And here's the explanation:
Isolated in the middle of the Pacific Ocean and at over 11,000 feet above sea level, the upper north face of Mauna Loa volcano is an ideal location to make measurements of atmospheric carbon dioxide that reflect global trends, not local influences such as factories or forests that might boost or drop carbon dioxide within their vicinity. The CO2 sensors at Mauna Loa are positioned such that they sample an incoming breeze direct from the ocean, unaffected by human activities, vegetation or other factors on the island. (The Mauna Loa Observatory is high enough that the incoming breeze rides above the thermal inversion layer.)
Volcanoes are considerable sources of carbon dioxide themselves. However, the sampling location was chosen to be normally upwind of Mauna Loa's vent, and Keeling perfected methods for detecting and correcting intervals when the wind blew the wrong way.
Measurements at about 100 other sites have confirmed the long-term trend shown by the Keeling Curve, although no sites have a record as long as Mauna Loa.