Climate change and international migration: Exploring the macroeconomic channel

GDP per capita (GDPc) levels have consistently been found to be positively related to immi- gration [10, 11]; richer countries tend to attract more immigrants. We will assume that this relation, as estimated from historical data, will remain stable in the coming decades; as coun- tries grow relatively richer or poorer, they become relatively more or less attractive, respec- tively, for migrants, compared to other destinations. On the side of the origin country, the relation between GDPc and emigration is more com- plex and ambiguous. Many recent studies have highlighted a non-monotonous, inverted U- shaped, dependence of emigration rates on average income levels in the origin country, which has been called the “migration hump” [15] It has been explained as a combination of migration aspirations that decline with rising incomes, and a number of factors that impede emigration from very poor countries [16, 17]: For instance, international migration is costly, and in poor countries many people may not have the resources to cover these costs even if they would like to emigrate. Thus, emigration tends to be highest in middle-income countries where people can both afford the investment to move, and can still expect a substantial gain in income—or generally, in utility—from moving. Ref. [15] provides a comprehensive overview of this and other factors explaining the migration hump. It is, however, not fully clear whether the migration hump also reflects a process sometimes termed the migration “transition”: The movement of individual countries along an inverted

U-shaped trajectory of emigration rates as incomes gradually rise over the course of decades. Do countries track the migration hump as they grow richer (or poorer)? Available emigration data suggest that they do not, and that emigration rates may instead decline even in poor coun- tries during periods of economic growth [18]. However, the available data is limited in dura- tion and primarily reflects what we consider short-term variations in migration flows, on the order of five years (which is a typical interval of measurement at which global migration data are available). The migration “transition”, on the other hand, is thought to proceed in the long-run as countries grow wealthier in the course of many decades. It is thus currently not possible to determine whether individual countries’ emigration rates do follow the migration hump over time, meanwhile being perturbed by more short-term variability that follows differ- ent mechanisms; or whether the migration hump is a purely spatial phenomenon that can be explained solely by differences between countries with regard to other, exogenous factors [18]. This is however a crucial question, because climate change, as a long-term phenomenon, will have gradually growing impacts on countries’ economic development, and thus would affect the rate at which countries progress through the migration transition. Moreover, supposing that countries do undergo a long-term transition along the migration hump, it is still unclear whether relative or absolute incomes matter for this transition. Some

empirical analyses seem to indicate that the peak of the migration hump is associated with higher (real) GDPc values in more recent periods [15, 18]. That is, a certain value of real GDPc would have a country located on the declining branch (to the right of the peak) of the hump in the 1970s, but on the increasing branch (left of the peak) in the 2010s. This may be explained by arguments that as the world as a whole grows richer, it is relative deprivation, rather than absolute poverty, that motivates people to emigrate. However, the empirical evidence for this non-stationarity of the migration hump is weak, and there are also arguments for absolute (real) incomes playing a role both in migration aspirations and in the ability to afford the costs of migration [19]. Given these open questions surrounding one of the most important mechanisms driving migration globally, we test in our study three alternative assumptions about the relation between origin-country GDPc and emigration, using an innovative, non-linear migration model calibrated on historical migration flow data, as described below. The first assumption is that the migration hump is a purely spatial phenomenon, and that countries do not undergo a PLOS ONE Climate change and international migration: Exploring the macroeconomic channel PLOS ONE | https://doi.org/10.1371/journal.pone.0276764 November 16, 2022 3 / 25