Resolution according to CO2 measurements in ppm and the calculation of the annual mean (rounded up or down) from Global Monitoring Laboratory at Mauna Loa.
I will resolve to 529 if the market trades for more than a week at 529 (the upper end of the range) or to 480, if it trades for more than a week at 480 (the lower end of the range). I will then create a new market with a more appropriate range.
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
Similar markets:
I'm surprised not more people are betting max (530). Going by this article, even under the RCP4.5 scenario (green line), the 2100 CO2 ends up at ~530, and there's plenty of scenarios where it's much higher.