The Global Extinction Reduction IDL codes compare optical photometry from the twin Gemini North and South Multi-Object Spectrographs (GMOS-N and GMOS-S) against the expected worsening of atmospheric transparency due to global climate change. Data from the Gemini instruments are first reduced by DRAGONS (ascl:1811.002). GER then calibrates them against the Sloan Digital Sky Survey (SDSS) and Gaia G-band catalogs; image rotation and alignment is accomplished via identification of sufficiently-bright stars in Gaia. A simple model of Gemini and their site characteristics is generated, including meteorology, cloudy-fractions, number of reflections, dates of re-coatings modulated by rate of efficiency decay, together with response of detectors and associated zeropoints, and can be compared with the decline of transparency due to rising temperature and associated humidity increase.