Title: Amplification of the North American `Dust Bowl' drought through
human induced land degradation.

The `Dust Bowl' drought of the 1930s was highly unusual for North America,
deviating from the typical pattern forced by `La Nina' with the maximum drying
in the central and northern Plains, warm temperature anomalies across almost
the entire continent (1-5), and widespread dust storms. General circulation
models (GCMs), forced by sea surface temperatures (SSTs) from the 1930s,
produce a serious drought, but one that is centered in southwestern North
America and without the warming centered in the middle of the continent.
Here we show that the inclusion of forcing from human land degradation during
the period, in addition to the anomalous SSTs, is necessary to reproduce the
anomalous features of the Dust Bowl drought. The degradation over the Great
Plains is represented in the GCM as a reduction in vegetation cover (crop
failure) and the addition of a soil dust aerosol source.  As a result of land
surface feedbacks, the simulation of the drought is much improved when the new
dust aerosol and vegetation boundary conditions are included.  Vegetation
reductions explain the high temperature anomaly over the northern U.S. and the
dust aerosols intensify the drought and move it northward of the purely
ocean-forced drought pattern.  When both factors are included in the model
simulations, the precipitation and temperature anomalies are of similar
magnitude and in a similar location compared to the observations.  It is
concluded that human-induced land degradation not only led to the dust storms
of the 1930s, but also amplified the drought and these together turned a
typical SST-forced drought into one of the worst environmental disasters
the U.S. has experienced.