X Desertification A Deserts areas of low rainfall Arid less than 10 cm 4 in Semi arid less than 25 cm 10 inches X Desertification A Deserts B Drylands What life cares about Effective Moisture the ratio of precipitation PPT to potential evapo transpiration PET Hyperarid PPT PET 0 05 Arid PPT PET 0 05 to 0 20 Semi arid PPT PET 0 20 to 0 5 Dry sub humid 0 5 to 0 65 Humid PPT PET 1 X Desertification A Deserts B Drylands C Distribution 1 Sunbelt 2 Rainshadow 3 Continental interior 4 Coastal Adjacent to these real deserts are the sensitive drylands X Desertification A Deserts B Drylands C Distribution D Desertification the degradation of land in arid semi arid and dry sub humid areas Can be caused by human activity or climate change X Desertification D Desertification the degradation of land in arid semi arid and dry sub humid areas 1 Causes of Desertification Complex interplay of physical and ecological processes and developmental and socioeconomic forces For example a brief period of excess rainfall the rain follows the plow b over irrigation salanization c ground water depletion X Desertification D Desertification the degradation of land in arid semi arid and dry sub humid areas 1 Causes of Desertification Complex interplay of physical and ecological processes and developmental and socioeconomic forces For example 2 Drought rainfall significantly below normal for an extended period several years more than 25 less than normal Dryland regions of the world yellow These are sensitive to desertification D Desertification 4 The Sahel as an example climate climate change agricultural practices political process population D Desertification 3 The Sahel as an example a Sub saharan rainfall index shows drought from 1970 to 2000 b Impact on human activity in the Sahel D Desertification 3 The Sahel as an example a Sub saharan rainfall index shows drought from 1970 to 2000 b Impact on human activity in the Sahel c Possible explanations for the Sahel Remember what drives the monsoon circulation Strong seasonal pressure gradient between land and sea hot land mass Warm oceans to supply moisture Sufficient recycling of rainfall by plants evapotranspiration to allow moisture to reach deep into the interior D Desertification c Possible explanations for the Sahel Natural climate variability Monsoon activity controled by sea surface temperatures related to ENSO variability Inappropriate technology misguided foreign aid Aerosols produced by industrialization in Europe cool Sahel reducing strength of the African Monsoon d Positive feedbacks D Desertification c Possible explanations for the Sahel d Positive feedbacks Fuel wood gathering and stock overgrazing 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 the monsoon and deliver precipitation over the Sahel region D Desertification 4 The Sahel as an example 5 Desertification in the US 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 D Desertification 5 Desertification in the US The Dust Bowl of the 1930s Precursers Causes Consequences Changed farming practices Plowing techniques Shallow along contours Established windbreaks Crop rotation D Desertification 5 Desertification in the US The Dust Bowl of the 1930s Precursers Causes Consequences Changed farming practices Better Yes but Phoenix Arizona in the early 1970s