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).
We might also look to our experience with GCMs for some guidance as to how to evaluate the situation while we await an improved basis for numerical projection. GCMs are anchored in a broader range of observations than are ice sheet models at both the process and synoptic levels. But what would we do today if GCMs had failed several critical tests? With the climate already changing, we would likely not throw up our hands and say “let’s come back and reassess the situation once our numerical tools have improved”. More likely, we would increase our attention to paleoclimate analogs, a standard test bed for, and complement to, numerical modeling.
With the ice sheets already changing, the importance of analogs has come sharply into focus. Inferences from Eemian climate and sea level yield a wide range of estimates with regard to the climate changes that might result in widespread deglaciation of either or both ice sheets. Polar warming less than 5ºC (Overpeck et al, 2006) and global mean warming of 1ºC (Hansen 2004) or 2ºC (Oppenheimer and Alley, 2005) above recent temperature have been cited. Such global mean warming would almost certainly occur during this century, given forcing within the range of IPCC scenarios. In the southern hemisphere, the requisite circumpolar warming may occur more gradually, but a northern hemisphere trigger for WAIS deglaciation during the Eemian has also been proposed (Overpeck et al 2006). The information on potential rates of deglaciation is extremely sparse and its relevance to the future unclear. Potential rates of sea level rise equivalent to 1m/century (10mm/yr) have been suggested based on paleoclimate analogs (Overpeck et al, 2006) and by comparison to current ice discharge from West Antarctica (Oppenheimer 1998).
Filing the gap in knowledge between the risk (a significant probability of many meters of sea level rise) and the current reality (rapid local ice responses to local warming but small aggregate effect on sea level rise so far) will require a sharpened focus on all three fronts: observations, modeling, and paleoclimate assessment. Currently, the resources to do any one of these at the appropriate level are lacking. And because greenhouse gas concentrations and ice sheet loss are effectively irreversible, policy decisions need to be made based on the information in hand, which argues that deglaciation could be triggered by a modest warming.
E.Kintisch, Science 312, 1296-1297, 2006.
J.Hansen, Climatic Change, 68, 269-279, 2005.
M. Oppenheimer and R.B. Alley, Climatic Change, 68, 257-267, 2005.
M. Oppenheimer, Nature, 393, 325-332, 1998.
J.Overpeck et al, Science 311, 1747-1750, 2006.
A.Vieli and A.J.Payne, J. Geophys. Research, 110, F01003, doi:10.1029/2004JF000202, 2005.