As humanity experiences the early effects of a rapidly changing climate, policymakers focus on two primary responses: mitigation, or direct intervention such as reducing emissions, and adaptation, or modifying infrastructure or behavior to adjust to climatic change. Rising temperatures and sea level, increased coastal flooding and perturbation of weather patterns are among the changes likely to require an adaptive response. Projections, including those mentioned above, have become essential tools for planners seeking to anticipate the profound changes ahead.

A recent projection [Hinkel et al., 2014] found that, absent adaptation, 0.2 to 4.6 of Earth's population--tens of millions or more--could see yearly flooding by 2100, assuming a 25 to 123 centimeter (0.8 to 4 foot) rise in global mean sea level. That translates to yearly losses in gross domestic product of 0.3 to 9.3 percent. Adaptive measures, such as construction of dikes for coastal protection, would bring yearly investment and maintenance costs to $12 to $71 billion by 2100--likely a far smaller figure than the cost of the damages that would otherwise be unavoidable.

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