Sea ice is a geophysically important material that bears some similarity to the partially molten mantle. Newly formed sea ice contains a significant amount of salt as liquid brine in the interstices of a permeable ice matrix through which the brine can flow. Compositional convection drives fluid motion within porous sea ice. The convective circulation leads to the development of so-called 'brine channels,' liquid channels formed by a dissolution reaction. I discuss the physical mechanisms that create and sustain these channels. Some aspects of these mechanisms may apply to reactive channelization of magmatic flow through the mantle beneath mid-ocean ridges, for example.

Salt fluxes through brine channels are an important buoyancy forcing for the polar oceans, with implications for ocean mixing and deep-water formation. I develop a simple, semi-analytical Chimney-Active-Passive (CAP) model of convection in sea ice. I investigate the physical controls on the convective velocities, brine channel size, and salt fluxes. I test my predictions by considering previous laboratory observations of the propagation of dye fronts within the ice and salt fluxes from it. Finally, I take a one-dimensional, thermodynamic sea-ice model of the kind currently used in coupled climate models and parameterize porous convection within a one-dimensional model. The parameterization allows dynamic determination of physical properties and salt fluxes from sea ice. I argue that existing models are likely to overestimate substantially the peak buoyancy forcing of the polar oceans.