Climate change and international migration: Exploring the macroeconomic channel

migration transition as their income levels change on long timescales. To reflect this assump- tion in our model, we keep emigration rates constant at their average historical levels as we investigate future climate impacts on migration; casually speaking, we “freeze” the migration hump. We refer to this first assumption as CR, for constant rates. The second, alternative assumption, is that countries undergo a transition along the migra- tion hump with respect to their absolute real income levels; i.e., the migration hump function is stationary in terms of GDPc, and country-level emigration rates change as GDPc levels change. In our model, the migration hump is represented through a simple but effective approximation to a non-parametric regression of the empirical data (see section 2.2.4). We refer to this second assumption as T0 (transition, no change to migration hump function). Finally, the third alternative assumption is that the migration transition occurs with respect to relative income levels. To this end, we shift the migration hump function towards higher GDPc values over time, along with rising global average GDPc. We refer to this third assump- tion as TS (transition, shifted migration hump function). We investigate how future climate change affects global migration patterns in our model under each of these three assumptions. Given the lack of well-performing predictive models of migration, as well as the complexity of both migration and climate change and the large num-

ber of possible interactions between them, the contribution of our study is to expose a specific, indirect channel of climate impacts on migration; to disentangle it from other potential effects, and to demonstrate how it can be addressed by combining the most recent advances from the climate-economics and migration modeling literature. We do not present predictions of future migration, but a quantification of the effect that climate change may have on migration pat- terns via a specific channel—the impacts on macroeconomic development –, conditional on the current knowledge and uncertainties about the effects of national income as one of the most important migration drivers. 2.2 Data and models To estimate the potential effect of climate change on future migration, we first derive country- level temperature data from global climate model (GCM) simulations. In order to control for differences in the speed of global warming between different climate models and future green- house gas (GHG) concentration scenarios, we focus on a level of 3˚C global warming above pre-industrial conditions, rather than on a certain time period. The different climate models are therefore used primarily to represent different possible spatial patterns of warming. In a next step, we calculate the expected effect of climate warming on GDPc, following two alterna- tive methods from the recent climate-economic literature. GDPc projections from the Shared Socio-economic Pathways (SSPs; [20, 21]) serve as a baseline, from which climate change-

induced perturbations are calculated. Finally, a global migration model is used to calculate migration flows for both the baseline and the perturbed GDPc scenarios, under different assumptions. The effect of climate change is then obtained by comparing migration patterns between the baseline and the perturbed case. 2.2.1 Data. We use historical data on country-specific total population [22] and migrant stocks [23], defined by country of birth and residence, coming from the UN Department of Economic and Social Affairs. Data on future population growth rate is obtained from the SSP projections dataset [24]. Data on migration flows for calibrating the model is obtained from a recent version of global bilateral migration flows dataset [25]. These flows are reported by country of residence and destination. The data is not disaggregated by country of birth, and therefore does not report transit and return migration flows separately. On the other hand, PLOS ONE Climate change and international migration: Exploring the macroeconomic channel PLOS ONE | https://doi.org/10.1371/journal.pone.0276764 November 16, 2022 4 / 25