Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentrations are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air. (Figure source: Forster et al.2007).
Image from the National Climate Assessment. Source: [https://nca2014.globalchange.gov/highlights/report-findings/our-changing-clim... uploaded in CLEAN:Teaching Climate and Energy:Teaching Climate.
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