Climate migration, driven by sea-level rise (SLR), significantly amplifies demographic changes and accelerates population aging in origin areas. This study uses matrix population models, flood hazard models, and a migration model based on 40 years of environmental migration data to project U.S. county-level population distributions under various Representative Concentration Pathway-Shared Socioeconomic Pathway (RCP–SSP) scenarios. The results show that climate migration could lead to a demographic amplification of 8.6–28 million people (5.7–53 million), which is 5.3 to 18 times the number of actual migrants (0.4–10 million). This amplification occurs because younger populations are more likely to migrate, leaving older populations behind, which accelerates aging in origin areas. The study also finds that as the percentage of the population lost due to climate migration increases, the median age of the population rises, potentially by up to 10 years in highly impacted coastal counties. The research highlights the importance of considering demographic feedback loops, such as fertility and gravity effects, in climate migration models to fully understand the long-term demographic impacts. The findings suggest that climate migration could significantly alter population distributions and aging patterns, with implications for policy planning and adaptation strategies in both origin and destination areas. The study underscores the need for integrated models that account for multiple climate hazards and demographic processes to better predict and manage the effects of climate migration.Climate migration, driven by sea-level rise (SLR), significantly amplifies demographic changes and accelerates population aging in origin areas. This study uses matrix population models, flood hazard models, and a migration model based on 40 years of environmental migration data to project U.S. county-level population distributions under various Representative Concentration Pathway-Shared Socioeconomic Pathway (RCP–SSP) scenarios. The results show that climate migration could lead to a demographic amplification of 8.6–28 million people (5.7–53 million), which is 5.3 to 18 times the number of actual migrants (0.4–10 million). This amplification occurs because younger populations are more likely to migrate, leaving older populations behind, which accelerates aging in origin areas. The study also finds that as the percentage of the population lost due to climate migration increases, the median age of the population rises, potentially by up to 10 years in highly impacted coastal counties. The research highlights the importance of considering demographic feedback loops, such as fertility and gravity effects, in climate migration models to fully understand the long-term demographic impacts. The findings suggest that climate migration could significantly alter population distributions and aging patterns, with implications for policy planning and adaptation strategies in both origin and destination areas. The study underscores the need for integrated models that account for multiple climate hazards and demographic processes to better predict and manage the effects of climate migration.