Global Extinction: Gemini North and South GMOS Combined Photometry Relative to the Gaia Catalog, and Long-term Atmospheric Change

Effects of long-term atmospheric change were looked for in photometry employing Gemini North and South twin Multi-Object Spectrograph (GMOS-N and GMOS-S) archival data. The whole GMOS imaging database, beginning from 2003, was compared against the all-sky Gaia object catalog, yielding ~10⁶ Sloan $r^{\prime} $ -filter samples, ending in 2021. These were combined with reported sky and meteorological conditions, and versus a simple model of the atmosphere plus cloud together with simulated throughputs. One exceptionally extincted episode in 2009 is seen, as is a trend (similar at both sites) of about 2 mmag worsening attenuation per decade. This is consistent with solar-radiance transmissivity records going back over six decades, aerosol density measurements, and more than 0.2°C/decade rise in air temperature, which has implications for the calibration of historic data sets or future surveys.