Correlations between geomagnetic-field and climate parameters have beensuggested repeatedly, but possible links are controversially discussed.Here we test if weak (Earth-strength) magnetic fields can affectclimatically relevant properties of seawater. We found the solubility ofair in seawater to be by 15\% lower under reduced magnetic-field (20 muT) compared to normal field conditions (50 mu T). The magnetic-fieldeffect on CO(2) solubility is twice as large, from which we surmise thatgeomagnetic field variations modulate the carbon exchange betweenatmosphere and ocean. A 1\% reduction in magnetic dipole moment mayrelease up to ten times more CO(2) from the surface ocean than isemitted by subaerial volcanism. This figure is dwarfed in front ofanthropogenic CO(2) emissions.