Supplementary data to Wu et al (2023): Calculating global mean surface CO2 abundance from the WMO GAW in situ network

The World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) coordinates high-quality atmospheric greenhouse gas observations globally and provides these observations through the WMO World Data Centre for Greenhouse Gases (WDCGG) supported by Japan. The WDCGG and the National Oceanic and Atmospheric Administration (NOAA) analyse these measurements using different methodologies and site selection to calculate global annual mean surface CO2 and its growth rate as a headline climate indicator. This study applies a third hybrid method (named semi-NOAA) to observations from most WMO GAW stations and 3D modelled CO2 fields from the CarbonTracker Europe (CTE). The semi-NOAA analysis of the GAW observations produces global mean surface CO2 for 1980-2020 that agrees (r=0.999, RMSE=0.058 ppm) with the WDCGG analysis, and is ~0.329 ppm higher than the NOAA analysis and ~0.186 ppm lower (2001-2020) than the semi-NOAA analysis of the CTE global output. However, there is an excellent agreement on the global CO2 growth rate (r>0.903, RMSE<0.192 ppm yr-1, ME<0.025 ppm yr-1) between considered methodologies. The network with inclusion of continental sites improves early detection of biogenic emission change.

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