On developed coastlines, humans react to physical processes in coastal environments by stabilizing shorelines against chronic erosion and by taking measures to prevent destruction of coastal infrastructure during storms. While we are beginning to understand how physical processes affect human behavior, causality runs in the other direction as well. Over decades or longer, even localized anthropogenic shoreline manipulations influence large-scale patterns of coastline change as much as physical, climate-related forcing does. The long-range spatial and temporal spillovers of localized human actions in coastal environments, combined with widespread localized shoreline-stabilization and storm protection efforts, amount to unintentional geoengineering of our coastlines. In essence, investments in coastal engineering fail to consider tradeoffs that can unfold over long temporal or large spatial scales. A more purposeful geoengineering of coastlines requires a richer understanding of the two-way couplings between physical and human coastline dynamics, including efforts to reduce uncertainties in forecasting future scenarios for the coupled system. Steering toward preferred outcomes for our coastlines and coastal economies will involve coordination across local, state, and federal jurisdictions to mitigate spatial externalities that extend beyond local communities.