  In this work we present the results of a spectral observing campaign of Pluto
to search for temporal changes in surface composition on 1- to 3-year
timescales. Near-infrared spectra of Pluto were obtained from June 2014 to
August 2017 with the TripleSpec cross-dispersed spectrograph at the Apache
Point Observatory's 3.5-meter Astrophysical Research Consortium (ARC)
telescope. Observations were requested in order to obtain spectra of
approximately the same sub-observer hemisphere $\sim$14 months apart, thus
removing the effects of viewing geometry and rotation phase. Comparison of the
CH$_4$ (methane) band areas and band center shifts between each component of
these "matched pairs" revealed a surface in transition. Band areas for the 1.66
and 1.72 $\mu$m CH$_4$ absorption features exhibited a $>$5-$\sigma$ increase
between 2014-06-17 and 2015-08-19, corresponding to a sub-observer hemisphere
centered at $\sim$280$^{\circ}$E, with the latter date only 1 month after the
New Horizons flyby of Pluto. The majority of matched pairs were obtained of the
anti-Charon hemisphere, home to the bright, volatile-rich Sputnik Planitia, and
did not present statistically significant changes in CH$_4$ band areas. CH$_4$
band center shifts, which provide information on the mixing state of CH$_4$ and
N$_2$ in solid solution, were calculated between components of each matched
pair, with no significant band shifts detected. The favored explanation for
these combined results is the sublimation of more-volatile N$_2$ from the
northern latitudes of Pluto in the lead-up to northern hemisphere summer
solstice in 2029, leading to an increase in CH$_4$ concentration.
