Desertification

Desertification Deserts: areas of low rainfall….. Arid: less than 10 cm (4 in) Semi-arid: less than 25 cm (10 inches) Desertification: the degradation of land in arid, semi-arid, and dry sub-humid areas. Can be caused by human activity or naturally occurring phenomenon.

Desertification Causes of Desertification: Complex interplay of physical and ecological processes, and developmental and socio-economic forces. For example: brief period of excess rainfall “the rain follows the plow” over irrigation (salinization) ground-water depletion

Desertification …is the degradation of land in arid, semi-arid, and dry sub-humid areas. Causes of Desertification: Complex interplay of physical and ecological processes, and developmental and socio-economic forces. Drought: rainfall significantly below normal for an extended period. several years more than 25% less than normal

Drylands cover about 40% of Earth’s land surface Home to more than 38% of the total global population of 6.5 billion (about 2.4 billion) Land degradation is present on 10 to 20% of the global drylands Land degradation indirectly affects about 250 million people in the developing world

Dryland regions of the world (yellow) these are sensitive to desertification

BIOGEOMORPHIC the physical geography of arid lands transitional region between savanna and desert variability in precipitation natural vegetation

Desertification Excessive forest clearing and overgrazing livestock reduces vegetation cover. Two impacts: *Albedo increases: cooling the region in summer and weakening monsoon circulation. Less vegetation = less evapotranspiration. So less water vapor in the atmosphere to fuel precipitation over the region. Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space

From Lancaster, 2007

30% of the land in the U.S. is affected by desertification. Desertification in the US 30% of the land in the U.S. is affected by desertification.

Precursers 10-fold increase in population between 1860 and 1920. Deep plowing and monoculture destroyed soil structure and increased sensitvity to erosion. Additional factors: Great Depression: no $ for prairie farmers Out migration “Okies” True drought.

Shallow, rather than deep, along contours Established windbreaks Desertification in the US: The Dust Bowl of the 1930s. Changed farming practices & plowing techniques Shallow, rather than deep, along contours Established windbreaks Crop rotation Irrigation

Phoenix, Arizona in the early 1970s

How have each of these factors contributed to desertification?