Desertification Desertification the degradation of land in arid semi arid and dry sub humid areas It is caused primarily by human activities and climatic variations o o Does not refer to expansion of existing deserts o Occurs because dryland ecosystems which cover over one third of the world s land area are extremely vulnerable to over exploitation and inappropriate land use Poverty political instability deforestation overgrazing and bad irrigation practices can all undermine the land s fertility Over 250 million people are directly affected Some 1 000 mil 1 billion people in over one hundred countries are at risk These people include many of the world s poorest most marginalized and politically weak citizens Introduction o Drylands cover about 35 of Earth s land surface o Home to more than 38 of the total global population about 7 billion about 2 6 billion o o Land degradation is present on 10 to 20 of the global drylands Land degradation indirectly affects about 250 million people in the developing world Desertification o The 5 D s of the historical geography of desertification Land degradation in arid semi arid and dry sub humid areas resulting from various factors including climatic variations and human activities UNEP Drylands Drought Desiccation longer term drought Degradation Desertification Arid Regions Deserts o What controls the location of arid regions Subtropical high o General causes of aridity Atmospheric stability subtropical highs Rainshadow effects Upwelling cold currents off west coasts Prevailing winds parallel coasts Remoteness from sources of moisture Desertification The Beginning o Sahelian Desertification Drought in the Sahel 1970 1997 Consequences environmental social 1977 UN Conference on Desertification UNCOD held in Nairobi Kenya 1993 Convention to combat desertification CCD 2006 International Year of the Desert and Desertification Biogeomorphic The physical geography of arid lands Transitional region between savanna and desert Variability in precipitation Natural vegetation The Sahel and ancient stabilized sand seas ergs The nature of desert winds Harmattan Drought and dust Long term climatic change since post glacial times Short term drought and desiccation cycles Global warming consequences Atmospheric circulation changes Multi decadal variations in SST sea surface temperature Reduction gin the strength of the African Monsoon source Shifting of the ITCZ Inter tropical convergence zone Land atmosphere feedbacks Anthropogenic o o o o Climatic Grazing o Cultivation o Firewood o Salinization and waterlogging o o Migration forced unforced o A recent green of the Sahel Sahelian population is doubling every 20 years o o o Increasing rainfall Improved land management very limited increases in ag production Sahel precipitation was above the long term mean from 1915 through the late 30s Drylands Development Paradigm DDP Desertification is a growing threat worldwide o Two perquisites for successful interventions Ensure the local community is fully involved Combine modern technologies Aeolian Dust Dry arid and semi arid climates occupy 35 of land surface Wind has a large impact on erosion and shaping the landscape in arid locations o Soil erosion agriculture health Deserts are optimal environments for sediment transport In the desert winds can carry more sediment than any other geomorphological agents In addition to its contribution to soil erosion and loss of agricultural productivity Aeolian dust can transport biota nutrients and toxins very rapidly over large distances potentially affecting the health and productivity of terrestrial and ocean ecosystems and human populations alike Dust entrainment and transport climate change induced increases in drought will increase dust can feedback upon climate by increasing or decreasing temperatures by directly affecting radiative forcing Marine impacts of dust deposition of dust into the oceans provides critical nutrients to marine plankton which have potential to significantly reduce atmospheric carbon by sequestration and to produce climatically significant Hjulstrom curve flow velocity thresholds for the entrainment transport and deposition of granular particles The fluid threshold applies to wind moving over a surface when no particles are in motion Once sand movement has begun the impacts of grains colliding with the surface puts other grains into motion Less wind energy is needed to keep sand moving after some grains are in motion and the impact threshold is approximately 80 of the fluid threshold Slight increase in wind velocity exponential increase in sediment transport rate U U t 3 Three methods of Aeolian sediment transport Only the finest particles are transported by turbulent air Suspension May last for days Creep Saltation The Bodele Where is the dustiest place on Earth Covered in diatomite which when eroded is the source of huge amounts of dust The Harmattan is a dry and dusty Sahelian tradewind Blows south from Sahara into the Gulf of Guinea from November to March o o o o o o Characteristics of Dust Source Terrains Large basins of internal draingage Presence of deep and extensive alluvial deposits o o o Dry lakes playas o Dunes or sand sheets Environmental consequences of desert dust o Demise of Caribbean corals o Red tides toxic red algae o Amphibian diseases o o o Increased asthma cases Eutrophication in estuaries Presence of toxic chemicals Mercury Chromium Zinc etc Pesticides garbage burning o o Fe rich Saharan dust serves as a nutrient for plants in the upper canopy of the South American rain forest Long term consequences of desert dust Influence radiative transfer in the atmosphere and thus affect climate atmospheric chemistry Fe rich dust can fertilize the ocean s primary productivity thus impacting the global carbon cycle o o o African droughts and dust dust concentrations were sharply lower during much of the 20th century before 1970 Prospero and Lamb 2003 in Science o As desert soils age the supply of soil nutrients from minerals declines unless replaced by other inputs such as dust Clay minerals in Saharan dust with their high cation exchange capacities increase the nutrient holding abilities of these soils SHAMALS result from strong northwest winds that are funneled into the Persian Gulf by the mountains of Turkey and Iraq to the northeast and the high plains of Saudi Arabia to the southwest KATABATIC winds downslope winds flowing from high elevations of mountains plateaus and hills down