Monitoring carbon dioxide (CO2) for carbon capture, gas pipelines, and storage as well as early detection of CO2 leakage is important to mitigate greenhouse gas emissions and have a high atmospheric concentration for a long lifetime. Moreover, the main cause of the corrosion in natural gas pipelines is owed by CO2. Therefore, real-time and effective CO2 monitoring is essential to improve efficiency, reduce pipeline emissions, and improve the economics of the natural gas industry. In this paper, we propose and experimentally demonstrate a distributed CO2 sensor based on the measurement of evanescent wave absorption by using optical frequency domain reflectometry (OFDR). A coreless fiber is re-coated with tetraethyl orthosilicate (TEOS) through a dip-coating process with well-defined fabrication conditions. Rayleigh scattering OFDR system is optimized to provide high spatial resolution and large dynamic range to trace gas detection. The proposed distributed fiber gas sensor exhibits continuous real-time measurement of CO2 gas concentrations from 5% to 100% calibrated with nitrogen (N2) as a background gas. The results provide confidence that the proposed sensing technology represents a novel paradigm and holds a potential tool for the early detection of CO2 leaks with high sensitivity in a distributed fashion.