First global measurement of midtropospheric CO₂ from NOAA polar satellites: Tropical zone

Midtropospheric mean atmospheric CO₂ concentration is retrieved from the observations of the NOAA series of polar meteorological satellites, using a nonlinear regression inference scheme. For the 4 years of the present analysis (July 1987 to June 1991 , monthly means of the CO₂ concentration retrieved over the tropics (20°N to 20°S) from NOAA 10 show very good agreement with what is presently known. Not only the phase of the seasonal variations (location of the peaks) but also their amplitude and their latitudinal evolution match quite well recent in situ observations made by properly equipped commercial airliners measuring in an altitude range similar to the one favored by the satellite observations. Moreover, the annual trend inferred corresponds to the known increase in the concentration of CO₂ as a result of human activities. Also, the impact of El Niño-Southern Oscillation events is clearly seen and confirms analyses of in situ or aircraft observations and of model simulations. Forty-eight maps of monthly mean midtropospheric CO₂ concentration have been produced at a resolution of 15° × 15°. A rough estimate of the method-induced standard deviation of these retrievals is of the order of 3.6 ppmv (around 1%). The coming analysis of the almost 25 years of archive already accumulated by the NOAA platforms should contribute to a better understanding of the carbon cycle. A simulation of the extension of the method to the next generation high-spectral-resolution instruments, with very encouraging results, is presented.