This market resolves as the year in which the global montly mean CO2 level as reported by NOAA first reaches 450 ppm or more.

I will use the data as reported here to resolve this market. Note that calculating the global monthly mean takes a while; as of July 2023, the most recent data is from April 2023.

The current highest global monthly mean CO2 level is 420.54 ppm, as measured in April 2023. Given that global CO2 levels are increasing by about 2 ppm/year, we might reach 450 ppm by 2040. If we don't reach 450 ppm by 2100, this market resolves N/A.

A measured value of 449.99 ppm is not enough to cause a YES resolution, but a measured value of 450.00 ppm is.