Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2A nonlinearresponse of ice-shelfmelting to increasingocean temperaturesis a central tenetin the scenariofor abrupt climatechange arising fromocean–ice-shelfinteraction.of warm water in the vicinity of an ice shelf isa necessary condition for high melting, but it isnot sufficient by itself. Additional factors suchas the details of the bathymetry can be equallyimportant, as for example, a submarine sill canblock access of warm waters while a submarinecanyon can facilitate the exchange of warm,deep waters into a cavity beneath an ice shelf.Recent years have seen an increase in the collectionof bathymetric data around the Greenlandand Antarctic continental shelves, and in someinstances even beneath the ice shelves.4.1.2 Ice-Pump CirculationThe manner in which ocean waters circulatebeneath an ice shelf has loosely becomeknown as the “ice-pump” circulation (Lewisand Perkins, 1986). The circulation can bevisualized as dense, salty water (either cold orwarm), entering an ice shelf cavity and flowingtoward the back of the cavity, to the groundingline where the ice shelf first goes afloat on theocean. Here at the grounding line, the ice shelfis at its greatest thickness. Because the freezingpoint of seawater decreases as ocean depthincreases, the invading ocean waters have anever increasing thermal head with respect tothe ice as the depth of the ice increases. Thethermal head determines the amount of meltingat the grounding line. An end result of meltingis a cooled and freshened ocean water mass atthe grounding line. An empirical consequenceof the equation of state for seawater is that thiswater mass will always be less dense than thesource waters that originally fed into the iceshelfcavity. These light waters subsequentlyflow upward along the ice-shelf base as akind of upside-down gravity current, a flowfeature termed a plume. As the waters rise, thedepth-dependent freezing point also rises, andat some point the rising waters can actuallybecome supercooled with respect to the localfreezing point. In this instance some of themeltwaters refreeze to the base of the ice shelf,forming so-called marine ice, in contrast to themeteoric ice (also called snow/ice) that feedsthe ice shelf from the inland ice sheet. It is themanner in which ocean waters can melt the deepice and refreeze ice at shallow depths that hasgiven rise to the term “ice pump.” In the case ofwarm waters in the cavity beneath the ice shelf,the term ice pump is a misnomer, as there maybe no refreezing of ice whatsoever, just melting.These under-ice circulation processes areclearly important to the stability of ice shelvesor ice tongues, but it is difficult to yet predicttheir impact on Antarctica and Greenland in thecoming decades. Future changes in ocean circulationand ocean temperatures will producechanges in basal melting, but the magnitudeof these changes is currently not modeled orpredicted.4.2 Ice-Shelf Processes4.2.1 Ice-Shelf Basal MeltingA nonlinear response of ice-shelf melting toincreasing ocean temperatures is a central tenetin the scenario for abrupt climate change arisingfrom ocean–ice-shelf interaction. The nonlinearresponse is a theoretical and computationalresult; observations are yet inadequate to verifythis conclusion. Nonetheless, the basis of thisresult is that the melt rate at the base of an iceshelf is the product of the thermal head and thevelocity of the ocean waters at the base. Thegreater the thermal head or the velocity, thenthe greater the melt rate. A key insight fromthe theoretical and modeling research is that asthe ocean water temperature is increased, thebuoyancy of the plume beneath the ice shelf isincreased because greater melting is initiated bythe warmer waters. A more buoyant plume risesfaster, causes greater melting, and becomesmore buoyant. This positive feedback is a keynonlinear response mechanism of an ice-shelfbase to warming ocean waters.The susceptibility of ice shelves to high meltrates and to collapse is a function of the presenceof warm waters entering the ice-shelfcavities. But the appearance of such warmwaters does not actually imply that the globalocean needs to warm. It is true that observationalevidence (Levitus et al., 2000) doesindicate that the ocean has warmed over the pastdecades, and that the warming has been modest(approximately 0.5 °C globally). While this isone mechanism for creating warmer watersto enter a cavity beneath the ice shelf, a moreefficient mechanism for melting is not to warmthe global ocean waters but to redirect existingwarm water from the global ocean towardice-shelf cavities; however, ocean temperature58