Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 343

13.4.2 Vegetation and Dust Source Extent

The removal of vegetation exposes soil particles to a greater force by the wind,
potentially increasing mobilization (Chap. 5 ) . Conversely, increased vegetation
within the Sahel during recent decades is correlated with the reduced occurrence
of dust storms (Cowie et al. 2013 ). By changing precipitation, dust radiative forcing
alters the regional distribution of vegetation, feeding back upon the mobilization of
dust. The sign of this feedback and its quantitative importance remain unresolved.
A positive feedback between dust radiative forcing and climate may contribute to
observed climate anomalies including desertification within the Sahel (Yoshioka
et al. 2007 ), the North American Dust Bowl during the 1930s (Koven 2006 ; Cook
et al. 2008 ), and the multidecadal droughts within the Great Plains during the
Medieval Climate Anomaly (Cook et al. 2013 ).
Devegetation is represented by several effects within models (Koven 2006 ). The
area vulnerable to wind erosion is expanded, while surface roughness is decreased,
reducing the wind stress that must be exceeded for soil mobilization (Chap. 5 ) . In
addition, the areal expansion of bare soil reduces transpiration of soil moisture
into the atmosphere, increasing the turbulent flux of sensible heat at the expense
of the latent heat flux. As noted above, the increased sensible heat flux increases the
strength of wind gusts, leading to greater mobilization.
Understanding the feedback between dust aerosols and vegetation introduces the
challenge of specifying the strength of a new dust source, defined as the mass of
dust mobilized at a given wind speed. Source strength depends upon the availability
of easily eroded particles. Some models assume that newly exposed sources are
more prolific than existing sources because loosely bound soil particles that were
protected by roots and leaves are now exposed to the force of the wind (e.g., Tegen
et al. 1996 ). In contrast, existing sources may have been already stripped of their
most easily erodible particles. In the absence of an increase to the strength of
newly created sources, model dust loads seem unrealistically insensitive to large
expansions of source extent related to the disappearance of vegetation (Yoshioka
et al. 2007 ). African dust arriving at Barbados increases fourfold between wet
and dry decades. In contrast, Yoshioka et al. ( 2007 ) find that removal of Sahel
vegetation associated with an equatorward expansion of the Sahara by 2 ı latitude,
meant to represent the probable change between a wet decade like the 1950s and
the comparatively dry 1990s, increases dust emission from North Africa by only
about one-third. This discrepancy with respect to observations suggests that the
model underestimates the initial strength of the new sources by assuming that they
are no stronger than the original sources. Inferring the strength of new sources
from observations is complicated by concurrent changes to the wind threshold
for mobilization. Some models assume that mechanical disturbance of the soil by
cultivation reduces this threshold (e.g., Tegen et al. 2004 ). Other models raise the
threshold to account for the prevalence of cultivated sources in soils dominated
by clay, whose particles are subject to greater cohesion (Ginoux et al. 2012 ).