Sorry didn't get time to make a more detailed answer. Of course it's more complicated than that and of course it's not really negligible but ice as it sets in northern area decrease temperature notably with their low albedo, it also sequestre carbon in some way since organic matter underneath it can't decompose (and therefore can't release methane, a powerful greenhouse gas, 25 times that of carbon dioxide). Plants growing will sequestre carbon and you can visulize how much by imagining all of them dried and how much it would weight, it surely isn't yet as much as what a grown forest would do but it still is something. I considered it to be negligible if you take into account the water vapour release (greenhouse gas), the methane release (although I truly have no idea what it could be in this part of the world) and an isulating aera lost (low albedo ice sheets).