Where atmospheric concentration of CO2 is about now. What matters is less the absolute amount (which is bad enough, with preindustrial being 288 ppm), but the change.
These data come with the following qualifying statement:
Data shown may be measurements of air collected approximately weekly in glass containers and returned to GMD for analysis or averages from air sampled semi-continuously at a GMD baseline observatory. Circle Symbols are thought to be regionally representative of a remote, well-mixed troposphere. + Symbols are thought to be not indicative of background conditions, and represent poorly mixed air masses influenced by local or regional anthropogenic sources or strong local biospheric sources or sinks. A smooth curve and long-term trend may be fitted to the representative measurements when sufficient data exist. Data shown in ORANGE are preliminary. All other data have undergone rigorous quality assurance and are freely available from GMD, CDIAC, and