I will check,but I think that mainly they aree measuring them from trapped air in ice core (one link). CO 2is a greenhouse gas, that means it absorbs heat radiation at night, when earth gives heat up into the universe, and thus trapping additional heat at earth. CO2 levels today are either measured with mass spectrometers, by non-dispersive IR absorption, by Laser absorption or RAMAN scattering of Lasers; Mass spectrometer detectors usually have a dynamic range from one count/sec up to some billion counts per second; so are the most accurate ones, followed by laser absorption and NDIR methods, which have around 0.1 ppm accuracy. Problems as is with all measurements is to do proper sampling and prevent some kind of bias concerning sample or sampling inlet (means you should not measure CO2 near the road or cement production site; most popular backgroiund measurement sites have to b outsite heavy populated areas (like Finland, Canada, Hawaii etc.) I think some satellite of the ESA or NASA has gone online last year to make direct measurements of a whole variety of gases over the whole globe.