The UN in conjunction with Geneva-based Internal Displacement Monitoring Center (IDMC) an organization which monitors the environmental and social causes of displaced settlements, just published a new report that puts climate change (by far) as the leading cause of fast-onset displacement.
The report, "Monitoring Disaster Displacement in the Context of Climate Change" notes that the numbers are on the rise, and that does NOT include slow displacement factors like drought and sea rise, both of which are also linked to climate change.
In 2008, a total of 20 million people were forced to leave their homes due to climate-related causes. Had it not been for the Sichuan earthquake which displaced 15 million people, climate change would have been responsible for 90% of all home displacements on the planet.
4.8 million were displaced due to political conflict and violence, about 1/4 the number of those displaced by climate change.