For many modeling studies of climate migration, scenarios of socioeconomic and climatic quantities are required as input. Consistency of migration assumptions between scenarios and models is key to avoiding partial or double counting of migration. The Shared Socioeconomic Pathways (SSP) include assumptions on international migration whose effect on quantities of interest remain largely implicit. In a study published in 2021, my co-authors and I explicitly quantify the effect of international migration on income and inequality projections by comparing the original SSP quantifications to ones with zero migration. In doing so, we develop projections along the 5 SSP narratives of bilateral migration and remittances flows. The central objective of this contribution is to highlight where and to what extent migration plays a role in projections of income and inequality that are widely used in climate change research.

Bilateral migration and remittances flows in the SSP scenarios

Income projections without migration are obtained by removing two effects of migration on income dynamics: changes in population size and remittances sent to origin countries. Remittance estimates are obtained using migrant stocks derived from bilateral migration flow estimates obtained from a gravity model. Consistently with assumptions embedded in each SSP narrative, SSP5 is the scenario presenting the highest migration numbers. Yet independently of the scenario chosen, we find that migrants tend to move within continents, as highlighted by the dark colors along the diagonal. Migration corridors from Africa to Asia and Europe and from Latin America and the Caribbean to Northern America are also present in the projected migration patterns. International migrant flows remain limited to 0.3% of the population in the continent of origin and to the same proportion of population at destination. Associated remittances flows follow similar corridors, and amount to less than 10% of continental GDP for all SSP narratives.

On average, migration makes the world richer in all SSP narratives

We show that migration makes the world richer in all SSP narratives, with an increase of up to 10% in global per capita GDP under SSP4 (“Inequality”). Overall, the effect of migration and remittances does not modify the ranking of income levels among the SSP narratives: SSP5 (“Fossil-fueled Development”) is still the scenario with the consistently highest per capita GDP, and SSP3 (“Regional Rivalry”) is the one with the lowest. Depending on the narrative and location, effects of migration on income can be substantial, ranging from -5% to +21% at the continental level.

Effect of migration on inequality between and within countries

Migration and associated remittances are also found to decrease income inequality between countries. Our results indicate that, in general, migration is expected to play a significant role in regulating inequality between countries over the coming decades, and much more so in scenarios where strong income differences persist. Finally, international migration and remittances also affect inequality within-country. Here, we compute this effect on Gini coefficients, a commonly used measure of inequality that considers the population-weighted distribution of income within countries. Following Rao et al. (2019), we assess changes in inequality stemming exclusively from migration-driven changes in education drivers of inequality. Migration appears to have minimal effect on most large economies (USA, China, India, Brazil, or Russia). Many destination countries (Canada, Australia, Western Europe, Saudi Arabia), on the other hand, see their inequalities reduced with migration. For some origin countries (Mali, Burkina Faso, Sudan, Afghanistan), migration appears to strongly increase inequality. Yet, for most origin countries, the effect of migration stays limited to under 5% in either direction.