The Mauna Loa Observatory deals with atmospheric change, and according to NOAA (the National Oceanic and Atmospheric Administration) it is the oldest CO2 monitoring station in the world. The observatory is closed to the public.
At the time of writing the trails to Mauna Loa's summit are closed due to volcanic activity. Before planning a hike, visitors should check the national park website here: nps.gov/havo/planyourvisit/hike_maunaloa.htm