Ice Sheets and Sea Level Rise: Model Failure is the Key Issue

Guest post by Michael Oppenheimer, Princeton University

A plethora of research articles has appeared over the past year reporting new observations of the Greenland and West Antarctic ice sheets along with associated modeling results. RealClimate has reviewed the issues raised by these articles and attempted to clarify the sometimes conflicting inferences about the current mass balance of the ice sheets, as well as their future contributions to global mean sea level rise (see here and here).

Nevertheless, the issue still seems to perplex many journalists and others because there are two entirely distinct aspects of the sea level rise problem that are emphasized, depending on which scientists are speaking. On the one hand, these ices sheets are large enough to ultimately raise sea level by 7m and about 5m, for Greenland and West Antarctica, respectively. On the other, the recent observations that caused such a stir report a current contribution to the rate of sea level rise not exceeding ~1mm/yr from both ice sheets taken together. If this rate were maintained, the ice sheets would make a measurable but minor contribution to the global sea level rise from other sources, which has been 1-2mm/yr averaged over the past century and 3mm/yr for 1993-2003, and is projected to average 1-9mm/yr for the coming century (see IPCC Third Assessment Report).

The key question is whether the ice sheet contribution could accelerate substantially (e.g., by an order of magnitude) either in this century or subsequently. Sea levels were indeed much higher in the distant, warmer past but the timing of earlier sea level rise is very uncertain. From the point of view of societal and ecosystem adaptation, the timescale over which ice sheets might disintegrate, which may be on the order of centuries or millennia according to the two extremes posited in the literature, is crucial.

The complexity of bridging the gap between past and future trends is familiar to the climate community, which has dealt with the same issues with regard to global mean temperature. Ice sheets aside, continuation of past warming trends based on the roughly 100-year temperature record (0.05-0.1ºC/decade) would pose a significant but manageable problem for most countries. Projected future warming (0.15-0.55ºC/decade) based on increasingly reliable general circulation models, poses much more serious, even unmanageable challenges. But the state of ice sheet modeling is far different from the state of atmosphere-ocean modeling, as underscored by the recent observations. At this juncture, numerical modeling simply does not provide a credible basis for quantitative projection of ice sheet behavior in a warmer world.

The limitations of ice sheet models were revealed starkly by the collapse of the northern sections of the Larson B ice shelf in 1998 and 2002. Glaciers bounded by the landward edge of the ice shelf accelerated toward the sea while glaciers bounded by the more southerly section of the ice shelf, which remained intact, didn’t. Apparently, backpressure on glaciers from the abutting ice shelf provides a significant portion of the restraining forces keeping land-based ice in place, at least in some instances. The recent behavior of glaciers farther south in West Antarctica, and in Greenland, points to a similar dynamical response to ice-shelf fragmentation.

Many glaciologists regarded these observations as a clear test of the ability of ice sheet models to forecast dynamical changes in a warming ice sheet, a test the models failed. The long-standing inability of ice sheet models to reproduce the ice streams of West Antarctica, unexplained dynamical contributions to the mass balance of the Greenland ice sheet during the late 1990s and its apparent basal response in one location to surface melt-water reinforced this skepticism. The problem is threefold: the physics in the models is incomplete, the numerical problems are very difficult particularly in the neighborhood of the grounding lines where the land-based ice begins to float (Vieli and Payne, 2005), and observations remain sparse. It may take more than a decade, perhaps much longer, to bridge the gap in the model world because human and financial resources dedicated to the ice sheet problem are woefully inadequate (see Kintisch, 2006: Science 312, 1296, for a discussion of problems with the planned National Polar-Orbiting Operational Environmental Satellite System (NPOESS), a proposed platform for crucial future ice-sheet observations).

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