The carbon burden of migration: Inequality between and within cities

China’s large-scale cross-regional population migration has profoundly reshaped regional economic development and energy consumption patterns, thereby intensifying carbon intensity inequality across spatial dimensions. Within China's dual control framework for carbon emissions, understanding the mechanisms underlying carbon intensity inequality carries critical policy implications; however, the role of population migration in driving such inequality remains underexplored. Using panel data from 274 prefecture-level cities over 2008–2017, this study examines how population migration influences carbon intensity inequality through dual perspective of between cities and within cities, while unraveling its operational mechanisms. The findings demonstrate that population migration intensifies carbon intensity inequality both between and within cities, with amplified effects observed in low urban hierarchy, regions west of the Hu-Huanyong Line, and ordinary environmental protection status cities. Mechanistically, population migration exacerbates inequality indirectly through labor market reallocation and technological innovation, while environmental regulation counterintuitively amplifies this effect. Furthermore, significant spatial spillover effects emerge: population inflows into focal cities exacerbate intra-city carbon intensity inequality in neighboring cities. These findings advance our understanding of how migration shapes environmental inequality and provide important policy insights for designing spatially differentiated carbon reduction strategies.