The human population is embedded within an ecological context that supports, but also constrains, human action. As one type of human action, migration is an expression of the relationship between humans and the ecological context. Indeed, throughout recorded history, human population movements have been closely associated with ecological changes, both accumulative and acute. For example, human populations have been situated in the Nile River delta in Egypt for more than 5,000 years, waxing and waning in size and composition as a result of earthquakes, droughts, floods, and land erosion. However, acute environmental change has also caused population movements, as in the case of Hurricane Katrina in the US, which forced tens of thousands of people to emigrate from the city of New Orleans.