This study shows that if global temperatures rise by about 4 to 5 degrees Celsius in the course of the century, as they are projected to under business-as-usual growth in greenhouse gas emissions, the South Asian region could face a wave of migrants displaced by the impacts of climate change, including sea level rise and drought associated with shrinking water supplies and monsoon variability. It concludes that the resultant catastrophe may well be the greatest humanitarian and economic catastrophe that the developing world will face in the coming decades and demands and recommends policy steps to mitigate climate change and steer development in a sustainable direction.