Climate change and international migration: Exploring the macroeconomic channel

Existing projections are mostly limited to extrapolation of past statistical relationships between annual climate variables and aggregate net migration or outmigration for individual countries or regions (see [3] and ref. therein). In such studies, the mechanism that underlies a statistical relationship between migration and, say, annual temperature or rainfall variations often remains unclear. This is problematic because it is difficult to assess whether past relation- ships can be extrapolated into the future, or whether they will be altered by changes in other, non-environmental drivers of migration [4]. The few examples of projections that explicitly address such mechanisms include migration in Bangladesh due to future sea level rise [5], and Mexico–US migration due to climate-induced changes in crop yields [6]; both again limited to individual countries. There is thus a research gap related to projections of global migration under climate change based on a mechanistic understanding of the pathways connecting these two processes. Our study aims to contribute to closing this gap by addressing one specific pathway: The indirect effect of climate change on international migration via impacts on macroeconomic develop- ment. We provide, for the first time, a set of future projections of climate change impacts on global bilateral migration patterns acting through this macroeconomic channel. In doing so, we explicitly address existing theoretical and empirical uncertainties, by quantifying several

alternative assumptions about how macroeconomic changes affect migration dynamics. In the following section, we first discuss the theoretical and empirical basis for the effect of national income levels on migration, the related uncertainties, and how we reflect them in our study design; before turning to the technical description of data and models used. Section 3 presents the simulation results, and section 4 synthesizes the results and discusses strengths and limitations of our study. 2 Materials and methods 2.1 Effects of income levels on migration While there are many other important factors shaping international migration [7, 8], it has been recognised that economic conditions in the origin and the destination country—often measured by gross domestic product (GDP) or similar indicators—are fundamental [9–11]. On the other hand, robust findings in the climate-economics literature suggest that climate change will have significant effects on nations’ economic output, and that these effects will be distributed unevenly across the globe [12, 13]. Thus the question arises: how might such eco- nomic impacts of climate change affect global migration patterns? Addressing this question requires that we account for the economic impacts in both the ori- gin and the destination country, for any bilateral migration flow. Specifically, we must make assumptions about how future income levels, which may be quite different from today’s, will affect international migration. Unfortunately, such assumptions are not easy to constrain with

past data, given the short duration of observational migration flow time series, and the high level of short-term variability in observed flows. Indeed, existing migration models cannot pre- dict the observed temporal dynamics in bilateral flows [14]. Besides the short observational period and the incoherent temporal variability of flows, this is also related to the fact that spa- tial differences tend to be much larger than temporal changes. Lacking predictive models of changes in migration flows, the approach we take is to con- struct transparent scenarios of migration under climate change, conditional on certain assumptions about the long-term temporal dynamics of migration. While the temporal signals in observational data are incoherent, there are some clear spatial patterns that emerge from global migration flow data, and it is plausible that some of the mechanisms explaining these spatial patterns also hold valid over time. Specifically, on the side of the destination country, PLOS ONE Climate change and international migration: Exploring the macroeconomic channel PLOS ONE | https://doi.org/10.1371/journal.pone.0276764 November 16, 2022 2 / 25 funding/funding-opportunities/funding- programmes-and-open-calls/horizon-2020_en Author: K. F. Project and grant number: ISI-Access 16QK05 Funder: German Federal Ministry of Education and Research (BMBF) URL: https:// www.bmbf.de/bmbf/de/home/home_node.html The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared

that no competing interests exist.