Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.2.3.4 Summary of Changes in WMGHGs ====

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In summary, CO <sub>2</sub> has fluctuated by at least 2000 ppm over the last 450 Myr ( ''medium confidence'' ). The last time CO <sub>2</sub> concentrations were similar to the present-day was over 2 Ma ( ''high confidence'' ). Further, it is certain that WMGHG mixing ratios prior to industrialization were lower than present-day levels and the growth rates of the WMGHGs from 1850 are unprecedented on centennial timescales in at least the last 800 kyr. During the glacial-interglacial climate cycles over the last 800 kyr, the concentration variations of the WMGHG were 50–100 ppm for CO <sub>2</sub> , 210–430 ppb for CH <sub>4</sub> and 60–90 ppb for N <sub>2</sub> O. Between 1750–2019 mixing ratios increased by 131.6 ± 2.9 ppm (47%), 1137 ± 10 ppb (156%), and 62 ± 6 ppb (23%), for CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O, respectively ( ''very high confidence'' ). Since 2011 (AR5) mixing ratios of CO <sub>2</sub> , CH <sub>4</sub> , and N <sub>2</sub> O have further increased by 19 ppm, 63 ppb, and 7.7 ppb, reaching in 2019 levels of 409.9 (± 0.4) ppm, 1866.3 (± 3.3) ppb, and 332.1 (± 0.4) ppb, respectively. By 2019, the combined ERF (relative to 1750) of CO <sub>2</sub> , CH <sub>4</sub> and N <sub>2</sub> O was 2.9 ± 0.5 W m <sup>–2</sup> (Table 2.2; Section 7.3.2).

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