Abstract Upper atmospheric long‐term trends have been characterized through the analysis of the ionospheric ion temperature (Ti). Previous studies used Ti observations from various incoherent scatter radar (ISR) facilities located at different latitudes. In this paper, we analyze Arecibo Observatory's (AO) ISR (18°20’N, 66°45’W) data sets from 1985 to 2019 to detect Ti long‐term trends as a function of altitude from ∼140 to ∼677 km. We empirically modeled the responses of Ti to the known forcings of solar activity, geomagnetic activity, and the annual and semi‐annual oscillations. The Ti trend is determined through least squares fitting to the residuals of the Ti, which were estimated by removing the empirically modeled Ti from the observed Ti. Since the ions and neutrals are closely coupled, our results indicate that the upper atmosphere/ionosphere over Arecibo has been cooling over the 35 years studied. Above 350 km, a latitudinal dependency is seen by comparison of all ISR estimated Ti trends, which agrees with the earlier reported results. These observed cooling trends exceed the magnitude expected by the modeling studies from increased greenhouse gas (GHG) concentrations. These excess coolings are as high as −1.2 K/year below 320 km altitude, where an increase in GHG dominates. Nighttime cooling trends in the altitude of ∼320–400 km might be caused by the increasing GHG concentrations and magnetic field variations since the trends of AO‐ISR match with the Whole Atmosphere Community Climate Model eXtension simulations.
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