Experts estimate that by 2050 some 200 million people will be displaced by environmental problems
Already today an estimated 25 to 27 million migrants are believed to have fled their homes predominantly due to environmental issues or disasters.
Environmentally-motivated migration is expected to feature poorer people, more women, children and elderly, from more desperate environmental situations
Today most people on the run are "economic migrants" - mostly young men.
One serious concern is that criminal networks will systematically prey on migrating women - this is already happening in Bangladesh where women whose men have been killed by cyclones are trafficed for slavery, prostitution, smuggling etc.
Also see my recent Three cases of migration and conflict: Hurricane Katrina, Bangladesh and The Dust Bowl for some academical and historical perspectives.