With increased climate change effects, the expectation in coming decades is that populations living along some coastal regions will have to move to higher grounds to avoid catastrophic flooding of cities and homes. However, how many people will need to move and the specific regions likely to be most affected is still not certain. Now, a new study attempts to produce a more precise estimate of population displacement caused by climate change and attempts to integrate mitigation and adaptation attempts in a more specific regional context.
27% of the world’s population lives in coastal areas
Impacts on human migration due to climate change are likely already occurring in the Americas, the Middle East, and Sub-Sahara Africa. Nevertheless, this may only be the beginning and we may need more precise estimates on migration in the decades to come.
Over 27% of the global population currently lives in coastal areas and this number may increase in the near-term. This population is also highly vulnerable to storm surges and flooding as sea levels rise. One recent study estimates ~35 million migrants until 2100 may have to move due to coastal flooding threats.
Such populations estimates do not account for adaptive strategies and such studies do not typically apply spatial assessment to determine where internal migrants may move to. Many studies tend to be global in nature or look at coastal regions equally rather than based on how adaptation, geographic, and social factors may affect migration. In coming years, it will be critical to more accurately determine areas where internal migrants within countries may move in response to rising sea levels.
Predicting coastal flooding
In a new study, assessments on migration incorporate the effects of coastal adaptation policies that incorporates protection, housing, and managed strategies of placing migrants in given locations. This helps to show trends on areas that are likely to receive or be affected by migration. The approach includes using a gravity-based population downscaling model to make the spatial estimates. The Mediterranean region is used as the test case to show where along the wide area populations are likely to move.
This model assesses migration until 2100 and incorporates adaptive strategies to refine the estimate. The work also includes population and urbanization projections that incorporate socioeconomic pathways across the region.
The final modeling results are not one single projection but rather a range of projections based on various scenarios of adaptation and specific choices made. For instance, scenarios look at accommodation strategies to facilitate migration, more protective measures that limit migration in some areas and attempt to apply more adaptive measures (e.g., sea walls protecting against flooding), and even more protection scenarios which utilize protection estimates based on socioeconomic characteristics across the region. In addition to socioeconomic and policies incorporated across raster landscapes, the work includes physical geographic data.
The work incorporates 50- and 25-year periods to determine areas at risk from regular coastal flooding. Coastal wetland data from the Global Lakes and Wetlands Database (GLWD) across 30 arc seconds is also used, which can affect the degree to which areas flood.
20 million people affected by sea level rise by 2100
Overall, likely scenarios show potentially 20 million people will be forced to be internal migrants by 2100. The numbers do vary based on scenarios. Egypt is shown to be among the most vulnerable, as the low-lying Nile Delta has a high population and is already nearly sea level. Proactive adaptation scenarios that build protection along coastal regions to adapt to nature leads to high migration in the first half of the century, but the benefit is this declines later in the century until 2100, demonstrating a leveling off as populations adapt to new sea level realities. The most protective scenarios limit migration.
By limiting protection strategies, 70–86% of all migrants are projected to be urban, with this share decreasing if more protection is incorporated. This is because most adaptation strategies are likely to focus on urban areas, which will skew impacts in these areas. In general, migration is projected to be three times higher in the southern and eastern Mediterranean in comparison to the northern Mediterranean. Adaptation strategies may, however, reduce overall migration by factors ranging between 1.4 to 9. Hard protection in coastal areas may, in fact, lead to migration to other coastal areas rather than inland.
The coming decades will prove disruptive for global populations. This is particularly the case for coastal communities experiencing sea level rise. However, recent research shows that disruption could be significantly mitigated through adaptive measures that seek to preserve coastal communities. On the other hand, it could be more realistic for countries to combine adaptive strategies that preserve communities along coastal regions as well as move populations to inland and higher areas.
Countries will likely apply various strategies, depending on the resources available and what can be realistically achieved based on their geographic and socioeconomic circumstances. Now, we have better tools to help us estimate the impacts of migration as sea levels rise and how countries could perhaps best utilize their resources to help these populations.
 For more on recent global migration estimates in coastal regions due to climate-driven changes, see: Lincke, D. & Hinkel, J. Coastal migration due to 21st century sea-level rise. Earth’s Futur. 9, 1–14 (2021).
 For more on migration and global-scale effects of environmental change, see: Veronis, L., Boyd, B., Obokata, R. & Main, B. Environmental change and international migration. In Routledge Handbook of Environmental Displacement and Migration (eds. McLeman, R. & Gemenne, F.) Ch. 4, 42–70 (Routledge, 2018).
 For more on estimating migration using spatial and adaptation estimates based on sea level rise, see: Reimann, L., Jones, B., Bieker, N. et al.Exploring spatial feedbacks between adaptation policies and internal migration patterns due to sea-level rise. Nat Commun 14, 2630 (2023). https://doi.org/10.1038/s41467-023-38278-y