The collapse of the Aral Sea ecosystem is (arguably) the man-made environmental disaster of the 20th century.  Soviet mismanagement, water-intensive cotton production and population growth have all contributed the overtaxing of water resources in the Aral Sea basin, which has resulted in a the shrinking of the Aral Sea--it has lost more of the sea to an expanding desert than the territories of the Netherlands, Belgium and Luxembourg combined.  The health problems arising from this issues are large for the entire Aral Sea basin, which encompasses 5 Central Asian countries and it has profoundly changed (for the worse) the local climates. 

Via Seth Dixon