  We present an analysis of all currently available ground-based imaging of
Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal
changes in Neptune's mid-infrared ($\sim 8-25\mu$m) emission over time in the
years surrounding Neptune's 2005 southern summer solstice. Images sensitive to
stratospheric ethane ($\sim12\mu$m), methane ($\sim8\mu$m), and CH$_3$D
($\sim9\mu$m) display significant sub-seasonal temporal variation on regional
and global scales. Comparison with H$_2$ S(1) hydrogen-quadrupole
($\sim17.035\mu$m) spectra suggests these changes are primarily related to
stratospheric temperature changes. The stratosphere appears to have cooled
between 2003 and 2009 across multiple filtered wavelengths, followed by a
dramatic warming of the south pole between 2018 and 2020. Conversely,
upper-tropospheric temperatures -- inferred from $\sim 17-25$-micron imaging --
appear invariant during this period, except for the south pole, which appeared
warmest between 2003 and 2006. We discuss the observed variability in the
context of seasonal forcing, tropospheric meteorology, and the solar cycle.
Collectively, these data provide the strongest evidence to date that processes
produce sub-seasonal variation on both global and regional scales in Neptune's
stratosphere.
