Greenhouse-Gas Emissions from Public Transport in Urban Environments: A Systematic Review of Quantification Methods, Technologies, and Decarbonization Pathways

Public transport is central to urban decarbonization, yet its greenhouse-gas (GHG) performance varies with transport mode, propulsion technology, energy source, passenger utilization, route and terrain conditions, operating context, and accounting boundary. This systematic review synthesized worldwide quantitative evidence on GHG emissions attributable to public passenger transport. Scopus, Web of Science, and Transportation Research International Documentation (TRID) were searched through 9 September 2026. Primary quantitative studies reporting public-transport-specific GHG, CO2, CO2e, or carbon-footprint outcomes were eligible. Of 1616 records identified, 93 reports representing 92 underlying studies were included following PRISMA 2020 screening. Life-cycle assessment was the most common study design (42 studies), followed by direct measurement (33) and modeling/simulation (13). Buses dominated the evidence base (81 studies), and battery-electric or electricity-based pathways were examined in 45 studies. Sixty-nine reports provided a defensible principal quantitative value. Among 56 reports with an explicit comparator direction, 55 reported lower emissions for the evaluated public-transport option. Results were not pooled because functional units, system boundaries, technologies, energy pathways, and operating contexts differed substantially. Methodological confidence was high for 21 studies and moderate for 71. Overall, decarbonization performance was conditional on energy supply, utilization, infrastructure, and life-cycle assumptions, indicating that public-transport technologies should be evaluated as components of context-dependent transport-energy systems rather than as inherently low-carbon solutions. This review was not prospectively registered.