Concern about the human impact of climate change has led to predictions of how people living in areas vulnerable to drought, flood, and temperature changes will respond to such events. Early studies warned that climate change would lead to dramatic increases in human migration as households became unable to adapt to the impacts of climate change. More recently, empirical studies focused on observed climate events and trends have documented how migration flows vary as a function of both the severity of the event and the ability of the household to migrate, among other factors. In this paper, we provide a systematic review of this literature, based on a conceptual framework in which climate shocks (e.g., drought, floods, or temperature extremes) affect (a) household capability to migrate, by depleting household resources necessary for migration, and (b) household vulnerability in staying, by increasing the risk that a household falls (further) into poverty. In combination, these factors help explain four key patterns seen in the empirical literature: (1) climate-induced migration is not necessarily more prevalent among poorer households; (2) climate-induced migration tends to be more prevalent for long-distance domestic moves than local or international moves; (3) slow-onset climate changes (such as droughts) are more likely to induce increased migration than rapid-onset changes (such as floods); and (4) the severity of climate shocks impacts migration in a nonlinear fashion, with impacts influenced by whether the capability or vulnerability channel dominates.
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We define environmental migrants as per the International Organization for Migration definition (IOM, 2009): “Environmentally induced migrants are persons or groups of persons who, for compelling reasons of sudden or progressive changes in the environment that adversely affect their lives or living conditions, are obliged to leave their habitual homes, or choose to do so, either temporarily or permanently, and who move either within their country or abroad.” The term environmental migrants has largely replaced the term ‘environmental refugees’ (see Kibreab 2009; Etienne Piguet 2010; Biermann and Boas 2010).
The relatively recent development of this literature is highlighted by the sparsity of studies available for citing in previous reviews (see, for instance, Laczko and Aghazarm, 2009; Gray, 2009; Piguet, et al. 2011). While a large literature has existed for several decades on both theoretical models of migration (summarized briefly in Section 2) and on empirically determined general determinants of migration, studies that demonstrate causally the role of environmental factors in migration are much more recent.
This does not mean that household must be the migrating unit, however. For instance, households could send members to live temporarily or permanently for the sake of supporting the remaining household members.
This may occur before, during or after a weather shock. The process of changing expectations is important given shifting climate regimes, where a series of weather shocks may be indicative of a permanently more hostile climate. Dillon, Mueller, and Salau (2011), for example, present evidence of ex-ante diversification efforts in the face of potential drought in Nigeria.
We define developing countries as per the United Nations country classification system. This includes low-income, lower-middle income, and upper-middle income countries (per capita GNI less than $12,055 in 2019) of the World Bank country and lending groups.
While qualitative approaches are clearly immensely valuable for understanding migration decisions, we maintained an exclusive focus on quantitative empirical papers for consistency and comparability.
There are of course community or regional factors, beyond the household level, that affect the decision to migrate, for example, conflict and political instability. While we acknowledge these as important factors, the studies included in our review exploit differences at the household level or, at most, differences at a district level. Political instability, which affects entire countries or regions over long periods of time, is thus not explicitly considered.
This differs from the more common approach of considering household capability or vulnerability as a simple control variable, which provides a general characterization of whether richer or poorer households migrate but not in respect to weather shocks specifically.
The only departure from this trend is found in Mastrorillo et al. (2016), who find that large positive heat anomalies increase migration for both women and men, with a larger effect on women (although the statistical significance of the difference is not reported).
This may take the form of a distinction between rural and urban migration (the latter usually represented a longer move), or between within-district and out-of-district migration.
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The authors are grateful for the constructive comments from Christopher Paul, Erika Weinthal, Alexander Pfaff, and two anonymous reviewers.
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Kaczan, D.J., Orgill-Meyer, J. The impact of climate change on migration: a synthesis of recent empirical insights. Climatic Change 158, 281–300 (2020). https://doi.org/10.1007/s10584-019-02560-0