Resolution according to CO2 measurements in ppm and the calculation of annual means from Global Monitoring Laboratory at Mauna Loa Observatory.

Graphs of historic CO2 measurements at Mauna Loa are here:

https://gml.noaa.gov/ccgg/trends/mlo.html

The data for historic annual means are here:

https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_annmean_mlo.txt

Mean data will be rounded up or down for resolution.