claim: It’s usually taken to be the fact that as carbon dioxide concentrations in the atmosphere increase, the 1 per cent of CO2 that’s the heavier carbon isotope ratio c13 declines in proportion.
evidence: Increases in atmospheric concentrations of CO 2 and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have correspondingly strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century.