  Effects of long-term atmospheric change were looked for in photometry
employing the 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 ~10^6 Sloan
r'-filter samples, ending in 2021. These were combined with reported sky and
meteorological conditions, 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 deg C per decade rise in global air temperature, which has
implications for calibration of historic datasets or future surveys.
