Regulators are requiring that all the swipes, taps, and clicks be followed by their carbon cost, not approximations, but as accurate as a line in a balance sheet. This research presents the initial payment gateway to be able to append a carbon score to every ISO 8583 authorization and observe the speed and protection limits of card networks. The score is computed in parallel with routing of transactions; a Carbon Index Service (CIS), running together with the fraud filter, dynamically adapts to the instantaneous level of grid intensity and delivery mode (e-commerce versus in-person). Our analysis of a high-volume transaction dataset (Q4 2024) indicates that the system creates low latency (mean: 19 ms; 95th percentile [P95]: 36 ms) that is much less than 150 ms. The estimating error stays at 4.3% of the certified lifecycle data, whereas the availability of the system stays at 99.99%. The improved transaction data can be used to enable immediate climate receipts, machine-readable ESG (Environmental, Social, and Governance) reporting, and impact-incentive mechanisms that can guide consumers in selecting lower-impact options, without impeding the speed of settlement or protection of consumer data. These are demonstrations that real-time carbon accounting may be deployed on a national level measurement of the size of payments data sets, turning the idea of sustainability into an automatic component of the payments sector.