Geoscience Reference
In-Depth Information
elevations or latitudes. Today, many of these habitats have been reduced
to mere remnants—islands in the midst of human development—leaving
threatened populations with nowhere to go. Many are already on the verge
of extinction from other pressures, such as habitat loss, over-exploitation, and
environmental degradation.
a renewed dust bowl
As the climate warms and dries, the Southwest also faces an increase in atmo-
spheric dust levels from both natural and human causes. Under natural con-
ditions, the desert soils of the Southwest are stabilized by “desert crust”—a
mixture of soil, cyanobacteria, and lichens. h e cyanobacteria secrete a sticky
material that holds soil particles together and retains water and nutrients
important for the sparse desert vegetation.
As in the Great Plains of the 1920s, when farmers ploughed the soil too
deeply, leaving it vulnerable to wind erosion and leading to the Dust Bowl
drought of the 1930s, humans today are disrupting the natural protective
surface of desert crust with vehicles, livestock, housing developments, and
recreational activities. h ese ef ects are exacerbated during droughts (like the
decade-long drought between 1999 and 2009) because the bacterial i laments
between the soil particles become dry and more fragile, making them more
easily crushed. As microbes and lichen decline and the desert crust erodes,
the unstable soils are more susceptible to wind erosion, leading to an increase
in atmospheric dust.
In lakes near the desert Southwest, researchers have collected cores from
lakebeds and analyzed the sediments to calculate the levels of atmospheric
dust over the past several centuries. h e cores show a six-fold increase
in dust at er 1850, when livestock were i rst introduced to the West. h e
dust levels then declined somewhat at er the 1930s, when the government
passed the Taylor Grazing Act to reduce overgrazing in the West. But dust
levels have been on the rise in recent decades as the human population has
grown.
h e increased dust levels also have other unforeseen negative impacts on
hydrology in the West. For example, as dust settles onto snow in adjacent
mountains, it darkens the surface, lowering its rel ective properties and caus-
ing it to absorb more sunlight, which causes the snow to melt faster, leaving
less snow for the dry summer months.
