Carbon dioxide (CO2) is the primary greenhouse gas contributing to the enhanced greenhouse effect. Long-term observations of its atmospheric concentration represent an useful tool to evaluate the success of mitigation activities vs climate change. We present and discuss continuous long-term CO2 observations carried out at the very high mountain background station Plateau Rosa (Code: PRS, 45.93°N, 7.71°E, 3480 m a.s.l., North Western Alps, Italy). The PRS station has collected atmospheric concentrations of CO2 since 1989.

The station is located at a typical elevation of the free atmosphere upon a large snow–clad bare mountain plateau and far from urban and polluted zones. Owing to its high altitude and position, it is suitable for the background measurement of greenhouse gases. The PRS CO2 dataset currently covers about 27 complete years: the last 22 years, from 1993 up to now, are related to continuous monitoring.

The historical series of the monthly CO2 background concentrations was obtained by applying a filtering scheme, able to distinguish the background data from the data influenced by CO2 sources and sinks (for example, activities of photosynthesis). The growth rate is equal to 1.99 ± 0.04 ppm/year, and is comparable with the global trend (1.9 to 2.1 ppm/year) computed for the years 2002–2011 by IPCC and 2.02 ppm/year (for the last decade) reported in the WMO Greenhouse Gas Bulletin. The average value of the peak-to-peak annual cycle amplitude is 10.4 ± 0.9 ppm.