Natural resources modulate the nexus between environmental shocks and human mobility

In the context of natural resource degradation, migration can act as means of adaptation both for those leaving and those supported by remittances. Migration can also result from an inability to adapt in-situ, with people forced to move, sometimes to situations of worse or of the same exposure to environmental threats. The deleterious impacts of resource degradation have been proposed in some situations to limit the ability to move. In this contribution, we use remote sensed information coupled with population density data for continental Africa to assess quantitatively the prevalence of migration and immobility in the context of one cause of resource degradation: drought. We find that the effect of drought on mobility is amplified with the frequency at which droughts are experienced and that higher income households appear more resilient to climatic shocks and are less likely to resort to mobility as an adaptation response.

Introduction
Over the last 30 years, there has been a rising tide of predictions of mass human migrations, either forced or by choice, in the face of climate change1. While it is accepted that many people are currently displaced by climate-related disasters2, there is also a credible suggestion that migration might become less rather than more prevalent with future climate change3. The logic behind this latter possibility is that the impacts of climate change are likely to reduce the assets of vulnerable populations, impeding their ability to move4.

From a theoretical perspective, the role played by natural resources as a mediator between environmental shocks and migration decisions is complex5. The so-called “environmental scarcity” hypothesis poses that risks associated with environmental shocks and their variability make migration more likely, with households reallocating to compensate for potential losses in natural capital. The “environmental capital” hypothesis, on the other hand, sees natural capital as a resource providing income that in turn may support (long-distance) migration decisions. To the extent that population immobility in the context of environmental shocks and climate risks can be explained by a lack of resources, access to income streams from natural capital appears important to explain migration patterns and thus the potential emergence of trapped populations6. Natural resources play a mediating role in both determining who is vulnerable to climate change and who is able to afford to migrate away. The evidence of the extent of the phenomenon of immobility is restricted to selected regions or nations7,8,9,10,11. Some studies dealing with immobile populations exist12,13. These studies explore the sensitivity of international migration, measured through bilateral flow data, to temperature increases in origin countries. The analysis reveals that, while for middle-income economies increased temperatures are associated with increased migration rates to other countries, in poorer countries the reverse is observed13. The literature finds that, in response to temperature shocks, international migration flows tend to decrease for roughly 5 years before they increase for more than 20 years. In this study, we use the relative changes in measures of subnational migration in response to environmental stress (drought) to identify immobile populations and quantify the effect of environmental shocks on internal migration.

A schematic contextual framework to understand how resource scarcity and abundance intersect with migration processes is presented in Fig. 1. This diagram shows that resource scarcity and resource abundance can both lead to disruptive impacts on human populations and their well-being. Different causal pathways are presented in this regard and conflict can be a result of these divergent mechanisms. Demography links to environmental factors through metrics such as carrying capacity (which can itself change with adaptive technologies) and may lead to resource scarcity. On the other hand, resource abundance without appropriate distributive mechanisms can lead to inequality and hamper economic development, which is in turn also linked to migration movements. Conflict can occur within the migration nexus through the combination of tribalist impulses ensuing from resource scarcity or distribution failures, particularly in kleptocracies, as well as through the kinetics of physical movement and clashes with existing populations. Containing these scenarios to avert conflict requires us to overcome the challenges of governing an inherently complex system and also the security imperatives which any conflict dynamic can instigate. Two overarching challenges frame this set of relationships: political governance at the macro level and security and management concerns at the micro level.

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