Climate change mitigation and carbon neutrality have become global priorities, necessitating the reduction of greenhouse gas emissions in the livestock sector. However, the practical implementation of direct methane measurement is limited by high recording costs and the need for specialized equipment. This study investigated carcass weight by age (CWA) as a biologically meaningful proxy trait for carbon emission intensity in Hanwoo cattle. A reference population of 25,964 and a KPN population of 1,444 animals genotyped with the Hanwoo 50K SNP chip were utilized for genetic evaluation. The CWA-derived proxy trait was defined as the inverse of CWA (CWA⁻¹), representing the production time required per unit of carcass output. Genetic parameters and genomic estimated breeding values (GEBV) were estimated using a single-step genomic BLUP (ssGBLUP) framework. The heritability of CWA⁻¹ was estimated at 0.343, which is comparable to or higher than conventional carcass traits such as carcass weight (0.327) and marbling score (0.291). Mean genomic prediction accuracy reached 0.783 in the reference population and 0.699 in the KPN population. Genome-wide association analysis identified significant genomic regions on chromosomes 4, 6, and 14, with candidate genes including TOX, CALCR, and LCORL associated with growth and production efficiency. Genetic trend analysis showed a consistent decrease in mean GEBV over birth years, indicating a favorable genetic tendency toward improved efficiency and reduced emission intensity. Incorporating the CWA⁻¹ trait into the selection index demonstrated that environmental improvement can be pursued without sacrificing conventional genetic gain. These results suggest that CWA-derived traits serve as practical and candidate proxy trait for achieving both production efficiency and environmental sustainability in Hanwoo cattle.