Large fluxes of CO₂ from sea ice into the atmosphere have recently been reported. The driving forces behind these fluxes have, however, still remained a mystery. Here we propose a simple model of the carbonate system in sea ice to explain the high partial pressures of CO₂ (pCO₂) that can drive fluxes from sea ice to the atmosphere. The pCO₂ in sea water of a fixed composition decreases with temperature. Brine salinity is dependant on temperature: the lower the temperature the higher the salinity. Brine temperature (Tsubb) and brine salinity (Ssubb) are described by the freezing point relation Tsubb = Tsubf(Ssubb). The increase in salinity is associated with an increase in dissolved inorganic carbon (DIC) and total alkalinity (TA). High pCO₂ values (>1000 muatm at salinity Ssubb > 70) are caused most of all by the variation of equilibrium constants with salinity and to a lesser extent by the joint increase of DIC and TA. Increasing TA and decreasing temperature both have the opposite effect on pCO₂. Further increase of pCO₂ may be achieved by the precipitation of carbonate minerals.