The disclosure relates to enhancing alkalinity of brine, e.g. seawater, using bipolar membrane electrodialysis (BPMED) without removing divalent cations that otherwise cause scaling. In one embodiment, a BPMED is employed wherein the brine volumetric flow rate through a basification compartment is greater at a given current density than that through a brine compartment which increases the pH of the brine output while keeping it below the precipitation pH. In one embodiment, the spacer located in the basification compartment is thicker than spacers elsewhere in the BPMED so as resist membrane distortion due to the increased hydrostatic pressure in the basification compartment given the greater volumetric flow. The brine output having increased alkalinity can be returned to the ocean to mitigate acidification and enable capture of atmospheric carbon dioxide.