Reliable statistics on stocks or flows of irregular migrants, the well-being of migrants in irregular situations, or the extent to which they have access to services such as health and education, are generally not available. Irregularity does not refer to the individuals but to their migratory status at a certain point in time. Changes in national laws and policies can turn regular migration into irregular migration, and vice-versa. The status of migrants can change during their journey and stay in the country of transit/destination, which makes it difficult to have a comprehensive picture of irregular migration and the profiles of irregular migrants. Terms such as “irregular”, “undocumented”, and “unauthorized” are used interchangeably in this section.
Quantifying environmental migration is challenging given the multiple drivers of such movement, related methodological challenges and the lack of data collection standards. Some quantitative data exist on population displacement within a country, and to a lesser degree across borders, due to natural hazards. However, for migration due to slow-onset environmental processes, such as drought or sea-level rise, most existing data are qualitative and based on case studies, with few comparative studies. While data gaps persist, research methodologies are constantly being improved.