68 
For 2021 (the most recent year for which ﬁnal numbers are available) concentrations using the same observational products and methods as in AR6 WGI are: 415 ppm CO2; 
1896 ppb CH4; and 335 ppb N2O. Note that the CO2 is reported here using the WMO-CO2-X2007 scale to be consistent with WGI. Operational CO2 reporting has since been 
updated to use the WMO-CO2-X2019 scale.
the past six decades, with regional differences (high conﬁdence). In 2019, 
atmospheric CO2 concentrations reached 410 parts per million (ppm), CH4 
reached 1866 parts per billion (ppb) and nitrous oxide (N2O) reached 332 ppb68. 
Other major contributors to warming are tropospheric ozone (O3) and 
halogenated gases. Concentrations of CH4 and N2O have increased to 
levels unprecedented in at least 800,000 years (very high conﬁdence), 
and there is high conﬁdence that current CO2 concentrations are 
higher than at any time over at least the past two million years. Since 
1750, increases in CO2 (47%) and CH4 (156%) concentrations far 
exceed – and increases in N2O (23%) are similar to – the natural 
multi-millennial changes between glacial and interglacial periods over at 
least the past 800,000 years (very high conﬁdence). The net cooling effect 
which arises from anthropogenic aerosols peaked in the late 20th century 
(high conﬁdence). {WGI SPM A1.1, WGI SPM A1.3, WGI SPM A.2.1, 
WGI Figure SPM.2, WGI TS 2.2, WGI 2ES, WGI Figure 6.1}