of the century. The Bodele winds show a doubling in
the number of February days with wind speeds exceeding
11 m/s by the end of the century (
p
+
0.8 m/s, minimum
+
The soil particle size distribution used in many model
simulations are necessarily fictitious as the source re-
gions are extremely remote. There have nevertheless
been commendable efforts to recover better particle size
data from remote regions using remote sensing (e.g.
Christopher and Jones, 2010) as they have to map sub-
basin scale dust sources (Bullard
et al.
, 2008).
Like several other components of arid zone geomor-
phology, projected dust emissions under climate change
suffer from the degree of spread in climate model precip-
itation projections. In the case of the Sahel and Sahara,
this is a particular difficulty given the degree divergence
of precipitation futures for this region, with some mod-
els predicting a wet future and others a dry future (Cook
and Vizy, 2006; Hoerling
et al.
, 2006). The Bodel
´
eDe-
pression, the world's largest source of mineral aerosols
(Washington
et al.
, 2003), for example, lies just a few
hundred kilometres north of the northernmost fringe of
the Sahel grasslands - less than a gridbox in some global
climate models. The Bodele is currently therefore crit-
ically close to being stabilised by vegetation and small
changes to the current regime are unlikely to be captured
with sufficient accuracy by the climate models.
0.01 %). Given the
cubic sensitivity of dust mobilisation to wind speed, these
results point to the possibility of a substantial increase
in dust output from the world's largest mineral aerosol
source towards the end of the twenty-first century.
In the 5 years leading up to 2010, much of the effort
in dust modelling seems to have gone into refining and
calibrating climate models against observed data rather
than extended efforts on long-term climate change pro-
jections of dust (e.g. Yoshioka
et al.
, 2007). Much of
=
