In case you missed it, an audit of the U.S. Department of Defense’s installation climate resilience from last year, conducted by the Government Accountability Office, found that “installations have not consistently assessed risks from extreme weather and climate change effects or consistently used projections to anticipate future climate conditions.” One of those conditions is sea level rise that will affect multiple coastal installations (see here and here). Sea level rise will not only affect the physical infrastructure on these installations, it will also potentially lead to the inland migration of portions of the populations who live in the surrounding communities – some of whom form part of an installation’s work force. Depending on how far away and how many migrants move, their loss will degrade an installation’s ability to continue to function at an acceptable level over time.
Migration is an adaptive response to climate change. It is characterized by people leaving an area, people arriving/resettling in another area, usually nearby, and the flow between the two. Thus, climate-induced sea level rise not only affects coastal zones and populations, but also the resident populations of inland areas and populations on the move.
Historical migration patterns are unlikely to be followed by climate migrants. For example, flooding can occur across a large coastal area affecting a large population. The flooding can be severe as when a hurricanes strike, affecting many individuals simultaneously, or can be from nuisance flooding, where less than 100% of a population is affected by the cumulative effects of multiple, short-term, non-storm related events.
Robinson et al. (2020) present a population-level framework to assess “the broader impacts of climate change on population by explicitly considering the effects on migration directly affected by climate change and indirectly affected [emphasis in the original] by the change in migration patterns induced by climate change.” Indirect effects can be both positive—increased experience and skills and negative—greater job competition; more costly housing; increase income inequality. The framework focuses on models of sea level rise and its effects on migration within the lower 48 states in the U.S. Two sea level rise scenarios are used—a medium scenario where the rise totals 3 feet by 2100 and a high scenario where the rise totals 6 feet by 2100. Interim rises over time of one foot are also included in each scenario. Population projections for those affected and unaffected by flooding are made for every census block across the U.S. for each of these incremental rises in sea level and the forecasted dates when they occur. The probability of migration draws on recent migration data to determine whether someone leaves an area to travel to another.
Robinson et al. (2020) found that “the primary destination of climate migrants are counties just inland of their origin, but climate migrants also move farther towards large cities that offer more opportunities.” Under the medium scenario, at 2100, 24 million people in the U.S. will be indirectly affected by incoming migration, whereas in the high scenario, at 2100, 120 million people will be affected by incoming migration. The east coast versus the west coast will see the largest indirect effects of migration given the larger population that will be directly affected by sea level rise, particularly the southeast. Thus, cities will see the largest influx of migrants, continuing the historic trend of greater urbanization in the U.S.
Department of Defense coastal installations need to model not only the direct effects of sea level rise on their infrastructure, but what happens to the work force who live in the surrounding communities. If conditions continue to deteriorate, either from large, regional storms that instantly devastate large coastal areas or through incremental rises in water levels, no amount of funding for structural solutions to rising water levels will help to ameliorate the migration of populations containing some or all of the work force away from the coast. For those who migrate further, such as to more distant cities, their loss to the work force will be permanent. These migrating populations will change the areas to which they migrate both positively and negatively.
Dr. Marc Kodack is Senior Fellow at the Center for Climate and Security and former Sustainability and Water Program Manager in the Office of the Deputy Assistant Secretary of the Army for Energy and Sustainability.
* This post is part of the Council on Strategic Risks’ “Responsibility to Prepare and Prevent” Blog Series, designed to increase the tempo and scale of relevant and useful analysis during a time of crisis