How do environmental stressors influence migration? A meta-regression analysis of environmental migration literature.

Background: The amount of literature on environmental migration is increasing. However, existing studies exhibit contradictory results. A systematic synthesis of the environment-migration relationship is much needed.

Objective: This study summarizes research findings, calculates the effect sizes of environmental stressors, identifies publication bias, and investigates heterogeneous environmental effects on migration.

Methods: We collected 3,380 estimates from 128 studies published between 2000 and 2020 to explore the environment-migration relationship and performed weighted instrumental variable regression to unveil the heterogeneous environmental effects on out- and net migration.

Results: The majority of environmental stressors were not important predictors of out- and net migration. Among the results showing environmental impacts on migration, 58% and 68% reported that environmental stressors increased out- and net migration, respectively, while 58% reported that environmental stressors decreased in-migration. The overall environmental impact on migration was small; however, disaster-related stressors showed a medium effect, and rapid-onset stressors had a stronger impact than slow-onset ones. Multivariate meta-regression analyses demonstrated that environmental stressors were more likely to trigger internal migration than international migration and that developed countries were less likely to experience out-migration. Rapid-onset environmental stressors did not increase out-migration but played an important role in decreasing net migration toward environmentally stressed areas. Meanwhile, we also found a publication bias toward studies showing a positive relationship between environmental stressors and migration in the previous environmental migration literature.

Conclusions: Environmental stressors may affect migration; however, the environmental effect depends on migration measurements, environmental stressors' forces and rapidity, and the context in which migration takes place.

Contribution: This study contributes to migration studies by synthesizing and validating the environment-migration relationship and enhancing our understanding of how and under what circumstances environmental stressors may affect migration.