The increase in global temperature recorded since the end of the 19th century is associated with an increase in the atmospheric greenhouse gases concentration, especially with the fossil fuel produced CO2.Apart from the anthropogenic influence, the concentration of atmospheric CO2 is affected by natural processes such as photosynthesis and soil respiration, the sea-atmosphere CO2 exchange, and natural seasonal fluctuations. The proposed project will attempt to determine the sources and sinks of CO2 in the atmosphere using the isotopes of carbon, radioactive 14C and stable 13C (relative to 12C, ?14C, ?13C). Isotope 14C is a tracer of the atmospheric CO2, and is a unique way of determining carbon of fossil origin since fossil fuels do not contain 14C and their influence decreases ?14C in the atmospheric CO2. However, it is not possible to quantify all contributions from natural processes using ?14C alone, which affects the calculation of the fossil contribution. Therefore, ?13C is additionally used as a tool.The aim of the project is to monitor the share of CO2 from different sources in the atmosphere. For this purpose, concentration, ?14C and ?13C of atmospheric CO2 will be monitored in Croatia at 2 locations with a continental climate (urban Zagreb and ''clean'' Puntjarka, Medvednica mountain) and 3 locations with a Mediterranean/mountainous climate (urban Rijeka and ''clean'' Porer cliff near Medulin and Parg, Risnjak mountain). Additionally, ?14C and ?13C of the sea dissolved inorganic carbon (DIC) at Porer and in Rijeka will be monitored.Sampling at Porer cliff is particularly significant because data on ?14C and ?13C above sea surface CO2 and of DIC are very rare on a global scale. We expect that the project will be interesting for everyone involved in the carbon cycle and environmental research and protection. The project plans to employ a postdoctoral fellow and a number of disseminations, such as final workshop.