UK government's chief scientist suggests that without proper government migration management, it is likely that trace amounts of CO2 will cause "widespread humanitarian disasters on an unprecedented scale"

BBC News - Climate change migration warning issued through report

The government's chief scientist, Professor Sir John Beddington, who commissioned the study, said that environmental change would hit the world's poorest the hardest and that millions of them would inadvertently migrate toward, rather than away from, areas that are most vulnerable.

"[These people] will be trapped in dangerous conditions and unable to be moved to safety," he said.
...
The issue, according to Professor Beddington, was to ensure that the migration was properly managed - otherwise, he said, it was likely there would be widespread humanitarian disasters on an unprecedented scale.

List of wars and anthropogenic disasters by death toll - Wikipedia, the free encyclopedia

World War II [40-70 million deaths]

Humanitarian crisis - Wikipedia, the free encyclopedia

A humanitarian crisis (or "humanitarian disaster") is an event or series of events which represents a critical threat to the health, safety, security or wellbeing of a community or other large group of people, usually over a wide area. Armed conflicts, epidemics, famine, natural disasters and other major emergencies may all involve or lead to a humanitarian crisis.