This paper analyses a 15-year long atmospheric CO2 mixing ratio
record measured at a mid-continental, low-elevation station
(Hegyhatsal, Hungary) to reveal the effect of regional climate
variability. While the long-term trend and the temporal
fluctuation of the growth rate of CO2 mixing ratio follow the
global tendencies to a large extent, the shorter-term variations
show special features. We present the distorted seasonal cycle
caused by the seasonality in the atmospheric vertical mixing and
the tendentious change in its shape, which can be attributed to
the gradual warming and to the resulted prolongation of the
growing season. The decreasing summer diurnal amplitude and the
decreasing seasonal amplitude in the mixing ratio, furthermore
the higher than average summer CO2 mixing ratio growth rate in
the first period of the measurements (1994-2003) with generally
rising temperature and decreasing precipitation are explained as
the consequence of the reduced activity of the biosphere in the
influence area of the station and that of the reduced biomass
under environmental conditions getting increasingly
unfavourable. The explanation is supported by the co-located
tall tower surface-atmosphere CO2 exchange measurements and by
the crop yield statistics of the dominantly agricultural region
around the station
