What’s the difference between glacier or ice sheet surface mass balance and total mass balance?

Surface mass balance is the difference between the precipitation (rain and snow) that has accumulated on the upper surfaces of glaciers and ice sheets and what has been lost due to melt and eventual runoff and evaporation. It does not include ice lost in the lower margins due to calving and thinning from contact with warm ocean waters. The surface mass balance defines the net mass gain or loss of the ice in contact with the atmosphere only.

Total mass balance is the difference between total mass gains and total mass losses, which includes ice lost in the lower margins due to calving and thinning from contact with warm ocean waters. The total mass balance includes mass gains and losses at the surface in contact with the atmosphere (surface mass balance) as well as the mass gains or losses of the ice in contact with the ocean.