Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeBox 2.2. Mass Balance, Energy Balance, and Force BalanceThe glaciological analyses which we summarize here can all be understood in terms of simple arithmetic.To determine the mass balance, we add up all the gains of mass, collectively known as accumulation and dominatedby snowfall, and all the losses, collectively known as ablation and dominated by melting and calving. The differencebetween accumulation and ablation is called, by long-established custom, the total mass balance, althoughthe reader will note that we really mean “mass imbalance.” That is, there is no reason why the difference shouldbe zero; the same is true of the energy balance and force balance.The mass balance is closely connected to the energy balance. The temperature of the glacier surface is determinedby this balance, which is the sum of gains by the absorption of radiative energy, transfer of heat from theoverlying air, and heat released by condensation, and losses by radiative emission, upward transfer of heat whenthe air is colder than the glacier surface, and heat consumed by evaporation. A negative energy balance meansthat the ice temperature will drop. A positive energy balance means either that the ice temperature will rise orthat the ice will melt.Ice deformation or dynamics is the result of a balance of forces, which we determine by arithmetic operationscomparable to those involved in the mass and energy balances. Shear forces, proportional to the product ofice thickness and surface slope, determine how fast the glacier moves over its bed by shear deformation wherethe ice is frozen to the bed, or by basal sliding where the bed is wet. Spreading forces, determined by ice thickness,are resisted by drag forces at the glacier bed and its margins, and by forces transmitted upstream from itsfloating tongue or ice shelf as this pushes seaward past its margins and over locally shoaling seabed. The sum ofthese forces determines the speed at which the ice moves, together with its direction. However, we must alsoallow for ice stiffness, which is strongly affected by its temperature, with cold ice much stiffer (more sluggish)than ice near its melting point.The temperature becomes still more important when we consider basal drag, which is high for a dry-based glacier(one frozen to its bed), but can be very small for wet-based glaciers where their beds have been raised to themelting point by heat conducted from the Earth’s interior and frictional heat generated on the spot. Once thebed is at the melting point, any further gain of heat yields meltwater. One of glaciology’s bigger surprises is thatlarge parts of the ice sheets, whose surfaces are among the coldest places on Earth, are wet based.The varying pressure of basal meltwater on the moving ice can alter the force balance markedly. Its general impactis to promote basal sliding, by which mechanism the glacier may flow much more rapidly than it would by sheardeformation alone. Basal sliding, in conjunction with the presence of a porous reservoir for meltwater wherethe bed consists of soft sediment rather than rock, plays a major role in the behavior of ice streams.There are subtle links between the mass balance and the force balance. The ice flows from where there is netaccumulation to where there is net ablation, and the changing size and shape of the glacier depend on the interplayof dynamics and climate, the latter including the climate of the ocean.45