Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 344

Constraining the strength of newly created sources will require untangling the full
range of changes to the mobilization process by devegetation.
The historical record indicates the presence of new dust sources during dry
climates like the Dust Bowl (e.g., Hansen and Libecap 2004 ). This drought was
preceded by a La Niña event in the equatorial eastern Pacific that typically reduces
precipitation during winter months over the southwestern United States. However,
the Dust Bowl is unique among droughts in the historical record for its exaggerated
amplitude and persistence compared to the unexceptional ocean forcing as well
as the unusual northward displacement of the drought into the Great Plains. This
is illustrated by contrasting the observed and SST-forced precipitation anomalies
in Fig. 13.9 (left and center columns, respectively), reproduced from Cook et al.
( 2009 ). One hypothesis is that the modest drought induced by La Niña withered
crops that had recently replaced more resilient natural grasses. Devegetation led to
wind erosion and aerosol radiative forcing that amplified and sustained the drought
over the new sources created by cultivation (Koven 2006 ). This amplification is
supported by model experiments that show a reduction in precipitation in the
vicinity of the new dust sources (Koven 2006 ; Cook et al. 2008 ). To constrain
the strength of the new sources, Cook et al. ( 2008 , 2009 ) used measurements
of downwind deposition (Hansen and Libecap 2004 ) and showed that the source
expansion brought the anomalies of temperature and precipitation into better
agreement with observations compared to the effect of equatorial Pacific SST by
itself (Fig. 13.9 , right column).
Models suggest that the drought of the Dust Bowl was exacerbated by human
disturbance, but natural “megadroughts” within the Great Plains of North America
have been inferred from tree-ring measurements throughout the past millennium
(Cook et al. 2007 ). During the Medieval Climate Anomaly (MCA), there is geologic
evidence of expanding sand dunes within the Great Plains, implying increased dust
mobilization (e.g., Jacobs and Mason 2007 ; Miao et al. 2007 ). Cook et al. ( 2013 )
show that new dust sources amplify the reduction of precipitation initiated by remote
forcing from the equatorial Pacific, increasing not only the magnitude of the drought
but its persistence, consistent with the tree-ring record.
These studies show that new dust sources can significantly amplify and prolong
precipitation anomalies. What remains to be quantified is whether the regional
extent and strength of the new dust sources assumed by these studies are realistic.
Models of vegetation and its response to climate anomalies are needed to investigate
the entire feedback loop. For the example of the MCA, a vegetation model is
needed to demonstrate that mild forcing from the equatorial Pacific can lead to the
areal expansion of bare soil that is vulnerable to wind erosion. This requires that
vegetation models exhibit realistic sensitivity to climate perturbations. Some models
show large sensitivity. For example, anthropogenic climate change simulated by one
model converts the Amazon into a desert that triples the global dust load (Woodward
et al. 2005 ). Vegetation models give source extent, but additional information about
source strength is needed, which depends upon soil mechanics and soil conservation