Abstract
<title>Abstract</title> <p>International trade records name an exporter and an importer, but the supply chains behind a traded product often draw on production in other countries. Here we quantify third-country carbon: CO₂ emissions generated outside both trade partners, but required by the supply chain that links them. Using the Eora multi-region input-output table, we trace these emissions for 189 economies, 26 sectors and 35,532 directed trade routes from 2000 to 2021. Third-country carbon reached 1.85 Gt CO₂ in 2021, or 27.7% of trade-embodied emissions. After rapid growth from 2000 to 2011, this third-country layer persisted at a high level. It is strongly directional, spread across many trade routes, concentrated in a few upstream production sources and rooted in energy and material sectors. Structural decomposition shows that improvements in carbon coefficients reduced third-country carbon by about 2.0 Gt CO₂ over 2000–2021, whereas trade expansion and production-network restructuring added about 1.85 Gt and 0.91 Gt. These findings shift trade-carbon accounting from endpoint responsibility towards path and source-based responsibility, with implications for carbon border policies, product disclosure and supply-chain decarbonisation.</p>