The major concern is the slowing or stopping of the AMOC, the major deep water current that runs through the atlantic. Since one of the only places on earth that can make very cold and salty deep water is off the coast of greenland, and the glacier on greenland have been melting, there is much concern that this freshwater run off will cause the water near greenland to be more bouyant, causing deep water circulation to slow down or halt altogether. The consequences of this are speculative but none of them are good, as the AMOC brings important oxygen to deep water and slowing that down could cause the deep ocean to get progressively more oxygen depleted and acidic.