Carbon cycle involves exchanges between four reservoirs : oceans, biosphere, lithosphere and atmosphere. The natural cycle of these exchanges is perturbed by human activities, and a good understanding of the carbon cycle is needed to produce realistic scenarii for futur changes.

The LSCE has a unique approach combining studies of the carbon cycle in the different reservoirs for past , present , and future time, and numerical simulation as well as direct observations.

For instance , the Ramces team develops and manages a network of about 15 sites of atmospheric measurements distributed around the world. Major greenhouse gases like CO2 (carbon dioxide) and CH4 (methane) are investigated, but also carbon isotopes like 13C and 14C .

Very high precision measurements allow to detect changes in atmospheric concentration of the different gases. Measurements are also used to validate atmospheric models, which in turn allow to interpret measured concentration changes.

Several researchers develop at the LSCE inverse models which calculate from measured concentrations CO2 fluxes emitted or absorbed at the surface of continents and oceans. Satellite measurements are now  incorporated in these models.

The LSCE develops observations and models of the ocean role in the carbon cycle ,and in particular of the coupling with the biogeochemical cycles involved in carbon exchanges (role of zoo and phytoplankton...). The "Geochimie tropospherique" team organizes expeditions for measuring oceanic emissions of carbonated species (volatil organic components, carbon monoxide).

Modelisation of ocean acidification and future evolution of oceans is also developed.

Biosphere through photosynthesis and respiration plays a major role in the carbon cycle. Biospheric models of the LSCE are used to calculate the time evolution of different  vegetation types  (forest, meadow, agriculture..)

LSCE is also a recognized leader for its climate-carbon simulations. These complex models provide responses to questions about the future equilibrium for exchanges between ocean and atmosphere, in the framework of future climatic changes.

Paleoclimatic researches also involve the carbon cycle. Past atmospheric CO2 and CH4 can be measured in air bubbles trapped in ice cores. Models study the role of CO2 in particular during glacial - interglacial transitions.