Evolution of measured radiances at 75°N (top panel) and 70°S (bottom panel) from 2007 (northern winter) to 2016 (mid-spring). The sharp variation in the radiance between 228 cm^-1 and 231 cm^-1 in the spectra measured in August 2014 is a spurious noise feature which could not be eliminated during the calibration process. From 2007 to 2016, radiance at high southern latitudes has strongly increased, whereas it has decreased at high northern latitudes.