In 1997, the Kyoto Protocol introduced carbon trading using a single global metric: carbon dioxide equivalence (CO₂e), based on its Global Warming Potential over 100 years (GWP100). This standard equates the warming effect of various greenhouse gases to that of CO₂ over a century. Since then, one tonne of CO₂e (tCO₂e) has remained the cornerstone of global carbon accounting.